Implants and their prostheses replace teeth. They therefore have to participate fully in oral function. A substantial part of oral function is related to occlusion when the teeth and their replacements make contact. This module will examine the ability of implants and their prostheses to withstand occlusal forces and offer guidelines for occlusal design.

After completing this ITI Academy Module, you should be able to recognize characteristics of occlusal forces and biophysical differences in how teeth and implants respond to them, discuss the potential impact of occlusal forces on complications and failures of implants and their prostheses, apply literature guidelines for occlusal management in implant therapy.

The module will first discuss the characteristics of occlusal forces and the respective biophysical differences in response to load between teeth and their periodontal ligament outlined in red, and between implants and their osseointegration with bone outlined in yellow.

The magnitude and direction of occlusal forces are difficult to quantify. As a rule, occlusal forces arising from opposing contacts during normal physiological function, such as chewing and swallowing, have limited duration and magnitude. These can be considered to be within a physiological range. On the other hand, forces arising from parafunctional activity, such as bruxism and as demonstrated by the simultaneous anterior and posterior contact pattern of this dentition in lateral excursion, can be considerably magnified and prolonged. This is especially the case during sleep when there is no conscious, protective feedback even from the proprioception provided by teeth and their periodontal ligaments.

This table demonstrates the differences between teeth and implants which means periodontal ligament versus osseointegration: Teeth and implants have different scopes for axial and lateral movement: the range of axial movement for a healthy tooth is 25 to 100 μm axially and 56 to 108 μm laterally. In contrast, the movement of an implant is limited to that of bone. This range is around 3 to 5 μm axially and 10 to 50 μm laterally.

Occlusal feedback from teeth is provided by mechanoreceptors in the periodontal ligament. This is referred to as proprioception. With no periodontal ligament around implants, this type of occlusal feedback is missing. Even so it would appear that mechanical stimulation of mechanoreceptors elsewhere in peri-oral muscles, ligaments and jaw joints give rise to a compensatory type of occlusal feedback. This feedback is less than for teeth, but still better than for complete dentures.

Characteristics and Biophysical Differences, Key Learning Points: Occlusal force varies and can reach considerable magnitude and duration during parafunction. The biophysical response to occlusal force is different for implants and teeth. This affects capacity for movement and sensory feedback.

Having established that occlusal force varies and can reach considerable magnitude and duration and that the ability of implants to respond to force is different to that of teeth, learning objective two examines the evidence for the impact of occlusal force on complications and/or failure of osseointegration, crestal bone levels, implants themselves, implant components - referred to as mechanical complications and failures, and implant prosthesis - referred to as technical complications and failures.

What is the impact of occlusal forces on osseointegration? During healing, occlusal loading may be a factor that can lead to failure. But the risk diminishes as the healing period progresses and secondary stability increases, as illustrated here. Once the implant is fully integrated, the role of occlusal forces as a cause of failure lessens.

Occlusal overload as a cause of loss of integration is not proven but cases have been reported. In the clinical case shown here an implant in position 47, which had been in function for 7 years, loosened and had to be removed. With no marginal bone loss, no peri-implant inflammation and no pocketing detected on clinical or radiographic examination the diagnosis was occlusal overload of a posterior end implant supported prosthesis in dense bone with less resilience. Continuing management of the occlusal scheme on implant prostheses is important to reduce risk of failure from occlusal overload.

What is the impact of occlusal forces on progressive crestal bone loss? Although in-vitro and in-vivo studies have attempted to demonstrate a link between excessive occlusal force and progressive crestal bone loss around implants, to date there is no clinically established relationship. There is emerging evidence, however, that overload through parafunction coupled with inflammation - as in this patient case with ongoing periodontal disease and peri-implantitis - may result in acceleration of peri-implant crestal bone loss.

What is the impact of occlusal forces on the implant itself? The reported incidents of fracture of the implant itself amount to less than 1% over five years. This data predates more recent developments to further increase implant body strength - particularly for narrow diameter implants - through use of stronger implant body alloys. In the case illustrated by this radiograph, a narrow diameter implant has fractured secondary to a combination of labial bone loss, parafunctional occlusal interference and crestal bone fulcrum at the weakest level of the implant body. Increased risk of implant fracture should be considered in similar clinical situations.

What is the impact of occlusal forces on mechanical complications and failure of implant components? Problems with implant components including implant abutments are referred to as mechanical complications and failures. Long-term studies and systematic reviews report loosening and failure of abutments and screws. Abutment and screw loosening shows a high cumulative incidence of 8.8% over 5 years. The 95% confidence interval is 5.1 to 15%.

Most studies and reviews reporting on mechanical complications and failures include different types of implant-to-abutment connections. With the general move towards more stable, internal implant/abutment connections, this incidence is expected to decrease.

What is the impact of occlusal forces on technical complications and failure of prostheses? Problems with implant prostheses are referred to as technical complications and failures. Implant-supported prostheses have a significantly higher incidence of chipping, fractures and loss of retention than tooth-supported prostheses. Part of the buccal cusp on this 16 implant-supported crown fractured after three years; the patient had a hard protective occlusal appliance but rarely used it. The reported figures over 5 years based on systematic reviews are up to 38.7% for implant-supported prostheses against 15.6% for tooth-supported prostheses.

Common complications with implant prostheses include chipping, fractures and loss of retention. The reported incidence over 5 years is shown in this table along with the corresponding 95% confidence interval. The incidence of chipping and fractures is 3.5%, the incidence of loss of retention is 4.1%. Wear is also a frequent complication and it is material-dependent.

Impact of Occlusal Forces on Complications/Failures, Key Learning Points: Implants, osseointegration and crestal bone levels together with implant components seem to withstand occlusal forces associated with normal physiological function. The risk of technical complications with implant prostheses is significantly higher than for teeth. Occlusal overload can prevent achievement of osseointegration. Occlusal overload as a cause of loss of an integrated implant is not proven but cases have been reported. Occlusal overload in combination with peri-implant inflammation may be detrimental to crestal bone levels.

The module will now focus on guidelines for occlusal management. The guidelines presented here are based on literature suggestions on how to reduce the potential impact of occlusal forces on implants and their prostheses as a result of the biophysical characteristics of osseointegration. The guidelines are divided into three points: Occlusal risk assessment - illustrated here with the use of the ITI SAC Classification for assessment of case complexity. Recommendations for occlusal design for implant prostheses including positions and tensions of contacts - illustrated here by hold of shimstock foil. Ongoing occlusal management including protection by hard occlusal splints and regular evaluation.

Risk assessment includes accurate and complete information about the entire occlusion from the outset. This is also important for both treatment and medico-legal documentation. The information includes high quality dental impressions and casts and accurate recording of occlusal relationship. The images demonstrate analog and digital impressions and the different types of casts. The analog casts have been orthodontically trimmed to allow them to be positioned accurately in static occlusion in a number of different positions to aid observation. The combined information is needed to implement the recommended occlusal guidelines.

The ITI SAC Classification includes evaluation of three aspects of occlusion, as listed in this table. For each aspect, the degree of difficulty can be low or high. The first aspect is the occlusal scheme, that is, the pattern of contacts between opposing teeth in a given occlusion. In the presence of anterior guidance provided by contact between anterior teeth in dynamic occlusion, the occlusal degree of difficulty is considered low, but in its absence the difficulty is high. This is because the anterior guidance can separate the posterior teeth in dynamic occlusion and thereby avoid potentially damaging forces from lateral dynamic contacts on posterior implant prostheses. Second is the extent of involvement of the planned prosthesis in occlusion. With minimal involvement, the degree of difficulty is low, whereas the involvement of an implant prosthesis in anterior guidance is regarded to have a high degree of difficulty. The third aspect is occlusal parafunction. If this is absent the degree of difficulty is low. If it is present the degree of difficulty is high. It should be noted that the SAC Classification considers an occlusal re-organization with introduction of a new occlusal scheme a complex procedure with a high degree of difficulty which requires appropriate experience and expertise.

The next topic focuses on the recommendations for occlusal design. In the absence of an implant-specific, evidence-based concept, the literature recommendations for occlusal design on implant prostheses in Static Occlusion aim at axial loading through centered contacts to avoid potentially damaging stress to the prostheses through lateral loading. The first recommendation is for occlusal contact positions as can be seen on the graphic sections of the maxillary arches. The occlusal contact positions for implant restorations can be seen in the left image, whereas the right image for comparison shows occlusal contact positions for teeth. The occlusal contacts on the implant prostheses are centered and limited to the occlusal fossae. The contact patterns used for fixed prostheses on natural teeth have contacts on maxillary palatal cusps and in the occlusal fossae. Note the absence of contact between the anterior teeth in static occlusion. This is to avoid loading on anterior teeth in static occlusion and is often referred to as "long centric" or "horizontal freedom". This freedom should be within the range of movement of natural teeth to ensure stimulation during dynamic occlusion. While there should not be any contact on anterior implant prosthesis in Static occlusion, there may be light contact on canines in natural teeth.

The second and third recommendations for static occlusion are to design the implant prostheses with flatter cusps and wider occlusal fossae to increase horizontal freedom and thereby further reduce risk of stress from lateral forces. The image demonstrates the centered blue contact between the occlusal fossa of the maxillary implant prostheses against the mandibular buccal cusp on the natural tooth. The image also illustrates the flatter cusps of the implant prostheses compared to the steeper cusps of the opposing natural tooth. The wider occlusal fossa in the implant prosthesis is shown by a green line.

Where implants are located among natural teeth, the fourth recommendation in Static Occlusion is to reconcile the difference in axial movement of the implants with that of the teeth. This is to allow for the greater range of axial movement of the opposing teeth from first light contact through to firm closure compared to the range of an implant opposing a tooth or an implant opposing an implant. It is referred to as ‘graded axial contact’ and it is assessed in the clinical situation with shimstock foil. The extent to which an occlusal contact is holding shimstock is graded into "Hold", "Just pull through" or "Pull through easily".

For grading of occlusal contacts this table lists differences in shimstock hold between first light closure and firm closure for tooth opposing tooth, implant opposing tooth and implant opposing implant. For tooth opposing tooth, the shimstock should hold in first light closure. For implant opposing tooth with only half the potential axial movement in later firm closure, the shimstock should just pull through. For implant opposing implant with minimal scope for axial movement, the shimstock - in first light closure of the teeth - should pull through easily. In firm closure where the potential for axial movement has taken place for teeth and implants alike, the shimstock can and should hold equally for all contacts.

This is a graphic illustration of graded contact in static occlusion. The image shows contact in light closure. Shimstock should hold where tooth opposes tooth; this is indicated by a green dot. Where implant opposes tooth the shimstock should just pull through; this is indicated by a yellow dot. Where implant opposes implant the shimstock should pull through easily; this is indicated by a red dot. This is to allow for the greater range of axial movement of teeth compared to implants. This image shows contact in firm closure. Here there is no further scope for axial movement of teeth or implants so shimstock can hold equally between all opposing contacts as indicated by all green dots.

In dynamic occlusion, the recommendations can be divided into implants in the anterior regions and in the posterior regions. For implants in the anterior regions, the aim should be to use adjacent teeth with proprioceptive feedback where possible for guidance. The clinical and graphic example of implants in positions 12 and 22 shows a fortunate situation where the natural adjacent upper central incisors and canines can provide guidance, as illustrated by the green lines in the illustration.

For implants in the posterior regions, there should be no lateral working or non-working contacts on their implant prostheses. This is illustrated in the clinical example where there is no contact between the implant prostheses 15 and 14 in lateral excursion to the right. The corresponding contact patterns are seen in the occlusal view in the illustration where contacts in lateral excursion to the right are shown as green lines and confined to guidance by the anterior teeth 13, 12 and 11. The contacts on the implant prostheses 15 and 14 are restricted to centered occlusal blue contacts in the occlusal fossae in static occlusion only. This is similar to the recommendations for protection of individual teeth with doubtful structural integrity.

The following specific recommendations are offered in the presence of parafunction: First, consider the number of implants, their respective sizes and the possibility of splinting implant prostheses together to increase overall and mutual stability. The implants in the radiograph may be less vulnerable to loss of integration if the three prostheses had been splinted together. Second, avoid risky surgical and prosthodontic solutions such as combined immediate placement and loading and therefore thirdly, adopt a conventional loading protocol allowing at least eight weeks for the period of osseointegration.

The fourth recommendation is to use retrievable implant restoration designs, for example, screw-retained, as seen in this clinical example where the implant crown in position 36 has an occlusal access hole to the abutment screw. The fifth recommendation is to be cautious with cantilevers and the lateral load that could follow in bruxism. Finally, the sixth recommendation is to provide a hard resin protective occlusal appliance for use during sleep. The occlusal appliance seen in situ in the clinical image has been adjusted to the guidelines for a mutually-protected occlusion to reduce the risk of unfavorable stress on the teeth and implants in the opposing arch.

Continuing management must include regular evaluation of the occlusal design. Positional changes to the teeth over time, together with occlusal attrition can result in heavier contacts on implant prostheses and return of working and non-working contacts in dynamic occlusion. In this image of the maxillary arch, return of working side contacts in dynamic occlusion are indicated by the red arrows. These contacts will need to be removed to reinstate anterior guidance only by the group function between teeth 13, 12, and 11. A clinical example of such green line working side contacts is seen in this image and the situation after their removal in the right image. If not removed, these working side contacts could lead to potentially damaging lateral forces and loading. Note that the distobuccal cusp of the 16 prosthesis has already fractured. It is therefore important to carry out necessary adjustments on a regular basis.

Ongoing management also includes regular evaluation of hard protective occlusal splints and adjustments as necessary. The image of the lower occlusal splint shows the contact pattern of a mutually protected occlusal design with even blue posterior occlusal stops in static occlusion and anterior green lines from canine guidance and incisal rise in dynamic guidance. This occlusal scheme should be checked and the occlusal splint should be inspected for signs of wear from parafunction as seen here.

Guidelines for Occlusal Management, Key Learning Points : Risk assessment includes evaluation of occlusal involvement of planned implant prostheses based on complete and accurate information on the occlusion. In the absence of an evidence-based specific concept for occlusion in implant therapy, guidelines are based on empirical experience and consensus recommendations. Guidelines aim at reducing risk of occlusal overload and associated complications of implants and their prostheses. Specific recommendations apply in the presence of bruxism. Occlusal design and construction require complete, accurate and appropriate casts and records. An occlusal implant scheme requires ongoing monitoring and maintenance.

Learning Module "Occlusion on Fixed Implant Prostheses", Summary: Occlusal forces vary and teeth and implants differ in their response to them. Occlusal overload can prevent achievement of osseointegration. Occlusal overload as a cause of loss of an integrated implant is not proven but cases have been reported. Occlusal overload in combination with peri-implant inflammation may be detrimental to crestal bone levels. The risk of technical complications with implant prostheses as a result of occlusal forces is significantly higher than for teeth. Occlusal implant schemes require continuing monitoring and maintenance.