Change in Bone Mineral Density after Total Ankle Replacement: Two Year Follow Up

Abstract

Category: Ankle, Ankle Arthritis Introduction/Purpose: Bone strength about the components is an important factor in Arthroplasty survival. This importance has been studied in total hip & knee replacement. However, there is paucity of literature Where little has been published regarding the quality of the bone and bone mineral content postoperatively surrounding Ankle Arthroplasty and circumstances surrounding the development of stress fracture. This prospective study used a method to quantify Bone Mineral Density BMD in different regions of the surrounding bones adjacent to tibia and talar components of uncemented Mobility ankle prosthesis, and to study the effect of loading on local ankle bone in the presence of un-cemented three components ankle replacement, by analyzing the BMD of the areas around tibia and talus before and after Mobility total ankle replacement. Methods: Twenty three Ankles (7Females, 15 Males) who underwent a Mobility Total Ankle Arthroplasty (TAA) between March 2008 and April 2009 were included prospectively in this study. They underwent a Bone Density Scan using the DEXA HOLIGIC Scanner with the specific method designed for measuring BMD. Seven areas (R1-R7) around the prosthesis were studied (Please see attached figure). Scans were carried out pre-operatively and repeated at 1 and 2 years post-operatively. The results were also compared with clinical outcomes, using (AOFAS scores) both pre, 1 and 2 years post-op. Results: Mean age of patients was 63.3 years (SD 9, range 43 to 80). AOFAS scores showed significant improvement (from 28.8 pre-op to 78.7,76.9 at 12 and 24 months post-op respectively; p<0.05). BMD within the lateral malleolus decreased significantly from 0.5g/cm2 to 0.42g/cm2 (17%, P < 0.01), at one and two years postoperatively - R2. There was an insignificant increase in Mean BMD at medial side metaphysis of tibia increased by,7% (+0.07 g cm -2, R6), but this just failed to reach statistical significance, mean BMD within medial malleolus decreased slightly from 0.67g/cm2 to 0.64 g/cm2 at the same period which was statistically insignificant. There was little insignificant increase in BMD in tibia just proximal to implant (R7) and at talus (R5). Conclusion: Stemmed implant causes changes in Tibial BMD around prosthesis. There was stress shielding over the lateral malleolus resulting in decrease BMD in lateral malleolus and to a lesser extent of the medial malleolus. The increase in BMD at tibial metaphysis indicates an increase in mechanical stresses at that region, which may explain occasional postoperative stress fracture of medial malleolus or medial sided ankle pain. There is no further change in BMD from year 1 to year 2 following TAR.