Development of a novel radiographic angle to assess the calcaneal tuberosity enlargement in insertional Achilles tendinopathy

Objective: This study investigated morphologic features of the calcaneal tuberosity in a group of normal controls and developed a radiographic angular metric, the Pathologic Achilles Insertion Angle (PAIA), with the goal of assisting in the insertional Achilles tendinopathy (IAT) evaluation. Methods: On lateral weight-bearing radiographs, calcaneal tuberosities of 33 control feet were analyzed to develop a mathematical algorithm-based Standard Circle to describe the ideal contour of each tuberosity. Results: Offsets of the circle center and the circle radius were scaled according to their dimensions and the weight-bearing features of the calcaneus (Algorithm 1). Then, enlarged calcaneal tuberosities of 58 feet with different severities of IAT were plotted, and their respective ideal Standard Circles were generated. An angular measurement algorithm (Algorithm 2) was developed to describe how to rotate each enlarged tuberosity to align with its ideal contour using the weight-bearing point as the apex. Thus, the PAIA angular metric provided the amount of the enlargement. Conclusion: Pathologic Achilles Insertion Angle is a novel radiographic metric that considers morphological and biomechanical features of the calcaneal tuberosity and is able to quantitatively evaluate the enlargement of the calcaneal tuberosity in IAT. Level of evidence IV; Case series; Therapeutic studies - investigating the results of treatment.