There is generalized agreement that excessive stress to the bone-implant interface may result in implant overload and failure.1 Such stress may adversely affect an implant during the healing phase or while in function. If this occurs during initial osseous healing after surgery, the result may be implant mobility rather than rigid fixation. Early implant loading failure occurs shortly after uncovering of the implant. The implant appears to exhibit rigid fixation, and all clinical indicators are within normal limits. However, once loaded following fabrication of a prosthesis, the implant becomes mobile. Early loading failure may affect 2% to 6% of implants, and as many as 15% of restorations fail as a result of this problem.2,3 Excess load on a final restoration after successful implant integration can result in implant failure. In addition, fracture of the implant body is a long-term complication often associated with cantilever restorations or other offset forces on the restoration.4
Inadequate osseous healing, crestal bone loss, abutment screw loosening, porcelain and/or implant component fracture, and loosening of the prosthesis can all be related to the amount of stress applied to the implant/prosthetic system.5 Therefore, diagnosis and management of excessive forces on implants are essential to reduce complications.
STRESS AND FORCE
Stress is defined as force divided by the functional area over which the force is applied. Therefore, stress is directly related to force. Force may be evaluated in terms of magnitude, duration, type, direction, and multiplication factors.4 Different clinical conditions affect force on an implant in different ways, and an increase in any of these factors has the potential to increase complications. Early in treatment planning, the potential forces that will be exerted on the prosthesis should be evaluated and considered in the overall treatment plan to minimize the risk of complications. If it is anticipated that the force on the implant(s) will be greater than usual, the treatment plan should be modified to decrease the stress on the system.
NORMAL BITE FORCE
The greatest natural forces exerted against teeth, and by extension against implants, occur during mastication. These forces are primarily directed perpendicular to the occlusal plane in the posterior regions, are of short duration, occur only during brief periods of the day, and range from 5 to 44 pounds per square inch (psi) for natural teeth. A force of 21 psi is needed to chew meat, and 28 psi to chew a raw carrot.6 The actual time during which chewing forces are applied to the teeth is about 9 minutes each day.7
Parafunctional forces on teeth or implants are characterized by repeated or sustained occlusion, and have long been recognized as harmful to the stomatognathic system.8 The causes of parafunctional force include bruxism, clenching, and tongue thrust. Parafunction may be categorized as absent, mild, moderate, or severe. Bruxism and clenching are the most critical factors to evaluate prior to implant reconstruction. Long-term success is not predictable in the presence of severe bruxism or clenching. This does not mean that patients who brux or clench cannot be successfully treated. However, a careful pretreatment evaluation of these conditions, and patient education as to the possible resulting complications and treatment, need to be established before treatment is initiated.
Bruxism is defined as the unconscious gnashing, grinding, or clenching of the teeth.9,10 The literature often does not identify clenching (force with little movement) and bruxism as separate entities. Therefore, in this article bruxism is defined as nonfunctional grinding of teeth in a horizontal direction, which is the most significant biomechanical stress factor. The forces involved are in excess of normal physiologic masticatory loads. The maximum biting force of patients who brux is greater than average.7 As a result of constantly exercising the muscles of mastication, patients who brux develop a greater maximum bite force than patients who do not brux. For bruxers, loads on the teeth may exceed 500 psi. A 37-year-old patient with a long history of bruxism recorded a maximum bite force greater than 990 psi (four to seven times normal).11 Bruxism may affect the teeth, muscles, joints, bone, implants, and/or the prostheses.12 These forces may occur while the patient is awake or asleep, and may be present for several hours per day. Bruxism and clenching are the most common oral habits, and may occur to some degree in over 80% of the population.13
|Figure 1. Tooth wear is the best method to evaluate bruxism. The “pathway of destruction” informs the dentist about parafunctional activity. The posterior wearing of teeth indicates a loss of anterior guidance during excursions.|
The most direct way to diagnose bruxism is to evaluate the wearing of teeth (Figure 1). Nonfunctional wear facets on occlusal surfaces may occur on both natural or prosthetic teeth. Attrition of the anterior teeth appears on the incisal edge, especially in the mandible and maxillary canines, and there may be notching of the cingulum in the maxillary anterior teeth. Isolated wear of an anterior tooth is not as much of a concern if all posterior teeth contacts can be eliminated in excursions. Tooth wear is particularly significant when found in the posterior region. Posterior wear patterns are more difficult to manage because these are usually related to a loss of anterior guidance in excursive movements. If the posterior teeth are in contact during mandibular excursions, greater forces are generated.14 Consequently, prior to restoration with an implant-retained prosthesis, the occlusal plane and anterior incisal guidance may need to be restored to eliminate posterior contacts during mandibular excursions.
|Figure 2. Anterior wear on the maxillary lateral and mandibular cuspid is mild.||Figure 3. Patients may brux beyond the incisal edge position. The engram pattern is shown by matching wear patterns on the incisal edges of the maxillary lateral and mandibular cuspid. The mandibular first premolar has a working contact in this engram pattern, and the excess force at this position is responsible for the cervical abfraction. The “pathway of destruction” should be modified, so the excess force will not be delivered to the mandibular implant reconstruction.|
Patients who brux often exhibit mandibular excursions that do not correspond to border movements of the mandible. As a result, the occlusal wear is very specific, and primarily on one side of the arch, or even on only a few teeth (Figures 2 and 3). When a habit is regularly repeated and persists after the stimuli ceases, it may be called an engram. Hence, an engram is a definite and permanent trace left by a past stimulus,15 and usually remains after treatment. If the restoring dentist re-establishes incisal guidance on teeth severely affected by an engram bruxing pattern, the incidence of complications on these teeth will be increased.16
The most common complications associated with teeth restored in this “pathway of destruction” are porcelain fracture, uncemented prostheses, and root fracture.17 The amount of wear on teeth is a measure of this condition, as well as its severity (mild, moderate, or severe). Severe bruxism modifies normal masticatory forces in magnitude (higher bite forces), duration (hours rather than minutes), direction (lateral rather than vertical), type (shear rather than compression), and magnification (4 to 7 times normal).5
IMPLANT FATIGUE FRACTURES
The increase in force magnitude and duration on implants as a result of bruxism is a significant problem. Materials have a fatigue curve, which is related to the intensity and frequency of the force.18 There is a force of sufficient magnitude that one cycle causes a fracture (eg, karate blow to a piece of wood). However, if a force of lower magnitude repeatedly strikes an object, it will also fracture. The wire coat hanger that is bent does not break the first time, but repeated bending will fracture the material. This is not because the last bend was more forceful, but because of fatigue. A bruxing patient is at greater risk for implant fracture over time because the magnitude of the force will increase as the muscles become stronger and the number of cycles accumulates. The chance of an untoward outcome will increase if the force cannot be reduced in intensity and duration. Therefore, once the dentist has identified the source(s) of additional force on the implants, the treatment plan must be altered in an attempt to minimize the adverse effect on the alveolar bone, implant, and final restoration.
The cause of bruxism is multifactorial, and may include occlusal disharmony.13 When an implant reconstruction is considered in a bruxing patient, occlusal analysis is warranted. Premature contacts and posterior contacts during mandibular excursions increase the potential for excess force on teeth or implants. The elimination of eccentric contacts may allow recovery of injury to the periodontal ligament and return of normal muscle function within 1 to 4 weeks.14 However, elimination of occlusal disharmony does not necessarily eliminate bruxism.19,20
A night guard fabricated for the maxillary teeth can be a useful diagnostic tool to evaluate the influence of occlusal disharmony and its relationship to nocturnal bruxism. A night guard that promotes even occlusal contacts around the arch in centric-related occlusion, and provides posterior disclusion with anterior guidance in all excursions of the mandible, can be helpful in this regard.21 This device may be fabricated with 0.5- to 1-mm colored acrylic resin on the occlusal surface. If the patient wears this device for 1 month, the influence of the occlusion on the bruxism habit may be directly observed. The night guard establishes an even occlusal form, with anterior disocclusion. If the colored acrylic is not worn through, the parafunction has been reduced and occlusion on the guard is a significant factor. Therefore, occlusal reconstruction or modification is warranted. If the 1 mm-thick colored acrylic on the night guard is ground through, an occlusal adjustment will have little influence on decreasing this habit because the proper occlusion on the guard failed to reduce the habit.
|Figure 4. If a posterior sextant of implants supports a fixed prosthesis in the maxilla, a soft reline material is placed around the crowns for stress relief and to decrease the force on the implants.|
Unlike teeth, implants do not extrude in the absence of occlusal contacts. As a result, in partially edentulous patients, the night guard can be relieved around the implants, so the natural teeth bear the entire load and the implant prosthesis is taken out of occlusion in centric and excursions when the night guard is in place. As examples, when the implant restoration is in the maxilla, the night guard is hollowed out so no occlusal force is transmitted to the implant crown(s). When the restoration is in the mandible, the occluding surface of the guard is relieved over the implant crown(s) so no occlusal force is transmitted to the implants. Teeth, with their periodontal membrane, are better able to cope with stress than implants. A mandibular posterior cantilever on a full-arch implant prosthesis may also be taken out of occlusion with a maxillary night guard. When a posterior sextant of implants supports a fixed prosthesis in the maxilla, a soft reline material is placed around the crowns for stress relief and to decrease the impact force on the crowns (Figure 4). When full-arch implant prostheses are opposing each other, the night guard is fabricated so only anterior teeth contact during centric occlusion and excursions. This reduces the amount of muscle force when bilateral posterior regions are out of occlusion, and therefore, decreases the force on the implants.
The functional surface area of each implant support system is primarily related to the width and the design of the implant. Wider root form implants have a greater area in contact with bone than narrow implants of similar design. Occlusal stresses are greatest at the crest of the ridge. As a result, the width of the implant is more important than the length of the implant once a minimum length has been obtained for initial fixation.22 If forces are increased because of moderate to severe bruxism, augmentation of bone width may be indicated to allow placement of implants with a diameter that is increased by 1 mm or more.
Implant macrodesign may affect surface area even more than increasing the implant width. A cylinder-(bullet) shaped implant provides 30% less surface area than a threaded implant. A threaded implant with 10 threads has more surface area than an implant body with five threads, and deeper threads provide a greater surface area than shallow threads.
Natural teeth are narrower in the anterior regions of the mouth compared with the posterior regions, and the amount of force on the teeth is lower in the anterior regions. The natural teeth increase in diameter in the premolar regions and then again in the molar regions, as the amount of force increases. In addition, the number of roots present on teeth increases moving posteriorly. This results in a 300% increase in surface area for molars compared to anterior teeth. The length of the roots of the teeth does not increase when examined from anterior to posterior. Unfortunately, most implant designs only increase 25% to 50% from the smallest to largest diameter.22 Therefore, although wider implants have greater surface area, their width increase is less than that observed with natural teeth.
|Figure 5. The internal hex implant has a thin outer body wall, which makes it 40% weaker than an external hex implant.|
The size of the implant directly affects its strength. The formula to describe the resistance to bending fracture for a solid metal of diameter R is π/4 R4.18 A component twice the diameter is 16 times stronger. Hence, larger diameter implants not only have more surface area to dissipate force, they also are much more resistant to fracture. The resistance to bending fracture for an open tube equals π/4 (Ro4 – Ri4), where Ro is the outer diameter and Ri is the inner diameter.22 This means that an increase in the interior opening of an implant body decreases its strength to the fourth power. Therefore, the internal hex implant (with its larger inner diameter) is 40% weaker than an external hex implant of similar size (Figure 5). The use of larger diameter implants with an external hex connection is strongly advocated for patients who exhibit severe bruxism.
|Figure 6. A fatigue curve plots the magnitude of stress on the Y axis and the number of cycles on the X axis. The greater the stress (force/area), the fewer the number of cycles before fracture.
Titanium alloy (Ti6A1 4V) is four times stronger than grade 1 CP Titaniurn (Ti1) and twice as strong as grade 3 (Ti3). Moderate to severe bruxing patients are at less risk for complications when the implant body and prosthetic components are made of titanium alloy.
|Figure 7. A maxillary implant-supported fixed partial denture (metal occlusals) opposing a cantilever fixed partial denture in the mandible, supported by nine grade 1 titanium implants. Both cantilevers broke off from the prosthesis. The patient decided to wear the restoration as presented, because it still replaced eight teeth (first premolar to first premolar).|
|Figure 8. A few years later, this bruxing patient fractured five of six implant bodies. All six implants were removed. Because the last implant had already been cycled as many times as the other five, it was at increased risk of fracture in the near future.||Figure 9. This implant fixed prosthesis has posterior cantilevers and has lost incisal guidance. The posterior teeth now demonstrate occlusal wear. The teeth should be replaced on the prosthesis and incisal guidance re-established to decrease the excursive forces during parafunction.|
|Figure 10. One year later, the cantilever fractured distal to the last implant. Instead of replacing the teeth on the restoration, the entire prosthesis must be refabricated.|
The strength of the implant body is also related to the material (grade of titanium) from which it is fabricated. There is no clinical difference in implant-bone contact between any of the grades of titanium. Titanium alloy grade 5 is four times stronger than conventionally pure (CP) titanium grade 1 and twice as strong as grade 3 CP titanium (Figures 6 through 8). Most orthopedic devices are fabricated from titanium alloys to decrease the risk of fracture. Therefore, all implant components used in patients with severe bruxism should be fabricated from titanium alloys to decrease the risk of fracture.
If bruxism has caused a loss of anterior guidance during excursions, the anterior teeth should be restored to re-establish the proper incisal guidance and avoid posterior interferences during excursions (Figures 9 and 10). The posterior teeth separate when the incisal guidance is steeper than the inclination of the condylar eminence. However, Weinberg has shown that the steeper the canine guidance, the greater the forces in that region.23 Therefore, the incisal guidance should be as shallow as possible, yet steep enough to separate the posterior teeth.
The prosthesis should be designed to improve the distribution of stress on the implants. The surgeon should insert the implants perpendicular to the curves of Wilson and Spee, and the restoring dentist should, when possible, place centric vertical contacts aligned with the long axis of the implant body.5 The restoring dentist should also indicate the desired contacts on the opposing cast, and communicate precise requirements to the laboratory technician. The posterior occlusal tables may be narrowed from the lingual aspect in the maxilla or the facial aspect in the mandible to prevent inadvertent lateral forces, to decrease the forces during mastication, and to leave greater space for the tongue or cheek. Enameloplasty of the cusp tips of the opposing natural teeth is often indicated to help improve the direction of vertical forces.
Bruxism is a potential risk factor for implant failure. Excessive force is the primary cause of late implant complications. An appreciation of the etiology of crestal bone loss, failure of implants, failure to retain implant restorations, and fracture of components will lead the practitioner to develop a treatment plan that reduces force on implants and their restorations. The forces are considered in terms of magnitude, duration, direction, type, and magnification.
Once the dentist has identified the source(s) of additional force on the implant system, the treatment plan is altered to contend with and reduce the negative sequelae on the bone, implant, and final restoration. One viable approach is to increase the implant-bone surface area. Additional implants can be placed to decrease stress on any one implant, and implants in molar regions should have an increased width. Use of more and wider implants decreases the strain on the prosthesis and also dissipates stress to the bone, especially at the crest. The additional implants should be positioned with intent to eliminate cantilevers when possible. Greater surface area implant designs made of titanium alloy and with an external hex design can also prove advantageous.
Anterior guidance in mandibular excursions further decreases force and eliminates or reduces lateral posterior force. Metal occlusal surfaces decrease the risk of porcelain fracture and do not require as much abutment reduction, which in turn enhances prosthesis retention. The retention of the final prosthesis or superstructure is also improved with additional implant abutments. Night guards designed with specific features also are a benefit to initially diagnose the influence of occlusal factors for the patient, and as importantly, to reduce the influence of extraneous stress on implants and implant-retained restorations.
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19. Sheikholeslam A, Riise C. Influence of experimental interfering occlusal contacts on the activity of the anterior temporal and masseter muscles during submaximal and maximal bite in the intercuspal position. J Oral Rehab. 1983;10:207-214.
20. Holmgren K, Sheikoleslam A, Riise C. Effect of a full-arch maxillary occlusal splint on parafunctional activity during sleep in patients with nocturnal bruxism signs and symptoms of craniomandibular disorders. J Prosthet Dent. 1993;69:293-297.
21. Rateitshak KH, Wolf FW, Hassel TM. Periodontology. 2nd ed. New York, NY: Thieme; 1989:332-333.
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Dr. Misch is the director of the Misch Implant International lnstitute, a hands-on training format for prosthetics and/or surgery (including bone grafting) for implant dentistry. For more information, visit www.misch.com or call (248) 642-3199. He is also cochairman of the International Congress of Oral Implantologists, the world's largest implant organization. For more information on the ICOI, visit www.icoi.org.