Mechanical changes in the Achilles tendon due to insertional Achilles tendinopathy

Insertional Achilles tendinopathy (IAT) is a painful and debilitating condition that responds poorly to non-surgical interventions. It is thought that this disease may originate from compression of the Achilles tendon due to calcaneal impingement. Thus, compressive mechanical changes associated with IAT may elucidate its etiology and offer clues to guide effective treatment. However, the mechanical properties of IAT tissue have not been characterized. Therefore, the objective of this study was to measure the mechanical properties of excised IAT tissue and compare with healthy cadaveric control tissue. Tissue from the Achilles tendon insertion was acquired from healthy donors and from patients undergoing debridement surgery for IAT. Several tissue specimens from each donor were then mechanically tested under cyclic unconfined compression and the acquired data was analyzed to determine the distribution of mechanical properties for each donor. While the median mechanical properties of tissue excised from IAT tendons were not significantly different than healthy tissue, the distribution of mechanical properties within each donor was dramatically altered. In particular, healthy tendons contained more low modulus (compliant) and high transition strain specimens than IAT tendons, as evidenced by a significantly lower 25th percentile secant modulus and higher 75th percentile transition strain. Furthermore, these parameters were significantly correlated with symptom severity. Finally, it was found that preconditioning and slow loading both reduced the secant modulus of healthy and IAT specimens, suggesting that slow, controlled ankle dorsiflexion prior to activity may help IAT patients manage disease-associated pain.