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Weight-bearing-line analysis in supramalleolar osteotomy for varus-type osteoarthritis of the ankle.

BACKGROUND
We determined the preoperative and postoperative passing points of the mechanical axis of the lower limb at the level of the tibial plafond using a new method involving a full-length standing posteroanterior radiograph that includes the calcaneus (a hip-to-calcaneus radiograph) and correlated them to the clinical results after supramalleolar osteotomy for ankle osteoarthritis.


METHODS
We reviewed the hip-to-calcaneus radiographs of fifty lower limbs of forty-one patients treated for lower limb malalignment at our institution. The mechanical axis point of the ankle was the point at which the mechanical axis divides the coronal length of the plafond, expressed as a percentage. Four independent observers performed all measurements twice. Supramalleolar tibial osteotomy was performed in twenty-seven ankles (twenty-four patients) to treat moderate varus-type osteoarthritis of the ankle. The mean follow-up period was 2.8 years (range, two to 5.3 years). Clinical assessment was based on the American Orthopaedic Foot & Ankle Society (AOFAS) scale.


RESULTS
Interobserver and intraobserver reliability in identifying the mechanical ankle joint axis point were very high. The mean postoperative mechanical axis point was 50% (range, 13% to 70%) in ankles for which the preoperative point was ≤0%, whereas the mean postoperative point was 81% (range, 48% to 113%) in ankles for which the preoperative point was >0%. The mean change in AOFAS score was significantly less for patients with a preoperative point of ≤0% than for those with a preoperative point of >0% (p=0.004). Improvement was significantly greater in ankles with a postoperative mechanical ankle joint axis point of ≥80% than in ankles with a postoperative mechanical ankle joint axis point of <60% (p=0.030).


CONCLUSIONS
Traditional tibial correction resulted in great variation in the locations of the postoperative mechanical ankle joint axis point. In ankles with the preoperative point more medial than the tibial plafond, the point was insufficiently moved to the lateral side, and the clinical outcomes were less satisfactory.