Abstract
A significant source of failure for external fixation devices is loosening of the fixation implant. As bone competence drops with ageing or disease such as osteoporosis, the risk of loosening is likely to increase. However it is not clear how fixator configuration should be adapted to minimise loosening in weaker bone. The aim of this study was to assess the effect of bone competence on the yielding of bone tissue surrounding fixation implants, and thereby inform the selection of fixator configuration to minimise loosening. External fixation of the tibial midshaft using half-pins and Illizarov wires was modelled using finite-element analysis. Half-pin configurations of two and three stainless steel and titanium pins pins were assessed. Illizarov wire configurations of two and four wires were studied, over a range of wire tensions. Bone competence was varied by changing the cortical thickness and elastic properties of the bone fragments to approximate: a) young, high-density bone, b) middle-aged, mid-porosity bone and c) old-aged, severely porous bone. Bone elastic properties were taken from a recent study of cortical bone conducted by the authors. The interaction between implants and bone was modelled with contact analysis, enabling realistic separation. Implant loosening was included using a bone-specific, strain-based yield criterion. Regions where bone tissue yielded were identified as likely sites of loosening. In all cases loading was applied to simulate a one-legged stance.
Half-pin fixation
Increasing the number of half-pins from two to three produced an approximate 80% reduction of yielded bone volume in all age groups. The volume of yielded bone increased with ageing, approximately three times greater in old-aged bone than in young bone. In the young and middle-aged cases yielded bone never penetrated the full cortex. Contrastingly, the full cortex was yielded in the old-aged bone fragments for both two- and three-pin fixation. In all cases the volume of yielded bone was greater at the pin(s) nearest to the fracture gap. The use of titanium pins increased the volume of yielded bone around half-pins by approximately 1.7 times. These results suggest bone competence, number of half-pins, location of half-pins and half-pin material all significantly influence implant loosening.
Illizarov wire fixation
Increasing the number of Illizarov wires reduced the volume of yielded bone by approximately 60% in all age groups. The volume of yielded bone increased with ageing by a factor of approximately 2.0 times from young to old bone. Bone yielding never progressed through the entire cortex; it reached a maximum of 70% of the cortical thickness in two-wire fixation of the old-aged bone fragment. This is a possible reason for the lower rate of loosening in Illizarov wire fixation as compared to half-pin fixation. Increasing wire tension reduced the volume of yielded bone. These results suggest that bone competence, number of wires, wire tension and wire arrangement significantly influence loosening.
