Abstract
Introduction: For plate osteosynthesis (OS) many surgeons use a rigid fixation which prevents callus formation. The present paper applies biomechanical laws and a FE analysis for optimal screw placement to turn a rigid plate OS into a dynamic and biological OS.
Methods: A Finite Element Analysis was performed. The bone was modeled as a cylinder with an outer diameter of 30 mm and an inner diameter of 22 mm. An E-modul of 18 GPa was assumed for cortical bone. A DC steel plate was modeled with a preload of 300 N for each screw. Fracture motion and stress on the screw head was calculated for different screw placements and a load of 300 N angulated at 30 deg.
Results: The number of screws did not influence fracture motion. This could only be controlled by the distance of the first screw to the fracture site, the use of a lag screw and the material of the plate. When one screw hole was omitted close to the fracture site, motion doubled. Using A lag screw reduced fracture motion dramatically. The stress was greatest at the screw closest to the fracture site.
Conclusions: In order to achieve a dynamic plate OS with callus formation a long plate with a minimal amount of screws and no lag screws should be used. To adjust the flexibility of the OS, the distance of the first screw to the fracture site is the most crucial parameter. Additional screws do not influence the stiffness. The stress is highest at the screw head close to the fracture site. This screw is endangered for fatigue failure. To reduce the stress on this screw it must not be placed oblique and also not eccentric. However, the last screw has little stress and should be placed oblique to increase the pull out strength.
The abstracts were prepared by Professor Jegan Krishnan. Correspondence should be addressed to him at the Flinders Medical Centre, Bedford Park 5047, Australia.
