Association Between Individual Calf Muscle Volume And Achilles Tendon Enthesis Morphology

Abstract

Although chronic Achilles tendinopathy (AT) is one of the most frequent injuries in sport, it is generally difficult to treat as the etiology and background to the pain mechanisms has not yet been clarified. Anatomically, as calf muscles travel distally, the AT internally rotate, consequently the initially posterior fibers of the soleus insert mainly on the medial aspect of the AT enthesis whereas those of the gatsrocnemuis insert mainly on the lateral aspect. Recently, it has been suggested that the AT undergoes non-uniform deformation under in vivo loading conditions PURPOSE: We investigated whether there are regional differences in calf muscle volumes, in the trabecular architecture of the calcaneus or in the histology of the AT enthesis that could suggest unequal force transmission from muscle to tendon and from tendon to bone. METHOD: We studied 12 cadavers (age 74 ± 7 years). For both legs, individual calf muscles (lateral gastrocnmius (LG), medial gastrocnemius (MG) and soleus (SL) were dissected and their volumes measured. Fine saw cuts were made in the sagittal plane, either side of the midline of the calcaneus, so that approximately 8-mm-wide strips of tissue were sampled from the insertion site. Each strip contained the distal part of the tendon and its insertion, together with the superior tuberosity of the calcaneus. Trabecular architecture was analysed from X-rays taken with a Faxitron radiography. Histological sections were made at the enthesis and regional differences in the thickness of the uncalcified fibrocartilage and the subchondral plate were evaluated. RESULTS: The radiological studies showed that the quantity of bone and the apparent trabecular thickness in the calcaneus were greatest in the center. The thickness of calcified fibrocartilage tissue was significantly greater in the central part of the enthesis than medially (P = 0.04) and laterally (P = 0.03). Uncalcified fibrocartilage was significantly thicker medially than laterally (P = 0.02). Muscle volume was highest (P <0.05) for the SL, followed by the GM, and GL. CONCLUSION: Overall, the results indicate that mechanical stress at the AT enthesis is asymmetrically distributed and greater on the medial than on the lateral side. Thus, we suggest that the functional anatomy of the AT is closely related to regional variations in force transmission, which in turn relates to the site of pain and pathology in AT.