Radiostereometric analysis of the double-coated STAR total ankle prosthesis

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Abstract
The designs of total ankle prostheses have changed in recent years in order to give better performance. Only a few studies of these ankle prostheses have been published, however, and none on micromotion. We evaluated 5 patients with rheumatoid arthritis and 5 with osteoarthrosis, 4 (3-5) years after arthroplasty with the double-coated STAR prosthesis. Clinical examination included AOFAS hindfoot score. Standardized a-p and lateral radiographs were taken and RSA analyses were done at regular intervals. There was no difference in results between ankles operated on due to rheumatoid arthritis and due to osteoarthrosis. A rapid initial migration was observed for the tibial components at 6 weeks, but thereafter all but 1 implant seemed stable. The migration pattern for the talar component was similar. Rotation around the 3 axes was observed for the tibial components at 6 weeks, but not thereafter. The talar components became stable for rotation around the longitudinal and sagittal axes, but not around the transverse axis. 8 out of 10 ankles were painless. The median total AOFAS score was 83 and the median range of motion was 32 degrees. None of the 20 components had changed position and there were no signs of bone resorption. Provided the indication is adequate and the prosthesis has been implanted correctly, the double-coated STAR ankle prosthesis will have a satisfactory fixation to underlying bone.