David R. Burns, OMDAssistant Professor

A Review of Attachmentsfor Removable Partial

Denture Design: Part 1.Classification and Selection

lohn E. Ward, ODS, MSOAssociate Professor

Department of Removable ProsthodonticsVirginia Commonwealth UniversitySchool oí DentistryBox S66 MCV StationRichmond, Virginia 23298

An attachment is a mechanical device, other than a claspassembly, that functions as a direct retainer. Attachments forremovable partial denture treatment are reviewed and amethod for classifying different types of attachments isprovided. Attachments are categorized as precision orsemiprecision, depending upon the method of manufacture;internal or external, according to their intracoronal orextracoronal location relative to the abutment tooth; andrigid or resilient, as determined by the amount of movementallowed between the component parts. They are alsoclassified by design. The advantages and disadvantages ofattachment use as well as indications and contraindicationsare considered. Additionally, the conventional clasp-typedirect retainer is compared to attachments. Int J Prosthodont1990:3:98-102.

A ttachments have always been surrounded byan aura of mystery, primarily because of a lack

of knowledge and experience. Not all practitionersmay consider tbe use of attachments as essential,but a basic understanding is useful and important.Tbe purpose of this two-part paper is to present thefundamentals of attachments for removable partialdentures. Part 1 defines attachments and discussestheir function and indication. Part 2 discusses treat-ment planning and attachment selection.

Definition of Attachment

By definition, an attachment is a mechanicaldevice for the fixation, retention, and stabilizationof a dental prosthesis.' For removable partial den-ture prosthodontics, it is a mechanical device, otherthan a clasp, that functions as a direct retainer.^ Asthe direct retainer, it must provide: (1) support—resistance to movement of the prosthesis toward thetissue; (2) reíe/ii/on—resistance to movement of theprosthesis away from the tissue; (3) reciprocation—counteraction of the forces exerted by the retentivecomponent; (4) siafe;7/zaf/on—resistance to horizon-tal movement of the prosthesis, and (5) fixation—resistance to movement ofthe abutment tooth away

from the prosthesis and movement of the prosthesisaway from the tooth. Additionally, the direct retainershould be passive when the prosthesis is in its ter-minal position. An attachment derives its functionstiirough closely fitting, coupling parts. It incorpo-rates one component into the removable partialdenture and the connecting component is tradi-tionally incorporated into a cast crown or fixed par-tial denture (Fig 1). Recent advances in resin-retained prostheses have led to the introduction ofthe resin-bonded connecting component that isluted directly onto the enamel of the abutmenttooth.

Classification of Attachments

Attachments may be classified in a number ofways.'" They may be classified as cither precisionor semiprecision, depending on the method of fab-rication and tolerance of fit. Precision attachmentshave prefabricated, machined components withprecisely manufactured metal-to-metal parts withclose tolerances. The fabrication methods for semi-precision attachments yield a less precise tolerance.These may be either manufactured patterns (madeof plastic, nylon, or wax) or hand waxed.

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Attachments for Removable Partial Oentures, Part 1 Burns/Ward

Anachments are classified according to their rela-tionship to the abutment tooth. If the attachment isincorporated within (he body of the abutment toothintracoronally, it is an internal attachment; whenlocated extracoronal ly, it is an external attachment.Neither type of attachment is applicable to all cir-cumstances. Selection of an internal or externalattachment is based on design considerations for theprosthesis and the anatomic morphology, location,and position of the abutment tooth. Internal attach-ments have the advantages of maintaining forcesmore in line with the long axis of the tooth andhaving a more desirable resistance to vertical andlateral forces, while external attachments requireless reduction of the abutment tooth.

Attachments are also classified as either rigid orresilient. Rigid attachments are those that theoreti-cally allow no movement of their component partsduring function. However, even under the best ofconditions, minute movement of the prosthesis willoccur when occiusal forces are applied. The amountof movement will increase with wear of the com-ponents.

Resilient attachments provide a defined amountand direction of movement of their componentparts, permitting movement of the denture basetoward the tissue under function, while theoreticallyminimizing the amount of force being transferred tothe abutment teeth.'' Thus, the resilient attachmentacts as a "stress director,"'* Resilient attachmentsmay provide a hinged motion, allowing movementalong one plane ¡Fig 2), or a rotary motion, allowingmovement along many planes (Fig 3), The precisionintracoronal attachments ate usually designed tofunction as rigid attachments, while the extracoronalattachments are usually resilient.

Rigid intracoronal attachments provide all of thenecessary functions of a direct retainer." The resil-ient extracoronai attachments, in contrast, do notalways supply suitable support and bracing becauseof their resilient nature. This represents a point ofcontroversy, because for the resilient attachmentsto maintain their ability to move freely in all planeswithout binding or torquing the teeth, the connec-tion between the components of the resilientattachments must be the only contact between theremovable partial denture and the teeth. When thispremise is followed, the removable partial denturederives little more than retention from the abutmentteeth, while support, bracing, and stability arederived primarily from the residual ridge. Therefore,some believe that additional components must beincorporated into the removable partial denturedesign to provide the necessary functions of a directretainer and follow sound principles of prosthodon-

Fig 1 An attachment derives its tunction through closely fit-ting, coupling parts. One component of the attachment isincorporated into the removable partial denture. The con-necting component is incorporated into the abutment tooth.

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Fig 2 Hinged resilient attachment allowing movement onlyalong the sagittai piane, thus restricting lateral movement.(Dalbo attachmeht, APM-Sterngoid, San Mateo, California.)

T f ^•^' ^ilowing movement alongattachment, Preat Corp, San Wateo, Cail:

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Burns/Ward Attachmeni! hu Removable Parlial Derilures, Part I

tics. Specially designed rests and guiding planes onsurveyed crowns contacted by the major connectormay be used to supply support and bracing for theprosthesis" (Fig 4), The rests and guiding planes alsoprovide the positive relationship between the rigidframework and teeth necessary to evaluate the fitof the framework and the relationship of the denturebase to the residual ridge. Unfortunately, when thisfeature is incorporated, the movement of the pros-thesis is more restricted, but proponents feel thatthe benefits of such a design outweigh some lossof movement of the prosthesis.

Finally, attachments are classified according todesign, and there are many designs and combina-tions. The following are examples: The key and key-

Fig 4 Rests and guiding planes incorporated in surveyedcrowns are contacted by tne removable partiai denture majorconnector and provide increased support and bracing. Thisdesign also provides a method for evaiuation of the fit of thefrarnework to the teeth.

way design is representative of the rigid type (Fig5]. The ball and socket is a multidirectional resilientdesign incorporating a ball freely moving within asocket (Fig 6], The bar attachment design consistsof a prefabricated metal bar of specific dimensionsand shape that extends across an edentulous areajust superior to the tissue of the residual ridge. It ispermanently attached to cast crowns or resinbonded to the enamel of the abutment teeth (Fig7). Retention is usually gained with a precisely fittedclip incorporated intotheacrylic resin of the denture(Fig 8).

Deciding on Attachment Use

Attachments have a number of desirable qualitiesthat indicate tbeir use in place of conventionalciasps. The primary indication is esthetics.^ Con-ventional clasp assemblies and rests may be visibleand unesthetic, whereas the attachment is con-cealed within the contours of the abutment toothor within the body of the removable partial denture.

Another appropriate indication for the use ofattachments is for divergent abutment teeth withhigh survey lines. The use of conventional claspswould require the placement of clasp arms high onthe tooth, or lowering of the height of contourthrough tooth modification or placement of sur-veyed crowns. Although attachment use may alsorequire crowns, the preparations do not need to bemade parallel to one another. This is because thepath of placement of the removable partial dentureis determined by parallel placement of the attach-

Fig 5 (Left) Key and keyway (semipre-cision) attachment design.

Fig 6 (Right) Ball and socket design.

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AltachmeiKs for Removable Pardsl Oentures, P.Hrt [i urns/Ward

ments within the surveyed crowns, independent ofthe parallelism of the crowns themselves.'"

A major advantage of the use of attachments isthat the point of force application to the tooth ismore apical than for occlusal or incisai rests, thusshortening the lever arm and decreasing torquingforces. Attachments may also allow better cross-arch force transmission and stabilization than clasps,but this is determined by the type of attachmentused, the number of guiding surfaces, and thedesign and adaptation of the framework and attach-ment.

There are a number of negative aspects to theuse of attachments,' In general, whenever a con-ventional clasp can be used it is the retainer ofchoice. The use of attachments requires additionalexpense to the patient, for both the crowns or resin-bonded retainers on the abutment teeth as well asthe attachments themselves. Poor dental motivationand manual dexterity of the patient may result inearlier failure than with the use of conventionalclasping. Repairs or alterations are difficult or impos-sible with some attachments.

Short clinical crowns contraindicate the use ofattachments. A minimum of 4 mm of vertical spaceis necessary for most attachments." Therefore, aminimum of 6 mm of clinical crown would be nec-essary to retain the attachment without overcon-tour. This occlusogingival length is required so thatthere is adequate space hetween the plane of occlu-sion and the gingiva for placement of both theattachment and the prosthetic teeth. Likewise, forattachments that rely on frictional resistance forretention, the occiusogingival length is important inproviding enough length of parallel contacthetween the components of the attachment to en-able adequate retention. It is possible, however, toprovide supplemental means of retention, such aslocks, retentive clasp arms, etc. Placement of attach-ments in the incisor and canine areas can also bedifficult because of limited faciolingual tooth width(Fig 9). The anatomy of the abutment tooth and theipace requirements for the attachment must beconsidered. Adequate space between the pulp andthe normal contours of the tooth is necessary forthe intracoronal component of an internal attach-ment. If the pulp of the abutment tooth is large,preparation of the tooth for a crown plus additionalreduction for placement of an internal attachmentmay necessitate root canal therapy. This may notpreclude the use of attachments, but may be anindication for the use of an extracoronal attachment.

Biologic conditions that contraindicate a conven-tional removable partial denture also preclude theuse of attachments.'« These include poor periodon-

Fig 7 Bar attachment design.

Fig S Clip incorporated into the denture base iocks tightlyover the bar. providing retention.

Fig 9 Placement of attach-ments in the incisor andoanine areas can be difficultbecause cf limtted faciohn-guai tooth width.

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tal health of abulmenl teeth, poor tissue quality orquantity, poor crown-to-root ratio, and enrlodonticand restorative considerations.

The greatest deterrent to the use of attachmentsis their complexily. The close tolerances demandcoordination between the denture base, partial den-ture framework, and supporting tissues. The treat-ment therefore becomes considerably more difficultto plan, accomplish, and maintain.'- The use ofattachments requires a thorough knowledge of basicprosthodontic principles, appropriate traininj; andexperience with the particular attachmenl used,technical skills, and clinical ability and judgment.

Summary

An introductory review of attachments, includingclassification, parameters for use, advantages, anddisadvantages, has been presented. Pari 2 discussestreatment planning and attachment selection forremovable partial denture treatment.

References

1. Acidemy of Denture ProstbeMts: CImsiirv iit Prii<,tho-dont,c Terms, ed S SI Louis, CV Mosby Co, I %7.

2. Tregiiskei |N, Ward |E: Removable Partial Denture CiinialStudy Manual. Richmond. Virginia, Virginia Commiin-Wfîallh University, Sthool (if Dentistry, I9H11, p 1.

i. Becerra C, Mattniee M: A Llawiiication ot pff-ciiron•íltüchmenls. / Prasthet Dent i9li7,Sfl: i22 -127.

4. Preiskel HVW: Prctision .lilathments: Uws and abuses. IPrasl/ierDenf 197),10.491 492.

5. Henderson D, McCivney GP, Caîlleberry D|:McCraci<en's Removable Partial Prosthodiintics. ed 7. SiLouii, CV Mosby Cr>, 1985, p 79.

6. Preiikcl tlW: Precisian Attachments in Prosthotiontii.s\Vol I. The Applications of Intr.tcoronai anri ExtracoronaiAttaciiments. Chicago, Quintessence Publ Co, 19it4, pUlS.

7. Blatierfein L: Tbe use of ihe semiprecision resi in remov-able parricii dentures. / Proslhet Dent I969;22:.î{)7-112.

8. Singer 1: improvemonis in predsion-ailached removablepartial dentures. / Prosthet Dent 1567;! 7:69-72.

9. Cunningham DM: inditalions and conlraindicalions forprecision ailachmenls. Denl C/m Nor//) Am 1970;14:595-fjlll.

10. Lorencki SF: Planning precision attachment restoration;./ Proslhet Dent 1969:21 :.';06-.';08.

11. Preiskei HW: Precision Attachments in Prosthodontics:Voi !. Tiie Applications of Intracoronal and ExtracoronaiAttachments. Chicago, Quintessence Publ Co, 1904, p32.

12. Sthuyier CH: An analysis of Ihe use and relative value ofIhe precision atlachment and the clasp in partial denturepicinning / Proslhet Dent 195Í:^:711-714

Literature Abstract-

Stability in the Correction of Dentofacial Deformities: AComprehensive Review

Postoperative stability of skeletai segments repositioned during orthognattiic surgery was reviewed.Factors ttiat atfected the stabiiity of orthognathic surgery procedures inciuded the type and duration ofmasiilary-mandibular fixation, the number ot skeletal sections involved, condylar dispiacement followingsurgery, ttie use cf presurgicai orthodontics, ttie direction of movement of the bone, the posterior faceheight, and the tension of the suprahyoid muscuiature. The resuits showed that maxiilary superiorrepositioning tends to be a stable operation, but maxillary inferior repositioning has a tendency tc beunstabie. witti the upper lip generaliy lengttiening postoperatively. The greater the mandibularadvancement, the greater the tendency for relapse. Ttie clinicai implications of various stabilizationtechniques are not fuiiy understood.

Welch TB. J Oral Ma'illotac Surg 1989:471142-1149 References: 9S Repeints: Or Welch, Department ol Oral andMaiillotacial Sjrgery, Loma Linda Uriiversity, Loma Linda. California 9235O.-Srepíien Wagner. DDS. Abstract andBook Revietv Editor, Albuquerque, New ttiexico

a\ of Pros t h odontic 4 1 0 2 Volume 3, Nymber I, 1990