THE BIOMECHANICS OF THE PC-FIX INTERNAL FIXATOR

Conventional fracture treatment implants, i.e. bone plates and intrame dullary 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 rea ctive forces created by compression in the areas of contact, while the torsional and the axial forces are most frequently transferred by bon e 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 implan t to the bone or by chance fit. Vascular damage in these areas of dire ct contact has been shown to account for most of the implant-related d amage to the bone. The Point Contact Fixator (PC-Fix: an internal plat e and screw fixation system, the function of which is based on similar mechanical principles to the external fixator) is a general purpose i nternal fixation system in which the mixed mode of load transfer has b een eliminated in favour of screw-only transfer, made possible by lock ing of the screw head into the ''plate'' of the PC-Fix. The possibilit y to contour the implant and to use the screws without any special pre paratory steps gives the PC-Fix the versatility of conventional platin g systems, while practically eliminating their major drawback of direc t 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 a llowed a further reduction of damage to the bone by the use of monocor tical screws. A theoretical comparison of the mechanisms of load trans fer between the Point Contact Fixator and the Dynamic Compression Plat e has been demonstrated by in vitro experimental data. The bone-implan t construct is generally stronger in the case of the internal fixator provided the bone treated can withstand implant yielding loads. The me chanical conditions at the fracture site for sub-yield levels of loadi ng are dominated by the technique of application and are expected to b e comparable for the two implants. It appears that most of the advanta ges of the PC-Fix observed to date in in vivo experimental fracture tr eatment can be explained by the reduction of implant-related damage to the bone without any compromise on the mechanical function of fractur e stabilization.