Abstract
Introduction: There is limited literature available looking into circumstances surrounding the development of stress fracture of the medial and lateral malleoli after ankle replacement. We present the preliminary results of a prospective study examining the effect of ankle replacement upon local bone mineral density and the phenomenon of stress shielding.
Aim: To assess the effect of ankle replacement loading of the medial and lateral malleoli, by analysing the BMD of the medial and lateral malleoli before and after Mobility total ankle replacement.
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 malleolus and lateral malleolus 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 improved from 0.58g/cm2 to mean 6 months postoperative BMD of 0.59g/cm2 and 0.60g/cm2 at 12 months. The mean preoperative BMD within the lateral malleolus decreased from 0.40g/cm2 to a mean 6 months postoperative BMD of 0.34g/cm2. However the BMD over the lateral malleolus increased to 0.36g/cm2 at 12 months. The mean alignment of the tibial component was 88.5° varus (85° varus to 94° 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 TAR 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.
The abstracts were prepared by Mr Matt Costa and Mr Ben Ollivere. Correspondence should be addressed to Mr Costa at Clinical Sciences Research Institute, University of Warwick, Clifford Bridge Road, Coventry CV2 2DX, UK.
