Morphologic and morphometric features of the calcaneal insertions of the superficial digital flexor tendon in the horse

introduction: Accurate description of the calcaneal insertions of the superficial digital flexor tendon (SDFT) is lacking and inconsistent. The aim of this study was to undertake morphologic and morphometic evaluations of these structures to assist in elucidating their functional and pathogenic roles in displacement of the SDFT from the calcaneal tuber. Method: Dissections were performed on 10 normal cadaveric hindlimbs. The anatomy was photographed to allow measurements at repeatable locations and differences in SDFT dimensions at the various locations were compared using a paired student t-test. Results: This study demonstrated that the calcaneal insertions of the SDFT are independent from the overlying tarsal insertions of the biceps femoris and semitendinosus, which blend into the plantar surface of the fibrocartilaginous cap (FCC) of the SDFT before inserting dorsal to the insertion of the SDFT on the calcaneal tuber. The lateral insertion of the SDFT is larger in cross-sectional area (median: 219 mm²) at its origin from the FCC than its medial counterpart (median: 159 mm², p = 0.004) and has a more complex fibre alignment. The lateral site of attachment of the SDFT on the calcaneal tuber is dorsolateral to the insertion of the gastrocnemius tendon and is larger (median: 525 mm²) than the medial insertion (median: 428 mm², p = 0.036), which inserts distal to the insertion of the gastrocnemius tendon. Conclusion: The features identified in this study suggest that the calcaneal insertions of the SDFT are complex and their morphological and morphometric differences are likely to contribute to clinical lesions identified at this site.