A simplified model for the short-below-knee stump.

Abstract The Patellar Tendon Bearing (PTB) prosthesis is in use all over the world. A major problem in the fitting of the prostheses is the transmission of forces between stump and socket, especially for the Short-Below-Knee (SBK) amputee. A theoretical study of forces and moments might help in improving the design for a better and more adaptable prosthesis. In this work the test of 3 variations of a theoretical model of the SBK stump is reported. The models are characterised by circular symmetry and they are simple enough to allow for calculations to be made. Five equilibrium equations are proposed in each case, relating the forces and moments as a function of geometric parameters such as the diameter and length of the tibia. The calculations are carried on for the whole stance-phase period. To define and evaluate the geometric parameters 44 samples of complete joints or tibia bones were measured. The results were used as the possible range of change and the equations were solved for this whole range on a computer. The best geometric parameters (giving minimum forces) were found. The results show the theoretical importance of the total length of the stump, well known from clinical practice, the best theoretical location for the main transmission areas and the role of the fibula in the stump.