Biomechanics of internal fixation of femoral neck fracture by spongious compressive screws

Purpose: Osteosynthesis of femoral neck fracture with three spongious screws is incompletely investigated problem, especially from biomechanic aspect. It is this author's view that standard way of screw osteosynthesis with screws in triangle pattern with distally oriented triangle basis is less rigid than the opposit. The hypothesis was that stability of such osteosynthesis was dependent on screws position inside the femoral neck, and biomechanic model experiment was examined to confirm it. The purpose of work was to establish stability difference between osteosynthesis with screws in triangle pattern with basis oriented caudally and a triangle with basis oriented cranially. Material and Methods: Investigation was done in Laboratory for Biomechanics, Department of Anatomy, School of Medicine Faculty, University ofZagreb, on seven macerated specimens of human femoral bone divided in subject and control group. Specimens were exposed to static bending and torsion force moments before and after osteotomy and osteosynthesis in increments which did not cause plastic deformation. The degree of elastic deformation was measured by laser beam deflection on millimeter paper. It was the basis for deflection in degrees and "rigidity" of every specimen counting. Results: All results were compared within and between groups and presented in tables and diagrams schematically. Before the experiment, all specimens were measured, geometrically defined and radiographed. Radiography was also done after osteotomies and osteosyntheses. Theoretical part of work was based on Pauwels' analyses and theorems of static and dynamic forces in fracture gap of femoral neck. In this regard, analysis of best screws position was done with the purpose of distraction and neutralization of shearing forces. Conclusion: Both parts of the study confirmed the hypothesis that osteosynthesis of femoral neck fracture with spongious screws in triangle pattern with proximally oriented basis was superior to osteosynthesis with screws in triangle pattern with distally oriented basis, probably because of better distraction and neutralization of shearing forces.