A redefinition of bilateral sagittal osteotomy (BSO) advancement relapse

Abstract

M a n y bilateral sagittal osteotomy (BSO) wire osteosynthesis studies have examined relapse and ascribed significance to a number of variables that seemingly cause relapse. 1-11 These studies have generally isolated condylar sag (proximal fragment positional change) as the major source of BSO relapse. The source of condylar sag has not been defined. No study has postulated whether condylar sag occurs because the condyle is not seated into the glenoid fossa at surgery, or some force moves the condyle out of the fossa after placement. Other cited factors associated with relapse have been large advancements, s'7'~2'13 upward and forward (counterclockwise) advancements, 8'1°'14 and high mandibular plane angle. 10,15 It is not clear whether these additional variables are independent of condylar sag or are the cause of condylar sag. The wire osteosynthesis condylar sag relapse model only partially explains early B point relapse (surgery to osteotomy union). It is totally inadequate in explaining late (greater than 9 months) B point relapse, which sometimes occurs. Screw osteosynthesis studies have also examined multiple causes of B point relapse. Magnitude of advancement has had the greatest correlation with relapse associated with this fixation technique. 16 The placement of larger, more numerous and different patterns of screws into the osteotomy site or the use of some form of skeletal suspension have been suggested to stop relapse. 16 The source of screw fixation relapse is not condylar sag, but is assumed to be osteotomy slippage. This accounts for the emphasis on larger screws and greater screw number. Comparison of relapse between rigid and wire osteosynthesis has shown consistently that less relapse occurs with rigid fixation, at least in the short term. 17,18