RPD Manual 2013-2014 Final

Removable Partial Dentures

Laboratory Manual &

General Course Information

Sandra L. McCarthy, D.D.S. Associate Professor

Rosemarie Zartman, D.D.S., M.S. Associate Professor

Thomas W. McKinney, D.D.S. Associate Professor, Retired

Texas A&M University Baylor College of Dentistry

Department of Restorative Sciences Division of Removable Prosthodontics

Dallas, Texas 2014

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CONTENTS CHAPTER 1: What Is a Removable Partial

Denture and When Is It Indicated? 1-1 CHAPTER 2: Removable Partial Denture

diagnosis and Treatment Planning 2-1 Steps in the treatment of an RPD patient 2-1

CHAPTER 3: RPD Classification 3-1

Applegates Rules for applying the Kennedy Classification 3-2

CHAPTER 4: Selection of abutment teeth 4-1 CHAPTER 5: Surveying of Abutment Teeth 5-1

Technique of Surveying 5-1 Angle of Cervical Convergence 5-2

CHAPTER 6: RPD Component Parts 6-1

Major Connectors, Direct Retainers, Bracing and Stabilizing Components, and Rests 6-1 Minor Connectors, Indirect Retainers, Denture Base, and Tissue Stop 6-2

CHAPTER 7: Component Parts: Rests 7-1

Occlusal Rests 7-1 Incisal Rests 7-2 Cingulum Rests 7-3

CHAPTER 8: Component Parts: Bracing,

Stabilizing, and Reciprocal Components 8-1 Guide Planes 8-1 Minor Connectors 8-2 Bracing Arms 8-2 Reciprocal Components 8-3

CHAPTER 9: Component Parts: Direct

Retainers 9-1 Clasp Assembly 9-1 Clasp Materials 9-1 Direct Retainer Arm Shape 9-2 Direct Retainer Position on tooth 9-3 Basic Circumferential Clasp Assembly 9-3 Combination Clasp Assembly 9-5 Multiple Clasp Assembly 9-6 Hairpin Clasp Assembly 9-6 Half and Half Clasp Assembly 9-6 Embrasure Clasp Assembly 9-7 Ring Clasp Assembly 9-8 Back Action Clasp Assembly 9-8

CHAPTER 10: Component Parts: Connectors 10-1

Maxillary Major Connectors 10-1 Finish Lines 10-1 Anterior-Posterior Strap (Bar) 10-1 Palatal Strap 10-2 U Shaped 10-2 Palatal Plate 10-2 Palatal Linguoplate 10-3 Full Palatal Connector 10-3

Mandibular Major Connectors 10-3 Lingual Bar 10-3 Lingual Plate 10-4 Interrupted Lingual Plate 10-4 Double Kennedy Bar 10-4 CHAPTER 11: The Surveyed Crown As An

RPD Abutment Sequence of procedures 11-1 Drawings of Surveyed Crown Wax-up 11-3 Photos of Surveyed Crown Wax-up 11-4 Surveyed Crown, Immediate denture, and RPD project 11-6 Grinding posterior teeth 11-11 Windowing the maxillary record base 11-11 Waxing around RPD denture teeth 11-12 Preparation and Wax-up Sequence of surveyed crown 11-12 Immediate denture surgical guide 11-12 - 11-14

CHAPTER 12: Fabrication of The RPD Metal

Framework 12-1 Master Cast 12-1 Block-out of Undesirable Undercuts 12-1 Placement of Relief Wax 12-2 Placement of Wax Pattern Component Parts 12-2 Sprueing of The Wax Pattern 12-3 Rough Casting 12-4 Fitting the Framework on the master cast 12-4

Work authorization to the dental laboratory 12-5

CHAPTER 13: RPD Design & Project

Information 13-1 Pattern of Partially Edentulousness For RPD Design Exercises 13-2

Treatment Planning Forms 13-3 13-10 Cases With Clinical Information 13-11 - 13-16 Lecture Schedule Laboratory Schedule Project Work Cards

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Figure 1-1

Figure 1-2

Chapter 1

What is a Removable Partial Denture and When is it Indicated

Prosthodontic treatment for partially edentulous patients can be provided in many forms: (1) fixed partial dentures, (2) removable partial dentures, (3) dental implants, or (4) a combination of any or all of these. When the dentist preforms a differential diagnosis for a partially edentulous patient, there usually will be one best treatment plan; however, the patient may present conditions which will require the dentist to formulate alternate treatment plans. These conditions may include: (1) medical contra indications for certain treatments (e.g. implants), (2) esthetics (cases of traumatic loss of anterior teeth and soft and hard tissues where a fixed partial denture would be unaesthetic), (3) cost, and (4) patient prejudice against certain treatment forms (can include any routine dental procedure).

The success of any dental treatment is dependent upon

that treatment being correctly performed for a patient who is a proper candidate for the procedure. Many times when treatment fails, it is because of improper diagnosis, not the mechanical or technical quality of the treatment. An example of this is the placement of a fixed partial denture on abutment teeth with inadequate periodontal support. This FPD may be beautifully designed, prepared, fitted, and cemented; however, it fails because of improper diagnosis of the inability of the periodontium to withstand the stresses of the restoration. A removable partial denture could possibly have been a successful restoration because of the ability of the framework to provide cross-arch bracing and stabilization of abutment teeth. In many cases, abutment teeth with less than adequate periodontal support for a FPD may be acceptable abutments for an RPD.

The underlying philosophy that must permeate

the diagnosis and planning for dental treatment is one where decision-making is based on facts, not on prejudice. One kind of treatment is not inherently better than another type (e.g. a FPD is not inherently better than an RPD, or an implant is not inherently better than a FPD).

So if the differential diagnosis determines that a remov-

able partial denture is the indicated treatment, just what is a removable partial denture? A removable partial denture is a dental prosthesis that restores one or more but not all of the natural teeth and/or associated parts and is supported by the teeth and/or mucosa and can be removed from the mouth and replaced at will (Fig. 1-1). It is often referred to as an RPD. An RPD is composed of many parts (components) and must be bio-engineered to be non-abusive to both the remaining teeth and soft tissues. Proper selection of the various component parts will allow the dentist to place an RPD in the mouth of the partially edentulous patient so that it is supported, retentive, esthetic, functional, and comfortable.

The type of RPD that will be discussed in this manual has a cast metal framework that provides support and retention for the missing teeth and soft tissues (Fig. 1-2).

There is a basic difference between a fixed par- t i a l d e n t u r e a n d a r e m ov a b l e p a r t i a l d e n t u r e :

1) a fixed partial denture replaces a missing tooth/teeth by connecting it/them to retainers (crowns) that are cemented to abutment teeth. This type of tooth replacement is not removable by the patient.

2) a removable partial denture replaces a tooth/teeth by attaching it/them to a metal framework that is supported

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and retained by abutment teeth (and in certain situations supported by soft tissues also) and is removable by the patient.

An RPD is composed of two basic parts: the metal framework and the base(s) which attache(s) the prosthetic teeth to the framework. The typical metal framework consists of the following component parts:

1. Rests 2. Direct retainers 3. Indirect retainers 4. Bracing, stabilizing, & reciprocal components 5. Minor connectors 6. Major connector 7. Tissue stop (distal extension cases) 8. Retentive cribbing or mesh

When an RPD utilizing a metal framework is fitted to

the patients remaining teeth, it may be completely tooth supported or it may be supported by both teeth and soft tissues. Fig. 1-3 shows a maxillary RPD that is supported by abutments that are both mesial and distal to the edentulous spaces; therefore, it is an all tooth supported RPD. Figure 1-4 shows an RPD that is tooth supported on the patients right side and tooth/soft tissue supported on the patients left side. The tooth/soft tissue supported side is referred to as a distal extension base (there is no distal abutment for support). This situation means that the soft tissues of the ridge provide the support for the base as it extends to the distal. The amount of good support depends on the quality of the ridge tissues (firm vs. soft and spongy). If both posterior areas are tooth/soft tissue supported, the RPD is referred to as a bilateral distal extension RPD. The importance of distinguishing between tooth supported and tooth/soft tissue supported is that the absence of a distal abutment tooth (support tooth) allows a fulcrum line to be established around which rotation of the RPD can occur.

Figure 1-5 shows a unilateral (1 distal extension base) RPD and the axis of rotation (fulcrum line) around which rotation can occur. Since the posterior teeth are not supported by an abutment tooth at the distal extent of the base, occlusal forces (arrow A. in photo) would tend to cause the base to move in a gingival direction. If the base moves in a gingival direction under occlusal forces (arrow A), the parts of the RPD that are on the opposite side of the fulcrum line (arrow B) will move away from the teeth (visualize a seesaw with one end going up and the other end going down). It is this rotation under strong forces (primarily occlusal) that can produce detrimental torquing forces against the abutment teeth as well as the soft tissues.

One of the fundamental responsibilities of the dentist is to understand the principles of three dimensional movement and torque control in the distal extension RPD and to be able to design an RPD that properly incorporates these principles. Methods of controlling undesirable forces present in the distal extension RPD will be discussed more in detail later in the manual.

Figure 1-4

Figure 1-3 Figure 1-5

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Chapter 2

Removable Partial Denture Diagnosis and Treatment Planning

The work of many people over a considerable period of time has resulted in a body of knowledge relating to the diagnosis and treatment planning for removable partial dentures. There has also arisen a mind-set in some practitioners which states that removable partial denture treatment is second-class dentistry. Therefore, it is incumbent upon the student of restorative dentistry to not let dental procedure prejudice become a part of treatment planning. When a patient presents with conditions that contraindicate the placement of a fixed partial denture, the removable partial denture is often the treatment of choice. This is particularly true if dental implants are not a viable option for the patient. Even though patients present with dental conditions which indicate placement of dental implants, many patients cannot afford them or they may not be medically appropriate candidates for these implants. The dentist must always study the facts of the case before making any decisions about treatment or before any discussion with the patient concerning treatment choices.

The primary purpose of this manual is to present a

simple and straight forward approach to the diagnosing and treatment planning for removable partial dentures. It is hoped that the student will be able to use this approach to diagnose and treatment plan for most simple cases. The logic of creating a treatment plan and then implementing it in a sequential manner is that this approach will result in the vast majority of cases being successfully completed, even by the novice practitioner. The single biggest factor in unsuccessful RPD treatment frequently is the dentists failure to properly obtain all of the necessary information (clinical and radiographic exams, accurately made, surveyed, and mounted study casts, evaluation patients expectations for esthetics, comfort and function, etc. ) prior to begin- ning treatment. Even experienced practitioners must guard against the impulse to look in the mouth, see teeth missing, and start to work making a removable partial denture.

The information in this manual is presented in a manner

to try to help the student learn to logically and sequentially diagnose and treatment plan for patients requiring a removable partial denture. The obvious starting point in this procedure is to visit with the patient to obtain information about his/her past dental experiences, current dental prob- lems, expectations of treatment, dental fears and concerns, economic concerns, etc. It occasionally will become obvious that the patient is placing expectations on you that you will not be able to achieve. This patient will be a problem if treatment is begun. Treatment should not be started if there is not a reasonable expectation of success: neither the patient nor the dentist will benefit under these circumstances.

In the text, An Atlas of Removable Partial Denture

Design, Stratton and Wiebelt present the steps in the treatment of an RPD patient. These steps allow the dentist to proceed from diagnosis and treatment planning to successful delivery and follow-up care for the patient. The basic steps are listed below.

Steps in the treatment of an RPD patient.

1. Collection of information 2. Diagnosis and treatment planning 3. Mouth preparations: phase I 4. Formulation of the final RPD design 5. Mouth preparations: phase II 6. Fabrication of the RPD 7. Recall and maintenance

If the student will follow this sequential approach to

RPD treatment, the vast majority of cases can be treated successfully. Failure to adhere to this basic approach will introduce more and more chances for errors to occur during treatment. These errors frequently will be compounded, which may result in the treatment having to be re-done.

When the diagnosis and treatment planning are begun for a partially edentulous patient, frequently the first question that the dentist has to answer is: is a fixed partial denture indicated or is a removable partial denture indicated? It is generally accepted that we must apply Antes rule to the abutment teeth remaining before deciding if a fixed partial denture is indicated. Antes rule states that for a fixed partial denture to be properly supported, there should be an amount of periodontal ligament surrounding the abutment teeth equal to or greater than the amount that surrounded the teeth to be replaced. If this rule cannot be applied to the patients situation, then a fixed partial denture is contraindicated. If the dentist elects to ignore Antes rule and places a fixed partial denture anyway, one or both abutment teeth may be lost because of the excessive stresses from the restoration.

A prime example of this situation is the placement of a 2xx5 FPD on abutments with a moderate amount of bone loss. The amount of periodontal support provided by teeth #2 & #5 is inadequate to resist forces from the opposing teeth. Either or both of the abutments may be lost because the dentist misdiagnosed what type of replacement was indicated. If an RPD had been placed instead of the FPD, the metal framework would have spread stresses to more abutment teeth. The abutment teeth on the opposite side of the arch would have provided lateral bracing and stabilization so that the lateral forces are resisted by teeth other than

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#2 and #5. The cross-arch bracing provided by the RPD metal framework is one of the reasons that the RPD is the indicated method of tooth replacement in this type of case. If tooth #31 in Figure 2-2 represents a somewhat periodontally compromised abutment tooth (has bone loss), then placing the RPD shown in Figure 2-1 would add stability to the tooth by means of the rigid metal framework. If the dentist made

the choice of placing an FPD from #28 to #31, then no lateral bracing on the distal abutment would be provided. In many cases where this type of restoration is placed, the lifespan of the restoration is short and results in the patient losing the distal abutment tooth. This is an example of the necessity for thorough evaluation of the patient and consideration of all aspects of the tooth replacement process.


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Chapter 3

RPD Classification

When the decision has been made that a removable partial denture is indicated, plans have to be made for the design of the framework of the RPD. There are many factors for the dentist to consider when designing and fabricating a removable partial denture. One of the factors that helps to determine the design for the RPD framework is its classification. In 1923, Dr. Edward Kennedy developed a system of classifying partially edentulous arches into four (4) different groups based on the pattern of missing teeth. Since each of

these groups has unique characteristics, knowing these differences will aid the dentist in developing his/her design for the RPD. Below is the list of the classifications as well as a photograph of an example of each of the classifications. On the following page is the list of Dr. Oliver C. Applegates eight (8) rules that he developed to help dentists apply the Kennedy classifications to individual cases.

Class I: Bilateral edentulous areas located posterior to the remaining natural teeth.

Class II: A unilateral edentulous area located posterior to the remaining natural teeth.


Class III: A unilateral edentulous area with natural teeth remaining both anterior and posterior to it.

Class IV: A single, but bilateral (crossing the midline), edentulous area located anterior to the remaining natural teeth.


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Applegates Rules for applying the Kennedy Classification:

Rule 1. Classification should follow rather than precede any extractions of teeth that might alter the original classification.

Rule 2. If a third molar is missing and not to be replaced, it is not considered in

the classification.

Rule 3. If a third molar is present and is to be used as an abutment, it is considered in the classification.

Rule 4. If a second molar is missing and is not to be replaced, it is not considered

in the classification: (for example, if the opposing second molar is likewise missing and is not to be replaced).

Rule 5. The most posterior edentulous area (or areas) always determines the

classification.

Rule 6. Edentulous areas other than those determining the classification are

referred to as modifications and are designated by their number.

Rule 7. The extent (or size) of the modification is not considered, only the numberof additional edentulous areas.

Rule 8. There can be no modification areas in Class IV arches: (another edentulous

area lying posterior to the single bilateral area crossing the midline would instead determine the classification).

Note: See examples of various classifications and modification spaces on pages in the latter part of the manual.

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Chapter 4

Selection of Abutment Teeth

Selection of abutment teeth:

Antes rule must be applied when making clinical decisions regarding teeth to be used as abutments. Because of the loss of periodontal support, many teeth can have a crown to root ratio that is far less than ideal. In some cases, there can be an actual reversal from the ideal ratio. If these teeth are used as abutments for a fixed partial denture, leverage placed on them may result in soreness, mobility, or even eventual loss of the teeth. Therefore, it is imperative that you be conservative in selecting abutment teeth onto which you plan to place extra stresses.

Abutment teeth used for removable partial dentures

are also placed under extra stress; however, teeth with some degree of bone loss may be viable abutments for a removable partial denture when cross-arch bracing of a major connector helps to provide stability by spreading the stresses over many teeth as in Class III cases as shown in Figure 4-1. In this case, the rigidity of the major connector around the arch braces teeth on both sides of the arch so that when the restoration is seated, no abutment tooth can move independently. It is possible to successfully treat this type of partial edentulousness with an RPD restoration even when there is slight clinical mobility of abutment teeth.

Beware of using small-rooted premolars as terminal

abutments for a distal extension (no abutment tooth posterior to the edentulous space) RPD. The small amount of periodontal support can easily be overcome by the additional torquing stresses from the RPD. In cases where the root support of the premolars is basically intact, they may be able to withstand the extra stresses of the RPD without clinical consequences. The case shown in Figure 4-2 is an example of a first premolar being the terminal (last) abutment on the patients left side. Since this is a bilateral distal extension RPD, the bases have the ability to move in all directions when forces are applied to them. Because of the length of the extensions to the distal, considerable force can be applied to the abutment teeth. As will be discussed in the chapter on direct retainers, it is essential that torquing forces be minimized on abutment teeth.


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Chapter 5

Surveying of Abutment Teeth

The process of determining where the maximum height of contour (bulge) is located on an abutment tooth is called surveying. A line is scribed onto the tooth using a dental surveyor (Figure 5-1). This instrument has a vertical spindle that holds various tips. When a carbon rod (piece of pencil lead) is placed in the spindle and its side is moved across the abutment tooth surface on the study cast, a line marking the maximum height of contour is made (Figure 5-2). This line is called the survey line. It is critical that the side, not the end of the carbon rod touch the tooth. Figure 5-2 shows lines from both the side (grey rod) and the end (dashed outline) of the carbon rod. It is obvious that one of these lines does not represent the maximum bulge of the tooth surface. If you mark the tooth with the end of the carbon rod, you will erroneously be lead to believe that the area below the line is in undercut and can provide retention for a direct retainer. That is not the case!

When a tooth is properly surveyed, the area of the tooth

above the line is called the supra bulge and the area below the line is called the infrabulge (Figure 5-3). All areas of the tooth below the survey line are in undercut. The amount of undercut varies from one location to another on the tooth and will change if the angulation of the tooth is changed and re-surveyed.

Technique of surveying:

The general procedure for surveying a cast is to place it in the cast holder, securing it tightly. The plane of occlusion of the study cast is aligned so that the vertical rod (spindle) is perpendicular to it. The analyzing rod can be placed in the spindle and used to evaluate the relative position of various parts of the cast (teeth surfaces, soft tissue undercuts, etc.).

When the cast is determined to be in the desired position, the analyzing rod is replaced with the carbon marker (backed by the metal support). The cast is moved across the platform of the surveyor so that the carbon rod marks the maximum convexity of the axial surfaces of the teeth. It is important to have the side of the marker, not the end, in contact with the tooth surface as has been discussed previously (Figure 5-2).

Figure 5-1

5-2

To make it possible to return the cast to the same exact position in the surveyor, a procedure referred to as tripod- ding is done. This involves placing the carbon rod in the surveyor spindle and locking its vertical position such that three widely separated horizontal marks can be placed on the cast. These marks define a plane which can be re-established by returning the cast to the surveyor and aligning the marks using the analyzing rod locked in the spindle.

After surveying has been accomplished, the dentist now

knows where undercuts exist on the abutment teeth. How- ever, he/she does not yet know the amount of the undercut. The amount of undercut is measured by placing the undercut gauge of the desired amount (0.01, 0.02 or 0.03) in the vertical spindle of the surveyor. The vertical shaft of the gauge is placed against the surface of the tooth (it will touch at the maximum convexity, which is the survey line) and the head of the gauge on the surface of the tooth where the amount of undercut is to be determined (yellow arrow in Figure 5-4). Typically the amount of undercut will be 0.01 for cast chrome cobalt alloys and 0.02 for non-precious wrought wire direct retainers. If gold wrought wire is used, because of its increased flexibility, more undercut may be needed.

In the typical circumferential cast Chrome Cobalt alloy

direct retainer, the tip of the direct retainer is placed on the abutment tooth so that the survey line divides the retainer arm into 2/3 above and 1/3 is below the line (Figure 5-5).

Figure 5-5 The Angle of Cervical Convergence:

When it is determined that the required amount of undercut exists, the dentist must determine how rapidly the direct retainer arm will flex into its terminal position (into the undercut). Since teeth have different axial contours, each one will have a different curvature relative to the vertical rod of the surveyor (either the analyzing rod or the undercut gauge shaft).

The angle of cervical convergence is defined as the angle formed between the tooth and the vertical rod of the surveyor which touches the tooth at its maximum convexity. The light viewed between the surface of the tooth and the vertical rod of the surveyor allows the dentist to see the relative size of this angle. This space is indicated by the yellow arrow shown in Figure 5-6.

Figure 5-4

Figure 5-6

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Figure 5-7 shows two different teeth superimposed over each other. Each has a different angle of cervical convergence, yet the same undercut gauge is being used to measure the same amount of undercut on both teeth. If this is the 0.01 undercut gauge, then where the head of the gauge touches each of the teeth is 0.01 of undercut.

The importance of the angle of cervical convergence

to the clinician is that the effectiveness of the direct retainer placed into the undercut is influenced by the vertical distance the retainer arm must travel on its way into or out of the undercut. As Figure 5-7 shows, the tooth with the smallest angle of cervical convergence (B) requires the retainer arm to travel over twice the vertical distance to its terminal position as on the tooth with the larger angle (A).

The two retainer arms are required to flex the same

amount laterally during the seating or unseating process. The effectiveness of the direct retainer is directly related to its resistance to flexing laterally as vertical movement (unseating) of the RPD is attempted (sticky food, patient attempt to remove, muscles acting on the borders, etc.).

When the angle of cervical convergence is small, the re-

tainer arm can move vertically a significant amount without flexing laterally very much at all. It is this ability to move vertically while the direct retainer is still in the correct amount of undercut that may create a clinical problem of chronic vertical movement during function, especially from sticky food.

Figure 5-7 also shows that on the tooth with the larger

angle of cervical convergence (A), the direct retainer arm has to enter the undercut very rapidly: it almost snaps into place. Since there is no gradual flexing of the retainer arm during its seating, its removal may be very difficult because it has to flex even more suddenly over the maximum convexity of the tooth. The clinician should find an average between these extremes to insure that the direct retainers are effective, yet are easily removed when desired, especially by the patient.

Figure 5-7

Remember this important fact: retention of the RPD onto the abutment teeth is because of the direct retainer arms being forced to flex into the undercut on the teeth during the seating of the restoration. Friction is not the retentive force that holds the RPD onto the teeth. Once the RPD is seated, the direct retainers should rest passively against the teeth.

In cases where abutments do not have sufficient undercut for retention of the RPD, restorations called surveyed crowns can be placed on these abutments. In Chapter 11, the information for surveyed crowns is presented as well as the laboratory exercise in which a surveyed crown is indicated as part of the treatment plan (Projects #10 and 11- I/RPD). The surveyed crown itself is Project #15.

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Chapter 6

RPD Component Parts: Overview

A removable partial denture is made up of many component parts, each of which serves a specific purpose. Each of the components has to be applied properly in the design of

1. Major connector: the unit of the partial denture that connects the parts of the prosthesis located on one side of the arch with those on the opposite side.

2. Direct retainer: a clasp or attachment applied to an

abutment tooth to retain a removable partial denture in position.

an RPD for it to function in a biomechanical sound manner. Below and on the following page are the component parts and their required function. 3. Bracing and stabilizing components: those components

that prevent the RPD from moving in the horizontal plane under masticatory forces.

4. Rests: the unit of a partial denture that rests on a tooth

surface to provide vertical support. These rests are placed on natural teeth or cast restorations in areas called rest seats.

Figure 6-1

6-2

RPD Component Parts

5. Minor connector: arises from the major connector and connects other parts of the RPD to the major connector.

6. Indirect retainer: a part of a removable partial

denture that assists the direct retainers in preventing displacement of distal extension denture bases by functioning through lever action on the opposite side of the fulcrum line. Indirect retainers are always rests placed in prepared rest seats.

7. Denture base: the part of the RPD that rests on the supporting soft tissues and attaches the teeth to the framework of the RPD.

8. Tissue stop: supports the retentive cribwork during

packing of the acrylic resin. This is its only function.

Figure 6-2

7-1

Chapter 7

Component Part: Rests

A rest is the component part of an RPD that rests on a tooth surface to provide vertical support. These rests are placed on natural teeth or cast restorations in areas called rest seats. The primary purpose of rests is to prevent the RPD from moving in a gingival direction when forces are applied to it. The rest seats are prepared in natural teeth by using diamond instruments in a high speed hand piece. After their proper shape is accomplished, the enamel is smoothed and polished. When rest seats are placed in castings, they are prepared in the wax pattern before the casting is made.

A rest is one of the parts of a clasp assembly shown in

Figure 7-1. This type of clasp assembly is called a circumferential clasp assembly because it encompasses more than 180 degrees of the tooth. There are other designs of clasp assemblies also which will be discussed in the chapter on direct retainers.

Rests areas on abutment teeth can be located in three

basic areas: (1) occlusal surfaces, (2) incisal edges, or (3) cingulum areas. The anatomy of each of these areas will define the shape of the rest preparation.

Occlusal rest: The occlusal rest is typically placed on

either the mesial or the distal of the abutment tooth. It is placed adjacent to the edentulous areas unless the RPI clasp assembly is being used. In RPI cases, the rest is always placed on the mesial of the tooth if it is the terminal abutment for a distal extension RPD. The occlusal rest is spoon shaped and occupies approximately 1/2 the width between the cusp tips of the tooth in a bucco-lingual direction. Every aspect of the preparation is rounded and blended onto the occlusal surface of the tooth. There are no line angles present in a correctly prepared occlulsal rest! Therefore, it is not possible to determine the exact outline of a properly prepared occlusal rest.

In Figure 7-2, the shaded area (indicated by the arrow)

represents the deepest part of the rest preparation. Because this area is deeper than the marginal ridge it prevents the rest from moving out of the preparation when occlusal forces are applied to the RPD. The occlusal forces are directed along the long axis of the tooth when no lateral movement is al- lowed. Sound abutment teeth can withstand significant vertical forces; however, slight lateral forces chronically applied to teeth can result in tooth mobility. Most patients will be comfortable during this procedure if a gentle touch is used.

It is considered unwise to place occlusal rests in the

box area of class II amalgam restorations. Cutting into the amalgam restoration will significantly weaken it and increase the probability of its fracture under the rest. The

illustration in Figure 7-3 shows the most common area of fracture. Replacing the restoration after the RPD has already been fabricated is extremely difficult and unpredictable. It is highly recommended to place occlusal rests only in natural tooth enamel or in preparations in castings.

Figure 7-1

Figure 7-2

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When the incisal rest is placed on either the mesial or the distal of the tooth, the floor of the preparation should slant toward the middle of the tooth (Fig. 7-6) and toward the facial (Fig. 7-7). The junction of the lingual surface and the floor should be rounded so that no line angle remains in this area as is shown in Figure 7-8. If a sharp line angle is left in this area, fitting the framework to such a sharply defined edge becomes difficult. A point to remember is that the RPD framework is usually cast of a non-precious metal which usually does not fit as accurately as a gold casting would.

Figure 7-3

Incisal rest:

The incisal rest can be placed on either the mesial or the distal or even in the center of a tooth if it has lost its tip due to abrasion. It is common in older patients for anterior teeth to be flattened on the incisal edges or cusp tips (Figure 7-4). If the rest is planned for placement on a canine with a flattened-off cusp tip, the preparation is commonly placed in the center of the tooth. It should be 1.5 mm deep and 2.5 mm wide and should be rounded as is shown in Figure 7-5. The floor of the preparation should be slanted slightly toward the facial of the tooth so that the rest will remain seated during function. Any incisal-lingual line angle should be rounded slightly.

Figure 7-5

Figure 7-4

Figure 7-6

7-3

prepared. Most mandibular canines do not have cingula of sufficient size to allow cutting a sufficiently deep ledge into the tooth. The absolutely minimum facial-lingual measure- ment for the ledge is 1.0 mm. Less than this can result in the rest being unstable and likely to slide off of the tooth during application of occlusal forces to the RPD.

In cases where the mandibular canines are the terminal

abutments (most distal tooth) for a Class I RPD, it is essential that the ledges be of sufficient faciolingual dimension to assure that the metal framework will not slide off of the teeth. When vertical stability is not provided, the framework can move gingivally and result in excessive and detrimental forces being applied to the soft tissues. When rests are cut into teeth without adequate cingula, the enamel will usually be penetrated resulting in the possibility of sensitivity and/or caries. If esthetics will allow, incisal rests can be used in these cases. However, placement of incisal rests increases the leverage on the abutment teeth and must be taken into consideration when designing the RPD.

Figure 7-7

If no cingulum exists on a mandibular canine, and you cannot place an incisal rest because of esthetic concerns, a surveyed crown can be placed on the abutment tooth. In the survey crown, a rest preparation with an ideal shape, location, and size can be created.

Figure 7-8

Cingulum rest:

The cingulum rest is located in the bulged area of the cingulum so that a ledge (Figure 7-9) can be created to support the framework. Most maxillary canines (and cen- tral incisors) have adequate cingula into which rests can be

Figure 7-9

8-1

Chapter 8

Component Parts: Bracing, Stabilizing, and Reciprocal Components

Bracing, stabilizing, and reciprocal components are the parts of the framework that tend to prevent movement in the horizontal plane. Framework parts that work in this manner are: (1) guide planes, (2) minor connectors, (3) lingual coverage, and (4) bracing arms. These parts of the metal framework are against vertical surfaces of teeth which results in their preventing movement in the horizontal plane.

Guide planes:

Guide planes are prepared proximal surfaces along

which minor connectors slide during seating or unseating of the RPD (Figure 8-1).

Figure 8-2

Figure 8-3

Figure 8-1

The ideal is to prepare these surfaces parallel with each other so that they guide the RPD to place. The more guide planes that are present, the more definite the path of seating and unseating of the RPD will be. The well prepared Class III RPD (all tooth supported) (Fig. 1-2 ) will have only one path of insertion and removal. This very definite direction of seating or unseating makes all other components work more predictably and is one of the reasons for making these tooth alterations. Figure 8-2 shows the abutment teeth prior to being prepared for guide planes. Figure 8-3 shows the abutment teeth after preparation for guide planes. Other than increasing the definiteness of the path of insertion and removal, another benefit is that guide plane preparation eliminates some of the undercut on the proximal surfaces of the abutment teeth. This space cannot be filled by the acrylic resin or metal of the RPD since it is in undercut. Elimina- tion of this space helps reduce the space for food impaction between the teeth and the RPD. Also, in cases where anterior teeth are being replaced, esthetics are improved by elimina- tion of these proximal spaces.

Guide planes should be placed adjacent to the edentulous areas in both tooth supported as well as distal extension cases. The guide plane is prepared starting at the marginal ridge and extending gingivally approximately 2.5 - 3.0 mm. Guide planes do not involve the entire proximal surface of the abutment tooth. They are flat vertically, but curved bucco-lingually (Fig. 8-4).

Figure 8-4

8-2

Minor Connectors are the parts of the framework that arise from the major connector and connect other parts of the RPD to the major connector.

Minor connectors will have some contact with the axial

surfaces of abutment teeth and thus provide resistance to movement in the horizontal plane. Minor connectors should be placed so they are as inconspicuous as possible to the tongue (Fig. 8-5). Generally, they should: (1) be placed in concavities (interproximal areas), not on convexities, (2) connect to the major connector at 90 for maximum strength, and (3) not be placed such that they will become wedges between the teeth as could occur in distal extension cases where vertical and horizontal movements of the bases are possible.

Bracing and stabilizing components include major connectors that have extension of the metal onto the lingual surfaces of teeth. The major connector in Fig. 8-7 shows lingual coverage of teeth #s 20-28. This much metal in contact with vertical surfaces of teeth provides significant resistance to lateral movements of the framework from strong forces.

Figure 8-7

Figure 8-5

Another example of minor connectors is the retentive elements for attachment of the acrylic resin to the RPD framework (Fig. 8-6).

Figure 8-6

Bracing arms are a part of clasp assemblies that contact axial surfaces and resist horizontal movement from masticatory forces. They cover much less area of the tooth than lingual coverage and are always placed in areas that are not undercut. Fig. 8-8 shows three teeth with bracing arms and tooth #11 with lingual coverage. All of these areas of metal/tooth contact provide resistance to horizontal movement of the RPD.

Figure 8-8

8-3

Reciprocation is the provision of a lateral bracing action on the abutment tooth during the forced flexing of the direct retainer arm into or out of the undercut on the abutment tooth. Fig. 8-9 shows a surveyed crown with a direct retainer on the buccal side and a bracing/reciprocal arm on the lingual side. Since the direct retainer arm is forced to flex in the buccal direction during seating, there is a lateral force generated against the abutment tooth in the B. lingual direction (arrow A). Because the surveyed crown has been specially prepared on the lingual side, there is a A. long vertical surface along which the bracing arm can slide during seating and unseating. As long as this bracing arm is in contact with the crown before the direct retainer arm contacts the buccal surface, then reciprocation (bracing against movement) is provided for the tooth (arrow B).

Another benefit of this type of design for the RPD is that

when the RPD is completely seated, there is a more natural shape to the abutment tooth. Since this special preparation is on the lingual, the tongue is more likely to be comfortable when the restoration is in place. A.

A bracing arm prevents lateral (movement in the horizontal plane) after the RPD is completely seated. A bracing arm can effectively provide bracing action, but still not provide reciprocation. In other words, a reciprocal arm will be a bracing arm, but a bracing arm may not be a reciprocal B. arm.

In Fig. 8-10, the bracing arm (A) is located so high on the lingual of the abutment tooth after it is seated that it cannot provide reciprocation. It contacts the tooth only as the entire clasp assembly seats. Also, it will leave the surface of the tooth immediately when the direct retainer arm (B) starts to move out of the undercut. In this case, there can be no reciprocation since the bracing arm is not in contact with the surface of the tooth during seating or unseating.

In Fig. 8-11, the lingual surface of this premolar is

slanted so much to the buccal that creating a flat guiding surface along most of the lingual surface is impossible. The area where the vertical line touches the tooth could be flattened off, but it would not be wide enough vertically for the bracing arm to contact the tooth soon enough to provide reciprocation during the seating or unseating process.

As has been previously stated, for the lingual bracing

arm to provide reciprocation, it must be in contact with the lingual surface throughout the entire seating or unseating process. The tooth shown in Fig. 8-12 can be re-contoured on the lingual to provide for reciprocation since the surface will be long in an occluso-gingival direction. Frequently, when unmodified teeth are used as abutments, the design integrity of the RPD is compromised. Modification of healthy natural teeth is required for proper placement of a removable partial denture.

9-1

Chapter 9

Component Parts: Direct Retainers

Direct retainers are the parts of the RPD framework that are applied to abutment teeth to retain a removable partial denture in place. The direct retainer is one of the parts of a clasp assembly. The clasp assembly is composed of four parts: (1) minor connector, (2) rest, (3) bracing arm, and (4) direct retainer (Fig. 9-1).

Figure 9-1

There are many different types and shapes of direct retainers. Each of them will have a use in certain circumstances. There are also different metals from which direct retainers can be fabricated: (1) non-precious Chrome-Co- balt alloys, and (2) gold alloys. These metals can also be of different characteristics: (1) cast or (2) wrought. Prior to deciding what type of direct retainers to use, the dentist must consider the material as well as the shape of the direct retainer that he/she thinks is indicated for each of the abutment teeth. Certain unique situations will be discussed later in the manual.

There are two basic decisions for the dentist to make

when selecting which direct retainers to use for a particular case: (1) material and (2) shape. Each of these categories has multiple considerations that play a part in the overall decision making process and are discussed in the following material.

Materials:

Cast Chrome Cobalt alloy: this material is a very rigid, light weight, and inexpensive material and is generally the material used in most RPD frameworks today. When cast direct retainers are used, they are an integral part of the RPD framework and because of their shape and stiffness, they are placed into 0.01 of undercut on the abutment tooth.

Wrought wire (non-precious): this material is supplied to the technician as a piece of straight wire. It comes in various gauges (high gauge number means smaller diameter). The technician must bend the wire to fit the exact shape of the abutment tooth. It takes a skilled technician to properly fashion this type of direct retainer and correctly place it on the framework. Wrought wire is formed by forcing ingots of cast metal through hardened metal dies that get progressively smaller and smaller until the desired diameter is reached. This process elongates the grains of the metal causing them to overlap much like the individual fibers in a thread do. This overlapping of grains produces a material with increased flexibility and resistance to fracture failure. Since wrought wire has the benefit of being less likely of fracturing during adjustment, some dentists choose this material rather than a cast metal. This is an especially important consideration because direct retainers frequently are adjusted as part of the fabrication, delivery, and recall process. Multiple adjustments can work-harden any metal resulting in it fatiguing and fracturing. Wrought wire direct retainers are less prone to this type of failure than are cast direct retainers. The most common size of wire is 18 Gauge and is placed into 0.02 of undercut on the abutment tooth. On small abutment teeth where the length of the retainer is shorter, 19 Gauge wire may be used. These direct retainers are usually electro-soldered onto the finished and polished framework. Type IV gold: prior to the development of non-precious chrome cobalt alloys, gold was used for most RPD frameworks. This type of gold alloy is more flexible than comparable non-precious shapes and sizes. When an RPD framework is made of Type IV gold alloy, it is common for the direct retainers to be cast as an integral part of the whole framework. The direct retainers cast of Type IV gold alloys are more flexible that non-precious alloys and can be placed into slightly more undercut than cast non-precious direct retainers. Three distinct disadvantages are present when using gold for the entire RPD framework: (1) the cost, (2) the weight of the metal, and (3) the thickness required for sufficient rigidity of the framework. Even for patients where cost is no factor, the bulk and weight of the RPD may make it uncomfortable. Wrought wire (gold): when the dentist is making the RPD framework from gold, then any wrought wire

9-2

direct retainers usually will be made from gold wire. Because gold is more flexible than the same size of non-precious wrought wire, it can be used in a deeper undercut and sometimes is the choice even when the framework is made of chrome cobalt alloy. One advantage of using more flexible direct retainers is that they can be placed into more undercut and therefore will be closer to the gingival areas on abutment teeth which results is improved esthetics. There is also less tendency for torquing abutments when the forces are closer to the center of support in the periodontium.

Shape:

Circumferential: the direct retainer arm originates in the occlusal portion of the tooth and approaches the undercut (area below the survey line) from that direction (arrow in Fig. 9-2). The entire clasp arm is in contact with the surface of the tooth. The surface of the clasp arm in contact with the tooth is relatively flat, but the outer surface is convex. Generally, the cross-sectional shape is approximately 1/2 round.

Figure 9-3

Figure 9-4

Figure 9-2

Bar: this type of direct retainer arm approaches the undercut from the gingival direction (arrow in Fig. 9-3). Approximately the last 2 mm of the clasp arm are in contact with the surface of the tooth; the remainder of the arm is relieved away from the underlying soft tissues. The arrow in Fig. 9-4 shows an I bar direct retainer arm that is dangerously placed into soft tissue undercut. When this retainer is seated, it is likely to contact the gingival tissues and damage them. The arrow in Fig. 9-5 indicates where contact of the I bar direct retainer is occurring with the buccal frenum. This tissue will be severely abused by this direct retainer! These types of direct retainer clasp arms are contraindicated when underlying severe soft tissue undercuts exist or when the vestibule is of insufficient depth to accommodate the metal.

Figure 9-5

9-3

Figure 9-6

Cast circumferential direct retainers

There are many shapes of circumferential clasp assemblies. Each will have a direct retainer arm, a bracing arm, a rest, and a minor connector (Fig. 9-1). When considering where to place the bracing and the direct retainer arms, the abutment tooth is divided into three areas: the occlusal 1/3, the middle 1/3, and the gingival 1/3 (Fig. 9-6).

The direct retainer arm should terminate (end) below

the survey line so that 2/3 is above and 1/3 below the survey line (Fig. 9-7). This will place the flexible terminal 1/3 of the arm into the undercut of the abutment tooth. Since the metal of the direct retainer arm must be forced into the undercut, the framework is retained onto the abutment tooth until sufficient unseating force lifts it off of the tooth.

The bracing arm is placed on the opposite side of the

abutment tooth from the direct retainer. It is usually larger and more rigid than the direct retainer and is not placed into undercut, but is placed just above the survey line in the middle 1/3 of the tooth (Fig. 9-8). The function of a bracing component is to resist movement of the RPD in a horizontal direction once the RPD is seated. Some teeth, especially mandibular molars, must be recontoured on the lingual to lower the survey line so that the bracing arm can be placed in the proper location in the middle 1/3 of the tooth.

Fig. 9-9 shows a tooth before modification with the

survey line high on the tooth. Fig. 9-10 shows the tooth after enameloplasty of the tooth to lower the survey line. The bracing arm placement in Fig. 9-9 is too high, whereas the bracing arm in Fig. 9-10 is correctly placed in the middle 1/3 of the tooth. One of the benefits of modifying the tooth to lower the placement of the bracing arm is that it will be less bothersome to the tongue. If the bracing arm is placed on the maximum convexity (unmodified tooth) it is placed very close to the occlusal surface of the tooth where the lateral border of the tongue will rest against it. This placement advertises the presence of the clasp arm unnecessarily to the tongue and may result in the patient being unable to wear the restoration.

9-4

Factors which affect the flexibility of direct retainers: it is important to remember all of the factors to be considered when making the choice of what direct retainer to use.

1. Length of clasp arm (molar vs premolar) 2. Diameter of clasp arm (wrought wire) 3. Cross-sectional form (shape) of clasp arm (half-

round vs. round) 4. Material used for clasp arm (cast vs wrought

wire, CoCr vs gold)

that the direct retainer on the terminal abutment tooth ( # 20) is made of wrought wire to reduce the tendency for torquing from the distal extension bases movement.

Different types of circumferential clasp assemblies:

Basic Circumferential Clasp:

Figure 9-12

Figure 9-11

Indications for use:

1. Tooth supported cases (Fig. 9-12). The arrows indicate teeth which have circumferential clasp assemblies on them. Since this is an all tooth supported case, there is no movement around an axis which would produce torquing or abutment teeth.

2. When the direct retainer is on the distal abutment (Fig. 9-13) of the tooth-supported side of a Class II, Mod I RPD. The fulcrum line passes through the rest on this abutment and through the rest on the terminal abutment of the distal extension side. Since this direct retainer is on the tooth where the axis is located, there is less probability that torquing would occur on the tooth. Note

Figure 9-13

9-5

Combination clasp assembly:

This clasp assembly has the same configuration as the basic C clasp except that the cast direct retainer arm is replaced with a wrought wire direct retainer arm. Its name is derived from the fact that a combination of materials is used in the clasp assembly. Fig. 9-16 shows a wrought wire direct retainer arm that has been soldered onto the framework.

The use of wrought wire creates a direct retainer which

has different flexure characteristics than a cast direct retainer arm. The cast direct retainer is designed to be flexible (within clinical limits) in a bucco-lingual direction only (Fig. 9-14). Because the occluso-gingival dimension is considerably greater than the bucco-lingual dimension, the metal cannot flex much in the vertical direction.

abrasive surface to reduce the diameter toward the exposed end. Tapering the wire will result in increased flexibility because of reduced cross-sectional size and is the only way that the retainer actually does what its theory prescribes. Indications for use: 1. Terminal abutments for distal extension cases. The

rationale for its use is: by placing the rest on the distal of the tooth and the direct retainer tip toward the mesial, the wrought wire can flex vertically to prevent the abutment tooth from being torqued when the distal extension base moves under an occlusal load (Fig. 9- 16).

2. Abutments on the opposite side of the fulcrum line from a distal extension base. Use in this manner reduces the upward forces generated on the abutment tooth when the distal extension base moves under an occlusal load (Fig. 9-17).

3. Cases where esthetics will be improved by placing the direct retainer in deeper undercut (closer to the gingiva). Since wrought wire must be placed into 0.02 of undercut, the metal will be more gingivally placed and in some cases will be less visible, especially if the lip line is low during smiling (maxillary).

Figure 9-14

Wrought wire is round with equal occluso-gingival and bucco-lingual dimensions; therefore, it can flex in all directions equally (Fig. 9-15). Because of the increased flexibility of wrought wire, direct retainers made of it must be placed into greater undercut than the stiffer cast CoCr (Cobalt chromium alloy) direct retainers which function well when placed into 0.01 of undercut. Wrought wire direct retainers should be placed into 0.02 of undercut for effective retention. Failure to accurately measure undercuts will frequently result in the dentist making an RPD that will not stay seated during function.

Figure 9-16

Figure 9-15

One of the characteristics of a properly fabricated wrought wire direct retainer is that it should be tapered toward the terminal end (the part in undercut). This tapering is accomplished by placing a short piece of the straight wrought wire in a slow speed handpiece and running it against an

Figure 9-17

9-6

Multiple Clasp: (Figure 9-18)

This type of clasp assembly is basically two circumferential clasp assemblies with their bracing arms connected together. By having the direct retainer arms facing each other as in Fig. 8-18, the undercut on each tooth becomes cumulative and can be clinically retentive even when less than ideal undercut is present on either tooth.

around from the distal to engage undercut in the mesial. It is necessary to have the length from origin to termination so that the clasp arm is flexible enough to engage the undercut safely.

Indications for use: 1. When the undercut is proximal to the edentulous area

of an abutment tooth. A common example of this is the mandibular second molar that has tipped forward into the space of a missing first molar. In this situation, the second molar will usually lose its undercut on the distobuccal. It is difficult to use this direct retainer on a tooth that is short occluso-gingivally.

2. Tooth modification (such as lowering the survey line by enameloplasty or placement of a survey crown) to accommodate the inflexible origin portion of the clasp arm may have to be made. The rigid portion of a direct retainer arm must always be above the survey line.

Figure 9-18 Indications for use:

1. When additional retention is needed; usually on tooth-

supported cases. 2. When undercut areas are adjacent to each other.

Hairpin Clasp: (Figure 9-19)

This type of clasp assembly uses a direct retainer arm which normally originates in the mesial and loops

Figure 9-19

Half and Half clasp: (Figure 9-20)

This type of clasp assembly is a holdover from earlier times when unilateral (Nesbitt) RPDs were made. These did not involve the other side of the mouth and frequently were used to replace a single tooth. In todays medico-legal setting, the small unilateral RPD is totally contraindicated. Aspiration or swallowing of these small unilateral removable partial dentures has been a serious consequence of their use in the past.

Figure 9-20

9-7

Embrasure Clasp assembly: (Figure 9-21)

Figure 9-21

This type of clasp assembly is basically two circumferential clasp assemblies connected together. This assembly is used where no interproximal space is available and requires a great deal of space for proper placement without interfering with opposing teeth. It is very common for under-preparation of the teeth to be done by the dentist; therefore, insufficient space for the metal will frequently result in breakage of the clasp assembly. In many cases, enameloplasty of the opposing teeth will be required to have the required space. This is perhaps the most difficult clasp assembly to successfully place. As the occlusal view in Fig. 9-22 shows, there is a large mass of metal in the embrasure area. This is the area where reduction of the teeth must produce a trough all the way from the buccal aspect to the lingual aspect. Once this trough is created, the corners of the trough must be round for the bracing and the direct retainer arms to wrap around the teeth. Remember: extensive tooth reduction is required for this clasp assembly.

Accurately mounted study casts are a necessity when

planning and placing this type of clasp assembly. It is critical in cases where opposing natural teeth are in tight occlusion. The contacts of maxillary and mandibular buccal cusps are frequently in the areas where direct retainer or bracing arms (when retention is on the lingual) are placed. Without significant tooth reduction of these opposing cusps, placing this clasp assembly will cause huge occlusal interferences. The typical response to this interference is to grind the metal where the arrows are pointing. Commonly, the consequence of this grinding is excessive thinning of the metal which results in fracture of the clasp arms.

Figure 9-22 Indications for use: 1. In situations where there are no modification spaces

on the opposite side of the arch. An example of this is the Class III case with no modification space. A common location of this clasp assembly would be on the molars.

2. Another situation is the Class II case that has no

modification space. Teeth on the opposite side from the one distal extension base would have proximal contacts with each other; therefore, it is less likely that torquing of the teeth would occur even though a fulcrum line is present (distal extension case).

3. In cases where undercut is minimal on an individual

tooth, but is enhanced by combining with another tooths undercut (they oppose each other). Such an example might be using the mesial of a first molar and the distal of a second molar.

9-8

Figure 9-25

Ring Clasp Assembly: (Figure 9-23)

This clasp assembly is designed so that the direct retainer arm can approach undercut in areas that would be inaccessible in a direct path. It can be an effective retentive assembly; however, there is a large amount of metal that has to encircle the tooth to arrive at the undercut area. It is important to remember that more metal/tooth contact there is the more difficult it is to accurately fit the assembly to the tooth. As can be seen in Fig. 8-23, it is necessary to brace the arm of metal with a minor connector to provide adequate rigidity to the metal as it wraps around the tooth.


1. Cases where undercut is adjacent to the edentulous

space, especially second molars.

2. Cases where the use of a hair-pin clasp assembly cannot be placed on the buccal surface because the tooth is too short occluso-gingivally.

Figure 9-24

BAR CLASP ASSEMBLY:

Figure 9-23 Back-action Clasp Assembly: (Figure 9-24)

This type of clasp assembly is an old design and does

not meet the requirements of a modern clasping philosophy. The long slender arm of metal is so long and flexible that its correct function is unpredictable. Other clasp assemblies can be used without the limitations of this design. It is included here so that you are aware of its use in the past in case you were to encounter it is an existing old RPD that your patient has.

Indications for use: there are none!

The bar type of direct retainer approaches the undercut on the abutment tooth from the gingival direction. The most common type of bar clasp is the I bar, but bars may have other shapes such as L, T. With the I bar, the only portion of this type of direct retainer that contacts the abutment tooth is toward the end (approx. 2 mm) of the bar. The area of contact is in the form of an ellipse such as the pad of your finger would make when pressed against a curved surface. The most common use of this type of direct retainer is in the RPI clasp assembly as shown in Figure 9-25. The three letters represent:

1. Mesial Rest 2. Distal Proximal plate 3. I bar

9-9

RPI Clasp assembly: Indications for use:

1. The primary use of the RPI clasp assembly is on the

terminal abutment for a distal extension RPD. Its use in this situation is to try to eliminate torquing forces on the abutment tooth. Notice that the placement of the I bar on the facial surface of the tooth is slightly mesial to the height of contour. Contact is made with the tooth in the mesial-buccal-gingival quadrant (Fig. 9-26). By placing the contact in this location, movement of the distal extension under occlusal forces will cause the I bar to tend to move away from the surface of the abutment tooth. This movement away from the tooth surface is because the tip of the I bar will move in a direction that is perpendicular to the axis of rotation as is shown in Figure 9-27. When this movement happens, contact with the tooth will be either lessened or lost which results in no torquing forces being applied to the abutment tooth.

Figure 9-26

Figure 9-27

Figure 9-28 If the I bar is placed in the distal part of the tooth as is shown in Figure 9-28, when it tends to move perpendicular to the axis of rotation, it will be forced against the tooth and there can be no release of the torquing forces. For this release of torquing forces to occur, the rest through which the fulcrum line passes must be placed on the mesial of the abutment tooth. If the rest is placed on the distal of the tooth, the rotational movement of the I bar will be in a more upward direction. Since the I bar is in undercut, when it tries to move vertically, it would become tighter against the tooth. When evaluating the movement of parts of an RPD, keep in mind that in an RPD with a fulcrum line (axis of rotation), movement of any part will occur in a plane that is perpendicular to the axis. The axis can be straight across the arch or at any diagonal angle. Each differing position of the axis of rotation will present different directions of movement of the various parts of the RPD. Some other things to consider about the unique RPI clasp assembly are: 1. As long as the I bar remains in the undercut of the abutment tooth (Figure 9-29), uplifting forces in the edentulous area will try to make the RPD rotate around a point near the tip of the I bar. If the I bar stays in position, then the rests that are anterior to it will try to move downward. The rests are metal and are against hard tooth structure, so they cannot move downward. This is the basic principle of indirect retention. If the upward forces in the edentulous area are strong enough, then the direct retainer (I bar) starts to move upward out of the undercut (Figure 9-30). When this happens, the most anterior rest then becomes the pivot point around which rotation will occur. Even the mesial occlusal rest of the RPI assembly will start to move away from the abutment tooth.

9-10

Figure 9-29

use of a mesial occlusal rest. By placing an incisal rest, this clasp assembly is not considered to be an RPI. The pier abutment (second premolar) does not have any occlusal rests placed on it; however, it does have proximal plates and possibly lingual coverage that will provide some degree of lateral stability to the RPD. If occlusal rests are placed on an isolated tooth such as a pier abutment, it is easy to overload the tooth and possibly damage its periodontium. If the quality of the supporting ridge tissues is low (soft and spongy) then the forces against the isolated pier abutment can be even damaging enough to cause the loss of the tooth. Some experts suggest the placement of a fixed partial denture from the canine to the second premolar and then placing an RPI clasp assembly on the distal FPD abutment. This approach will usually increase the stability of the abutments; however, it does add considerably to the cost for the patient.

NOTE: there are two contra indications for using a

bar type of direct retainer:

Figure 9-30

1. Shallow vestibule: as shown in Fig. 9-32, the bar must have sufficient vertical space to drop below the gingival margin of the abutment tooth before it turns upward toward the undercut area on the tooth. If the bar touches soft tissue (the buccal frena in Figure 9-32) irritation of the tissue will result in a failure of the prosthesis. Failure to realize the shallowness of the vestibule is a common cause of failure when using the RPI clasp assembly. Study cast impressions frequently distort soft tissues and are not a reliable source of soft tissue location during function.

2. Another situation where the RPI assembly can be used is when there is a pier abutment such as a second premolar as shown in Figure 9-31. In this situation, the canine is used for vertical support with a mesial incisal rest instead of a mesial occlusal rest. Remember that the definition of an RPI clasp assembly indicates the

Figure 9-32

Figure 9-31

2. Severe soft tissue undercuts: the bar must not contact any of the underlying soft tissues; therefore, it has to be relieved from contacting the most convex areas of the soft tissues (which will usually be the attached gingival areas). To not contact these convex tissues, the lowest portion of the I bar (red arrow in Figure 9-33) may be significantly away from the soft tissues when there is severe undercuts below the attached gingival tissues.

9-11

becomes a torquing machine. Also, a direct retainer on tooth #28 will place torquing forces on the tooth when occlusal forces are placed on teeth # 18 and 19.

Figure 9-33

The I bar will protrude facially into the lips or cheeks causing irritation of the tissues. If an I bar is fabricated into the soft tissue undercut, during seating of the RPD, the lower portion of the I bar can actually remove a portion of the attached gingival tissues because of its rather sharp edge.

A great deal of care must be used in evaluating the patients intraoral anatomy to determine if a bar type of clasp assembly can be used successfully. Do not rely on the study cast as a source of the functional shape of moveable intra oral soft tissues!

Some patients cannot tolerate the contact of the I bar of an RPI clasp assembly with the moving tissues of the lips and the cheeks. Use of circumferential type of clasp assemblies places less metal into contact with the moving tissues and are generally better accepted by many patients. Use these bar clasp assemblies only when there is a definite need for them.

Discussion of leverage:

Because of the different types of support (tooth vs soft

tissues) present in the distal extension RPD, movement around an axis can (and usually does) occur.

Figure 9-34 shows an axis through the distal rest on

tooth #20 and the mesial rest on tooth #31. It is around this axis (fulcrum line) that torquing of the abutment tooth # 20 can occur when the distal extension on the patients left side moves either gingivally or occlusally. The strongest forces are generated during application of occlusal forces. The lever at work in this situation is a Class 1 lever with the force applied to the tooth by the part of the direct retainer arm that is in undercut. The rest acts as the fulcrum, the teeth in the base as the effort source, and the direct retainer as the resistance factor. Since the direct retainer is in undercut on the abutment tooth, its resistance to movement is provided by the tooth. This situation creates an RPD that in effect

Figure 9-34

Figure 9-35

Fig. 9-35 shows the three classes of levers where:

E = Effort (force) R = Resistance = Fulcrum Remember that when a lever is at equilibrium, the dis-

tance through which the effort acts multiplied by the effort must equal the distance through which the resistance acts multiplied by the amount of resistance.

D x E = D x R E R

For the example in Figure 9-36, if the effort (force) is

10 lbs., the formula for equilibrium would be:

Figure 9-36

9-12

It becomes obvious that the ratio of the distances through which the effort and the resistance work is dependent on the location of the fulcrum point. When we design a distal extension RPD, the fulcrum line is determined by where we place the rests on the abutment teeth.

A common example of the Class 1 lever is the seesaw.

A common example of the Class 2 lever is the wheel barrow. A common example of the Class 3 lever is the fishing pole. The location of the fulcrum (in the RPD it is the terminal rests: the most posterior rests) will determine if detrimental forces will be applied to abutment teeth. In designing an RPD, it is desirable to use lever action that does not magnify forces on abutment teeth.

Figure 9-37 shows a Class II RPD with its fulcrum line.

The rests on teeth # 11, 12, and 14 are acting as indirect retainers. The distance (B) from the fulcrum line to the rest on # 11 is slightly shorter than the distance (A) from the application of upward force (sticky food, for example) to the fulcrum line. This is an example of a Class I lever that is effective in providing indirect retention; this means that the distal extension base is prevented from lifting away from the ridge by the rests on teeth # 11, 12, and 14. The cingulum rest on tooth # 11 the most effective of the 3 rests since it is farther from the fulcrum line than the rests on # 12 and 14.

In Figure 9-38, the distance (A) from the fulcrum line to

the indirect retainer (cingulum rest on # 22) is much shorter than the distance from the fulcrum line to the application of upward force (B). In this situation, indirect retention will not be as effective as in Figure 9-37. The factor that must be considered in this kind of lever action is what is called mechanical advantage. Mechanical advantage of a lever is defined as: the ratio of the distance through which the effort acts divided by the distance through which the resistance acts. The lever shown in Figure 8-37 has an effort distance of 4 and a resistance distance of 2: therefore the MA would be 2. In effect, this means that one pound of effort can balance two pounds of resistance: this is directly related to the length of the lever through which the forces are acting.


INDIRECT RETAINERS:

The concept of indirect retention has already been men-

tioned; however we should consider some important aspects of how to properly provide it. Indirect retainers are rests in prepared rest seats placed on teeth on the opposite side of the fulcrum line from the distal extension of an RPD. Examples of this are shown in Figures 9-37, 38 and 39. It is tempting to think that the lingual plate against the mandibular anterior teeth would provide more indirect retention than the rest on the cingulum of # 22 in the example shown in Figure 9-38. The lingual coverage may provide some indirect retention, but it does not provide ideal indirect retention. Ideal indirect retention is provided only when there are rests placed

Figure 9-39

9-13

in prepared rest seats (occlusal, cingulum, or incisal) inteeth on the opposite side of the fulcrum line from the distal extension base or bases.

Important considerations about Indirect Retainers are:

1. They should be placed as far away from the fulcrum line as reasonably possible. When they are too close to the fulcrum line, they become ineffective.

2. Do not place them on the incisal edges of teeth unless there is no other choice. Always inform the patient of your intention to place metal in highly visible locations on teeth. A good study model with the RPD design drawn on it will be an invaluable aid in showing the patient where you plan to place metal of the RPD framework. Almost never will it be indicated to place incisal rests on maxillary anterior teeth!

3. They are always placed in rest preparations. 4. Lingual coverage of anterior teeth alone does not

provide ideal indirect retention. The placement of the lingual plate on the unprepared lingual surfaces of the teeth does not produce a stable metal/tooth contact. Pressure of the metal against the teeth can cause a forward force on the teeth that over time may result in the teeth moving facially and the RPD becoming unstable.

5. Their use is designed to prevent distal extension bases from lifting in a rotatory manner away from the supporting ridge tissues. This lifting away from the supporting tissues can be caused by gravity, muscle action, sticky foods or a combination of all of these.

10-1

Chapter 10

Component Parts: Major Connectors

The major connector is the component part of the RPD that connects the parts of the prosthesis located on one side of the arch with those on the opposite side.

MAXILLARY MAJOR CONNECTORS:

There are many different maxillary major connectors.

The choice will depend on many factors such as: (1) need for cross-arch bracing, (2) means of attaching missing teeth, (3) provision of additional support (palatal plate or full palatal connectors), (4) presence of a torus, (5) location of the missing teeth, etc. One of the differences between maxillary and mandibular major connectors is that maxillary connectors MUST be in contact with the underlying palatal soft tissues. The mandibular major connector does not touch the underlying soft tissues because the tissues are too thin and delicate to withstand pressure from the RPD framework. The maxillary major connector is placed against the underlying soft tissues for two reasons: (1) to prevent the tongue from feeling the edges and (2) to prevent food from being forced between the metal and the soft tissues during chew- ing and swallowing. To make the edges disappear from the tongue, they are beaded (Figure10-1). This bead becomes slightly depressed into the soft tissues of the palate when the framework is seated and allows the edges of the metal to become flush with the soft tissues. The beading is accomplished by carving a 1/2 round trough into the master cast along the outline of the major connector prior to the fabrication of the framework by the laboratory technician. It is a very similar process to carving a posterior palatal seal, except that beading is much smaller. Beading cannot be successfully placed over thin non-resilient tissues such as a palatal torus. Placement over these tissues will result in pain and possible ulceration of the tissues.

When a maxillary RPD has a distal extension base, the major connector and finish line must extend to the pterygo- maxillary notch. The example shown in Figure 10-2 does not have the finish line extending all the way to the notch. All RPDs must also have finish lines for the metal and the acrylic resin to butt up against each other. This finish line will allow the resin to have sufficient thickness to help maintain its shape and position on the framework. Figure 10-1 shows an internal finish line and Figure 10-2 shows an external finish line. All finish lines should have a shape that allows for a 90 junction of metal and acrylic resin.

Figure 10-2

The following photographs show examples of typically shaped maxillary connectors. ANTERIOR-POSTERIOR STRAP (BAR)


10-2


1. Class I and II arches in which excellent abutment support and residual ridge support exist.

2. Long edentulous spans in Class II, Mod I arches. 3. Class IV arches where anterior teeth are to be replaced.

In these cases, the anterior part of the major connector would look like the anterior part of the framework in Figure 9-5. There would be a finish line following the anatomy in the area lingual to the anterior teeth to be replaced.

4. In the presence of an inoperable palatal torus that does not extend to within 7-8 mm of the junction of the hard and soft palates.

PALATAL STRAP: (Figure 10-4)


1. Bilateral edentulous short span spaces in a tooth-borne

RPD (Class III). 2. Possibly in a Class II, Mod I case where the distal

extension area is relatively small. U SHAPED CONNECTOR: (Figure 10-5)


1. Cases where an inoperable palatal torus extends to the

junction of the hard and soft palates. Less than 7-8 mm of space would be considered too close to the soft palate to place the posterior bar of an A-P Strap connector.

2. Class IV cases where there is a high palatal vault: there will be a large surface area of vertically oriented tissue for the metal to contact thus providing additional lateral support.

PALATAL PLATE: (Figure 10-6)


1. Class I cases where the residual ridges have not resorbed

significantly, thus will provide good support. 2. V or U shaped palates. 3. Strong abutments. 4. More than 6 anterior teeth remaining. 5. No interfering palatal torus. Torus is the singular and

tori is the plural of the bony growth in the middle of the palate.


Figure 10-6

10-3

PALATAL LINGUOPLATE: (Figure 10-7)

Figure 10-7 Indications for use:

1. In most cases where only anterior teeth remain. 2. In the absence of a large pedunculated torus. 3. When residual ridges have undergone extreme vertical

resorption (because palate will be covered with acrylic resin just as it is in a complete denture).

4. When terminal abutments have some degree of bone loss and slight mobility indicating a need for additional support (from acrylic resin in the palate).

FULL PALATAL CONNECTOR: (Figure 10-8)


1. The same as for the palatal linguoplate. 2. Note: Extensive use of metal in the palate makes

fitting and adjusting the RPD more difficult than if acrylic resin is used in this area. Also, relining of the metal is very difficult. The trade-of of metal vs acrylic resin in the palate is that metal is thinner, follows the natural contours more closely, changes temperature more rapidly, and feels more natural to the patient. The resin acts as an insulator from temperature changes and is also considerably thicker than the metal which can be a problem for patients with a significant gag reflex.

MANDIBULAR MAJOR CONNECTORS:

There are fewer kinds of mandibular major connectors. One significant difference between major connectors is that maxillary connectors are placed into intimate contact with the palatal tissues, whereas mandibular connectors are always relieved from contact with the underlying soft tissues. Most soft tissues of the mandibular lingual are too thin and delicate to withstand any contact from the framework metal. During fabrication of the framework, the laboratory technician uses 24 gauge relief wax to create space under the pattern for the major connector of the mandibular RPD. There is no relief wax used in the maxillary connector fabrication since it must have intimate contact with the underlying soft tissues to be acceptable to the patients tongue. LINGUAL BAR: (Figure 10-9)

Figure10-8 Figure 10-9

10-4


1. Cases where there is sufficient vertical space between the functional position of the lingual sulcus and lingual frenum tissues and the gingival margins. The minimum space for correct placement is considered to be 7 - 8 mm. The bar is 1/2 pear shaped and should be 3-4 mm in height and the space above it to the gingival margins should be a minimum of 4 mm.

2. Cases where placement of a lingual plate would be difficult because of overlapped and crooked anterior teeth. There still is a minimum space requirement to prevent encroachment of the delicate gingival tissues.

LINGUAL PLATE: (Figure 10-10)

be covered with metal in the absolute minimum amount necessary to satisfy all of the structural and functional requirements of the RPD.

INTERRUPTED LINGUAL PLATE: (Figure10-11)

Figure 10-11



Figure 10-10

1. Same as for lingual plate, but where large diastemas are present that would allow metal to show from the facial view.

2. Use only when absolutely necessary since the interruptions tend to be bothersome to the tongue and food impaction may be a problem. If possible, the metal fingers should be placed in rest preparations on the teeth to maximize stability.

1. Where the lingual sulcus soft tissues or lingual frenum are in close proximity to the gingival margins of the anterior teeth (less than 7 - 8 mm.).

2. Where excessive vertical resorption of the residual ridges has occurred in long distal extension cases. The metal against the lingual surfaces of the teeth provides some resistance against movement of the extension bases.

3. Cases where periodontally weakened teeth are present: the metal framework provides some degree of splinting of these teeth.

4. Where future replacement of teeth is likely. 5. Where the philosophy of the dentist is that for tongue

comfort, the lingual surfaces of the RPD should not have multiple openings. This is an example of a difference between minimal coverage and coverage for tongue comfort philosophy. The minimal coverage philosophy espouses that soft and hard tissues should

DOUBLE KENNEDY BAR: (Figure 10-12)

Figure 10-12

10-5


1. Where a lingual plate is indicated, but there would have to be extensive blockout of undercuts where anterior teeth are severely overlapped. The same vertical space limitations exist on the lingual as for the lingual bar.

2. When tooth contacts are still present, but there are large interproximal spaces present such as after periodontal surgery: metal from a lingual plate would show through these spaces and be unesthetic. This connector has the same tendency to be bothersome to the tongue because of the distinct band of metal that is high on the teeth (in the tip of the tongue area). This band of metal is thicker than the superior margin of the lingual plate connector and therefore is more noticeable to the tongue.

11-1

Chapter 11

The Surveyed Crown As An RPD Abutment

In many cases where an RPD is planned, abutment teeth do not meet the criteria for proper abutments. This can include: teeth with no undercuts present, large existing amalgam or composite restorations, existing full coverage crowns (especially PFM with porcelain occlusal surfaces and without proper axial contours), and tipped, tilted, or rotated teeth. In these cases, it will be necessary to alter the abutment tooth by means of a restoration that meets the criteria of the RPD design. Generally, this restoration will be a full coverage surveyed crown. The term surveyed means that after the full contour waxup is completed, the axial surfaces of the crown will be evaluated and modified using a dental surveyor. Occasionally, the term surveyed onlay is used. However, since the onlay does not extend into the axial areas of concern for an RPD clasp assembly (especially the undercut areas), this is a misnomer and is incorrect.

When an abutment tooth requires a full coverage

restoration, the full coverage restoration will allow certain portions of the metal RPD framework to be placed into recessed areas in the crown which will required modifications to the tooth preparation. Additional reduction must be done in the rest area as well as the area where the bracing/reciprocal component will be located within the normal co

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RPD Manual 2013-2014 Final