Pull-out strengths for a range of screws inserted into the calcaneus: a preliminary study.

The holding strengths of AO screws in the calcaneus were tested with a view to incorporating them into a new design of ankle prosthesis aimed at reducing the incidence of loosening. Comparative studies were performed to determine the optimal type and number of screws and their optimal location and thread length. Pull-out tests with 4.5-mm cortical and 6.5-mm cancellous screws were performed on screws inserted into the posterior and middle facets of the calcaneus, these being the two most suitable places for screw fixation. The cancellous screws showed much higher holding strength than the cortical screws. There is room for only a single screw in each facet. The results indicated that a screw of 16-mm thread length should be inserted into the middle facet and a screw of 60-mm thread length into and through the posterior facet. The high holding strengths suggest that screw fixation could be a viable alternative to commonly used methods for anchoring the lower prosthetic component. Further tests are to be conducted which simulate the different types of loads occurring during gait and other activities. These will allow decisions to be made about the benefits of the proposed screw fixation method. This work is part of a long-term project which aims to produce a new ankle prosthesis that does not suffer the problems of loosening associated with present current designs. A strong screw fixation of the talar component may help to improve stability. The results show that a good fixation into the calcaneus can be achieved, and suggest that further work based on this approach is worthwhile.