Abstract
In a bilateral sagittal split osteotomy (BSSO) mechanical irritation of the inferior alveolar nerve (IAN) (e.g. by chiselling) should be avoided to prevent neural damage. A modification of the Obwegeser–Dal Pont operation technique was studied by splitting 100 pig mandibles ex vivo . An additional osteotomy at the caudal border of the mandible was used to facilitate the sagittal split by means of a locus of minor resistance. The chisel was inserted distal to the second molar and far away from the IAN. The mandible was split by torque. The modified technique reduced the required torque to split the mandible about 30% compared with the original technique (paired t -test, t (69) = −12.89; p < 0.05). 75% of all mandibles split by the modified technique were classified as bad splits compared with 100% using the original technique using the same protocol without the additional osteotomy.
Damage to the inferior alveolar nerve (IAN) is one of the most frequent complications of bilateral sagittal split osteotomy (BSSO) . Sensory deficits at least 1 year after the operation are reported in 13–40% of all patients. Attempts to solve this problem have been made since its introduction in 1953 by O bwegeser and T rauner . Hunsack, Epker and Dal Pont altered the basic technique to prevent unfavourable bone splitting (bad splits), to avoid pseudarthrosis and to avoid the IAN .
Partial lip and chin numbness is caused by axonotmesis , anaesthesia by neurotmesis. Although, in most cases, it recovers within a year , sensibility is still a problem in orthognathic patients after surgery. Other infrequent problems are unfavourable splits and pseudarthrosis. Unfavourable or bad splits are often related to insufficient osteotomies, which can be related to anatomical aberrations, or to the use of superficial osteotomies in an attempt to protect the IAN . Improved preoperative diagnostics did not improve the complication rate .
The aim of this animal cadaver study was to examine the effect of an additional osteotomy of the caudal border of the mandible in a BSSO conducted using the Obwegeser–Dal Pont technique. The technique was modified to maintain a safe distance from the neurovascular bundle during the entire operation. By creating an additional locus of minor resistance the authors anticipated a favourable split despite more superficial chiselling of the remaining osteotomy lines.
Materials and methods
The Obwegeser–Dal Pont operation was standardized in the following way. Each jaw was measured and major anatomical structures were marked ( Fig. 1 ). Unlike the human mandible, there are three mental foramens in pig mandibles. The middle mental foramen was chosen as the point of reference. The mandibular body was divided into three segments. The lingual osteotomy was made above the mandibular foramen through the cortical bone. The buccal cut through the cortical bone was performed on the perpendicular line between the first and the second segment distal to the mental foramen. The two osteotomies were connected by a third osteotomy along the oblique line. By chiselling, the crestal osteotomy was deepened to two-thirds of the distance between the mental foramen and the alveolar ridge. An 18 mm wide osteotome was inserted distal to the last molar tooth no deeper than two-thirds of the distance between the mental foramen and the alveolar ridge. The mandible was sagittally split by torquing with the osteotome in one direction only. In the left mandible, the osteotome was wrenched clockwise and there was a counter clockwise rotation in the right body of the mandible. In the modified operation technique, an additional osteotomy, 5 mm deep, was added at the caudal border of the mandible ( Fig. 2 ).
A test system made from stainless steel was constructed, which contained a fixation for one half of a pig mandible and a torque gauge. The mandible was fixed rigidly at the condyle and at the mental part of the jaw. The distal part of the mandible was moveable in all three dimensions. The torque gauge holder was freely moveable, so it was always possible to put the osteotome in the right position. The digital torque gauge HTG2-10 (IMADA, Toyohashishi, Japan) was used to record the force ( Fig. 3 ) necessary to split the mandible. It ranged between 0 and 10 N m with a resolution of 0.01 N m. The accuracy of the torque gauge was ±0.5% and the measuring frequency was 33 times per second. The gauge was connected to a PC over a RS232 cable for data recording and processing using ZLINK 2 software (international edition, version 2.02E delivered by IMADA).
100 Pig mandibles ( Sus scrofa domestica , Schlachthof Bochum GmbH, Germany) were available in two batches of 30 and 70. 30 mandibles were used to validate the test system, and 70 were used to study the different operation techniques.
To validate the test system, 30 adult pig mandibles were halved along the median line and both halves were used for the split osteotomy. The operation technique was randomly assigned to the Obwegeser–Dal Pont technique or the modified technique implementing the caudal border osteomy. 70 pig mandibles were used to compare the different splitting procedures. The mandibles were halved and randomly chosen to be split using the Obwegeser–Dal Pont technique or the modified technique.
Results
The first batch of 30 pig mandibles was used to validate the test system, including 16 mandibles that were split according the Obwegeser–Dal Pont technique and 14 that were split with the modified technique. The correlation between both sides in the same mandible for the modified technique was 0.91 with an average torque of 1.74 N m (SD: 0.34) on the left side and 1.74 N m (SD: 0.36) on the right side. For the Obwegeser–Dal Pont technique the correlation was 0.96 with an average recorded torque of 2.22 N m (SD: 0.32) on the left side and 2.27 N m (SD: 0.33) on the right side.
The average correlation in both techniques was 0.96. The hypothesis of different torques was rejected significantly (paired t -test, t (29) = −1.06; p > 0.05). In general the aberration was 6% using the same split technique on both sides in the same mandibular body. The maximum aberration was 9%, the minimum 1%.
Another 70 mandibles were used to compare both methods in the same mandible. An average torque of 2.12 N m (SD: 0.57, max: 4.25 N m, min: 0.90 N m) was measured in the Obwegeser–Dal Pont procedure and an average torque of 1.51 N m (SD: 0.46, max. 2.62 N m, min. 0.55 N m) was measured in the modified technique. This technique reduced the torque needed to split the jaw by 30% compared with the classic BSSO technique. The torque loss with the additional caudal border osteotomy was strongly significant ( t (69) = −12.89; p < 0.05) in the paired t -test.
Comparing the split results of both techniques in the standardized operation protocol there was a 100% split through the mandibular canal in the Obwegeser–Dal Pont technique ( Fig. 4 , type A). Using the modified technique, 75% of the mandibles fractured through the caudal border ( Fig. 4 , type B), the remaining 25% of the cases split according to type A fracture lines.
