The biomechanics of the PC-Fix internal fixator

Abstract

Conventional fracture treatment implants, i.e. bone plates and intramedullary nails, but also a great variety of special devices, rely on a mixed mode of load transfer between the bone segments and the implant. The bending moments and transverse forces are usually balanced by reactive forces created by compression in the areas of contact, while the torsional and the axial forces are most frequently transferred by bone screws, directly, or by screw-dependent friction between the implant and the bone. In all cases, contact between the implant and the bone is required and is achieved either by careful adaptation of the implant to the bone or by chance fit. Vascular damage in these areas of direct contact has been shown to account for most of the implant-related damage to the bone. The Point Contact Fixator (PC-Fix: an internal plate and screw fixation system, the function of which is based on similar mechanical principles to the external fixator) is a general purpose internal fixation system in which the mixed mode of load transfer has been eliminated in favour of screw-only transfer, made possible by locking of the screw head into the “plate” of the PC-Fix. The possibility to contour the implant and to use the screws without any special preparatory steps gives the PC-Fix the versatility of conventional plating systems, while practically eliminating their major drawback of direct contact to the bone. Locking of the screw head in the PC-Fix permits transfer of the moments between the screw and the implant. This has allowed a further reduction of damage to the bone by the use of monocortical screws. A theoretical comparison of the mechanisms of load transfer between the Point Contact Fixator and the Dynamic Compression Plate has been demonstrated by in vitro experimental data. The bone-implant construct is generally stronger in the case of the internal fixator provided the bone treated can withstand implant yielding loads. The mechanical conditions at the fracture site for sub-yield levels of loading are dominated by the technique of application and are expected to be comparable for the two implants. It appears that most of the advantages of the PC-Fix observed to date in in vivo experimental fracture treatment can be explained by the reduction of implant-related damage to the bone without any compromise on the mechanical function of fracture stabilization.