A Formula to Predict the Magnitude of Achilles Tendon Lengthening Required to Correct Equinus Deformity

Achilles tendon lengthening (ATL) is one of the most commonly performed procedures in paediatric orthopaedic surgery. An appropriate adjustment of the amount of ATL is crucial to avoid insufficient or excessive lengthening. However, there is currently no effective method to preoperatively calculate the tendon length needed for equinus deformity correction. Thus, in this study we evaluated the accuracy of a calculation using a mathematical model based on the law of cosines.

A total of 16 feet of 14 patients who were scheduled for ATL surgery due to equinus deformity were included in the study. ATL surgery was performed using a standard Z-plasty technique. Calculation of the amount of ATL using the law of cosines, and assessments of intraoperative lengthening of the tendon, were performed in a double-blind manner. The extent of lengthening resulting from the two methods was then compared.

The mean ATL determined intraoperatively was 23.67 ± 8.7 mm, and that obtained using the cosine-based method was 22.49 ± 8.6 mm. Thus, the new method showed excellent statistical agreement with the actual lengthening performed during surgery.

The required dimension of ATL can be calculated preoperatively using the mathematical formula presented here. The advantages of this approach are that it allows accurate tendon lengthening and reduces the size of the surgical incision.