Direct measurement of the Total Contact Cast wall load

Abstract

The offloading mechanism of a total contact cast (TCC) is attributed to the redistribution of weight-bearing pressure across the entire plantar surface of the foot and by increasing the plantar surface contact area. However, contact area data and regional pressure data suggests this is not entirely the case. The other proposed offloading mechanism is by transfer of load to the cast walls of the TCC. Various studies have assessed this mechanism by indirect methods and have reported that the difference in plantar load due to the cast walls was 30%. The aim of this Thesis was to measure cast wall load directly to further understand the offloading mechanism of plantar pressure reduction of the TCC in the treatment of forefoot plantar ulceration. In Study 1, a proof of concept investigation was performed to determine the feasibility of a new method to directly measure the load between the cast wall and the lower leg interface using capacitance sensors. The cast wall load was able to be measured directly and that the anterodistal and posterolateral-distal regions of the lower leg were identified as the two areas of greatest load. The combined average measured load per step upon the walls of the TCC equated to 23-34% of the average plantar load of the opposite foot, which is similar to previous indirect estimates of load transfer (30-36%). In Study 2, the highest areas of shank loading (anterodistal and posterolateral-distal regions of the lower leg) were directly measured in 17 patients with plantar forefoot ulceration and diabetes. It was determined that the walls of the TCC bear considerable load. Reduced plantar contact area in the TCC compared to the shoe-cast suggests that the foot is suspended by the considerable load bearing capacity of the walls of the TCC which contributes mechanically to effective plantar pressure reduction and redistribution.