Computer Tomography assessment of the fusion rate after posterior arthroscopic subtalar arthrodesis

The purpose of this study was to assess the fusion rate on CT examinations and to correlate clinically the functional result with the degree of bone fusion in the subtalar joint after posterior arthroscopic subtalar arthrodesis (PASTA). Fourteen cases, from 36 to 84 years old, were retrospectively followed-up for a minimum of one year (range 12-92 months). A CT scan had been systematically performed at the six-month follow-up visit. The CT scans were examined in sagittal 2-mm-thick reformatted slices, measuring the length of the joint surface and the length of the fused portion of the joint space on each image. At six months, the average fusion ratio was 39 +/- 19% (range 0-69%). Fusion defined by a fusion ratio superior or equal to 33% on the CT scan was observed in 11 cases. One patient had a delayed union and required a revision of fixation. One patient had a bilateral nonunion. Mean average AOFAS score improved from 51 +/- 10 to 77 +/- 9 at last follow-up. Compared to open procedures, the posterior arthroscopic fusion seems to offer a promising alternative. However, our results suggest that the fusion rate following PASTA is not as favourable as reported in previous studies. Factors such as adequate compression and stable fixation provided by the screws together with the surgeons' experience with this demanding technique are of the utmost importance. A 33% CT fusion ratio threshold could accurately discriminate between clinical stability and instability.