Implant Occlusion

International Dental Journal (2008) 58, 139-145

Medium- or long-term failure of endosseous dental implants after osseointegration, when

it has occurred, has been associated in the great majority of cases with occlusal overload.

Overload depends ultimately on the number and location of occlusal contacts, which to a

great extent are under the clinician’s control. Much of our current understanding of occlusal

contacts in this context is based on concepts derived from non-implant-borne prosthetics

and has not been rigorously tested. The present article reviews occlussal contact designs

and offers occlusion strategy guidelines for the main types of implant-borne prostheses.

© 2008 FDI/World Dental Press

0020-6539/08/03139-07

Guidelines for occlusion strategy in implant-borne prostheses. A review

Key words: Endosseous dental implants, overload, failure, implant-borne prosthetics

Benito Rilo, José Luis da Silva, María Jesús Mora and Urbano SantanaSantiago de Compostela, Spain.

doi:10.1922/IDJ_1893Rilo07

Osseointegration has been one of the most important therapeutic advances in recent years, greatly facilitating the placement of single or multiple prostheses. The medium- and long-term results obtained to date are very promising, and there is a continuing effort to improve these techniques through the development of different types of implant with modifications of surface area, surface texture, morphology, etc. Nevertheless, there remains a small risk of implant failure, and these failures can be divided into two major groups: early failures closely related to the surgical procedure and late failures manifesting after osseointegration has occurred.

Late failure of osseointegration may occur for sev-eral reasons such as host resistance, plaque build-up and occlusal overload1,2. Some of these problems (such as plaque build-up) are readily resolved, while others (such as occlusal overload) are more complex. There is an extensive literature supporting the view that oc-clusal overload may provoke peri-implant bone loss3-9 and eventually osseointegration failure. However, some authors have argued that bone loss and osseointegration failure are principally due to biological causes20, notably infection, and that there is no firm evidence to support a negative effect of prolonged periods of non-axial loading on implant osseointegration11,12. Certainly, the relative importance of mechanical and microbial factors in determining the time-course of bone loss around implants remains to be properly resolved13. What ap-

pears clear is that occlusal overload may at least cause mechanical complications14-18 such as fracture of the ceramic, decementation of the prosthesis and loss of abutment retention.

Various factors and situations that can give rise to oc-clusal overloading have been reported16, including exces-sive crown-to-implant length ratio19, over-sized occlusal surfaces20, unfavourable direction of axial forces21,22, and cantilever effects23,25. Undoubtedly all these factors may cause occlusal overloading, whether acting alone at high intensity or together at lower intensities;; however, all act via a single route, i.e. via occlusal contacts. Thus any damage due to occlusal overloading will be highly dependent on the number and location of occlusal contacts. Both variables can of course be controlled by the dentist;; in other words, our occlusion strategy and occlusal adjustments will have a major influence on the risk of occlusal overloading, both in normal patients and in patients showing parafunctional behaviours (bruxism, clenching, etc.) that worsen prognosis8,15,19,26.

It would be of interest to have broad guidelines on the optimal number and location of occlusal contacts for each type of implant-borne prosthesis, though of course adjustable in view of the specific characteristics of each individual patient. As a first approximation we would suggest that patients can be divided into two major groups. First, we have patients with many teeth and few implants;; in such cases, occlusion should be

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designed so that the teeth bear the full occlusal load, and transmit mainly lateral loads to the implants. In these cases there is no need to modify the pre-existing occlusion, unless there are symptoms of dysfunction requiring modification27. Thus restoration aims to be in harmony with existing mandibular relations;; in other words, the restoration will be designed such that the occlusal contacts of the other teeth are unaffected28,29. Second, at the other end of the spectrum we have pa-tients with many implants and few teeth, or no teeth;; in these patients the occlusion will be designed such that the implants receive all the load. In such cases a reorganisational approach is appropriate30, since the new restorations will lead to different occlusal patterns that need to be anticipated before rehabilitation is started.

To develop practically useful guidelines it is impor-tant to consider dentition status in greater detail, and to consider the type of prosthesis to be fitted. As far as we are aware, there have been few previous reports aimed at offering dentists recommendations on occlusal considerations in implant rehabilitations1,16. We reviewed occlusal contact designs in implant rehabilitations, considering type of implant/prosthesis and dentition status (Table 1).

It is important to distinguish between contacts in static occlusion (when the mandible is closed and sta-tionary) and dynamic occlusion (when the maxilla is moving in relation to the mandible). In static occlusion, the ‘ideal occlusion’ paradigm is generally accepted, for all implant/prosthesis types, as for tooth-tooth

occlusion31,32. In other words, the clinician aims for the maximum intercuspation position to coincide with centric relation, with the mandible free to move slightly forwards from this position in the sagittal and horizontal planes (i.e. freedom in centric occlusion). In lateral ex-cursion movements, occlusion goals will be more varied, and will be discussed below.

Single-tooth implantsWith this type of implant, the aim should be to ensure that occlusal loads are directed as much as possible along the longitudinal axis of the implant, since bone height and/or width are often insufficient for placement of the implant in the most appropriate position. In addition, it is important to ensure that loads are small33 and to this end if there is a tooth contact of light or medium intensity in maximum intercuspation position, a clear-ance of 30µm should be left between the occlusal face of the implant and the opposing arch34. This clearance aims to compensate for the different biomechanics of the tooth and the implant35,36 and to avoid overloading of the implant, since under heavy loads the tooth may intrude into the alveolus, whereas the implant-borne prosthesis will not intrude into the bone. Failure to leave this occlusal clearance will expose the prosthesis to ex-cessive loads, and unfortunately the patient will typically not be aware of the overloading due to the absence of periodontal ligament and limited tactile sensitivity in the bone implant’s alveolus37-39. The aim is thus to ensure

Table 1 Occlusal guidelines for the major categories of implant-supported prosthesis

Dentition status Prostheses type Occlusal contacts

Maximum Intercuspidation Position Excursive Movements

Light intensity Maximum intensity

Partially edentulous

Single missing tooth Single-tooth implant Clearance 30µm Contact No contact

Partially edentulous with distal

tooth abutment

Fixed prosthesis Clearance 30µm Contact No contact

Unilateral free-end

Canine present Fixed prosthesis Clearance 30µm Contact Canine guidance

Canine absent Fixed prosthesis Clearance 30µm Contact Group function

Bilateral free-end Fixed prosthesis Contact Contact Group function

Anterior partially edentulous Fixed prosthesis Clearance 30µm Clearance 30µm Contact protrusion only

Partially edentulous with distal

implant abutment

Implant/tooth-supported prostheses Clearance 30-50µm Contact No contact

Completely edentulousFixed prosthesis Contact Contact Mutually protected

Occlusal balance

Overdenture Contact Contact Occlusal balance

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Rilo et al.: Guidelines for implant-borne prostheses

that the implant is not subjected to load during light or moderate dental contact, and that during high-intensity contacts the implant and the teeth contact simultane-ously (Figure 1). In protrusive and lateral movements the occlusal face of the implant should not be loaded, in order to minimise the transverse forces that can act on implants of this type;; researchers using finite-element models have indicated that non-axial horizontal forces in particular give rise to high stresses around the neck of the implant40. Minimisation of transverse forces can be achieved by performing selective adjustments on contacts marked on articulator paper, so that only the natural teeth (not the implant) participate in occlusal guidance. In addition, it may also be of interest to re-duce the size of the occlusal face and/or the inclination of cusp slopes in the denture41-43.

Fixed partial denturesOcclusion goals for dentures of this type will vary depending on location (anterior or posterior) and on whether it has a uni- or bilateral free end. In what fol-lows we discuss the different situations as grouped by Kennedy classes, in order of decreasing number of natural teeth.

Kennedy Class III

As with single-tooth implants, natural teeth located anterior or posterior to the edentulous space will allow clearance of 30µm between the occlusal surface of the implants and the opposing teeth during light- or moder-ate-intensity contacts. Loading should be as axial as pos-sible, and there should not be contacts during protrusion

or lateral excursions32. Anterior or canine guidance may minimise the potential destructive stresses on posterior implants and it has been suggested that working-side contacts should be placed as far anterior as possible to minimise leverage34.

Kennedy Class II (unilateral free end)

Achieving an axial direction of loading tends to be more difficult due to bone resorption processes. A clearance of 30µm should be left in low- to moderate-intensity occlusion, so that the natural teeth can intrude and so that there are contacts with both teeth and implants during maximum-intensity occlusion, thus distributing the loading more evenly. Contacts during protrusive movements should be avoided. In the case of laterality movements the appropriate response will depend on type of guidance. If a canine is present (i.e. unilateral free end with absence of premolars and molars), canine guidance needs to be established, disoccluding the pros-thesis during working and balancing movements44. If no canine is present, it is necessary to establish group func-tion, aiming to achieve the maximum contact possible during working movements, with the aim of distribut-ing the load over all the implants (Figure 2). Splinting implant crowns may also be beneficial, with the aims of favourably distributing implant loadings, minimising the transfer of horizontal loads to the bone-implant interface, and increasing the bone surface45.

Kennedy Class I (bilateral free ends)

In this case if we leave a clearance between the teeth and the implants in low-intensity occlusion it is very possible

Figure 1. Single-tooth implant. Teeth-implant contacts: a) During light or moderate intensity. b) During high-intensity.

a b

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that this will overload the natural front teeth. Thus the approach should be similar to mutually protected oc-clusion: contacts should be established on the implant in low- and medium-intensity occlusion at maximum intercuspation position, and the incisors should be left without contact or with only slight contact. There is one controversial point: if canines are present, these will contact with their antagonists in maximum in the inter-cuspation position and the protrusion movement will be guided by the natural teeth without involvement of the implants. In lateral movements, if canines are present we can opt for canine guidance;; if they are not present or are periodontally compromised, group function can be established on the implant prosthesis33.

Kennedy Class IV

This is a bridge in the anterior sector, so that the oppo-site considerations apply. There should be no contacts in the anterior sector in maximum intercuspation and the loads should be borne by the posterior sectors of the natural dentition. If an implant is located in the ca-nine position the clinician will have to decide whether it participates or not in the corresponding working move-ment during lateral movements. If the natural teeth have good support, either canine guidance or group function may be appropriate;; occasionally, it may be advisable to allow canine guidance to be established over an isolated implant. The protrusive movement should be guided by the anterior sector, independently of whether the loads will be borne only by the implants (thus the planning stage should include the number and length of implants necessary), or whether they can be borne by both teeth and implants. As a general rule, the posterior teeth should be disoccluded for at least two incisive contacts on each side of the midline46.

Full-arch fixed dentures Two types of occlusal scheme can be considered for this type of prosthesis: mutually protected occlusion and occlusal balance.

Mutually protected occlusion47,48 is the most widely used approach, especially when the opposing arch is of natural teeth. This approach is based on concepts de-rived from the ‘gnathological school’ and thus the pos-terior sector receives loads in maximum intercuspation while a slight clearance is maintained in the anterior sec-tor. In contrast, in the anterolateral sector the implants in the incisor and canine positions should disocclude the posterior sectors during lateral movements both on the working and balancing side (it is not recommended that all load should be borne only by the implant in the canine position).

Occlusal balance49,50 (very useful when both arches have been rehabilitated) is a concept derived from the complete denture that aims to balance the action of the muscles on both sides simultaneously, and thus to balance forces and stress on the two sides of the dental arch. This approach can be defined as a “condition in which there are simultaneous contacts of opposing teeth or tooth analogues on both sides of the opposing dental arches during eccentric movements within the functional range”51. The number of contacts is maximal in maximum intercuspation and during lateral excursions there are simultaneous working and balancing contacts (Figure 3). Although the major disadvantages of this ap-proach are evident (greater technical complexity, more time-consuming), it appears to be the best approach in terms of stability and even distribution of loadings among implants. For natural teeth, a drawback of this type of occlusion is uncertainty about the position of the teeth, and thus of contacts, over time;;31 however, this is not a problem with prostheses permanently an-chored with implants.

Figure 2. Partially edentulous, unilateral free-end. a) Canine present. Canine guidance.

b) Canine absent. Group function.

a b

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Figure 3. Occlusal balance. Simultaneous working and balancing contacts during lateral excursion.

a b

Figure 4. Implant/tooth-supported prostheses. Teeth-implant contacts: a) During light or moderate intensity. b) During high-intensity.

Implant-retained overdenturesDespite occasional reports to the contrary, occlusal balance seems to be the most advisable approach when rehabilitation is of the two arches. This may be more complex to perform when the opposing arch bears natural teeth due to the difficulty of obtaining all lat-eral contacts. In such cases simultaneous balancing and working contacts for some if not all teeth should be sufficient for stabilisation of the prosthesis in biome-

chanical terms: a contact on the balancing side, together with one or various contacts on the working side may imply a reduction in mandibular leverage52. In cases of pronounced resorption of the upper maxillary bone lin-gualised occlusion has been proposed33,53 with contacts established only between the upper palatal cusps and the mandibular central fossae leaving a slight clearance between the buccal cusps;; this approach makes the im-plant loadings more axial, and simplifies the procedure. Another option in cases of extreme maxillary resorption is to establish posterior crossbite, thus achieving more axial implant loading.

Implant/tooth-supported prostheses

This is the most controversial type of implant-prosthe-sis. It is generally accepted that it is not an ideal situation, due to the different biomechanical behaviours of tooth and implant, so that the use of some sort of ‘stress-breaker’ has been recommended such as an interlock or a telescopic crown54-56. Despite this, follow-up studies have indicated good results and biomechanical studies have not observed stress gradients even when stress-breakers are not used, and independently of whether rigid or non-rigid connectors are used57-62. Given that the normal location for this type of prosthesis is in the posterior sector with one or various implants distal, and the supporting tooth mesial, and that there have been no detailed scientific studies of occlusion strategy for this type of prosthesis, we suggest the following guidelines: leave a clearance of about 30-50µm between the occusal face of the implant and the opposing arch with the aim of reducing the moment of the force produced at the

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start of the contact. If the contact is on tooth mesially and implant distally and the opposing arch is similar, due to very rapid intrusion of the tooth even under very light loads (1N may provoke a tooth displacement of 10µm), the situation is analogous to that of an im-plant-supported prosthesis with cantilever, which may lead to overloading of the implant. With the separation proposed the tooth intrudes into, and reaches its apical most position in, the alveolus;; the loads are then dis-tributed between tooth and implant and the prosthesis functions with two fixed abutments (Figure 4). The supporting tooth should have excellent bone support. Rotational movements should be avoided, and there should be no lateral contacts. Under these conditions occlusal overloading is not expected.

ConclusionsThis article has presented occlusal guidelines for the major categories of implant-supported prosthesis with the aim of reducing the risk of occlusal overload. There is widespread consensus about the pathogenic effects of occlusal overload on peri-implant bone resorption, and thus on osseointegration. However, solid informa-tion is not currently available about how loadings differ between the different types of prosthesis, and how load-ings can be modified by adjustment of occlusal contacts. This lack of solid scientific evidence is particularly acute for certain types of prosthesis, such as bilateral free-end prostheses and implant/tooth-supported prostheses.

Much of the information available is derived from occlusal concepts for non-implant-supported dental prostheses and in general, discussions about occlusions are based on personal experience rather than on scien-tific studies63. The clinician must thus be very careful when establishing dental contacts and planning the most appropriate occlusal scheme for each particular case. The guidelines presented here may be useful to this end.

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Correspondence to: Dr Benito Rilo, Prosthodontics Department,

Faculty of Medicine and Odontology, Santiago de Compost-

ela University, 15705 Santiago de Compostela. Spain. Email:

[email protected]

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Implant Occlusion