Occlusion in Implant Dentistry

Australian Dental Journal 2008; 53:(1 Suppl): S60–S68 doi: 10.1111/j.1834-7819.2008.00043.x

Occlusion in implant dentistry. A review of the literature of prosthetic determinants and current concepts
MD Gross*
*Department of Oral Rehabilitation, Tel Aviv School of Dental Medicine, Tel Aviv University, Israel.

ABSTRACT
Today the clinician is faced with widely varying concepts regarding the number, location, distribution and inclination of implants required to support the functional and parafunctional demands of occlusal loading. Primary clinical dilemmas of planning for maximal or minimal numbers of implants, their axial inclination, lengths and required volume and quality of supporting bone remain largely unanswered by adequate clinical outcome research. Planning and executing optimal occlusion schemes is an integral part of implant supported restorations. In its wider sense this includes considerations of multiple inter-relating factors of ensuring adequate bone support, implant location number, length, distribution and inclination, splinting, vertical dimension aesthetics, static and dynamic occlusal schemes and more. Current concepts and research on occlusal loading and overloading are reviewed together with clinical outcome and biomechanical studies and their clinical relevance discussed. A comparison between teeth and implants regarding their proprioceptive properties and mechanisms of supporting functional and parafunctional loading is made and clinical applications made regarding current concepts in restoring the partially edentulous dentition. The relevance of occlusal traumatism and fatigue microdamage alone or in combination with periodontal or peri-implant inflammation is reviewed and applied to clinical considerations regarding splinting of adjacent implants and teeth, posterior support and eccentric guidance schemes. Occlusal restoration of the natural dentition has classically been divided into considerations of planning for sufficient posterior support, occlusal vertical dimension and eccentric guidance to provide comfort and aesthetics. Mutual protection and anterior disclusion have come to be considered as acceptable therapeutic modalities. These concepts have been transferred to the restoration of implant-supported restoration largely by default. However, in light of differences in the supporting mechanisms of implants and teeth many questions remain unanswered regarding the suitability of these modalities for implant supported restorations. These will be discussed and an attempt made to provide some current clinical axioms based where possible on the best available evidence.
Key words: Implant occlusion, implants, dental occlusion, treatment planning. Abbreviations and acronyms: FPD = fixed partial denture; ICD = individual clinical determinant; MI = maximum intercuspation; OVD = occlusal vertical dimension; PDL = periodontal ligament; RCT = randomized controlled trials; TMD = temporomandibular disorder.

INTRODUCTION The intended function of implant-supported restorations is to restore missing teeth and lost elements of the dentition, to maintain or restore form, function and aesthetics and to optimize the longevity of the restored or remaining dentition. Implants and their bony housing need to be planned and placed to support the functional and parafunctional demands of occlusal loading. Occlusal restorative concepts that have evolved through complete denture and fixed toothsupported reconstruction are having to be rethought with the continuing development and advances in implant dentistry.
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In many cases clinicians appear to be applying paradigms transferred from occlusion in the natural dentition, basing treatments on continued empirical decisions in treatment planning and often appear to be experimenting with treatment. Recent concepts of replacing whole arches on four or three implants, angulating implants and supporting occlusal loads on bone substitutes are challenging former paradigms and the clinician is often in a quandary as to what is the most appropriate implant distribution, angulation and position, particularly if an evidence-based approach is considered to be the goal. In terms of evidence-based treatment planning and treatment, clinicians are sorely lacking in sufficient evidence at
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Occlusion in implant dentistry many levels and remain with more questions than answers. A preliminary consideration of the natural anatomy of the dentition, occlusion and alveolar support mechanisms is helpful to provide a perspective for planning and future function of implant supported restorations designed to replace missing elements of the dentition and alveolar housing. Principles of occlusion Posterior support Evolution of the dentition The human dentition appears to have evolved via omnivorous apes and various generations of hominids over a period of six million years. The molars of apes and hominids have not changed significantly. Canines no longer used for killing prey, have become smaller, while the incisors have remained similar in shape. Force distribution to the facial skeleton Posterior teeth crush and prepare food on mastication for digestion and stabilize the mandible for swallowing. The skull is designed with posterior intercuspation providing posterior occlusal support, with maximum force application at the zygomatic base above the first and second molars. In the maxilla, sinus and nasal cavities are located directly above the apices of the teeth with the forces of occlusion distributed peripherally along the facial aspect of the maxilla and premaxilla by in-plane loading. The teeth in the mandible are supported by periodontal tissues in alveolar bone surrounded by thicker mandibular cortices. All maxillary and mandibular teeth except for posterior mandible, generally have very thin buccal plates. Posterior areas of maximal loading tend to be more trabeculated than the anterior, particularly in the mandible. The anterior teeth are used for incision and food preparation with vertical and horizontal overlap varying between Class I, II and III relations in a wide range of normal distribution. Implant challenge to adaptation Restoring the dentition with titanium screw-shaped implants in the residual alveolar bone supporting fixed partial dentures (FPDs) has significantly challenged the adaptive potential of this complex system. Anterior splinted FPDs with distal cantilevers, implants in augmented sinuses, and multiple permutations of possible implant length, diameter, distribution, inclination and pontic options, are all additional challenges to the adaptive potential of the individual case, and to the diagnostic and prognostic abilities of the clinician
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faced with these multiple options, which are often not supported by adequate scientific research. Principal components of the occlusion and their interaction The occlusion may be viewed as consisting of three basic elements: posterior support, occlusal vertical dimension (OVD) and eccentric or anterior guidance.

Posterior teeth provide the posterior occlusal support that bears the often considerable forces of mastication, swallowing and occlusal parafunction, and maintains the occlusal vertical dimension. Eccentric guidance The eccentric guidance is the dynamic contact relation of the teeth as they slide voluntarily from maximum intercuspation (MI) to edge to edge relations in all excursions. This also guides reflex masticatory cycles into maximum intercuspation MI and is the site on which eccentric occlusal parafunction occurs. Parafunction Eccentric occlusal parafunction may generate extremely high and potentially destructive loads, sufficient to wear down the teeth, fracture crowns and roots, decement or break FPDs, dislodge or break abutment screws, fracture porcelain or superstructures, traumatize supporting bone and break implants. Considerations for planning need to focus on minimizing the potential destructive effects of this destructive behavioural phenomenon about which we know very little. Anterior guidance The degree of vertical and horizontal overlap determines whether the anterior teeth disclude the posteriors in protrusion and whether the working side discludes the non-working side in lateral and lateroprotrusive excursions. When the anterior teeth disclude the posterior teeth in all excursions, this has been termed ‘‘anterior disclusion’’ and ‘‘mutual protection’’. Mutual protection is described as the molars protecting the anteriors in MI and the anterior teeth protecting the posteriors in excursions. Mutual protection There is no phylogenetic evidence that this is a consequence of evolutionary specialization. Canines were for killing prey and incisors for tearing meat or
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MD Gross peeling fruit, and not for protecting molar teeth during occlusal parafunction. Neither does there appear to be any convincing evidence that Class II Division I, Class III and other dentitions lacking anterior disclusion, have higher morbidity or greater incidence of temporomandibular disorder (TMD), occlusal parafunction or tooth loss. This is relevant in planning implantsupported occlusions and the interaction of posterior and anterior teeth and implants. Mutual protection and anterior disclusion Mutual protection and anterior disclusion are purported to be desirable restorative occlusal schemes in tooth-supported fixed prosthodontics. Neuromuscular protective mechanisms and the mechanical advantage of a Class III lever are claimed to reduce occlusal loading, parafunction and TMDs. Applying these same principles to implants is problematic. Implants are more often than not supported buccally by thin buccal plates that do not have periodontal receptors and may be susceptible to cervical bone loss with occlusal overload. Considerations will vary between mixed tooth and implant-supported dentitions and between totally implant-supported fixed restorations. In mixed tooth and implant-supported dentitions, decisions need to be made whether teeth disclude implants, or whether implants or teeth and implants support excursive guidance, and whether restorations are independent or splinted. Local biomechanical considerations may outweigh the purported theoretical benefits of neuromuscular protection of anterior disclusion and mutual protection. Considerations of disclusion are complicated by full-arch splinting whereby anterior and posterior segments are no longer independent elements but part of a rigid structure with different biomechanical properties. Interacting prosthetic determinants Interaction of the various determinants of occlusion will also affect planning of implant location, dimensions, inclination, support and occlusal design. Aesthetic tooth display at rest and smiling determines the length of the maxillary crowns. Vertical dimension determines the interarch distance, crown ⁄ implant ratio and crown height space.1 Skeletal and residual anteroposterior and bucco-lingual ridge relations determine the degree of implant inclination, off-axis loading or need for bone augmentation. Planning of implant dimensions, distribution, inclination, support, superstructure design and occlusal schemes should therefore consider these various interactions. These requirements are case-specific, and should be designed to provide adequate posterior support at an appropriate occlusal vertical dimension with an eccentric guidance that optimally distributes the potentially destructive effects
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of excursive occlusal parafunction. The appropriate use of diagnostic preparations, radiographic and surgical guides, provisional restorations, and cross-mounting restorative and surgical modalities can aid in facilitating this challenging clinical task. Occlusal force distribution – teeth versus implants Teeth are suspended in the alveolus with periodontal tissues; may be displaced 25–100 lm vertically and 56–108 lm buccolingually and maintain the alveolus in response to customary functional loading. Excessive load causes trauma at the compression site with subsequent repair and widening of the periodontal ligament (PDL). Teeth with normal support respond to jiggling forces from occlusal overload with resorption, repair, widened periodontal space and increased mobility; in the absence of periodontal inflammation there is no apical loss of attachment. This is a reversible process. However, the combination of a traumatic lesion with periodontitis, causes increased irreversible bone loss. Implants are more rigidly attached to the bone and may be displaced 3–5 lm vertically and 10–50 lm laterally.2 The integrity of the interface is maintained in a steady state by bone ‘‘remodelling’’ as a continuous process of microtrauma and repair.3 Implants lack the adaptive facility of teeth to develop reversible increased mobility when loaded. Current concepts are mixed regarding the peri-implant response to occlusal overload. A phenomenon of fatigue microtrauma has been proposed as the process of cervical and progressive bone loss as bone ‘‘modelling’’ due to excessive occlusal load. When the rate of fatigue microdamage exceeds the reparative rate, cervical bone is irreversibly lost. Dynamic cyclic loading of dog and rabbit tibiae have shown cervical bone loss similar to saucerized cervical bone loss in the clinical situation.3,4 Occlusal overload with restorations in extreme superocclusion showed complete loss of integration in baboons,5 while smaller amounts of supraocclusion showed no bone loss.6 Static loading models with springs between adjacent implants showed no evidence of marginal bone loss at test or control sites with higher bone density and mineralized bone-to-implant contact adjacent to the loaded implants interpreted to be the result of adaptive remodelling to the applied force.7 One study in monkeys with repetitive mechanical trauma showed no histological effect on ligature-induced periimplant bone loss either in healthy or diseased implant sites after four months.8 In a recent beagle dog model, ligature-induced peri-implantitis with occlusal overload caused more marginal bone loss than peri-implantitis alone. In the presence of plaque-induced peri-implant inflammation overloading aggravated plaque-induced bone resorption, and increased bone loss on the buccal and lingual sides of the implant.9 Osseoperception and
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Occlusion in implant dentistry the presence of mechanoreceptors at the bone-implant interface have recently been shown, supporting the hypothesis for the presence of sensory feedback from loaded implants.10 Biomechanical studies Several physical and mathematical modalities are used to simulate occlusal loading. These include 2- and 3-D photoelastic models, strain gauge analysis and 2- and 3-D finite element analysis. While each has inherent advantages and weaknesses they essentially demonstrate points or areas of relative stress concentration in modelled superstructures, implants and supporting structures. Some quantify the degree of strain and relate these to calculated values of fatigue overload in bone deformation models. Some correlation is seen with animal and clinical outcomes as they all show an increased amount of cervical stress concentration with increased degree of load, off-axis inclination and bending moments. While not being directly applicable they have value in indicating relative degrees of biomechanical risk for differing loading situations.11 Splinting The question of splinting is relevant to a discussion of occlusion. Tooth connection particularly of mobile teeth has been considered advantageous in increasing the collective resistance of the splinted superstructure to lateral forces, which may be further enhanced by noncollinear or cross-arch splinting. This concept is challenged in various ways when considering splinting teeth to implants or splinting adjacent implants. Connection of teeth and implants has been confounded by the potential overload of the implant due to differential resiliency, by the problem of retrievability of a rigid connection, and the potential tooth abutment intrusion with a non-rigid connection. Implant connection particularly with unfavourable crown ⁄ implant ratio has been linked with increased torque loads and bending moments to the implant, abutment, crown and supporting bone. Mandibular flexure and retrievability for repairing damaged superstructures are considered when deciding on full-arch or segmental splinted units.12 In a systematic review of tooth to implant connections, intrusion of the abutment teeth occurred on non-rigid connection in 5.2 per cent of cases. Implant failures (mobility or fractures) occurred (3.4 per cent) in five years and 15.6 per cent after 10 years. Abutment teeth were lost (3.2 per cent) after five years and 10.6 per cent after 10 years. The survival rate of tooth and implantconnected FPD was 94.1 per cent after five years and 77.8 per cent at 10 years. The conclusions were that the freestanding solution is the primary option of choice. To avoid intrusion of abutment teeth, the connection,
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if made, should be rigid.13 In another review comparing implant success for implant versus implant-tooth supported FPDs the success rate was higher for implant support alone with 97 per cent for implant support, and 89 per cent respectively with no statistical difference for implant-tooth-supported FPDs.14 Some authors advocated separation of mandibular superstructures in the midline and cite mandibular flexure as a potential source of distal implant morbidity in full-arch restorations. However, many outcome studies reporting high success rates have not identified this factor or failed implants as a consideration in bone loss. Considerations for replacing posterior teeth with implant-supported fixed partial dentures Reduced posterior support When the posterior teeth become progressively lost, this situation may be restored with tooth-supported FPDs, or may continue to function with occluding premolars as a shortened dental arch. When the supporting bone is inadequate or abutment span too large, additional implant-supported units may be considered necessary. Loss of posterior support: multiple restorative permutations With loss of all molars and premolars, the restorative options available allow multiple permutations of implant arrangements, lengths, angulations and superstructure designs. Residual ridge height, bone density, maxillary sinus morphology and inferior alveolar nerve relations are significant determining factors. Options range from minimal single premolar implants or a single premolar implant connected to the adjacent natural canine, to increasing combinations of 2, 3, or 4 posterior implants. These may be adjacent, connected or single, or may be spaced for interposed pontics or for inclusion of distal or mesial cantilevers. Sinus augmentation, horizontal and vertical ridge augmentation further extends the range of clinical options. Biomechanical considerations arise with axial or non-axial implant inclination and crown-implant ratio. Changing paradigms obviate the need to provide molar support solely to avoid overload of the temporomandibular joints with the emerging acknowledgement of the shortened dental arch. The clinician is constantly faced with the dilemma of which of these multiple options to apply and must rely on the best available evidence at the time. Since many of these modalities are not isolated in clinical trials, this evidence is sadly lacking. Clinical decisionmaking must, as a result, be made more subjectively, taking into account patient health age, psychosocial
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MD Gross behavioural and socio-economic factors, and necessarily incorporate the cognitive and personal biases, education and experience of the individual clinician. Number of implants Studies of clinical outcome generally do not isolate the prosthetic and abutment variables. So while high partially edentulous FPD success rates of 95 per cent at 10 years are reported, the abutment distribution, number, length and diameter are not specified.14,15 One split-mouth study restoring mandibular Kennedy Class I cases compared posterior rigid short-span tooth to implant connected FPDs, with contralateral lone standing FPDs on two implants. Cumulative success rates at 10 years were 88.4 per cent for the tooth-implant FPDs with no outcome difference compared with the contralateral implant-supported FPDs. Sixty-nine implants were used at the outset.16 Another study showed no difference between 2 and 3 implants at five years. A systematic review of posterior restored quadrants with sinus augmentations reviewed 39 ⁄ 252 articles (3 RCTs) with 6913 implants in 2046 subjects. They showed overall implant survival rates of 92 per cent in the 39 articles where 96 per cent were roughened surfaces and 86 per cent were machined surfaces.17 Outcome results of posterior mandibular short-wide implants vary between studies ranging from 67–100 per cent. Co-variables of surgical technique, implant surface, bone volume and density, may obscure the effect of implant length and diameter. Studies after 1997 taking into account bone density and surface finish with microtexture versus machined co-variables, report comparable survival rates for short and standard length implants. The co-variable of implant surface from machined to microtexture significantly improved short and wide implant prognosis.18 Thus comparable success rates are shown for minimal and maximal options. Clinical decision-making will need to be guided by case-specific patient factors including psychosocial and socio-economic or subjective, such as individual patient preference regarding chewing efficiency, aesthetics and comfort. Prosthetic combinations with insufficient clinical outcome research include situations with a single maxillary distal implant connected to a natural canine or anterior FPDs, long-span posterior implant-supported restorations, implants as pier abutments in an FPD with peripheral natural tooth abutments, excessive crown implant ratio >1:1, extreme off-axis angulations >30° and varying bone factors. Occlusal vertical dimension (OVD) considerations Lost posterior support may result in posterior overclosure and loss of occlusal vertical dimension of
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occlusion. Restoration of posterior teeth may necessitate increasing OVD to increase posterior inter-ridge distance, increasing vertical crown space for prosthetic convenience and enhanced aesthetics. Alternatively, with severe vertical bone loss, excessive inter-ridge distance and unfavourable crown ⁄ implant ratio, considerations may be directed to decreasing the OVD with appropriate consideration of the reduced tooth display for aesthetics. When skeletal and aesthetic determinants predicate a severe anterior vertical overlap, the need to increase the occlusal vertical dimension and flatten excursive guiding inclines may be considered to reduce lateral loading vectors and should be weighed against the potential for an increased crown ⁄ implant ratio and the additional restorative commitment of restoring an entire dental arch. Excursive guidance Considerations for excursive guidance vary between mixed partially edentulous and fully edentulous implant supported modalities. In partially edentulous situations when anterior teeth remain with good bone support they may be used to disclude posterior implants in protrusion. Strong healthy canines or incisors can guide lateral movement discluding posterior implants and separating non-working contact conforming to tested tooth-supported conventional paradigms. When anterior implants are required to bear the protrusive excursive contact, questions arise as to how many implants are necessary, which lengths and diameters are indicated, how off-axis the implants may be inclined, and whether buccal bone augmentation is necessary. Similar considerations apply for lateral guidance when posterior implants must be employed for working guidance in the absence of suitable natural tooth guidance. Decisions must be made on whether to distribute lateral load over all working-side contact in group function, how far distally the group function should extend or where the traditional paradigm of anterior guidance be considered. Insufficient outcome studies are available to help with these decisions. Here too most outcome studies fail to isolate the relevant clinical parameters. Many clinical technique articles refer to aesthetic factors in anterior implant-supported restorations. Biomechanical studies show that off-axis loading and increased vertical overlap increases the facial loading vectors with increased cervical and facial stress concentrations. Other clinical studies show the presence of facial cratering together with interproximal bone loss. It is not clear whether the purported neuromuscular benefit ascribed to anterior disclusion and mutual protection in the natural dentition applies to anterior implant-supported protrusive guidance. Although an
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Occlusion in implant dentistry osseoperceptive mechanism is recognized, its contribution to an anterior neuromuscular protective function is unknown. In addition, the thin buccal bone support and different implant-bone interface connection and loading response, compared with the natural PDL, makes the concept of protrusive disclusion as a protective element for posterior teeth or implants dubious. This uncertainty applies also for canine guidance where buccal bone covering implants in the maxillary canine region would not appear to have the same biomechanical and sensory properties as with natural canines. When aesthetic and skeletal factors predicate, guidance on anterior implant-supported restorations may be unavoidable. In these cases multiple contact distribution, maximal implant length optimized bone resistance and splinting of adjacent implants should be considered to reduce potential morbidity. The Class III lever action of mandibular closure may play a role in reducing loading but each case would need to be planned according to individual clinical determinants (ICDs). It is often difficult to choose between a mild implant-supported anterior disclusion or a flat protrusive guidance with simultaneous posterior contacts. Adhering to traditional paradigms with a mild disclusion is tempting, however there is no evidence to support either approach, or many of the other clinical dilemmas outlined above. In Class II Division I or Class III skeletal relations, the protrusive and working guidance would benefit from flattened guiding inclines with optimal distribution of excursive load on as many abutments as possible, with smooth even guiding contacts to minimize unfavourable biomechanical risk. Bruxism should be diagnosed and addressed as a complicating and additional risk factor. The use of a full-arch night splint may be beneficial in reducing potential overload from nocturnal parafunction. In spite of the fact that some reviews show that bruxism has not been causally linked to supporting bone morbidity, its potential for creating complication in the superstructure and implant stack are very real. Thus common sense tempered with psychosocial and socio-economic patient factors must be the guiding determinants for decision making in treatment planning and occlusal design at present. Evidence and history of occlusal parafunction is a significant factor in the planning of excursive guidance and the creation of an occlusal scheme with abutment and bone support optimally designed to minimize the potentially destructive forces of bruxism. Flattening guiding inclines, increasing implant numbers and bone support, reducing occlusal vertical dimension to decrease crown-root ratio and minimizing porcelain occlusal surfaces without excessively compromising tooth display should be considered.
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Table 1. Single tooth implant-supported restoration – optimal criteria and current paradigms
Single tooth implant-supported restorations – posterior Axial posterior inclination at right angle to the occlusal plane is still the optimal paradigm (>30° controversial) Length ‡ 10 mm Diameter ‡ 3.75 mm Centred contacts (point centric or freedom in centric 1–1.5 mm) Narrow occlusal table Flat cusps Minimal cantilever No CR-MI slide (RC-IC syn. old), working, non-working or protrusive interferences Avoid creation of excursive guidance on single implant restorations.

Table 2. Posterior fixed implant-supported restoration clinical guidelines, current paradigms considerations and controversies. Considerations governed by case-specific individual clinical determinants
Posterior fixed implant-supported restoration Axial implant inclination at right angles to the occlusal plane when possible. Mesio-distal inclination and bucco-lingual angulation >30° is still controversial. Inter-implant distance to be not less than 3 mm. Number of implants may vary between 1–4 per quadrant. (The greater the number the smaller the biomechanical risk.) Splinting of adjacent implants is current practice (unconnected adjacent implants is still controversial). Lone-standing self-supporting implant segment is preferable. Crown ⁄ implant ratio >1:1 is biomechanically unfavourable. Rigid connection to adjacent teeth is less preferable but acceptable in small spans (risk of tooth abutment intrusion on non-rigid connection). Diameter ‡ 3.75 mm (smaller diameters increased risk factor). Length ‡ 10 mm (smaller wider implants at increased risk). Centred contacts in maximum intercuspation MI (point contact or freedom-in-centric). Restore in centric relation or established intercuspal relation according to conventional tooth-supported paradigms. Full-arch simultaneous contact (infra-occlusion of implant restorations in relation to teeth is controversial). Narrow occlusal table when possible. Steep cusps increase biomechanical risk and bending moments. Use bucco-lingual cross bite when necessary. Avoid cantilevers when possible (the greater the bucco-lingual, mesial or distal cantilever the greater the risk). Mesial cantilever is biomechanically more favourable than a distal cantilever. Infra-occlusion on cantilevered section reduces biomechanical risk. Excursive guidance on well-supported anterior natural teeth discluding posterior implant supported segment when possible. Single excursive contact on implant-supported restoration places restoration, implant-abutment-crown and supporting bone at greater risk (avoid posterior interferences). When working group function on posterior implant supported segment is selected, flattened cusps and smooth even contacts in group function are desirable to reduce biomechanical risk. Working guidance should separate non-working contact. Working guidance should be supported by optimal abutment buccal bone dimension to maximize lateral resistance and reduce biomechanical risk. Sufficient metal support for porcelain should be established. Use of full-arch night splint is recommended particularly if bruxism is diagnosed or suspected.
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MD Gross Fully edentulous considerations ˚ nemark’’ designs with interOriginal ‘‘ad modum Bra foraminal mandibular implant distribution and premaxillary maxillary distribution with distal cantilevers showed high long-term success rates and often mimicked the shortened dental arch. Extended options for sinus augmentation and increased number of posterior implants allow more posterior abutments with the anterior segment supported with anterior canine to canine pontics. This facilitates greater aesthetic control but may create an extended anterior cantilever. Alternative options of 4, 5, 6, 8 or 10 implants per arch are advocated and create treatment planning dilemmas between minimal and maximal options.19 Decision making for treatment planning must be based on psychosocial, psychophysiological and economic patient-specific factors and governed by patientinformed preferences for the available options. To this effect the urgent need for high level long-term outcome studies for appropriate evidence-based planning and determination of prognosis, has never been more pressing. In fully edentulous planning, the interaction of skeletal relations, residual ridge relations, vertical dimension, supporting anatomy, interarch distance and aesthetic factors of occlusal plane orientation, tooth display and lip support are significant determinants for planning implant positioning, support and occlusal scheme design. Their interaction must be established in advance with the appropriate application of diagnostic preparations to facilitate radiographic and surgical guides. Each case must be planned in accordance with its own particular combination of individual clinical determinants. The guiding treatment objectives must be those outlined as the guiding parameters for prosthodontics as restoring and maintaining, health, form, function, comfort and aesthetics. The guiding principles of occlusal restoration should be to create an appropriate posterior occlusion to support forces of mastication, swallowing and occlusal parafunction, at an optimum occlusal vertical dimension, with excursive guidance which is appropriate to the planned supporting base of integrated dental implants. Best available evidence (BAE) – hierarchy of evidence The aspiration and need for studies to provide the necessary evidence to answer the many clinical questions outlined above is clear. It is becoming more and more difficult to keep up with the ever growing published implant-related literature. Hierarchy of evidence Since clinical evidence is highly varied in scientific validity and clinical applicability, a hierarchy of

Table 3. Anterior fixed implant-supported restoration clinical guidelines, current paradigms considerations and controversies. Considerations governed by case-specific individual clinical determinants
Anterior fixed implant-supported prosthesis Minimal buccal bone of 2 mm thickness. Augmentation of buccal bone would appear to improve biomechanical resistance to facial loading but indications are as yet undefined (biomechanical durability and longevity of buccally augmented bone unproven). Lengths > 10 mm. Diameters < 3.75 mm sometimes unavoidable but at greater risk of interface component fracture. Crown ⁄ implant ratio >1:1 becomes biomechanically unfavourable with increased risk. No absolute criteria for contraindication based on clinical outcome data. Splinting adjacent anterior units is currently accepted paradigm. The suitability of wider implants with less buccal bone thickness but greater bone-implant surface area as opposed to less wide implants with greater buccal bone volume and lateral resistance is still under debate. Number of implants 2–6 (depends on bone dimensions width of arch and aesthetic factors. No evidence-base to define minimum acceptable number and dimensions). Vertical and horizontal overlap (overbite, overjet syn. old) – flatten or round-out protrusive and working guiding inclines to reduce lateral forces when possible (within limitation imposed by skeletal relations and aesthetic factors of tooth display and lip support). Contact in MI simultaneous with remaining posterior quadrants, skeletal and relations permitting (anterior MI contact in infraocclusion not substantiated). Selective excursive guidance – chose protrusive and working guidance according to the best biomechanical abutment distribution. Skeletal Class II Div I: mild retrognathia – flat lingual incisal platform within phonetic and comfort limitations. Severe retrognathia – protrusive guidance on mesial maxillary premolar inclines. Skeletal Class II Div II (increased vertical overlap, deep bite syn.old.) increased biomechanical risk when unavoidable. Raising OVD to flatten anterior guidance requires full-arch restoration at an increased iatrogenic biological risk and economic burden. Skeletal Class III flat protrusive guidance – mild anterior disclusion to slightly disclude posterior teeth or combine premolar protrusive contact according to case-specific clinical determinants. Conventional paradigms of protrusive anterior guidance separating posterior tooth contact for Ômutual protectionÕ are challenged. Stronger elements should bear the excursive guidance separating weaker elements when appropriate. Use of full arch night splint is strongly recommended particularly if bruxism is diagnosed or suspected.
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Occlusion in implant dentistry evidence categorizing validity has been presented. Prospective randomized controlled clinical trials (RCTs) give the highest levels of evidence followed by retrospective case series, animal studies, biomechanical studies, opinion-based reviews and case reports. Review papers Review papers are published to assimilate this ever changing knowledge base. Systematic reviews The highest level of review paper is the systematic review in which meta-analysis is used in assessing outcome studies including only prospective RCTs of the most stringent scientific rigour. The problem is that so few publications of this nature are available that the conclusions are often limited and of minor clinical usefulness. In many cases the available retrospective case series are reviewed.

Table 4. Fully edentulous fixed implant supported restoration clinical guidelines, current paradigms considerations and controversies. Considerations governed by case specific individual clinical determinants
Fully edentulous – fixed prosthesis Number of implants per jaw is controversial Maxilla: 6–8 implants acceptable, 4 implants controversial, 10 and more might be considered as over treatment? Mandible: 5–8 implants acceptable, 3–4 implants controversial, 10 and more might be considered as over treatment? Angulation: conventional paradigms of axial inclination at right angles to plane of occlusion. >30° inclination controversial. Potential support and biomechanical difference between mesio-distal and bucco-lingual inclinations. Occlusal vertical dimension initially determined according to conventional complete denture paradigms. These may need to be modified by individual clinical determinants of aesthetic display at rest and smiling, occlusal plane display, lip support, interarch distance, inter-ridge relations crown height space and crown to implant ratio. Cantilevers: distal cantilevers of 1 premolar unit, safe >1 premolar unit is controversial. Compressive resistance to load is provided by the two implants adjacent to the cantilever further implants do not participate in load distribution. Mesial cantilevered anterior segment is biomechanically potentially unfavourable when extended significantly anterior to the most mesial of the posterior supporting implants. Splinting cross-arch versus segmental. Conventional paradigms consider cross-arch splinting to provide composite resistance for supporting structures to lateral vectors of functional and parafunctional loading. A strain gauge study shows no difference between cross-arch and segmental splinting for both fixed and removable superstructure. Splinting increases bending moments (still controversial). Crown ⁄ implant ratio >1:1 increased biomechanical risk. Minimize vertical overlap and flatten guiding cusp inclines. Occlusal schemes dependent on case specific Ôindividual clinical determinantsÕ ICDs (skeletal relation, implant distribution, occlusal vertical dimension, posterior support, interarch distance, crown implant ratio, segmental inclinations, aesthetic occlusal plane orientations, tooth exposure, lip support etc.). Protrusive guidance should be as flat as possible according to the individual clinical determinants. Anterior disclusion or flat protrusive group function will depend on ICDs. Working guidance as group function with optimal load distribution and flattened guiding inclines should separate non-working contact. Insufficient evidence available regarding the appropriateness of simultaneous working and non-working lateral guidance.

Table 5. Fully edentulous removable overdenture clinical guidelines, current paradigms considerations and controversies
Implant-retained overdentures Use conventional complete denture paradigms for aesthetics, occlusal planes, occlusal vertical dimension, centric relation intercuspation and bilateral balanced occlusion Plan support to be combined tissue and implant supported or tissue supported and implant retained integrating support factors into occlusal loading scheme. Connected implants with bar versus single stud attachments is controversial particularly in the maxilla. Bar retained components are implant supported. Stud retained segments may be made tissue supported with suitable relief. Balanced occlusion is advocated to avoid denture base displacement in the final gliding occlusal phase of mastication following bolus reduction. (If attachments prevent denture base displacement the need to achieve full balance may be amended appropriately.) Lingualized occlusion may be considered to facilitate bilateral balance. For a single complete overdenture opposing the natural dentition, balance can be technically difficult to achieve. Attempt to achieve at least three point balance on lateral and protrusive excursion. Increase vertical dimension and alter plane relation to allow for vertical space for attachment housings and metal framework space if necessary. Decrease vertical dimension if interarch distance is excessive and poses a biomecnahical risk. Keep attachment height minimal to avoid unfavourable torqueing moments on implants. Horizontal axis of rotation of the denture base round anterior attachments is purported to reduce distal cantilever effect on loading of distal denture saddles. This and a lack of indirect retention causes distal denture displacement on anterior closure increasing need for protrusive balance With anterior and posterior implant supported attachments, enhanced retention and resistance reduces the need for balance to prevent distal base displacement.
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MD Gross General review The general review paper, of which this paper is an example, provides an overview choosing selective examples of studies of various categories and discussing their clinical applicability and relevance.20 The weakness is that such reviews tend to incorporate the opinions and bias of the reviewer which, while being appropriate, may vary between reviewers; a fact that should be borne in mind. Such general reviews may incorporate clinical guidelines as axioms, paradigms, concepts and current practice that are highly practical but are in many cases not evidence-based due to lack of available evidence. Clinical axioms and current paradigms Clinical factors and determinants relevant to planning and fabrication of posterior, anterior and fully edentulous implant-supported fixed restorations are shown in Tables 1–5. Restorations should be planned in advance on an articulator with a diagnostic set up, and radiographic and surgical guides used when necessary. Computerized guidance systems may enhance planning and surgical accuracy particularly with unfavourable ridge and anatomical relations. Clinical outcome studies in most instances do not isolate many of the prosthetic details listed in Tables 1–5, so at present, and until further research is available, clinical guidelines need to be based on biomechanical principles and the state-of-the-art of current therapeutic paradigms. REFERENCES
1. Misch CE, Goodacre CJ, Finlay JM, et al. Consensus conference panel report: crown-height space guidelines or implant dentistry – Part 2. Implant Dent 2006;15:113–121. 2. Kim Y, Oh T-J, Misch CE, Wang H-L. Occlusal considerations in implant therapy: clinical guidelines with biomechanical rationale. Clin Oral Implants Res 2005;16:26–35. 3. Hoshaw SJ, Brunski JB, Cochran GVB. Mechanical loading of ˚ nemark implants affects interfacial bone modelling and Bra remodelling. Int J Oral Maxillofac Implants 1994;9:345– 360. 4. Duyck J, Naert I, Van Oosterwyck H, et al. The influence of static and dynamic loading on marginal bone reactions around osseointegrated implants: an animal experimental study. Clin Oral Implants Res 2001;12:207–218. 5. Isidor F. Influence of forces on peri-implant bone. Clin Oral Implants Res 2006;17(Suppl 2):8–18. 6. Miyata T, Kobayashi Y, Araki H, Ohto T, Shin K. The influence of controlled occlusal overload on peri-implant tissue. Part 3: a histologic study in monkeys. Int J Oral Maxillofac Implants 2000;15:425–431. 7. Gotfredsen K, Berglundh T, Lindhe J. Bone reactions at implants subjected to experimental peri-implantitis and static load. A study in the dog. J Clin Periodontol 2002;29:144–151. 8. Hurzeler MB, Quinones CR, Kohal RJ, et al. Changes in periimplant tissues subjected to orthodontic forces and ligature breakdown in monkeys. J Peridontol 1998;69:396–404. 9. Kozlovsky A, Tal H, Laufer B-Z, et al. Impact of implant overloading on the peri-implant bone in inflamed and non-inflamed peri-implant mucosa. Clin Oral Implants Res 2007;18:601– 610. 10. Wada S, Kojo T, Wang Y-H, et al. Effect of loading on the development of nerve fibres around oral implants in the dog mandible. Clin Oral Implants Res 2001;12:219–224. 11. Sahin S, Cehreli MC, Yalcın E. The influence of functional forces on the biomechanics of implant-supported prostheses—a review. J Dent 2002;30:271–282. 12. Miyamoto Y, Fujisawa K, Takechi M, et al. Effect of the additional installation of implants in the posterior region on the prognosis of treatment in the edentulous mandibular jaw. Clin Oral Implants Res 2003;14:727–733. 13. Lang NP, Pjetursson BE, Tan K, Bragger U, Egger M, Zwahlen M. A systematic review of the survival and complication rates of fixed partial dentures (FPDs) after an observation period of at least 5 years. II. Combined tooth–implant-supported FPDs. Clin Oral Implants Res 2004;15:643–653. 14. Weber H-P, Sukotjo C. Does the type of Implant prosthesis affect outcomes in the partially edentulous patient? Int J Oral Maxillofac Implants 2007;22(Suppl):140–172. 15. Pjetursson BE, Tan K, Lang NP, Bragger U, Egger M, Zwahlen M. A systematic review of the survival and complication rates of fixed partial dentures (FPDs) after an observation period of at least 5 years. I. Implant supported FPDs. Clin Oral Implants Res 2004;15:625–642. 16. Gunne J, Rangert B, Glantz P-O, Svensson A. Functional loads on freestanding and connected implants in three-unit mandibular prostheses opposing complete dentures: an in vivo study. Int J Oral Maxillofac Implants 1997;12:335–341. 17. Del Fabbro M, Testori T, Francetti L, Weinstein R. Systematic review of survival rates for implants placed in the grafted maxillary sinus. Int J Periodontics Restorative Dent 2004;24:565–577. 18. Renouard F, Nisand D. Impact of implant length and diameter on survival rates. Clin Oral Implants Res 2006;17(Suppl 2):35– 51. ˚ nemark P-I, Svensson B, van Steenberghe D. Ten-year survival 19. Bra rates of fixed prostheses on four or six implants ad modum ˚ nemark in full edentulism. Clin Oral Implants Res Bra 1995;6:227–231. 20. Taylor TD, Wiens J, Carr A. Evidence-based considerations for removable prosthodontic and dental implant occlusion: a literature review. J Prosthet Dent 2005;94:555–560.

Address for correspondence: Professor Martin D Gross Head of Graduate Prosthodontics Department of Oral Rehabilitation Tel Aviv School of Dental Medicine Tel Aviv University Israel Email: [email protected]

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