Occlusal Considerations for Implant Restorations in the Partially Edentulous Patient

The type and frequency of complications associated with dental implants has changed during the past decade. As more-successful rates of osseointegration have resulted from improved surgical protocols and materials, the major complications have become restorative-related rather than surgery-related. Recent studies indicate that restorative complications with implant-retained restorations occur at rates of 10 percent to 77 percent over a three-year period. Many of the restorative complications can be minimized with careful treatment planning and coordination of care. However, because implants lack the stress release associated with a periodontal ligament, impact loading to restorative materials and the crestal bone remains potentially more damaging with implant-supported restorations. This article discusses the biomechanical implications of implant restorations and outlines occlusal considerations designed to decrease restorative complications.

Implant-retained prostheses have become a well-established option to treat the partially edentulous patient and often represent an improvement over traditional removable partial dentures. Improved support, a more stable occlusion, preservation of bone, and improved patient acceptance are reasons an implant-supported prosthesis should be considered.¹ Additionally, long-term oral health is often improved by using an implant because less-invasive restorative procedures are required for the remaining dentition.^2,3 Despite highly predictable techniques for achieving osseointegration and the recognized physiologic and functional benefits of an implant-supported prosthesis, restorative complications often occur.^4-7

Those complications occur with a reported frequency of 10 percent to 77 percent over a three-year period.^4-9 Complications are more common in screw-retained implant crowns rather than cemented restorations,⁸ and they occur more frequently with single-tooth implant crowns than with multiple-splinted units.⁶ Reasons for the high complication rate include anatomic limitations such as bone volume or quality, spacing issues such as excessive or inadequate horizontal or vertical restorative space, and the unique biomechanical loading that occurs with endosseous implants. Because implants lack the stress release associated with a periodontal ligament, impact loading to restorative materials and the crestal bone is potentially more damaging.^10-11 As a result of anatomic limitations and biomechanical differences, ideal implant placement and biomechanical loading is not always possible. Follow-up problems can include reversible complications such as screw loosening, screw fracture, or porcelain or acrylic resin fracture. Irreversible complications can include loss of bone, loss of osseointegration, or implant fracture.

This article will outline occlusal considerations in the partially edentulous patient when using dental implants. The emphasis will be on posterior occlusion because molars are the most commonly replaced teeth and posterior quadrants are the most common areas of restorative complications with implants. Masticatory forces, types of restorative complications seen with overloading implants, and the ways in which occlusal considerations can decrease restorative complications will be reviewed.

Masticatory Forces on Teeth and Implants

Understanding normal masticatory forces is important when treatment planning with dental implants. Masticatory forces developed by a patient with a quadrant restored with an implant-supported fixed prosthesis are equivalent to those of a patient with natural dentition.¹² However, forces passed to crestal bone and impact forces are greater with implant restorations than with natural teeth.^10,11 Occlusal modifications and restorative changes can decrease the stress on crestal bone and restorative materials.

Normal masticatory forces are mostly vertical and vary with age, gender, muscle mass, skeletal form, and measured position in the arch.¹³ Occlusal contact time totals about 17 minutes a day, eight of which occur during mastication.¹⁴ Most normal masticatory forces are vertical along the long axis of the dentition and average less than 70 Newtons with a typical Western diet.¹⁵ In general, the greatest forces would be expected in the first molar area of a young stressed male with a square-angle jaw chewing on a hard substrate. Conversely, lesser forces would be expected in a sedate elderly female with a long midface measured in the incisor area when chewing on a soft substrate.

Non-axial masticatory forces are generally much less than vertical forces and are generated from elliptical jaw motion and contact on inclines of cusp teeth. Non-axial masticatory forces vary with the chewing stroke and location in the mouth but are generally less than 50 Newtons in the buccolingual direction and 20 Newtons in the anterior-posterior direction.¹⁵ Patients lacking anterior disclusion, lacking proximal contacts for support, having a cross-bite, or having limited posterior natural tooth contacts would be at higher risk for generating great horizontal and anterior-posterior forces.

High-Risk Patients

Patients at high risk for excessive forces would be those with bruxing habits, defined as a nocturnal grinding that can include both a vertical and horizontal component.¹⁴ More than 20 percent of the population are clinically active bruxers at some point in their lives.^16,17 Identification of bruxers is important before treatment planning with implants. Studies have shown that bruxism often results in horizontal forces several times during a night for periods up to an hour. Electromyographic studies have shown that bruxing forces are generally at the level of the patient’s maximal biting force, approximately seven to eight times that of normal masticatory forces.¹⁸ In a study of 4,045 implants followed during a five-year period, all patients with fractured implants (eight patients, or 0.2 percent) had parafunctional habits; and bone loss preceded the implant fractures.¹⁹ In a three-year study of 1,279 fixtures, Quiryen determined that 27 percent of the fixtures in a high-risk group of individuals with parafunctional habits either failed or had more than 1 mm of bone loss during the second or third year of loading.²⁰ For the low-risk group, without parafunctional habits, the percent with failure or bone loss greater than 1 mm was 2.7 percent.²⁰ Bruxing is not an absolute contraindication to implant restorations, but caution and careful treatment planning would be essential in such cases.

Complications From Overloading Implants

Complications from overloading implants are often a result of unanticipated forces on restorations not designed to tolerate either the static or cyclic loading of the patient’s normal masticatory forces and/or parafunctional habits. Complications from overloading implants are more common in posterior restorations and increase dramatically when a prosthesis is nonpassive.⁶ Complications are more common in the maxilla than mandible and occur more frequently in screw-retained than in cemented restorations.⁸ The most serious biologic response to overloading is crestal bone loss. The most common restorative complications of overloading are screw loosening, screw fracture, and porcelain fracture.

Crestal Bone Loss

Crestal bone loss around endosseous implants is not fully understood but likely occurs for several reasons, some of which are related to occlusal loading.^21,22 Crestal bone is susceptible to bone loss because of the difference between implant and bone elasticity. Because titanium is so much stiffer than bone, a stress gradient occurs at the interface of the bone and implant, placing the crestal bone at risk for resorption. Additionally, the crestal bone is often immature at the time of initial prosthetic loading and susceptible to osteoclastic activity. In addition, the flex of the tooth or dampening effect from the periodontal ligament is not equivalent with a rigid implant or metal restoration, so more force is passed to the crestal bone. Human trials, animal studies, and finite element analysis studies support the theory that repetitive overloading beyond a physiologic threshold can cause microfractures and osteoclastic activity to crestal bone, resulting in bone loss.^6,19,23-27

Overloading of an implant can occur more easily, with fewer symptoms, and with more permanent damage than overloading of teeth. This is because implants do not have a surrounding supportive ligament that can provide increased proprioception, better distribution of forces, sharper pain perception, or adaptations to overloading, such as thickening of the periodontal ligament^28-29 (Table 1).

Screw Loosening

Screw loosening has been reported to occur with a three-year frequency of from 3 percent to 38 percent in screw-retained posterior restorations.^4,5,8,30,31 Screw loosening is more likely in single-unit restorations, occurs more often in the molar than premolar area, and can often be related to excessive loading. Screw loosening occurs when compressive occlusal forces are higher than the tension in the screw-implant assembly that holds the components together (the clamping force).³² Forces tending to separate the screw-implant assembly can be related to prosthesis design or misfit, excessive or off-axis forces that can occur from a premature occlusal contact or contact on a cantilever, an offset, or a steep cusp angle. The clamping force will also be decreased when an implant restoration is nonpassive.³²

When an abutment screw to an implant restoration loosens, it is important to check the fit of the implant/restoration interface with a radiograph. It is also important to verify that a proximal contact is not so tight that the restoration is not completely seated because of lateral pressure. The internal of the hex and screw threads should be inspected for damage.

Occlusal considerations and restorative modifications can decrease the incidence of screw loosening (Table 2). When checking the occlusion, make sure a prematurity does not exist. If steep cusp angles are found, they should be flattened, which will decrease the potential for exceeding the clamping force of the screw-implant assembly.^32,33 If the occlusal table is wide, narrowing it will decrease both the offset and the potential for exceeding the clamping force of the screw-implant assembly. Screw loosening occurs less frequently when a wide-diameter implant and a machined gold cylinder rather than a narrow diameter implant or castable wax sleeve is utilized. Replacing the titanium screw with a gold-alloy screw is reported to increase the clamping force or pre-load and has been found to be helpful but has not been documented by clinical studies. Additionally, a torque driver can be helpful to standardize the torque level.

Repeated complications such as screw loosening, screw fracture, or porcelain fracture are often warning signs that more-serious complications -- such as implant fracture or bone loss -- may occur. In a retrospective study, Rangert showed that reversible complications such as screw loosening occurred in more than 60 percent of the restorations before irreversible complications such as implant fracture occurred.⁶ When screw loosening or screw fracture have occurred, the use of a nightguard should be considered.

Screw Fracture

Screw fracturing usually occurs for different reasons than does screw loosening. While screw loosening most often occurs when occlusal compressive forces are beyond the threshold of clamping forces, screw fracturing usually occurs from excessive shear forces³⁴ (Figures 1A, B, and C). Screw fracturing often indicates that excessive lateral forces are occurring and should be considered a major warning sign of more-serious complications. Studies have shown a higher incidence of bone loss associated with restorations where screw fractures have occurred.⁶ A verification of fit and an optimization of occlusal factors should be completed as would be done for a patient with screw loosening. Careful consideration should also be given to major changes in the prosthesis. The sectioning of a cantilever, the addition of an implant and fabrication of a new prosthesis, or a prosthesis with a new design should be considered.

Materials Fracture

Materials fracture on implant-retained restorations is one of the more common complications that leads to refabrication of a prosthesis.³⁵ Material fractures can occur soon after loading but can also occur years after delivery because of material fatigue and deformation (Figures 2A and B). The incidence of materials fracture is higher with implant-retained restorations than the equivalent restoration on natural dentition for three major reasons. First, because implants lack the stress release possible with a periodontal ligament, impact forces are higher on implant-retained restorations; and fatigue, creep, and permanent deformation are more likely to occur. Second, with the small base for implants (3 to 6 mm) compared to the wider base of natural teeth (10 to 12 mm), even a well-designed metal frame for a porcelain-fused-to-metal restoration can have areas of unsupported porcelain that can fracture. Third, higher stresses within the implant-retained restoration can occur close to the screw-retained access hole resulting in fractures.

When selecting materials for an implant-retained prosthesis, options include various acrylic resins, gold, or porcelain. Transient shock absorption is best with a softer material like an acrylic resin, but static loading is not influenced significantly by the veneering material.^36,39 In individuals at low risk for bruxism or excessive force transmission, gold, porcelain, or acrylic resin are all viable options. Although some authors have determined that there is a reduced rate of fractures to abutments and screws when using acrylic resin,³⁸ the rate of acrylic resin fracture and the need for repair is significantly higher than when porcelain is used.³⁵ An important consideration is wear compatibility between the opposing arches.

Strategies to Minimize Overloading of Implants and Implant Components

Treatment Planning

Complications with dental implants are most often the result of inadequate treatment planning and lack of coordination of care. Considerations of bone density and volume, anticipated loads, and planned restorative design are all important to review before the number, length, and diameter of implants are determined. Treatment planning for an implant-supported restoration also includes identification of patients at high risk for developing excessive force to the implants, restorative designs and occlusal considerations to minimize and broadly distribute stresses, and an awareness that careful follow-up is required.

A complete history and detailed examination is important to identify patients at high risk for occlusal overload and/or restorative complications. Important considerations include how patients have lost teeth and whether they have a history of bruxism, clenching, or parafunctional habits. If a patient has lost teeth due to fracturing, he or she is at higher risk for overloading implants than if teeth were lost due to periodontal disease.³⁹ Additionally, if a patient’s maxillomandibular relationship includes excessive anterior-posterior or lateral discrepancies, e.g., a skeletal class II patient, he or she would be at increased risk for non-axial loading. Clinical signs or symptoms that warrant careful consideration include a square-angle jaw, large masseter muscles, lack of anterior disclusion, wear facets, fremitis, periodontal ligament thickening, or few posterior tooth-to-tooth stops.

Restorative and Occlusal Considerations

The fabrication of an implant-supported restoration in a posterior quadrant requires several modifications to the equivalent procedures on natural dentition. Reduction of stress to implants and restorative components is recommended and may be necessary even though the equivalent step is not as critical with natural dentition. A definitive restoration that includes a narrow occlusal table and shallow cusp anatomy is likely to decrease the forces on prosthetic components and crestal bone³⁴ (Figures 3A and B). Indexing and soldering have also been shown to decrease stress to crestal bone.⁴⁰ Splinting of single units in patients at risk for high occlusal loading should be considered as should judicial use of cantilevering. These recommendations apply to use of the traditional 3.75 mm diameter implant with an external hex and would be modified if wider platform or internal hex components were used.

A posterior implant-supported prosthesis with flat cusp angles stresses the bone, implant components, and restorative materials less than the equivalent prosthesis with steeper cusp anatomy. The more complex the occlusal surface, the greater the potential for wedging of food and increased lateral force transmission.⁴¹ Therefore, wide occlusal groves and fossae are recommended to decrease the potential for the wedging of food (Figures 4A and B). Additionally, anterior disclusion is easier to develop when posterior occlusal anatomy is shallow and the posterior occlusal plane is flat.

A narrower occlusal table should be considered in the diagnostic wax-up, provisional, and definitive restoration. Normally, a flatter and narrower occlusal table would invite tooth migration, but with an ankylosed implant restoration, that is not a problem. A narrower occlusal table also facilitates easier access for home care.

The splinting of posterior implants is important to consider for patients requiring distribution of forces. Although the total force passed to crestal bone will remain the same for a given load, stress distribution can be manipulated by splinting.⁴¹ Implant fractures as well as screw loosening occur less frequently when implants are splinted together.⁶ When splinting posterior units, indexing and soldering improves the fit and decreases the stress to the prosthetic components and crestal bone. It is often difficult to clinically evaluate the subgingival marginal adaptation of implant castings, so indexing and soldering is highly recommended. If the abutment screw on a multiple-splinted implants binds as it is tightened, it is often a sign that indexing and soldering is needed to provide passivity of the restoration.

Cantilevers significantly increase stresses to the crestal bone, and caution should be especially high when they are used opposite natural dentition. Lindquist and Ahlqvist found more bone loss around fixtures with long cantilevers.^21,22 Although traditional bone-anchor bridges used 10 to 20 mm cantilevers opposing an edentulous arch, modern research has shown stresses to be significant when opposed by the increased forces possible in natural dentition.

The cantilever length possible is influenced by biomechanical factors. The location of the cantilever, anticipated occlusal load, diameter, number, and surface characteristics of the implant, anterior-posterior spread, bone quality, and rigidity of the superstructure should be considered before cantilevers are planned (Figures 5A and B). If there is not sufficient rigidity, the anchorage unit closest to the load will be excessively loaded. Likewise, if the fit between the implant and prosthesis is not accurate, some of the implants will be excessively loaded while other units will not be loaded.

The delivery of an implant-retained posterior restoration involves first checking the proximal and occlusal contacts. It is imperative that contacts be adjusted until light proximal contacts occur so that the restoration is fully seated and the seating is verified with a radiograph. Occlusal contacts are checked next and the opposing dentition adjusted if it is believed that the centric contact can be brought closer to the long axis of the implant. The centric contacts are adjusted with light occlusal contact for two reasons. First, the opposing natural dentition is often compressed; and, secondly, progressive loading is possible when starting with light occlusion on an implant. Any occlusal prematurity will result in increased vertical and horizontal loading.

Lateral contacts are then evaluated. Whenever possible, anterior natural teeth should disclude the posterior implant crowns. When anterior disclusion is not possible, and lateral forces will knowingly be placed on the posterior implant crowns, it is advisable to treatment plan additional wide-diameter implants. A wider diameter implant is almost always an advantage due to increased surface area contact (approximately 25 percent for each 1 mm increase in diameter) and a broader occlusal platform. If lateral forces cannot be eliminated, they should be designed to equally distribute over as many teeth and implants as possible. If excessive forces are anticipated, either an alternative to an implant-supported prosthesis or a removable overpartial can be considered.

Posterior implant restorations that follow an arc are less prone to implant fracture and certain other restorative complications than a posterior quadrant that is restored in a linear configuration.⁶ It has also been shown that restorative complications decrease when three rather than two implants are used to restore a posterior quadrant.⁶ It therefore makes sense that when a posterior quadrant is restored, three implants in a nonlinear configuration be considered. Use of wide-diameter implants will often provide an equivalent benefit to the nonlinear configuration.

Restorations can be cemented, screw-retained, retained with a lingual set screw, or retained with substructure and attachment. Each method for retaining an implant restoration has indications, and occlusal relationships can influence the prosthetic choice. For example, the occlusion is often easier to control and axial loading more favorable with a cemented rather than screw-retained restoration because it is often easier to create a narrow occlusal table without the restriction in dimensions of a screw access hole. In the maxillary anterior, lateral movements are often smoother if not interrupted by a screw access hole. Wear compatibility to the opposing arch is often easier to establish with a cemented restoration because occlusal contacts are often directly over the screw access hole that is covered by composite and opposed by porcelain or metal.

In patients where a cantilever, wide off-set, or future modification in prosthetic design or prosthetic needs are planned or anticipated, a retrievable restorative option such as a screw-retained prosthesis should be strongly considered. Additionally, a screw-retained prosthesis is often necessary when minimal interocclusal space is present. Although reported complications are more common in screw-retained prostheses as opposed to cemented restorations, many of the reported complications represent the clinician’s early learning curve and product development. With improved machining of implant components, the improved pre-load possible with gold-plated screws, and an increased awareness of component limitations, screw-retained restorations have become very predictable and preferable to cemented restorations in many clinical situations.

When complications such as abutment screw loosening occurs with a cemented crown, a new restoration may be required. Additionally, soft tissue complications resulting from excess cement have been reported⁴² and are especially problematic when an implant-supported crown is cemented on an abutment with a subgingival margin where inflamed and/or unattached tissue is proximal to the implant. A screw-retained prosthesis is recommended when retrievability is a priority, and the use of an all-gold restoration is recommended when esthetics do not indicate a ceramo-metal restoration.

Monitoring and Follow-up

More follow-up is needed with implants than with traditional fixed prosthetic work. When a screw loosens, it is important to document the event so that appropriate steps can be taken if repeated screw loosening occurs. Likewise, it is important to monitor the bone level with bitewing radiographs twice in the first year because other signs and symptoms of potential overload are unlikely to be evident. The occlusion should also be evaluated yearly to make sure prematurities do not exist and that the anterior dentition has not worn, which could place more lateral stress on posterior implant restorations.

A nightguard should be fabricated for patients at high risk for occlusal overload. These would include patients with a history of bruxism or clenching, patients with lateral offsets or cantilever, or implant restorations that do not have a proximal natural tooth.

Conclusion

Implant restorations are a "high stakes" treatment requiring a substantial financial and time commitment. Unanticipated restorative complications can be disheartening for both patient and clinician. It is important to educate the patient about potential complications before starting an implant restoration. Many of the complications are related to force transmission and can be controlled with careful treatment planning, which often leads to a better distribution of forces. It is important to identify patients likely to overload the planned restoration and to be aware of warning signs that overloading may be occurring.

Authors

Donald A. Curtis, DMD, is a professor in the Department of Preventative and Restorative Dental Sciences at the University of California at San Francisco School of Dentistry.

Arun Sharma, BDS, MS, is an assistant clinical professor in the Department of Preventative and Restorative Dental Sciences at UCSF School of Dentistry.

Fredrick C. Finzen, DDS, is an associate clinical professor in the Department of Preventative and Restorative Dental Sciences at UCSF School of Dentistry.

Richard T. Kao DDS, PhD, is in private practice in Cupertino, Calif.

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To request a printed copy of this article, please contact: Donald A. Curtis, DMD, UCSF School of Dentistry, Department of Preventive and Restorative Dental Sciences, 707 Parnassus Ave.,

San Francisco, CA 94143-0758.

Legends.

Figure 1A. A radiograph of a 3.75 mm diameter implant replacing a molar in a patient with a bruxing habit.

Figure 1B. A radiograph showing the fractured implant after several appointments to tighten loose or broken screws.

Figure 1C. The fractured implant. Screw loosening and, in particular, screw breaking are warning signs that more serious problems may occur.

Figure 2A. Clinical example of fractured porcelain due to poor metal design that did not provide porcelain support.

Figure 2B. Laboratory example.

Figure 3A. Clinical example of a posterior restoration with a slight offset, shallow occlusal morphology, and a narrow occlusal table. The major benefits of a restoration with a narrow and shallow occlusal anatomy are improved access for oral hygiene and decreased potential for lateral loading.

Figure 3B. Another view.

Figure 4A. Laboratory example of a posterior restoration where shallow occlusal anatomy allowed anterior disclusion by natural dentition.

Figure 4B. Clinical example.

Figure 5A. Example of a patient with a cantilever.

Figure 5B. Another view. Anterior cantilevers are generally less problematic than a distal cantilever.