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Occlusal schemes have long been discussed in the dental literature. Various theories have been proposed, and the choice of occlusal pattern in fixed and removable prosthesis is not equivocal. Furthermore, the occlusal patterns in use are based on expert opinion and clinical experience rather than evidence-based studies.
The use of dental implants is common practice nowadays, and the dentist is required to design and create occlusal contacts on implant-supported restorations. Successful treatment calls for stable and long-lasting osseointegrated implants supporting functioning restorations. The main question is: What are the factors that challenge the osseointegration? The dental literature presents two main approaches. According to the first, loss of bone around the implant is primarily due to inflammation and infection proccessess.1,2 The second approach associates implant load with bone loss and implant failure.3-5
Implant-supported fixed dental prostheses are subject to occlusal forces of mastication, swallowing, and parafunction. The skeletal and muscular systems transfer the force to the prosthesis and the supporting one or more implants and to the supporting tissues. Overloading can lead to the mechanical failure of parts of the prosthesis or implants.6 Mechanical and structural solutions have been suggested in order to disperse the forces more correctly and evenly to overcome the load.7 As failures due to infection have been described at great length in the dental literature and are not in question, the focus in this review is on how normal load or overload may lead to loss of implant osseointegration.
In a 1994 editorial regarding dental implant research, Taylor8 states: “There is absolutely no objective evidence that occlusal design, concept, material, or pattern is of any consequence to the health of an osseointegrated implant.” Is that still so today?
The available literature dealing with this topic will be reviewed first to determine whether scientific evidence is satisfactory relating to the effect of occlusal load on osseointegration. Thereafter, recommendations found in the literature for specific occlusal schemes for implant-supported prostheses will be discussed and their validity will be examined.
Bone Response to Load
As the titanium alloys are several-fold harder than the cortical bone, when the implant is occlusally loaded, the force is transferred to the bone, primarily to the cortical bone.9,10 The bone tissues adapt mechanically and biologically to the applied load, which is transferred mainly by the muscles and leads to deformation of the bone, which reacts by continuous remodeling.11 Under regular occlusal load, the bone undergoes stable rearrangement. If the overload is greater than the bone can bear, fracture may occur. Frost12 suggested that overload, which remains below the fracture threshold, may increase the damage to the tissue, thereby increasing the creation of new bone. This theory is in accordance with findings showing greater bone density around loaded implants than around nonloaded ones.13-15 In studies with dog models, a greater bone-to-implant contact was observed after a 10-month load, although bone density did not increase.16 Furthermore, application of lateral force to implants with peri-implantitis caused an increase in bone density compared with that in nonloaded implants.17 It was found that when the load was small and did not cause a sufficient level of stress in the bone, loss of bone occurred. As for force direction, it was shown in vitro that a nonaxial load caused greater strain forces around the implant than axial forces.7 Although in vivo evidence regarding the survival of tilted implants and supported prostheses is accumulating,18 but no significant difference in marginal bone loss has been found between tilted and axial implants in either jaw after one19 and five20 years, respectively.
The significance of several factors was suggested in creating a load on the implants: cantilever, parafunction, premature contacts, cusp inclination, low bone quality, and a small number of implants.21
Cantilever
The implant-supported cantilever was shown to be associated with loss of bone around the implant and restoration failure.3,22,23
Duyck et al24 examined the occlusal load of cantilevers in 13 patients with implant-supported full-arch restoration. A 50-Newton force was applied to various points on the arch, the restoration supported by five to six implants and later by three to four implants. The results showed that loading of the restoration cantilevers led to compressive forces on the implant closest to the site of loading. As expected, stronger forces were observed on each implant when the number of implants was reduced. In 1988, Lindquist et al22 found greater bone loss around load-bearing implants with a distal cantilever in the anterior mandibular region. Clenching was considered the main reason for bone loss. In 1996, the same group presented different findings: Crestal bone loss in a 10-year follow-up was mainly due to smoking and poor oral hygiene habits and not necessarily caused by occlusal load.25 Upon examining mesiodistal cantilever length, more failure rates were found for cantilevers that were more than 15 mm long than for ones that were shorter.23 As expected, the number of contacts and their distribution greatly influenced force distribution. Examination of patients with mandibular implant prostheses versus full upper dentures showed that most forces were concentrated on the cantilever.26 Consequently, the present thinking is that the force distribution caused by the cantilever is not preferable to prostheses and implants. However, no nonequivocal evidence shows any association between the cantilever and bone loss around the implants.
Parafunction
Scientific evidence is contradictory regarding the connection between parafunction and bone loss around implants.27 Findings from a retrospective study revealed the association during the first year between crestal bone loss in both jaws of patients with parafunction and lack of anterior contacts.22 Bruxism was associated with technical complications but not with any effect on biologic failure.28
Naert et al29 suggested that occlusal load was the most probable cause of bone loss around implants. However, Engel et al30 examined 379 patients with implant-supported dentures and did not find any association between wear and horizontal bone loss. A prospective 15-year study did not show any association between failure and high occlusal loads.25 Despite reports of bone loss due to load, no causal association was found5 and a comprehensive review determined insufficient evidence to support the connection between bruxism and implant failure.27
Animal Studies
Several attempts were made to examine the effect of occlusal loading on the osseointegration of implants in animals. Using monkeys, Isidor31 investigated the effect of increased lateral load after loading. He found a loss of osseointegration in five of eight implants that suffered from lateral load, while the effect of oral hygiene on loss of bone support was not observed. Later, similar findings were obtained from histologic examinations. Miyata et al31 placed implants in four monkeys and overloaded on premature contacts of 100 µm, 180 µm, and 250 µm. The implants were examined clinically, radiographically, and histologically. Bone loss was observed in the 180-µm and 250-µm groups. Bone loss on application of exaggerated force on the implants is possible, even in the absence of inflammation.32 In a study by Hoshaw et al,33 when repeated loading was exerted on canine tibia implants, significantly greater bone loss was observed in nonloaded implants. Findings from a comparison between statically and dynamically increased load in rabbits showed that the dynamically increased load caused more bone absorption and the formation of a bone crater around the implants.34
Kozlovsky et al35 examined the effect of overload on bone level and bone-to-implant contact in the presence and absence of inflammation of the supporting tissues. Four implants were placed on both sides of the mouth in four dogs, and exposed after 3 months. Half the implants were loaded with premature contacts; the remainders were without premature contacts with the opposing natural teeth. During the 1-year follow-up, inflammation was induced on one side, whereas strict oral hygiene was maintained on the other side. On comparison with the overloaded implants, no difference in clinical parameters was observed between the inflamed and noninflamed implants. All the inflamed implants exhibited significant bone loss, overloading in combination with peri-implantitis causing significantly greater bone loss than peri-implantitis alone. The results showed that in the absence of inflammation, overload had a marginal effect on bone loss. In a similarly designed experiment in a monkey model, Hürzeler et al36 performed repeated orthodontic overload on implants in the presence and absence of inflammation and found no histologic difference in the supporting tissue of the two groups. An additional investigation in dogs supported these results. In implants loaded with premature contacts for 8 months, no difference in bone loss was observed between the loaded implants and controls when strict dental hygiene was maintaned.37
Differences Between Implants and Teeth
Natural teeth are supported by a flexible attachment apparatus with the ability to absorb stress and comply with the load; whereas an implant undergoes osseointegration and is tightly connected to the bone.38 The physiologic movement of the natural tooth is 25 µm to 100 µm, as opposed to only 10 µm to 50 µm in the case of implants.7 Upon load, the natural tooth undergoes two phases of movement. The first is nonlinear, due to the flexibility provided by the periodontium. This is followed by the elastic linear phase. The implant undergoes only elastic and linear movement, dependent on the flexibility of the bone.39 In addition, via the attachment apparatus, the natural tooth disperses the load throughout the root area; the load on the implant is concentrated mainly in the cortical region.
These differences create a significantly different response during overload. The natural tooth exhibits mobility, wear, pain, fremitus, and thickening of the periodontium,21 ie, indications of occlusal trauma. In implant restoration, mechanical failure is observed and, as mentioned above, some investigators claim that a loss of bone support occurs. There are also differences in proprioceptive perception and in the arch reflex spectrum.40 In addition, the awareness of the patient to occlusal interference in the two cases differs: whereas interference between opposing teeth was sensed from 48 µm, in the teeth opposite, the implants interference was sensed at 20 µm.41
Occlusal Concepts Suggested in the Literature
The picture obtained from the scientific literature is ambiguous and does not succeed in creating a causal connection between load and implant failure or even between overload and implant failure. A clear conclusion cannot be drawn regarding the effect of load, overload, cantilever, or parafunction on loss of osseointegation.
Several reviews deal with these aspects and attempt to provide analyses and recommendations.7,21,42-44
Taylor et al42 reviewed the relevant literature and concluded that no scientific evidence supported the association between occlusal factors and biologic results, noting that the theories and recommendations were based mainly on the expert opinion.
Carlsson43 maintained there was no scientific evidence in favor of a particular clinical scheme and suggested that good occlusion on implant-supported dentures complies with the old concept of therapeutic occlusion, as put forth by Beyron45 in the in 1950s. Carlsson43 also provided guidelines for therapeutic occlusion, which included ensuring an acceptable vertical facial height post-treatment, adequate interocclusal distance with the mandible in a resting position, well-distributed contacts in maximal intercuspation, and no soft-tissue impingement during occlusal contact.
In view of the above, Carlsson claimed that a successful implant-based prosthesis was possible, using simple and traditional registration methods. Furthermore, he maintained that occlusal forces are of marginal significance in treatment success, adding, “At present, it seems prudent to accept that principles and methods applied in conventional prosthodontics can in general be used also for implant prostheses.”
Kim et al21 reviewed at great length the differences between the attachment mechanisms of natural teeth and implants. As opposed to Carlsson’s work, the basic premise of the article was as follows: “Implants may be more prone to occlusal overloading, which is often regarded as one of the potential causes for peri-implant bone loss.”
Their analysis and recommendation were based on the above assumption. Several possible factors were mentioned as responsible for overload: cantilever, parafunction, premature contacts, large occlusion table, large cusp inclination, low availability and quality of the bone, and a small number of implants.
The article presented factors to consider for implant occlusion, with emphasis on three main considerations: increasing the support area, improving force direction and reducing force magnitude (Table 1).
Misch and Bidez7 also relied on a biologic rationale for the differences found between implants and natural teeth and propose the concept of “implant protective occlusion.” According to this concept, in order to reduce the danger to the prosthesis and implants, an attempt should be made to reduce the loads both on the parts of the prosthesis and on the implant-bone interface. The recommendations of occlusal load design include balancing occlusion in two stages—light contact and firm contact—because of the difference in vertical mobility between teeth and implants. At first, the implant-supported prosthesis should have light occlusal contact, with stronger contacts with the neighboring teeth, followed by application of greater occlusal pressure to obtain stronger contact between the implant-supported prosthesis and the opposing teeth.
They also recommended avoidance of premature contacts, freedom in centric occlusion, main contact being above the long axis of the implant, avoidance of lateral excursive contact, flat anterior contact, and posterior disarticulation while protruding.
Gross44 in an extensive review put forth considerations in choosing prosthetic determinants, based on the available scientific evidence. The article presented accepted current concepts, controversial concepts, considerations in planning, and the lack of agreement found in the literature for given clinical situations.
Most recommendations were based on clinical experience and the considerations for performing implant-supported prostheses were presented in detail. At the same time, questions were raised regarding implant number, diameter, length, and angle, as well as contact in movement and in centric occlusion.
In contrast to the other reviews, Gross questioned the application of principles relating to natural teeth and proposed that anterior guidance should not be performed automatically but each case should be considered on its own merits.
Discussion
A literature review shows that there is no clear scientific evidence for the effect of load and overload on implant survival and success. Evidence pertaining to the effect of load and parafunction is of a dual nature, both in animals and humans. Also, there is no scientific evidence for the advantage of a particular occlusal scheme or design in implant-supported prostheses. Naert et al46 in a recent systemic review came to the following conclusion: “The effect of implant overload on bone/implant loss in clinically well-integrated implants is poorly reported.” They added that the recommendations found in the literature are based on biologic rationale, clinical experience, and expert opinion.
The animal experiments relating to occlusal load dealt with a small number of animals, exceptional loads, and premature contacts versus a comparable situation to be found in humans, as well as contradicting results regarding the connection between overload and inflammation to bone loss. In addition, the question arises as to the applicability of the animal studies to humans.
The biologic difference between natural teeth and implants led, with time, to the notion that there should also be a difference in load distribution. Kim et al21 and Misch and Bidez7 presumed that such a biomechanical difference should affect the planning and execution of occlusal contacts. However, Carlsson43 maintained that the difference between implant and natural teeth occlusion is minor, as there was evidence for the long-term success of implant-supported prostheses in all sorts of occlusion schemes.
All authors adopt in their recommendations concepts borrowed from natural tooth occlusion, with adaptations, but without any scientific basis. As there are no proven alternatives, it seems logical to adopt those schemes, because they do apply and there are no significant complications. Patients following implant restoration exhibited mandibular movement and speed similar to those in subjects with natural teeth.47 Mastication patterns and coordination of masticatory muscles were also similar.48 It bears mention that also natural teeth occlusion is not based on proven investigation, but on theories, common sense, and clinical experience.
During the course of treatment, the dentist is required to choose the type of contacts for the prosthesis. The different clinical situations vary tremendously, and choosing the relevant recommendation for each situation may be difficult. Many published recommendations are based on clinical rationale. However, in some cases clinicians cannot follow them, and it seems that sometimes they are too cautious.
For example, while substituting an upper canine with an implant, Misch49 recommended including a natural tooth within the working movement in order to add a component of proprioception to the movement and for force distribution. The question that arises is: What occlusal contacts will clinicians provide when they cannot adhere to this recommendation, when they are forced with guidance on the canine’s implant crown? The dentist will try to do the best possible within the given situation. No evidence is available for the superiority of combined (tooth and implant) lateral guidance over implant canine guidance. The authors of this review believe that if the chosen occlusal pattern, canine protected in this example, proves itself during a period with a provisional restoration in function in a specific patient, one can proceed safely to the final restoration with the same occlusal guiding contacts.
Another example is full-arch implant-supported restoration. If following traditional tooth occlusal schemes, a dentist will probably provide anterior guidance with posterior disarticulation. Is there a rationale for creating simultaneous anterior and posterior contacts to spread or share the force evenly? Gross addressing the same stated: “Adhering to traditional paradigms with a mild disclusion is tempting, however there is no evidence to support either approach.”44
Although the magnitude of the forces exerted by the patients cannot be controlled, in accordance with basic mechanical principles, an attempt should be made to increase the supporting area and to distribute the force as far as possible in order to reduce the risk for the restoration.
Not only has occlusal load not been proven as a direct cause for bone loss, but overload too, according to the dental literature, is a minor contributing factor to failure. Overload has not proved to be involved in loss of osseointegration. On the contrary, in a patient with bruxism, it may be that the forces transferred to the bone are favorable and contribute to cervical bone remodeling and density and implant stability.11
In the introduction to this article, the authors quoted Taylor8 suggesting the lack of evidence for implant-supported occlusion. At present, almost 20 years later, no additional scientific evidence has been presented. Despite the paucity of evidence and nonequivocal guidelines regarding the effect of force on osseointegation, implant-based prostheses are a predictable treatment with a high percentage of success. In addition, many reported failures are mechanical restoration failures and loss of osseointegration is due to infection.
Conclusion
It may be concluded that the magnitude of implant load was not found to be of high practical importance. It may be assumed, therefore, that contact distribution between the prosthesis and opposing jaw play a substantial role in preserving the prosthesis, but have a lesser effect on implant survival and bone loss. That statement does not in any way mean that the patient’s comfort, function, and occlusal adjustments should be ignored. The fact that implant occlusion works so well in daily practice supports the adoption of occlusal principles from teeth and dentures until proven otherwise.
ABOUT THE AUTHORS
Gilad Ben-Gal, DMD, MSc, BMedSc
Graduate student, The Center for Graduate Studies in Prosthodontics, Department of Prosthodontics, Hebrew University-Hadassah Faculty of Dental Medicine
Jerusalem, Israel
Mordechai Lipovetsky-Adler, DMD, BMedSc
Clinical instructor, The Center for Graduate Studies in Prosthodontics, Department of Prosthodontics, Hebrew University-Hadassah Faculty of Dental Medicine
Jerusalem, Israel
Orith Haramaty, DMD
Clinical instructor, The Center for Graduate Studies in Prosthodontics, Department of Prosthodontics, Hebrew University-Hadassah Faculty of Dental Medicine
Jerusalem, Israel
Eldad Sharon, DMD, MSc, BMedSc
Clinical instructor, The Center for Graduate Studies in Prosthodontics, Department of Prosthodontics, Hebrew University-Hadassah Faculty of Dental Medicine
Jerusalem, Israel
Ami Smidt, DMD, MSc, BMedSc
Professor and Head, The Center for Graduate Studies in Prosthodontics, Department of Prosthodontics, Hebrew University-Hadassah Faculty of Dental Medicine
Jerusalem, Israel
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