Calf Muscle Contributions To Achilles Tendon Architectures And Injuries Using Finite Element Modeling

Abstract

Clinically the sites of Achilles Tendon (AT) overuse conditions occur primarily within the tendon mid-portion and the osteotendinous attachment. PURPOSE: We propose an anatomical and finite element model of the muscle-tendon-bone units that could provide a possible anatomic explanation for these 2 sites of injury. METHOD: Twelve cadavers (age 74 ± 7 years) were studied. In both legs, calf muscles (lateral gastrocnemius (LG), medial gastrocnemius (MG) and soleus (SL)) were dissected and their volumes measured. A finite element model based on a von Mises criteria of tendon coupled to a rupture index was developed to investigate the AT response to mechanical load. Three simulations were performed corresponding to three load cases (Force from GM= Force from GL = Force SL, Force from GM=1.5×Force from GL and Force from GL= 1.5×Force from GM). The SL load was kept constant during the three cases. Histological sections of the enthesis and thickness of both the uncalcified fibrocartilage and subchondral plate were evaluated. RESULTS: Muscle volume was highest for the soleus (p=0.02), followed by the MG (p=0.04), and LG (p=0.04). Uncalcified fibrocartilage was significantly thicker medially than laterally (P = 0.02). Finite element modeling predicted rupture propagation and final patterns depending on the load cases indicating that non-uniform stress in the AT can occur through modifications of individual muscle contributions (p=0.02). As the load increased, the pressure converged at the middle area between 4.6cm and 7.9cm from the AT enthesis. CONCLUSION: Our data suggest that the AT muscle-tendon-bone units are composed of anatomically distinct parts that undergo non-uniform mechanical loading. Finite element modeling can be used for identifying localized material property changes due to muscle atrophy or hypertrophy as well as in response to mechanical loading seen during normal gait and running.