Abstract
Introduction: There is limited literature looking into the circumstances surrounding the development of stress fractures of the medial and lateral malleoli after ankle replacement. We present the preliminary results of a prospective study examining the effect of total ankle replacement (TAR) upon local bone mineral density (BMD) and the phenomenon of stress shielding.
Aim: To assess the effect of TAR loading othe medial and lateral malleoli, by analysing the BMD of the medial and lateral malleoli before and after Mobility TAR.
Methodology: Ten consecutive patients undergoing Mobility total ankle replacement for osteoarthritis had pre-operative bone densitometry scans of the ankle, repeated at 6 and 12 months after surgery. The bone mineral density of a 2 cm square area within the medial and lateral malleoli was measured. The pre-operative and post-operative bone densitometry scans were compared. The relation between the alignment of the tibial component and the bone mineral density of the malleoli was also analysed.
Results: The mean preoperative BMD within the medial malleolus increased from a mean of 0.57g/cm2 to 0.58g/cm2 at six months and 0.60g/cm2 at 12 months postoperatively. The mean preoperative BMD within the lateral malleolus decreased from 0.39g/cm2 to 0.34g/cm2 at six months postoperatively. However the BMD over the lateral malleolus increased to 0.356g/cm2 at 12 months. The mean alignment of the tibial component was 88.50 varus (range 850 varus to 940 valgus). There was no correlation between the alignment of the tibial component and the bone mineral density on the medial malleolus (r = 0.09, p = 0.865).
Conclusion: The absence of stress shielding around the medial malleolus indicates that ankle replacements implanted within the accepted limits for implant alignment, load the medial malleolus. However, there was stress shielding over the lateral malleolus resulting in decreased BMD in the lateral malleolus.
Correspondence should be addressed to A.H.N. Robinson, BOX 37, Department of Orthopaedics, Addenbrooke’s Hospital, Cambridge University Hospitals NHS Trust, Hills Road, Cambridge. CB2 0QQ, England.
