M C Bone1, G Giddins2, T J Joyce2. 1. School of Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne, Orthopaedic Dept, Royal United Hospital, Bath, UK martin.bone2@newcastle.ac.uk. 2. School of Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne, Orthopaedic Dept, Royal United Hospital, Bath, UK.