Mandibular Advancement Using the Bilateral Sagittal Osteotomy Past, Present, and Future

Abstract

SUMMARY

Although the group of articles studying the sagittal split osteotomy is large and varied, it is possible to draw several conclusions regarding the present state of knowledge. First, several variables have been shown not to cause relapse within the limits of the patients studied. These variables include age, preoperative orthodontics, and mandibular plane angle. Likewise, several factors decidedly do have an influence on relapse. As various investigators have shown,²⁶'³⁰'⁵⁶ the magnitude of advancement is positively correlated with relapse, presumably secondary to stretch of muscles and other paramandibular soft tissues. Condylar displacement has also been shown by many authors^{15,17, 25, 28-30, 34, 39, 45, 59} to lead to relapse, along with a more general variable of proximal fragment position. In addition, as Schendel and Epker⁴⁵ reported (and the variability of relapse figures demonstrates), “different surgeons had predictably good or unacceptable results”— i.e., some unspecified technical variables are apparently at work.

Although identifying some of the factors responsible for relapse is an important step, it is only a first step. The next step is to find ways of preventing relapse by manipulating these factors. Skeletal fixation has been conclusively shown to reduce the amount of horizontal relapse and anterior facial height increase after sagittal osteotomies.^11,35 It is less clear what effect the devices used intraoperatively to position the proximal segment might have on relapse. Several investigators reported on such devices^15,22,31,41 and recommended their use, but only Raveh and coworkers⁴¹ had data to support such a recommendation. Rigid internal fixation was regarded as a panacea for the relapse problem for several years, but recent studies have shown that the role of this technique in the stability of the advanced mandible is not so clearcut. The present understanding is that when the amount of advancement exceeds 6 to 7 mm, there is a significant decrease in stability. It should be realized that with the different screws (self-tapping, nonself-tapping) and the different configurations of screws and plates used, rigid fixation cannot be considered a single technique.

Finally, some potential influences on stability have not yet been ruled in or out. The suprahyoid myotomy is a prime example. Indeed, clinicians and researchers have postulated for 20 years that “muscle pull” is one of the primary causes of mandibular relapse.^{16,20,21,36,38, 49} The extent of the contribution of muscle pull to instability, and the specific role played by the suprahyoid muscle group and its myotomy, is one of the prime questions left for the future. Another factor whose role is as yet unclear is the articular cartilage and its response, both short and long term, to increased loading such as is found after mandibular advancement. Huang and Ross²³ examined the amount of advancement and its influence on mandibular relapse in growing individuals. They found in all patients that growth ceased for 1 year, during which time neuromuscular adaptation presumably occurred. However, growth then resumed in 6 of the 11 patients having less than 10 mm of advancement. Those patients with greater than 10 mm of advancement showed no growth, with three of ten patients even showing continued decrease in mandibular length. This suggests that the articular cartilage was loaded with concomitant remodeling changes, and in the group with larger advancements, the loading was above the level that permitted mandibular growth. Copray and colleagues⁶ also investigated the influence of compressive forces on condylar cartilage, using rat embryonic cartilage in culture. They found that although continuous and intermittent compressive forces up to 3 g and 8 g, respectively, not only allowed but stimulated growth, forces above these levels caused deformation and necrotic shrinkage of the condyles.

What does the future hold for the bilateral sagittal osteotomy? It seems clear that the procedure will continue to be the main technique for surgical mandibular advancement. The principal advances that need to be made lie in the areas of determining the role of muscle stretch and adaptation and the response of the articular cartilage. Once these two factors are well understood, the bilateral sagittal osteotomy will be much more predictable.