Although occlusion and occlusal trauma on natural teeth have been studied extensively, there is limited literature regarding implant occlusion. The biophysiologic differences between a tooth and an implant make application of the occlusion literature for natural teeth to endosseous dental implants nearly impossible. Additionally, several challenges exist in studying implant occlusion, including its feasibility and the ethics of studying occlusion in human clinical studies. Thus, the majority of the available information regarding implant occlusion relies on the principles of engineering and mechanics to understand implant occlusion. The purpose of this systematic literature review was to describe the way occlusal forces may impact dental implants and their surrounding bone, to describe occlusal overload on implants and possible resulting complications, and to provide clinical recommendations for implant occlusion.
Materials and Methods
A literature search was completed using the PubMed database to create a systematic literature review that updates the understanding of occlusion on dental implants, the impact on the surrounding peri-implant tissues, and the effects of occlusal overload on implants. Additionally, information from the literature was used for the development of recommendations for occlusal schemes for various implant prostheses and designs. Two reviewers (R.S. and A.D.) searched the PubMed database manually using the terms “dental” and “occlusion” and several search terms and pairs of search terms, including, but not limited to, the words “implant occlusion,” “implant biomechanics,” “occlusal scheme,” and “occlusal overload.” In addition, a manual search of the following journals was conducted: The International Journal of Oral & Maxillofacial Implants, Clinical Oral Implants Research, and Implant Dentistry. Relevant articles from January 1950 to September 20, 2015 were considered under the condition that they were published in the English language. Figure 1 represents the selection process for articles included in this article.1
Tooth and Implant Responses to Occlusal Forces
Biophysiologic differences between the natural tooth and endosseous implant
An understanding of contrasting biophysiology of natural teeth and endosseous dental implants is necessary to understand how occlusal forces may impact each differently (Table 1). The most fundamental difference is their attachment or connection to the alveolus. Natural teeth are suspended in the socket and connected to the alveolar bone by the periodontal ligament (PDL), whereas an endosseous implant is directly connected to the bone through osseointegration (so-called functional ankylosis).5 This difference has several implications regarding biology as well as the biomechanics of occlusion.
The PDL functions as a shock absorber for the tooth.12 In addition, mechanoreceptors within the PDL send information to the central nervous system, allowing the detection of occlusal loads. An implant, which lacks the PDL, has shown less tactile sensibility and occlusal awareness.4,13 Hammerle et al4 showed that the natural teeth had an 8.75 times higher mean threshold for tactile sensibility than implants. Thus, occlusal overload is more likely to be detected in natural teeth, not implants, and to elicit a protective reflex to decrease the load.
Due to the presence of the PDL, a natural tooth has increased physiologic mobility under occlusal forces. A natural tooth can be displaced 25 to 100 μm in the axial direction and 56 to 150 μm horizontally.8 When occlusal loads are applied, the stress distribution diminishes along the root in the apical direction.8,9 The fulcrum of movement occurs at the apical third of the root and the tooth can respond to movement by rotation of the root.2,9 The dental implant is connected directly to the bone, eliminating space for physiologic movement. In contrast to a tooth, an implant can only be displaced 3 to 5 μm in an axial direction and 10 to 50 μm horizontally.8,12 Thus, while a tooth may adapt to movement through intrusion or slight rotation, the dental implant-bone interface may absorb all of the forces. Although forces are evenly distributed along the natural tooth, the forces are concentrated at the crestal bone level surround the implant.8 This process will be described in a later section, titled “Complications That May Be Related To Occlusal Overload.”
Differences between the natural teeth and implants also affect how occlusal forces impact the surrounding bone. For example, implants lack a fibrous attachment, and fibers around the implant are oriented parallel to the implant body. Contrarily, the fibers of the PDL are perpendicular to the root and are oriented to oppose an axial load.14 This is vital for the health of the tooth because vertically directed physiologic occlusal loads to not induce mobility, as lateral occlusal loads can.15,16 Without fibers oriented in toward to an axial load, an implant is more likely to be susceptible to lateral forces which create bending moments.6,17
The movement phases between the natural teeth and implants differ as well, impacting the response to occlusal loads of the surrounding bone.8 In a natural tooth, tooth movement is not linear. It begins with an initial phase, where the tooth moves within the boundaries of the PDL.7,18 Continued force involves the secondary phase, which involves elastic deformation of the alveolar bone. An implant lacks the initial, adaptive phase of movement. The implant moves in a linear and elastic fashion.
Mechanical loading on bone surrounding the tooth versus peri-implant bone
Wolff's law introduced the idea that bones are capable of adapting to mechanical stress.19 Frost20,21 further elaborated on this concept; he showed that the bone adaptation can occur in the form of anabolic or catabolic responses, depending on the amount of mechanical force applied.
Frost's Mechanostat model uses the concepts of stress and strain. In this model of bone, stress is the mechanical force on the bone over a given area and creates strain.22 Strain describes the deformation of the bone, specifically, its change in length over its original length. Although the amount of stress invariably impacts the amount of strain, the degree to which deformation occurs is determined by inherent properties of the bone.17 Frost22 describes strain in units of microstrain; 1000 microstrain is equivalent to 0.1% bone deformation.
According to Frost,20,21 low amounts of strain lead to catabolic bone reactions or disuse atrophy. However, some strain is required for bone remodeling. In this “steady state,” bone damage is balanced by repair and the deposition of new bone. However, continuing to increase the level of strain can lead to bone resorption and, eventually, to bone fracture.
Frost created his Mechanostat model based on the tibia, a long bone. Properties of the alveolar bone differ from this bone. Thus, the exact microstrain levels of the Mechanostat theory do not apply to the alveolar bone; however, the concept may be applicable. This would suggest that some level of occlusal loading is required to avoid disuse atrophy, that a range of occlusal load leads to healthy remodeling and that a threshold exists in which heavy occlusal forces can trigger bone resorption.
Some evidence supports the application of Frost's model to peri-implant bone.22–29 Melsen and Lang30 showed that bone apposition occurred around implants in monkeys at levels of 3400 to 6600 microstrain; however, net bone loss occurred after a threshold of 6700 microstrain. A normal level of occlusion, however, is associated with adaptive, bone remodeling,29 increased bone-to-implant contact,25 and enhanced osseointegration.31 Bone response appears to differ based on the type of loading studied: static loads lead to anabolic reactions, whereas cyclic loads show bone resorption at the crestal portion of implants.22
Similar to biophysiologic differences between teeth and implants translating to different responses of their surrounding bone to occlusal loads, their physical properties influence biomechanical responses. The modulus of elasticity describes stiffness or resistance to elastic deformity and is determined by both stress and strain.32 When stress is plotted against strain, the slope of the curve determines the modulus of elasticity. According to engineering principles, when the modulus of elasticity between 2 substances differs and one is loaded, stress is exerted where the first 2 materials come into contact.10,11 The modulus of elasticity of a tooth is very similar to the cortical bone.11 Thus, when a tooth is loaded, it will not create a large amount of stress at the crest interface. The modulus of elasticity of a titanium implant, on the other hand, is 5 to 10 times greater than the cortical bone.33 This supports the theory that crestal or marginal bone loss may occur in the presence of occlusal overload.
Generally, both natural teeth and dental implants should be in physiologic occlusion, which is described as “occlusion in harmony with the functions of the masticatory system.”34 If the occlusal scheme is not harmonious on natural teeth, occlusal trauma may occur. This may result in an adaptive response, such as thickened lamina dura or occlusal wear, or a traumatic response, including mobility or a widened PDL.35 In the context of implant occlusion, the appropriate term is occlusal overload. Occlusal overloading is the application of force to an implant, through either normal function or parafunctional habits, which leads to structural or biological damage.36 Occlusal overloading relates to damage to the prosthesis, abutment, implant structure, or the surrounding alveolar bone.
Although consensus exists on the general definition of occlusal overload, modifications of how the term “occlusal overload” is used in the literature vary widely. Some have stated that using the term overload for a dental implant is appropriate only when an implant is failing or has failed.37 Applying Frost's Mechanostat model, occlusal overload would refer to the level of microstrain that corresponds with a catabolic bone response. Melsen and Lang30 quantified this level of microstrain using dental implants in a dog model. Beyond 6700 microstrain, bone resorption occurred.30
The study of occlusal overload and interpretation of literature on the subject is difficult for several reasons. Occlusal forces, like all forces, can be described in the following 4 subjects: magnitude, duration, distribution, and direction.17 Studies such as Frost's take into account only 1 variable, magnitude.38 Additionally, while the occlusal load can be measured at the prosthesis or abutment level, mechanical measurements cannot be obtained from the bone-implant interface.38 Additional considerations such as confounders and risk of bias complicate the study of occlusal overload. Lastly, for obvious ethical reasons, clinical trials applying occlusal overload are unethical in humans. For this reason, occlusal overload on implants remains controversial.38–42 Despite this, occlusal overload has suspected associations with many implant complications, both biological and biomechanical. In fact, occlusal overload and peri-implantitis have been described as the 2 most common reasons for late (post-osseointegration) implant failure.42–45
Complications That May Be Related to Occlusal Overload
Occlusal overload has been suspected to be one of the contributing factors for marginal bone loss. Theoretically, this is possible. As previously mentioned, the stress distribution of an implant occurs at the crestal bone level.12 The difference in the modulus elasticity of bone compared with that of the titanium implant implies that forces are directed at the first area of contact, at the crestal bone.11 Microfractures in this area could in turn produce marginal bone loss. Varied results in the available literature have been described, ranging from a possible association, a possible relationship dependent on other factors, to no probable association.23,24,43
Kozlovsky et al46 found that dynamic occlusal overload created marginal bone loss, however, the extent was determined by the presence of inflammation. Without inflammation, the bone resorption did not occur below the implant neck. The presence of plaque-induced inflammation led to significantly greater bone loss, to the level of the implant threads. Some theorize that, if occlusal overload is indeed associated with marginal bone loss, the micromovements could lead to the development of peri-implantitis.47
Similar controversy surrounds a possible association between occlusal overload and the loss of osseointegration.29,48–50 The mixed results can be attributed to the complicated nature of studying occlusal overload, discussed previously. Differences in study design also challenge interpretation.
Occlusal overload has been regarded as a major cause of biomechanical complications,31 including screw loosening, prosthesis failure, and the fracture of screws, veneering material, or the implant.41,51–55 This is significant because these complications can be costly, time consuming, and some complications, such as implant fixture fracture, can lead to implant failure.31
Factors That May Cause Occlusal Overload
Recognizing factors that may cause occlusal overload is useful to prevent occlusal overload and suspected, related complications. Such factors include: large cantilevers, parafunctional habits/bruxism, steep cusp inclines, poor distribution of force (eg, limited contacts), interferences, and poor-quality bone.18,43
Recommendations for Physiological Implant Occlusion
Implant occlusion should aim to create a physiological, harmonious occlusion, to avoid occlusal overload, and to prevent unnecessary implant complications. As previously mentioned, occlusal forces, like all forces, can be described in 4 ways: magnitude, duration, distribution, and direction.17 Many of the goals of implant occlusion are based on these 4 factors.
In considering the direction of the occlusal forces, it is recommended to reduce shear (unaligned) forces and to aim for compressive (aligned) forces. In doing so, occlusion should create axial forces, rather than lateral or horizontal forces. Bone is stronger under compressive forces than shear forces.11,56 Nonaxial loading causes higher stress and tension around the crestal bone.22,57–59 In fact, Rangert et al60 found that a deviation of 15 degrees in a buccolingual direction contributed to occlusal overloading. Thus, aiming forces in an axial direction and reducing shear forces will protect the supporting, peri-implant bone.
If a buccolingual deviation of 15 degrees can contribute to occlusal overload, it is interesting to consider if the deviation of the implant that may be presented in All-on-Four cases may lead to occlusal overload. The All-on-Four concept allows for 20 to 30 degrees of deviation in the mandible or up to 45 degrees of deviation on the maxilla.61,62 This does not specifically refer to the buccolingual direction and could relate to mesiodistal deviation. This may also be important to factor into a treatment-planning decision, considering that Browaeys et al63 found marginal bone in 49.2% of patients with All-on-Four implants with a 20- to 30-degree deviation. Ramiglia et al64 also found an association between implant inclination and bone loss: buccal bone loss was associated with lingual and distal inclination on the mandible. The authors also found the ideal angle of insertion to be 79.1 degrees, suggesting that approximately 20 degrees of deviation is acceptable. Although other studies comparing tilted implants to upright implants have not reported significant differences in marginal bone loss, these studies did not consider buccal bone loss and did not use 3-dimensional radiographic analysis.65,66
Recommended Implant Occlusal Scheme for Single Implants and Fixed Partial Dentures Supported by Implants
A modified version of the mutually protected occlusal scheme leads to a harmonious implant occlusion (Table 2). The force distribution should be equal bilaterally and maximized on adjacent teeth.31,67–69 Light to medium occlusal contact in maximum intercuspation is recommended for the adjacent, natural teeth, with lighter contact or clearance between the occlusal face and opposing tooth.68,70 This is because the implant does not have a PDL support and does not have the vertical, physiologic mobility within the socket that a natural tooth has.
Anterior guidance is recommended in lateral and protrusive excursions. In lateral excursions, posterior teeth should avoid heavy forces in the lateral direction by discluding.18,71,72 Avoiding working and nonworking contacts on implant restorations is vital to reduce shear forces in a nonaxial direction.69,73 Premature contacts predispose implants to occlusal overload.18 Wide freedom (1–1.5 mm) for maximum intercuspation and centric relation are recommended to prevent premature contacts.74,75
There are clinical scenarios that require modified occlusion for excursive movements. If the canine has been replaced with an implant restoration, it should not be subjected to heavy lateral, shear forces.31 Recall that bone is weaker under shear forces than compressive forces and that shear forces lead to higher stress and tension around the crestal bone.11,22,56–59 Thus, occlusal forces should be aimed to be compressive, aligned, and axially directed.
Similarly, periodontally compromised anteriors or an anterior bridge in a Kennedy class IV patient should not be subjected to heavy anterior protrusion.70,76–78 In this situation, group function should be used. If an implant was placed in a palatal position (due to limited buccal bone), the implant is likely to undergo shear forces with occlusion. Here, Misch et al79 advise placing the teeth in crossbite to avoid nonaxial loading.
An understanding of how prosthesis design impacts the dental implant and surrounding bone is imperative for a physiologic and harmonious implant occlusion. Large cantilevers function as a lever arm, placing stress on the adjacent abutment. Large cantilevers have been associated with the production of shear forces,18 bone loss, and prosthetic failure.48,80 Shackleton et al81 found that prosthesis failure was more common in cantilevers >15 mm in length. A systematic review and meta-analysis by Torrecillas-Martinex et al82 found that minor complications, such as abutment screw loosening, were more common with cantilevered restorations than with noncantilevered restorations. Several authors speculate that prosthetic complications of cantilevered restorations are associated with nonaxial forces.60,83
A prosthetically driven implant placement will reduce shear forces and a cantilever effect on each implant. The use of a surgical guide is recommended for implant placement, whenever possible. Additional ways to eliminate shear forces include increasing the number of implants, maintaining an adequate crown height space (15 mm or more could create more microstrain),84 reducing the crown-to-implant ratio, minimizing vertical overlap,71 and obtaining a passive prosthetic fit.31
The anatomy of the implant crown can largely impact the direction, magnitude, and distribution of force that may be imposed on the tooth. Factors include the inclination of cusps, the size of the occlusal table, and the number of contact points with the opposing tooth. Finite element analysis has shown that high cuspal inclination increases the magnitude of forces on the tooth85 and that an increase of 10 degrees of cuspal inclination increases the bending moment by 30%.86 Opting for a reduced cuspal inclination protects the tooth from shear forces while decreasing force magnitude.40,70,87–89 A narrow occlusal table ensures that forces will be directed axially,80,90 prevents cantilever effects and bending moments,67,90 and reduces the magnitude of forces.91 In 1 study, narrowing the occlusal table by 30% reduced the magnitude of lateral forces by almost 50%.91 The most vital anatomical consideration for controlling the distribution of forces is the number of contact points on the implant crown. Posterior teeth do not have the same level of proprioceptive inhibition that anterior teeth have and thus can be subjected to unprotected force generated by the masticatory muscles.4,17,41 For this reason, increasing the distribution of force by including multiple contact points on multiple teeth is vital to protect the posterior occlusion. Studies have shown that there is increased stress on the bone when a dental implant has 1 contact point versus multiple contact points.92 Additionally, contact points should be centered on the crown to avoid cantilevers and bending moments.54
Individual patient considerations influence implant occlusion. For example, parafunctional habits or bruxism impact implant planning, restoration, and maintenance. Bruxism is associated with occlusal overload,93 marginal bone loss,48,80 mechanical problems,48,60,94 and implant technical and biological failure.48,95–99 In a Zhou et al99 meta-analysis, the odds ratio of implant failure for bruxers versus nonbruxers was 3.83. The prosthetic design is the key in bruxers, from correct implant placement to anatomical considerations so shear forces can be reduced. Additionally, because bruxers experience more complications and failures, frequent follow-up is advised. If parafunctional habits include nocturnal bruxing, an occlusal nightguard should be fabricated.
The patient's bone quality influences implant occlusion decisions. Lekholm and Zarb100 categorized bone into 4 types, depending on the amount of cortical and trabecular bone present. Type 1 bone is the densest, made entirely of the cortical bone. Type 4 bone describes bone that is mostly trabecular, surrounded only by a thin cortical layer. This has an impact on implant occlusion because the differing modulus of elasticity between dense, cortical bone and low density, trabecular bone.17 When substances have differing modulus of elasticity and one is loaded, stress will be placed where the substances meet. There is a larger difference in elasticity between titanium and trabecular bone than between titanium and cortical bone. Placing an implant in low bone density leads to higher levels of pure titanium implant failure.22 Jaffin and Berman101 found that 35% of pure titanium implants placed into type IV bone had failed after 5 years but only 3% of implants placed into types I to III bone had failed. Goodacre et al51 analyzed 7 studies that compared implants placed into different bone types, including the Jaffin and Berman101 study. The results of these 7 studies showed implant loss in 16% of implants placed into type IV bone, compared with only 4% lost when placed in types I to III.51 One possible solution is to use progressive loading of the implant or roughened the implant surface. This allows extended healing time and may increase bone density and reduce crestal bone loss.102,103 A roughened implant surface could shorten the time need for the osseointegration therefore minimize the chance of premature loading. Turkyilmaz et al104 found that computed tomography (CT) imaging could be used to determine the bone type before implant surgery. Thus, obtaining a CT image may help determine whether or not bone quality will help avoid, or promote, occlusal overload and possible implant failure.
Implant design may have an impact on how the surrounding bone reacts to occlusal forces. Screw-type implants have been shown to have higher bone to implant contact, thus, support, when compared cylindrical implants.105 A tapered implant design aids in reducing shear forces more than a parallel implant design.106–108 Regarding diameter, wider implants resist stress more than those with narrow diameter;109–111 however, length is not as great a consideration.111 A smooth collar is not recommended because it may increase shear forces; microthreads generate less shear force.112 Bone to implant contact is also increased by using square threads, an increased thread depth, increased thread pitch, and with coated implants (as compared with machined implants.).31,113–116 Although some theorize that splinting implants could distribute the amount of stress and strain placed on implants,117,118 studies have not shown splinting to decrease survival or bone loss in traditional or short implants.119–121 Thus, the authors do not feel there is enough evidence to recommending splinting to reduce the risk occlusal overload.
The aforementioned recommendations can aid in creating an implant occlusion that protects the implant, implant restoration, and natural dentition (Table 3). In some situations, implant occlusion may be compromised, for various reasons. However, monitoring occlusion is the clinician's responsibility. Occlusal changes can be expected and the possible consequences of occlusal overload make periodic evaluations imperative.41,67,79,88,122 Natural teeth tend to wear more than restorative materials, such as the implant crown.18 Thus, to avoid high occlusion on an implant restoration, occlusal adjustments may be necessary. Maintaining good force distribution and direction will help maintain the longevity of the implant.68
Due to the challenges of studying implant occlusion, particularly occlusal overload, minimal data is available. Although literature exists regarding the natural tooth and occlusion, the differences between the natural tooth and the dental implant alter the way that occlusal forces impact the bone surrounding them. The PDL of the tooth provides protection against occlusal force, whereas a dental implant lacks the proprioception and support of the PDL. At this time, the application of engineering and mechanical theory is crucial in understanding how implant design, implant placement, and prosthesis design impact implant occlusion. Considering the 4 characteristics of occlusal force (direction, magnitude, duration, and distribution) are important for placing and restoring an implant that will be harmonious with the adjacent natural dentition. This requires coordination from the clinician placing the implant and the clinician restoring the implant. If implant occlusion is not harmonious, it is possible that the implant can experience occlusal overload. Currently, this topic is highly debated, from the definition of the term to the possible complications that may result. At this time, several observations noted that occlusal overload may cause complications ranging from biomechanical failures to marginal bone loss or complete loss of osseointegration. Thus, it is vital for the clinician to keep implant occlusion in mind when placing or restoring an implant to protect the implant and surrounding peri-implant bone.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the paper.
The authors acknowledge and thank Dr William Carroll for his contributions to this article.
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