[Stress tests of reconstruction plates for bridging mandibular angle defects]. / Beanspruchungsuntersuchungen an Rekonstruktionsplatten zur Uberbrückung von Kieferwinkeldefekten.

AIM: The aim of the present study was to evaluate the mechanical stress in reconstruction plates used for bridging mandibular angle defects and in the screw-plate-bone interface with the finite element method. Additionally, the influence of reconstruction plate geometry, screw configuration, and screw diameter upon the mechanical stress distribution was determined. Suggestions for design improvements of the plate were derived from the results. MATERIAL AND METHODS: Based on the geometrical data of a human mandible, an angle defect bridged by a titanium reconstruction plate was generated and exposed to chewing force. The reconstruction plate was securely fixed by M 2.7 titanium screws. A variation of plate design, screw configuration, and screw diameter was carried out. The mechanical stress was calculated following the von Mises stress hypothesis. RESULTS: Using the standard plate the mechanical stress in all components exceeded by far the ultimate tensile strength. Possible clinical consequences could be a fatigue fracture of the plate, loosening of the screw, and irreversible damage of the bone leading to infection. Increasing the screw diameter by 50% would lead to a decrease of the mechanical stress by far more than 50%. An increase of the interface area between bone and plate and a triangular screw configuration diminishes the mechanical stress further, which may consequently allow a reduction of plate thickness with better adaptation to the actual jaw geometry. CONCLUSION: As a preliminary result the reconstruction plate could be thinned out in areas subject to less mechanical load.

Study by finite element method of the mechanical stress of selected biodegradable osteosynthesis screws in sagittal ramus osteotomy.

We tested the stability of the bilateral sagittal split osteotomy using four resorbable osteosynthesis screws (the PLLA screw introduced by Harada and Enomoto, the Isosorb screw, the BioSorbFX screw and the Lactosorb screw) which are all currently in clinical use. The distribution of stress in both the bicortically inserted screws and the adjacent bone of a computer-generated mandible was recorded by the three-dimensional finite element method. The stress of the materials under investigation was postulated to have reached threshold values for stability, and maximum chewing forces of 132 N (Harada and Enomoto), 117 N (Isosorb), 115 N (BioSorbFX) and 46.4 N (Lactosorb) were determined. As far as the postoperative chewing forces were concerned, all four screws were sufficiently stable at the osteotomy gap. Finite element modelling seems to be an appropriate method of investigating these clinical issues when the mechanical stress both in implants and in the adjacent bone is taken into account.

[Finite element assisted study of the mechanical stability of 2 selected osteosynthesis systems for mandibular osteotomy]. / FEM-gestützte Untersuchung zur mechanischen Stabilität zweier ausgewählter Osteosynthesesysteme bei der sagittalen Unterkieferosteotomie.

The aim of this study was to use 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. To this end, 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 or notified by the manufacturer. On condition that the materials were subjected only to their respective ultimate tensile stress, it was possible to neutralise the following masticatory force: 124.6 N with the miniplate and 167.5 N with the bicortical triangular screw configuration. The limitation of stress ensued from the peri-implant bone and not from the osteosynthesis material per se. The finite element method (FEM) appears to be suitable for simulating complex mechanical stress situations in the maxillofacial area, as also demonstrated by the fact that our data agree with those in the literature and with clinical experience. It will enable considerable savings to be made in terms of time and materials (animal experiments) in the future development of osteosynthesis materials and techniques.