Stress and Strain

The presence of fibrous tissue has long been known to decrease the long-term survival of a root form implant.¹ Excessive loads on an osteointegrated implant may result in mobility of the supporting device, even after a favorable bone-implant interface has been obtained.⁶ Although several conditions may cause crestal bone loss, one of these may be prosthetic overload.⁷ Excessive loads on the bone result in increased strain conditions in the bone.⁸ These microstrains on the bone may affect the bone remodeling rate and cause pathologic overload, which results in the loss of bone. The amount of bone strain is directly related to the amount of stress applied to the implant-bone interface. The greater the stresses throughout the implant-bone interface, the greater the risk factor for crestal bone loss and subsequent implant failure.⁹ Therefore the stress and strain relationship has been shown to be an important parameter for crestal bone maintenance and implant survival.

Forces applied to dental implants may be characterized in terms of five distinct (although related) factors: magnitude, duration, type, direction, and magnification.⁵ Each factor should be considered within the physiologic constraints on implant size. In addition, there are surgical and prosthetic considerations related to implant size. For example, the increase in stress to an implant body also increases the risk of abutment screw loosening or implant body fracture. As a result, a relative risk factor for different implant sizes may be established.

Force Type

Three types of forces may be imposed on dental implants within the oral environment: compression, tension, and shear. Bone is strongest when loaded in compression, 30% weaker when subjected to tensile forces, and 65% weaker when loaded in shear (Figure 9-1).¹⁹ Therefore an attempt should be made to limit shear forces on bone, because it is least resistant to fracture under these loading conditions. This is most important in regions of decreased bone density, because the strength of bone is also directly related to its density.¹⁴ An increased width of an implant may decrease offset loads and increase the amount of the implant-bone interface placed under compressive loads.

Figure 9-1 Bone is strongest under compression forces, 30% weaker to tensile forces, and 65% weaker to shear forces.

Force Direction

The forces to an implant body are typically greatest at the crestal bone interface.²⁰ Angled loads to the implant prosthesis produce angled loads to the crest module of the implant. The implant angulation is important to consider. For example, given an occlusal load with a 25-lb magnitude, the direction of the load has a significant effect on the magnitude of compressive and lateral load components. By increasing the angle of the load by only 6 degrees, the lateral component of that load is increased by 233%. In addition, every degree of angled load greatly increases the damaging shear load component experienced by the implants, which is the most damaging component of the load (Figure 9-2).

Figure 9-2 The force applied to an implant body with an angled load or angled direction of force is increased in direct relation to the force angle.

Angled loads increase the amount of shear loads to the bone, and the bone is weakest to shear-type loads²¹ (Table 9-1). Therefore, under ideal conditions, the implant body should be oriented to provide long axis compressive loads to the implant and to decrease shear loads to the crestal bone region. Occlusal loads in centric occlusion occur where the occlusal forces are usually the greatest. As such, the implant should be inserted perpendicular to the curve of Wilson and curve of Spee. Additionally, axial alignment places less stress on the overall implant system (i.e., abutment and abutment screw components) and decreases the risk of screw loosening and fatigue fractures of the implant or its components.

Table 9-1 Cortical Bone Strength Related to Angle of Load

The anatomy of the mandible and maxilla places significant constraints on the ability to surgically place root form implants suitable for loading along their long axis. Bone undercuts further constrain implant placement and thus load direction imposed on the implant. Most all undercuts occur on the facial aspects of the bone, with the exception of the submandibular fossa in the posterior mandible.²² Therefore implant bodies are often angled to avoid perforation of the lingual/facial bony undercut during insertion. Resorptive patterns after prolonged edentulism also exacerbates the normally occurring angulation challenges.²³

The maxillary anterior region does not permit an ideal implant position, even under ideal conditions. The natural maxillary anterior teeth are 12 to 15 degrees off the long axis of load, and the bone of the premaxilla is in a similar relationship after tooth loss. Therefore implants in this region are often positioned with a greater relative angle to occlusal loads than any other region.²¹ To decrease the effect of an angled load on the implant, the implant body may be increased in diameter.

Rationale for Longer Implant Length

The length of the implant is directly related to the overall implant surface area, when all other variables are constant. A 10-mm–long cylinder implant increases surface area by approximately 30% over a 7-mm-long implant and has about 20% less surface area than 13-mm–long implants (Figure 9-3).^30,31 As a result, a common axiom has been to place an implant as long as possible. The length of the implant used for prosthetic support often corresponds to the height of available bone in the edentulous site.

Figure 9-3 The length of a dental implant is directly related to its overall surface area. For every 3-mm increase in length, a cylinder implant has a 30% increase in surface area for shorter sizes and 20% for longer sizes.

Conventional thinking suggests that longer implants provide maximum functional surface area in a healed implant interface. However, when the length axiom is reevaluated, several challenges ensue. The available bone height is greater in the anterior regions of the mouth, especially the anterior mandible. Yet the bite forces are lower and the bone density is greater in these regions, especially in the mandible. The posterior regions have less bone height and the implant cannot engage the dense opposing cortical plate, because it is either nonexistent (maxilla) or beyond anatomical limits (neurovascular canal in the mandible). The posterior maxilla often has less height than the mandible, and the bone is less dense. As a result, poorer survival rates are reported for posterior implants, with the posterior maxilla associated with the highest failure rate (Figure 9-4).

Figure 9-4 The longest implants are most often placed in the anterior mandible, followed by the anterior maxilla. The shortest implants are often placed in the posterior maxilla or mandible (or cantilevered from anterior implants). The survival rate of implants is related to length and anatomical location.

Implants longer than 15 mm have been suggested to provide greater stability under lateral loading conditions. However, reports suggest that increasing the length beyond a certain dimension may not reduce force transfer proportionately.^20,32 Finite element analysis provides an analytical means to investigate the influence of implant length relative to functional surface area under such extreme loading conditions. Misch and Bidez placed cylindrical implants of different lengths in a computer bone model with ideal bone density and volume (see Chapter 11). The embedded implant body lengths were 5, 10, 15, 20, and 30 mm. A 10-mm crown height was designed above the level of bone, and a lateral force of 50 N was applied to the top of the 10-mm crown. For these loading conditions, the percentage of maximum stress was plotted against the percentage of embedded length (Figure 9-5). Peak stresses were not completely dissipated in the 5-mm-long implant model, with approximately 30% of the maximum stress still present at the apex of the implant. The 5-mm implant model, therefore, did not provide sufficient length for lateral force dissipation in spite of good bone density and volume. In the 10-mm implant model, 80% of maximum stress was dissipated in approximately 95% of the embedded length. For the 15- and 20-mm lengths, 80% of the maximum stress was dissipated in approximately 90% of the embedded length. For the 30-mm length, 80% of the maximum stress was dissipated in approximately 70% of the embedded length. Therefore the length of the implant in favorable bone quality and crown height may range from 10 to 15 mm, and 12 mm is usually ideal. All implant lengths exhibited 80% to 100% of the stress in the crestal 40% of the implant length. The results of this analysis point to the fact that the majority of the maximum stress generated by a lateral load can be dissipated as well by implants in the range of 10 to 15 mm in length, compared with implants in the range of 20 to 30 mm in length. In addition, the highest stresses were observed in the crestal bone regions, regardless of the implant length. This biomechanical analysis supports the opinion that longer implants are not necessarily better in good bone volume and density, such as that found in the anterior regions of the mandible. An implant length of 12 mm is usually ideal under most patient force and bone density conditions and up to 15 mm is suggested in softer bone types.

Figure 9-5 A computer bone model with implant lengths of 5, 10, 15, 20, and 30 mm was loaded with a lateral force of 50 N applied to the top of a 10-mm crown extension. Almost 80% of the stress was found in the crestal 40% of the implant, regardless of the length.

Bicortical stabilization, a rationale often cited for longer implants, is not needed in D1 bone, which is primarily homogenous cortical bone.¹³ Because bone overheating is a primary cause of surgical failure, attempting to engage the opposing cortical plate and preparing a longer osteotomy may result in overheating the bone when it is D1 or D2.³³ A threaded implant may not readily engage the denser bone of the apical cortical plate, and the implant threads may strip along the rest of the osteotomy, especially if D3 or D4 trabecular bone types. In addition, after the implant-bone interface is formed, excessively long implants do not transfer stress to the apical region, because most of the stresses are transmitted within the crestal 7 to 9 mm of bone (except in the softest bone types) and therefore are often not needed.²⁰ A further analysis relative to implant length is suggested, especially in good-quality bone.

D3 and D4 bone are primarily present in the posterior regions of the jaw, where less available bone height is observed compared with the anterior regions. Nerve repositioning is cited as an acceptable clinical treatment to facilitate placement of longer implants in the posterior mandible. However, this advanced surgical procedure represents an increased risk for paresthesia and is often not indicated, especially when other implant options are available.³⁴ To place the longest implants in the maxillary posterior regions, a sinus graft is often required. Sinus grafts have been shown to grow bone into the graft from the surrounding walls of bone, and bone does not form under the sinus mucosa. The apical end of longer implants will not benefit from the sinus bone graft procedure for some time, and the crest of the bone below the graft will remain at greater risk. Therefore increasing surface area primarily by length in the posterior regions of the jaws requires advanced grafting or nerve repositioning surgery and, in spite of this, does not benefit the primary regions of increased stress (i.e., the crestal bone region). However, implants from 12 to 15 mm in length are usually considered ideal for most situations.

Disadvantages of Short Implants

Different risk factors for implant longevity have emerged over the years. A review of the literature related to implant failure and implant length was published by Goodacre et al. in 2003.³⁵ In the majority of articles addressing implant length, implants 10 mm or smaller have increased failure rates. In the reports summarized, the failure rates of short implants were 10%, compared with a 3% failure rate of longer implants. The failure is even more apparent when the literature reviews implants smaller than 10 mm in the posterior regions of partially edentulous patients. Less than half the clinical reports had survival rates higher than 90%, and more than half of the reports had implant failure higher than 19.7% (Tables 9-2 and 9-3).^36–46

A review of several of the larger, multicenter reports of short implants is noteworthy. Minsk et al. reported the results of a training center in 1996, with 80 different operators using 6 different systems over a 6-year period.²⁷ Implants 7 to 9 mm in length were reported to have a 16% failure rate. The overall survival rate of all longer lengths was 95%. Winkler et al. published a multicenter report over a 3-year period in 2000.³⁷ The implant survival was directly related to the length of the implant. The 7-mm-long implants had a 26.4% failure rate, whereas 16-mm implants demonstrated only a 2.8% rate of failure. Implants of 8 mm had a 13% failure rate, whereas 10-mm implants in the report failed at a rate of 10.9%, and the 13-mm implants failed 5.7% of the time.

A multicenter report by Weng et al. in 2003 found 60% of all failed implants were 10 mm or less in length.³⁸ The overall failure rate of all implants in the study was 9%, yet the 7-mm implant failed 26% of the time and the 8.5-mm implant had a 19% failure rate³⁹ (Figure 9-6). Naert et al. reported in the literature on clinical outcomes of short dental implants.⁶ Implants shorter than 10 mm had a survival rate average of 81.5%, whereas longer implants had a survival rate higher than 95%.

Figure 9-6 The 7-mm implant failed 26% of the time and the 8.5-mm-long implant had a 19% failure rate. Nearly all the failures occurred within 18 months after the initial loading period.

(Data from Weng D, Jacobsen Z, Tarnow D et al: A Prospectiv multicenter clinical trial of 3: machined-surface implants: results after 6 years of follow-up, Int J Oral Maxillofac Implants 18:417-423, 2003.)

It should be noted the failure rates in most all these reports are not surgical failures or failures to osseointegrate. The failures associated with short implants often occurred after prosthetic loading. In other words, the surgical success was not affected by implant length. However, after the prostheses were in function, an increase in early loading failure was observed, especially within the first 12 to 18 months.

When an implant of 12 mm or more is unavailable without advanced surgical procedures to gain height, a shorter implant may be considered. However, four reasons linked to biomechanics may explain why the posterior short implant may have a higher failure rate after loading compared with longer implants⁴⁸: