[Osteosynthesis after sagittal division of the mandible. Biomechanical stability of various methods in a pig mandibular model]

Comparative evaluation of two osteosynthesis methods on stability following sagittal split ramus osteotomy

INTRODUCTION

The aim of this study was to employ the finite element method (FEM) to compare the stability of 2.0 mm titanium screws in a triangular configuration with that of a 2.0 mm titanium miniplate as osteosynthesis material following bilateral sagittal split osteotomy.

MATERIAL

A model of the mandible was produced, consisting of 19,854 elements and 4285 nodes. The mechanical parameters of the materials investigated were taken from the literature and notified by the manufacturer.

RESULTS

On condition that the materials were subjected only to their respective ultimate tensile stress, it was possible to neutralise a masticatory force of 1246 N (Newtons) with the miniplate and of 1675 N with the bicortical triangular screw configuration. The strain limit was determined by the peri-implant bone and not the osteosynthesis material.

CONCLUSION

The finite element method (FEM) appears to be suitable for simulating complex mechanical stresses in the maxillofacial area, as also confirmed by the agreement between our data and those in the literature, and with clinical experience. It should enable considerable savings to be made in terms of time, material and animal experiments in the future development of osteosynthesis materials and techniques.

Resorbable plate osteosynthesis of sagittal split osteotomies with major bone movement

This study evaluates resorbable miniplate osteosyntheses in sagittal split osteotomies with major bone repositioning. Two resorbable 2.0-mm miniplate systems, MacroSorb (Macropore, San Diego, Calif.) and PolyMax (Synthes, Oberdorf, Switzerland), were compared consecutively. Amorphous 70:30 poly-L/DL-lactide copolymer plates sustain continuous hydrolysis through water penetration into the implant body during the first 6 months in situ. This breaks copolymer chains into smaller particles, which later become degraded through phagocytotic cells. Eighteen patients, 10 women and eight men, 16 to 57 years old (average, 27 years) were examined. They had severe dysgnathia caused by congenital craniofacial malformations, systemic disorders, trauma, amelogenesis imperfecta, oligodontia, and other conditions, and they needed five 8- to 10-mm and 13 major 10- to 12-mm repositions. Twelve sagittal split osteotomies were fixed with 12 MacroSorb plates in six patients, and 24 osteotomies were filled with 32 PolyMax plates in 12 patients. Ten mandibular plate, screw, hard-tissue, and soft-tissue specimens were taken at 3, 6, 9, or 12 months postoperatively in secondary operations (e.g., dental implant placement).Follow-up ranged from 4 to 19 months; all osteosyntheses reossified. Four patients showed proximal fragments rotated up to 5 mm sagittally anteriorly and nonaligned burr holes on the postoperative radiogram, suggesting plate fractures or screw pullout. When plate fracture was noted, guided occlusion was maintained 4 weeks after surgery. Occlusal, radiologic, and skeletal results remained stable. After starting fixation with two plates on each side, no more plate fractures were seen. In three other patients, minor skeletal relapses up to 3 mm horizontally resulted. Local histologic inspection of specimens showed thorough osseous union. Screw remnants embedded in bone made screw pullout unlikely; rather, screw-head or plate fractures were found as multiple degraded particles. Microscopy showed a chronic foreign body reaction. Two patients (11 percent) developed a sterile fistula 3 and 4 months after surgery, draining implant debris. Here, the biopsies showed a granulocytic infiltrate that subsided clinically after excisional biopsy. The assignment of MacroSorb plates followed by PolyMax plates was done in an otherwise unchanged treatment protocol. Comparison of the number of patients in each group with stable osteosyntheses and regular healing showed no significant differences by Fisher's exact test (p = 0.1516); therefore, the authors focused on the combined results for both treatments. The current osteosynthesis systems showed sufficient stability for mandibular fixation after sagittal split osteotomy and repositioning more than 10 mm distant when two plates were applied to each side; however, 27 percent of patients had complications, including relapses. Disadvantages were the cost, breakability, diameter, and need to place the screws vertically to the plate, necessitating a bent instrument or transbuccal incisions.