A method for evaluating skeletal relapsing force during maxillomandibular fixation after orthognathic surgery: a preliminary report.

Although skeletal relapse after orthognathic surgery can be considered primarily the results of unbalanced tension in the surgically modified stomatognathic system (i.e., skeletal relapsing force [SRF] acting on the mandibular segment), no study of SRF has been reported and its nature still remains unclarified. This article presents a practicable method for evaluating SRF during the fixation period with the use of a foil strain gauge fixed to the upper arch wire between the central incisors, with a preliminary result after mandibular advancement by sagittal split ramus osteotomy to demonstrate the value of this method for studying SRF. The pattern and degree of SRF were shown to vary during the postoperative period and in accordance with experimental situations, including the mandibular resting posture, swallowing, clenching, and speaking. On day 3, a constant SRF of 550 gm existed even in the resting posture and was increased by 450 gm to a maximum of about 1 kg on swallowing; clenching and speaking also produced an increase. Thus it was inferred that SRF arose not only from surgically stretched perimandibular connective tissues but also from intermittent physiologic muscle actions. The constant SRF subsided gradually with time, and after day 33, swallowing as well as clenching no longer increased the SRF, which indicates that the critical period for postoperative skeletal stability was the first few weeks, during which muscle readaptation and primary bone healing occurred. It was concluded that this method of evaluating SRF, which is capable of immediately revealing any tendency to relapse or the progress of soft tissue adaptation and bone healing, could be used as an effective research tool.

Cause of early skeletal relapse after mandibular setback.

The present study was undertaken to examine the factors that might be responsible for the skeletal relapse occurring during the period of intermaxillary fixation after mandibular setback osteotomy. Fifteen patients, treated for absolute mandibular prognathism by modified sagittal split ramus osteotomy and fixation by skeletal suspension wiring, were evaluated cephalometrically by reference to the degree of postsurgical superior shift of the gonial region of the distal segment as a parameter of relapse since such a shift was evident despite the use of wiring. It was found that the degree of inadvertent anteroposterior rotation of the proximal segment at surgery, rather than the extent and pattern of surgical repositioning of the distal segment, was significantly correlated with the degree of shift. This result emphasizes the justification of preserving the proximal segment in its exact original anatomic site, in addition to the use of skeletal fixation, to ensure predictable stability after mandibular setback osteotomy.

Skeletal fixation versus skeletal relapse.

The use of skeletal fixation was evaluated for skeletal stability during the period of intermaxillary fixation following a modified sagittal split ramus osteotomy for mandibular prognathism. A combination of bilateral maxillary peralveolar wires and circummandibular wires in the canine region was used for the fixation. One group of patients with this method of fixation (S group) and a second group without the fixation (C group) were compared cephalometrically. Statistically significant differences existed in the amount and pattern of relapse; the fixation produced a significant effect on retention of the corrected chin position. As a consequence, downward and backward rotation of the distal fragment of the mandible and compensatory incisor extrusion were notably controlled. However, upward shift of the posterior end of the distal fragment occurred persistently even in the S group, causing considerable intrusion of the posterior teeth in comparison with the C group. This seems to indicate that tension, probably exerted by the pterygomasseteric sling, is important in postoperative skeletal instability.