Abstract
Introduction
There is no published series described change in bone mineral density (BMD) after ankle replacement. We present the results of a prospective study examining the effect of total ankle replacement (TAR) upon local bone mineral density (BMD).
Aim
To design a method and assess the effect of TAR loading on local ankle bones, by analysing the BMD of different area around ankle before and after Mobility TAR.
Methods
23 patients undergoing Mobility ankle arthroplasty for osteoarthritis had preoperative bone densitometry scans of the ankle, repeated at 1 and 2 years after surgery. BMD of 2 cm2 areas around ankle were measured. Pre- and postoperative data were compared. Results: Mean BMD within the lateral malleolus decreased significantly from 0.5g/cm2 to 0.42g/cm2 (17%, P > 0.01), at 1 & 2 years postoperatively. Mean BMD within medial malleolus decreased slightly from 0.67g/cm2 to 0.64 g/cm2 at the same period. However BMD at medial side metaphysic of tibia increased by 7%. There was little increase in BMD in tibia just proximal to implant and at talus.
Discussion and Conclusion
Absence of stress shielding around distal tibia, just proximal to tibial component and talus indicates that ankle replacements implanted within the accepted limits for implant alignment, load distal tibia and talus. However, there was stress shielding over the lateral malleolus resulting in decreased BMD in lateral malleolus. Increase BMD at tibial metaphysis, proximal to medial malleolus indicates an increase in mechanical stress which may explain occasional postoperative stress fracture of medial malleolus or medial side ankle pain.
