Strain behavior of the distal Achilles tendon: implications for insertional Achilles tendinopathy.

BACKGROUND: The changes associated with Achilles insertional tendinopathy (AIT) are seen in the anterior portion of the Achilles tendon near its calcaneal insertion. Strain behavior of this portion of the tendon was evaluated. HYPOTHESIS: The highest strains are in the anterior portion of the tendon. STUDY DESIGN: Controlled laboratory study. METHODS: Six cadaveric legs were tested. While moving through a functional range of ankle motion, strains in five different regions of the tendon insertion were tested. First, the load on the tendon was increased from 30 to 170 N in the starting, plantar flexed position. Then, the ankle was moved from a plantar flexed position into dorsiflexion. RESULTS: Strains in the posterior sites increased significantly (P < 0.001) as the movement into dorsiflexion occurred. This was significantly (P < 0.01) different than the anterior sites, which showed a trend toward decreasing strain. CONCLUSIONS: Although the anterior portion of the Achilles tendon is generally affected in AIT, relative strain shielding is seen in this portion of the tendon. These findings suggest that the role of repetitive tensile loads in the causation of AIT is more complex than often described. Clinical Relevance: These findings may explain the variable therapeutic response following measures aimed at decreasing tensile loads on the tendon. [ABSTRACT FROM AUTHOR]