The variability of prosthesis positioning and the resulting geometry of coxo-femoral joint

This work is focused on femoral prosthesis positioning inside bone and its consequences on stress distribution. Five synthetic femurs were employed: one femur was left unimplanted (NF), four femurs were implanted with two different prostheses (PF, A or B models); all implants were performed with specific instruments, by orthopaedic surgeons who usually perform that surgery. The CAD models of all femura were obtained from CT scans; a reference system was defined in NF in order to compare joint geometry between NF and PE The following parameters were considered: femur height, head offset, angle between femur axis and prosthesis shaft axis, angle between neck axis and femur axis. Each one has functional consequences: impaired art length produces discomfort and unbalanced muscle forces, a wrong offset determines a congruent variation in stress moduli; prosthesis orientation substantially modifies stress pattern inside bone. The differences between joint geometry in NF and PF and between two femurs implanted with the same prosthesis have been found to be significant. For example, depending on prosthesis models, an augmented height (up to 3.4mm) or a reduced height (up to 22.0mm) were observed, with respect to NF. Head offset differed of about 3.0mm between NF and PF; the angle between prosthesis shaft axis and femur axis reached about 4.30 (model A), or 7.00 (model B). These results suggest that orthopaedic implant design cannot be performed in a deterministic manner only, but also the variability due to manual and sensorial abilities of surgeons must be taken into account.