Stress Treatment Theorem for Implant Dentistry: The Key to Implant Treatment Plans

by Carl E. Misch, BS, DDS, MDS, PhD (hc)

Dentistry is a unique aspect of medicine, blending science and art form. Some aspects of the dental field emphasize the art form, as in dental esthetics, which deals with tooth color and shape to enhance a patient’s smile and overall appearance. In the partial or complete edentulous patient, the dental prosthesis is responsible for dental esthetics (Figs. 1-3). However, the dental laboratory technicians are largely responsible for the final esthetic result and they do not have the title of doctor. The primary reason the term doctor is applied to the dental profession is not a result of the art form — it is because of the dental sciences.

The dental sciences may be separated into a biological component and a biomechanical component. For dentists in general, the biological aspects of oral health are emphasized in our education. This makes sense because the most common complications related to the natural dentition are primarily of biological origins, with periodontal diseases, caries, and endodontic problems as examples.1-3 In fact, the doctorate we receive in dentistry is really a doctorate of biological sciences.

A combination of biological and biomechanical factors is responsible for the failure of tooth-supported fixed prostheses. For example, the four most common complications for three-unit fixed prostheses are (1) caries, (2) endodontic problems, (3) unretained prosthesis, and (4) porcelain fracture.3,4 The biological complications occur with greater frequency (11 percent to 22 percent), compared with the biomechanical (7 percent to 10 percent), but both aspects should be understood by the clinician. Yet, most often as dentists, we have few lectures on the biomechanics of dentistry with the exception of orthodontics related to tooth movement.

Compared with an implant, the support system of a natural tooth is better designed to reduce the biomechanical forces distributed to the tooth/restoration and the crestal bone region. The periodontal membrane, biomechanical design, nerve and blood vessel complex, occlusal material and surrounding type of bone blend to decrease the risk of occlusal overload to the tooth system.5 As a consequence, the biologic factors of caries and endodontic failure are a greater risk.

Implant dentistry primarily involves the replacement of teeth. When implant complications are reported, the vast majority of problems are related to the implant sciences. However, unlike natural teeth, the biological aspects of implant dentistry have fewer complications. On the other hand, biomechanical related problems may affect more than 30 percent of the implant restorations.6

Biomechanical parameters are excellent indicators of the increased risk because they are objective and can be measured. Stress is equal to the amount of the force divided by the area of load. Ways to decrease biomechanical stress are a constant concern to minimize the risk of implant system complications.7 The dentist can determine which condition presents greater risk and by how much the risk is increased. In other words, if a clinical condition creates an increased biomechanical stress to the implant-prosthetic system, the dentist should implement mechanisms to decrease the force and/or increase the area of load. Remember,the implant system is comprised of the occlusal porcelain, the cement or screw which retains the restoration, the abutment screw that retains the implant components, the crestal marginal bone around the implant, the complete bone-implant interface and the implant bodies themselves. The higher stresses may lead to complications in any aspect of this system.

BIOMECHANICAL COMPLICATIONS
The surgical and initial healing phase for implants is primarily related to biological aspects of healing and is very predictable. Most recent reports indicate the surgical phase of implants form a successful interface more than 95 percent of the time, regardless of the implant system used.8,9

Although the initial healing of the implant has very high success rates, an implant may fail shortly after it is loaded with the prosthesis. Before failure, the implant appears to have rigid fixation, and all clinical indicators are within normal limits. However, once the implant is loaded, the implant becomes mobile, most often within the first 18 months (Fig. 4). This has been called early loading failure.10 The most common conditions that cause this failure are when the bone is soft or the implant is less than 10mm long. Under either of these situations failure rate are 400 percent greater than when bone conditions and implant size are more ideal.11,12 The failure in soft bone and with short implants are from biomechanical related factors.13

FATIGUE FRATURES/COMPLICATIONS
The most common source of biomechanical stress to an implant system occurs during occlusal function. Most biomechanical complications do not occur as a result of a single force event, such as a car accident. Instead, they occur over time. Materials follow a fatigue curve, which is related to the number of cycles and the intensity of the force.7 There is a force so great that one cycle causes a fracture (i.e., a hammer hitting a glass window). However, if a lower force magnitude repeatedly hits an object, (and is greater than the endurance limit) it will still fracture (Fig. 5). The wire coat hanger that is bent does not break the first time, but repeated bends will fracture the material — not because the last bend was more forceful, but because of fatigue. Indeed, when the patient says he soaked his bread in coffee before he began to chew it and the porcelain/abutment screw/cement seal/cantilevered prostheses fractured, it may have been “the straw that broke the camel’s back” that caused the problem. The most common complications of implants and prostheses are related to biomechanical conditions related to fatigue.6

PROSTHESES/COMPONENTS FRACTURES
In a 2009 retrospective analysis by Kinsel and Lin of porcelain failures of metal ceramic crowns and fixed partial dentures supported by implants, porcelain fracture ranged from 0 percent to 53 percent of patients and was directly related to force factors14 (Fig. 6). For example, in this report 35 percent of patients with bruxism (and 19 percent of the implant crowns) experienced porcelain fracture with implant supported prostheses, while 17 percent of patients without bruxism had at least one porcelain fracture of porcelain units fractured. When implant prostheses opposed a denture, no fracture was observed. When implant prostheses opposed each other, 16 percent of the dental units experienced a fracture of porcelain. The higher forces in the implant system (including the occlusal porcelain) related to a dramatic increase in biomechanical complications. Note that the incidence of porcelain fracture, even in patients without higher force conditions are greater than observed with natural teeth.

Implant overdentures have problems of attachment fracture or complication (30 percent), removable prosthesis may fracture (12 percent) and in implant-supported fixed prostheses acrylic resin veneers may fracture (22 percent).6 Metal framework fractures also have been reported in an average of three percent of fixed complete prostheses and overdenture restorations may fracture with a range of 0 percent to 27 percent.

Prosthesis screw fracture has also been noted in both fixed partial and complete fixed prostheses, with a mean incidence of 4 percent and a range of 0 percent to 19 percent.6 Abutment screws are usually larger in diameter and therefore fracture less often, with a mean incidence of two percent and a range of 0.2 percent to eight percent (Fig. 7). Implant body fracture has the least incidence of this type of complication, with an occurrence of 1%. This condition is reported with more frequency in long-term fixed prostheses. For example, in a 15-year report, implant body fracture was the most common condition which lead to failure of the implant
system15 (Fig. 8).

Uncemented prostheses are the third most common cause of fixed prostheses failure on natural teeth.3,4 This condition is more common with implant abutments since they are more rigid and higher forces are transmitted to the cement interface. Uncemented restorations (or worse, when one or more crowns become uncemented and some abutments are still retained) occur most likely when chronic loads are applied to the cement interface, or when shear forces are present (as found with cantilevers). Cement strengths are weakest to shear loads. Zinc phosphate cement may resist a compressive force of 12,000 psi, but can only resist a shear force of 500 psi. A shear load is applied to the cement when a cantilever is present.

SCREW LOOSENING
Abutment screw loosening has been detected in an overall average of six percent of implant prostheses.6 Single-tooth crowns exhibited the highest rate of abutment screw loosening and in early screw designs and concepts averaged 25 percent. Recent studies indicate this ratio has been reduced in single crowns to an overall eight percent average, with multiple-unit fixed prostheses at a five percent average and implant overdentures at three percent. In other words, the greater the stress applied to the prostheses (single tooth versus overdentures), the greater the risk of abutment screw loosening.

Screw loosening may cause considerable complications. A loose screw may contribute to crestal bone loss, because bacteria are able to harbor in the open interface. When an abutment screw becomes loose on a cemented crown, the crown may need to be cut off the abutment to gain access to the abutment screw (Fig. 9).

Cantilevers increase the risk of screw loosening, as they increase the forces to the implant system in direct relationship to the length of the cantilever.16 The greater the crown height attached to the abutment, the greater the force applied to the screw, and the greater the risk of screw loosening (or fracture)17 (Fig. 10).

The platform dimension upon which the abutment is seated is more important than the autorotation hexagon height or depth dimension. Larger-diameter implants, with larger platform dimensions, reduce the forces applied to an abutment screw and change the arc of displacement of the abutment on the crest module.17 For example, in a report by Cho et al. abutment screw loosening over a 3-year period was almost 15 percent for the 4mm implant diameter but less than six percent for the 5mm implant diameter.18 Therefore, methods to decrease stress to the abutment screw may be used to decrease the incidence of complications related to screw loosening.

MARGINAL BONE LOSS
Crestal bone loss has been observed around the permucosal portion of dental implants for decades. It has been described after exposure and loading of successfully osteointegrated implants regardless of surgical approaches. It can range from loss of marginal bone to complete failure of the implant.19,20

The bone loss addressed from occlusal trauma is that portion beyond the crest module and below the first thread or rough surface condition of an implant body. In other words, bone loss around the implant body portion designed to distribute the occlusal load (Fig. 11). Many authors find a correlation of crestal bone loss to occlusal overload.20,23 Occlusal trauma is a condition that is more under the control of the dentist, compared to biological factors.24 The treatment plan, implant position, implant number and occlusal design may all affect the magnitude, direction and duration of the occlusal load.

EFFECT ON TREATMENT PLANNING
Any complex engineering structure will fail at its “weakest link” and dental implant structures are no exception. A general concept in engineering is to determine the causes of complications and develop a system to reduce the conditions that cause the problems. Natural teeth most often have biological complications. Implants may also have biological problems. However, unlike natural teeth, the most common causes for implant-related complications are centered on biomechanical stress. Thus, the overall treatment plan should (1) assess the greatest force factors in the system and (2) establish mechanisms to protect the overall implant-bone-prosthetic system.

As a consequence of these biomechanical complications, the evaluation, diagnosis, and modification of treatment plans related to stress conditions are of considerable importance. Therefore, once the implant dentist has identified the sources of additional force on the implant system, the treatment plan is altered in an attempt to minimize their negative impact on the longevity of the implant, bone, and final restoration.

The clinical success and longevity of endosteal dental implants as load-bearing abutments are controlled largely by the biomechanical milieu in which they function25 (Figs. 12 & 13). Stress-related conditions that affect the treatment planning in implant dentistry include the bone volume lost after tooth loss, bone quality decrease after tooth loss, complications of surgery, implant positioning, initial implant interface healing, initial loading of an implant, implant design, occlusal concepts, prosthesis fixation, marginal bone loss, implant failure, component fracture, prosthesis fracture, and implant fracture.26-30 Biomechanical parameters are excellent predictors of increased risks because they are objective and can be measured. One cannot only predict which condition presents greater stress, and therefore greater risk, but also how much the risk is or increased. As the biomechanical stress/risk is increased methods to decrease the force increase the area over which it is applied are employed to decrease complications. OH


Carl E. Misch is Clinical Professor and Director, Oral Implantology at Temple University, Philadelphia. Dr. Misch serves on the Board of Trustees at the University of Detroit Mercy where he is also an Adjunct Professor in the Department of Prosthodontics. He is Adjunct Professor at the University of Michigan, School of Dentistry in the Department of Periodontics/Geriatrics and Adjunct Professor at the School of Engineering in the Department of Biomechanics, at the University of Alabama at Birmingham. Dr. Misch has written three editions of Contemporary Implant Dentistry (Elsevier), which has become the most popular, book in dentistry and has been translated into 9 languages. He has published over 250 articles and has repeatedly lectured in every state in the United States as well as in 47 countries throughout the world.

Oral Health welcomes this original article.

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