Consideration of Biomechanical Stress in Treatment With Dental Implants

There are biological and biomechanical aspects of dental practice. Whereas the most common complications related to the natural dentition are primarily biologic in nature (eg, periodontal diseases, caries, and pulp pathology),1 both biological and biomechanical components are involved in the replacement of teeth with a fixed prosthesis. For example, the 4 most common complications observed for fixed prostheses are (1) caries, (2) endodontic problems, (3) loss of retention, and (4) fracture of the porcelain.2 The biological complications (caries and endodontic problems) occur with greater frequency than the biomechanical complications (loss of retention and fracture of the porcelain). The clinician must understand the concepts underlying both complications.

Figure 1. An implant-supported complete maxillary porcelain-fixed-to-metal restoration for a 40-year-old male.

Figure 2. The panoramic radiograph of the maxillary and mandibular arch of the patient in Figure 1 shows the support for the maxillary restoration and a hybrid complete mandibular prosthesis.

Implant placement almost always in-volves the replacement of teeth. As such, aesthetics is a major concern (Figures 1 and 2). However, when implant complications are reported, the vast majority of problems are related to biomechanical causes. Unlike what occurs for natural teeth, the biologic aspect of implant dentistry has relatively few complications. The initial healing of the bone-implant interface is largely a biologic phenomenon. Reports in the literature indicate that the surgical phase of implantology is successful almost 95% of the time, regardless of the implant system employed.3 Hence, the biologic aspect of this discipline can be considered as predictable.

Figure 3. An early loading implant failure occurred after 1 year of occlusal loading.

Figure 4. Porcelain fracture is a frequent stress complication, with an incidence of up to 7% within the first 7 years.
Figure 5. A single-tooth implant after 1 year of loading has marginal bone loss to the seventh thread. This is an occlusal overload complication.

Figure 6. The implant body from Figure 5 fractured. This is a stress-related complication.

The most common implant-related complications occur after the implant is loaded and can be considered biomechanical in nature. Biomechanical stress is equal to the magnitude of force divided by the area over which it is applied. Implant failure primarily occurs within 18 months of initial implant loading (Figure 3). These early implant loading failures occur most often in poor-quality bone (16% failure) and with shorter implant lengths (17% failure).4 Bone has 4 different densities, and the softest bone types are more than 50% weaker than the harder bone densities.5 Hence, early loading failures are related to the biomechanics of soft bone, since it will microfracture and resorb from the occlusal forces transmitted to the implants. Further, as compared to longer implants, short implants have been shown to have greater biomechanical stress at the bone-implant interface.6

Table. Biomechanical Complications of Implant Dentistry.4,7


1. Acrylic resin veneer fracture (22%)
2. Overdenture attachment fracture (17%)
3. Early implant failure (16% in soft bone or short implants)
4. Porcelain fracture (7%)
5. Prosthetic screw loosening (7%)
6. Acrylic base fracture of overdentures (7%)
7. Abutment screw loosening (6%)
8. Prosthetic framework fracture (3%)
9. Abutment screw fracture (2%)
10. Implant body fracture (1%)
11. Marginal bone loss around the implant

Other biomechanical complications are problematic but do not lead to implant failure. A review of the literature from 1981 to 2001 by Goodacre, et al found the percentage of restorations with complications for implant-fixed prostheses included acrylic resin veneer fracture (22%), abutment or prosthetic screw loosening (6% to 7%), porcelain fracture (7% [Figure 4]), and prosthesis metal fracture (3%).4 This review also reported implant overdenture mechanical problems, such as fracture of the attachments (17%) and fracture of the prosthesis (12%). Implant components and even implant bodies can also fracture, although this is not a common occurrence (Figures 5 and 6). In addition, crestal bone loss may be related to occlusal overload7 (Table). In sum, mechanical complications are more common than biologic complications.
Any engineered structure will fail at its weakest point, and dental implants and their restorations are no exception. A general engineering concept is to determine the causes of the complications, and develop a protocol to reduce the causative elements. The most common cause of implant complication is biomechanical stress. Thus, the overall treatment plan should:
(1) assess the largest forces on the system and (2) establish mechanisms to protect the total implant-bone-restoration system.

EFFECT ON TREATMENT PLANNING
The clinical success and longevity of osseointegrated dental implants as load-bearing abutments are largely controlled by the biomechanical factors under which they function. All restored implants function under stress, and mechanical stress is a risk factor for implant-restoration complications and failure.
Misch developed a theorem for implant dentistry based upon a concept of stress management.8-10 This concept focuses implant and restorative treatment on the biomechanical effects of mechanical stress.
Understanding the relationships between stress and implant complications provides a basis for consistent treatment planning. This concept organizes the elements of diagnosis and treatment planning into a specific sequence:
• prosthesis design
• consideration of forces
• bone densities of implant sites
• key implant position and number
• implant size
• available bone
• implant design.

Prosthetic Design
Partially and completely edentulous patients want teeth, not implants. The final result (the prosthesis) should be designed prior to placement of the foundation (implants). Most partially edentulous patients are treatment planned for a fixed restoration. The fixed restoration must consider the soft and hard tissues and whether or not a surgical approach is required to replace them.
Completely edentulous patients can be treated with either a removable overdenture or a fixed implant prosthesis. In either case, the loss of bone as a consequence of tooth loss and the need to reduce future bone loss are part of the treatment plan.

Consideration of Forces
Since stress equals force divided by the area over which the force is applied, the amount of force directly impacts the amount of stress. Several factors (since they are not the same for all patients) are to be considered, including bruxism, clenching, masticatory dynamics, crown height space, and arch position.
Some factors are more important than others. For example, a history of severe bruxism is the most significant factor.11 The next most important patient force factor is severe clenching.12 Crown height represents a vertical cantilever and is next on the scale, followed by masticatory muscle dynamics.13 The position of the implant in the arch is also important, since implants in the posterior region will be subject to forces that are 3 times greater than those that occur in the anterior region.14 Each patient is unique, and the clinician should evaluate each patient, and each risk factor, individually. As an overall force factor risk increases, the chance for overload increases, and the overall treatment plan should be appropriately modified by increasing implant number and/or size.

Bone Density
Misch has reported on the biomechanical properties of 4 different bone densities in the jaws.15 Bone strength is directly related to bone density.5 Dense cortical bone (D1) is 10 times stronger than soft, fine trabecular bone (D4). D2 bone is approximately 50% stronger than D3 bone. As a general rule, the bone is more dense in the anterior regions of the mouth versus the posterior regions.
The amount of implant-bone contact is directly related to the density of bone.15 Very dense bone (D1) will have the highest percent of bone in contact with an endosteal implant. The sparse trabeculae of bone often found in the posterior maxilla (D4) offers minimal contact with the implant body. Consequently, compared to dense bone, greater implant surface area is required to obtain the same amount of implant-bone contact. Therefore, with less bone contacting the implant body, the overall stress will increase. The implant surface area should be increased in regions of less trabeculated bone.


KEY IMPLANT POSITION AND NUMBER
Key Implant Position: From the perspective of managing mechanical stress, consideration of implant position is important. In a 1-unit or 2-unit prosthesis, an implant should be placed in each tooth position, with avoidance of a cantilever on the crown in any direction (ie, facial, lingual, mesial, and/or distal). For an implant-restoration complex replacing 3 to 4 teeth, the most important abutments are the terminal abutments. If a terminal abutment is not present, the cantilever thus created magnifies the stress on the rest of the implant-restoration complex. Canti-levers are a force magnifier and represent an important risk factor for complications in implant dentistry, including frequency of screw loosening, crestal bone loss, and fracture. In a 5- to 14-unit prosthesis, intermediary abutments are also important in order to limit the edentulous spans to the equivalent of less than 3 pontics. Three adjacent pontics are contraindicated because 3 pontics will flex 19 times more than a 2-pontic restoration.16 It is also suggested that longer edentulous spans be restored with implants in a staggered or offset position (tripod effect), or with the use of larger diameter intermediate implants.16
From a biomechanical perspective, an edentulous mandible may be divided into 3 sections: the anterior region (canine to canine) and the bilateral posterior regions (premolar and molars). An important consideration includes at least one implant in each region, or at least 3 key implants.17 An edentulous maxilla is divided into 5 regions: the bilateral canines, the bilateral posterior regions (premolars and molars), and the anterior region (lateral and central incisors). An important consideration is the need for a key implant in each region, or at least 5 implants.18 Additional implants (besides the key positions) are also usually needed, with the total number of implants determined by patient force factors and bone density.19
Implant Number: The overall biomechanical stress on the implant system may be reduced by increasing the area over which the force is applied.19 The most effective method to increase the surface area of implant support is by increasing the number of implants used to support a prosthesis. For example, Bidez and Misch demonstrated that forces distributed over 3 abutments result in less localized stress to crestal bone than 2 abutments.19 Therefore, the number of pontics should be reduced and the number of implant abutments should be increased whenever biomechanical forces are increased.
Force reduction studies apply only to implants that are splinted together. The retention of the prosthesis is also improved with an increase in the number of splinted abutments. This increase will decrease the incidence of unretained restorations. Splinted implants also decrease the frequency of abutment screw loosening and porcelain fracture. The amount of biomechanical stress to the system is reduced, and the marginal ridges of the implant crowns are supported by the connectors of the splinted crowns. This results in compressive forces rather than shear loads being applied to the porcelain and reduces the risk of fracture.20
Clinical judgment would suggest that it is better to err on the side of safety and place an additional implant to support the prosthesis, rather than err by placing too few implants. One implant too few may result in treatment failure, but an additional implant is rarely associated with a problem. Placing one implant for each lost tooth is usually not necessary for multiple missing teeth, regardless of the risk factors that are present. However, one implant for each missing tooth may be indicated when treating a large, young male patient with severe parafunction. For a full edentulous arch, more than 10 implants are rarely required. Further, fewer than 5 implants are rarely suggested.

 

Implant Size
As a general rule, shorter implants have higher failure rates than longer implants following loading.20 Therefore, the initial treatment plan should include implants that are at least 12 mm in length. In general, less dense bone requires longer implants as compared to more dense bone.15
The surface area of an implant is directly related to the width of the implant.21 Wider root-form implants have a greater surface area than narrow implants (of similar design), resulting in greater potential contact area with bone. Bone augmentation to increase the width of available bone may be indicated to allow placement of wider implants when a patients force factors are greater than ideal.19
It is interesting to note that the natural teeth in the anterior regions of the mouth are narrow, and the amount of force generated in this area is less than what is generated in the posterior regions. The natural teeth increase in diameter in the premolar region and further in the molar region. Surface area increases 300% when comparing the lower anterior teeth to the maxillary molars. However, the length of roots of natural teeth do not increase from anterior to posterior regions of the mouth.9

Available Bone
Once the previously noted aspects of the treatment plan have been determined, the bone at the potential implant sites is evaluated. If adequate bone is present to place the preselected number and size of the implants, implant surgery can be scheduled. If available bone is not present, bone augmentation or modification is required.22 If neither option is available, the treatment planning sequence is begun again. This begins with design of the prosthesis.
In the past, the available bone was the first condition evaluated to determine the number and position of implants.23 Without other considerations, this approach can lead to excessive stress on the prosthetic-implant-bone system and an increase in force-related complications.4

Implant Design

Implant macrodesign may affect surface area even more than an increase in size. 
Strong, et al24 have identified 11 different variables that affect the overall functional surface area of an implant. For example, a cylinder (bullet shape) implant provides 50% less surface area than a conventional threaded implant of the same length and width. A threaded implant with 10 threads per 10 mm in length has more surface area than one with 5 threads. A thread depth of 0.2 mm provides less surface area than an implant with 0.4-mm thread depth.25 Implant design may compensate for less-than-ideal conditions of implant size or number.6 Hence, implant design may be the most efficient method to increase surface area significantly and thereby decrease the overall risk of implant failure.

SUMMARY
The most common implant complications, whether associated with the implant or prosthetic restoration, occur as a result of biomechanical stress. These complications include early implant failure, fracture of the prosthesis, abutment or prosthetic screw loosening, implant crestal bone loss, and problems with overdenture attachments. An engineering approach to resolve biomechanical problems involves determining the nature of complications and then designing an approach to eliminate their underlying causes.
Treatment planning should incorporate methods to reduce stress and minimize its initial and long-term effects. The treatment plan is altered when forces are greater or bone is less dense than usual to minimize the negative impact of stress on the implant, bone, and restoration. Several parameters under the clinicians control can improve the transosteal environment relative to managing stress on the implant-restoration complex. The goal is to decrease the amount of force, or increase the implant-bone surface area, to decrease the chance of implant-restoration complications.

 

 


References

1. Burt BA, Ismail AI, Morrison EC, et al. Risk factors for tooth loss over a 28-year period. J Dent Res. 1990;69:1126-1130.

2. Goodacre CJ, Bernal G, Rungcharassaeng K, et al. Clinical complications in fixed prosthodontics. J Prosthet Dent. 2003;90:31-41.

3. Esposito M, Hirsch JM, Lekholm U, et al. Biological factors contributing to failures of osseointegrated oral implants. (I). Success criteria and epidemiology. Eur J Oral Sci. 1998;106:527-551.

4. Goodacre CJ, Bernal G, Rungcharassaeng K, et al. Clinical complications with implants and implant prostheses. J Prosthet Dent. 2003;90:121-132.

5. Misch CE, Qu Z, Bidez MW. Mechanical properties of trabecular bone in the human mandible: implications for dental implant treatment planning and surgical placement. J Oral Maxillofac Surg. 1999;57:700-706.

6. Misch CE. Short dental implants: a literature review and rationale for use. Dent Today. Aug 2005;24:64-68.

7. Misch CE, Suzuki JB, Misch-Dietsh FM, et al. A positive correlation between occlusal trauma and peri-implant bone loss: literature support. Implant Dent. 2005;14:108-116.

8. Misch CE. Part I: Diagnosis and treatment planning. In: Misch CE, ed. Contemporary Implant Dentistry. St Louis, Mo: CV Mosby; 1993:3-256.

9. Misch CE. Part I: Diagnosis and treatment planning. In: Misch CE, ed. Contemporary Implant Dentistry. 2nd ed. St Louis, Mo: CV Mosby; 1999:3-204.

10. Misch CE. Stress factors: influence on treatment planning. In: Misch CE, ed. Dental Implant Prosthetics. St Louis, Mo: CV Mosby; 2005:71-90.

11. Misch CE. The effect of bruxism on treatment planning for dental implants. Dent Today. Sep 2002;21:76-81.

12. Misch CE. Clenching and its effect on implant treatment plans. Oral Health. Aug 2002:11-21.

13. Misch CE, Goodacre CJ, Finley JM, et al. Consensus conference panel report: crown-height space guidelines for implant dentistry: part I. Implant Dent. 2005;14:312-318.

14. Bidez MW, Misch CE. Force transfer in implant dentistry: basic concepts and principles. J Oral Implantol. 1992;18:264-274.

15. Misch CE. Density of bone: effect on treatment plans, surgical approach, healing, and progressive bone loading. Int J Oral Implantol. 1990;6:23-31.

16.Bidez MW, Misch CE. Clinical biomechanics in implant dentistry. In: Misch CE, ed. Contemporary Implant Dentistry. 2nd ed. St Louis, Mo: CV Mosby; 1999:303-316.

17. Misch CE. Mandibular full-arch implant fixed prosthetic options. In: Misch CE, ed. Dental Implant Prosthetics. St Louis, Mo: CV Mosby; 2005:252-264.

18. Misch CE. Maxillary partial and complete edentulous implant treatment plans: fixed and overdenture prostheses. In: Misch CE, ed. Dental Implant Prosthetics. St Louis, Mo: CV Mosby; 2005:281-308.

19. Bidez MW, Misch CE. Issues in bone mechanics related to oral implants. Implant Dent. 1992;1:289-294.

20. Misch CE. Short versus long implant concepts: functional surface areas. Dent Today. Aug 1999:60-65.

21. Misch CE. Implant design considerations for the posterior regions of the mouth. Implant Dent. 1999;8:376-386.

22. Misch CE. Divisions of available bone in implant dentistry. Int J Oral Implantol. 1990;7:9-17.

23. Adell R, Lekholm U, Rockler B, et al. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg. 1981;6:387-416.

24. Strong JT, Misch CE, Bidez MW, et al. Functional surface area: thread-form parameter optimization for implant body design. Compendium. 1998;19:4-11.

25. Steigenga J, Al-Shammari K, Misch C, et al. Effects of implant thread geometry on percentage of osseointegration and resistance to reverse torque in the tibia of rabbits. J Periodontol. 2004;75:1233-1241.

 


Dr. Misch is clinical professor and director of oral implantology at Temple University School of Dentistry. He is also a clinical professor at the University of Michigan School of Dentistry and adjunct professor at the University of Alabama at Birmingham, School of Engineering. In addition, he is the director of the Misch International Implant Institute. Dr. Misch is co-chairman of the Board of Directors of the International Congress of Implantologists (ICOI), has published more than 200 articles, and is author of the books Contemporary Implant Dentistry (C.V. Mosby) and Dental Implant Prosthetics (Elsevier-Mosby). He can be reached at (248) 642-3199.

 





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