A Novel Measurement of Altered Achilles Subtendon Load Sharing 6–12 Months Following Rupture

Achilles tendon ruptures cause muscle-tendon structural and functional deficits that persist years after the initial injury. A healthy Achilles tendon contains three semi-independent subtendons that slide relative to each other during muscle contractions in healthy adults. However, such sliding decreases postinjury as load sharing-likely caused by intratendinous adhesions-increases between adjacent subtendons. This study quantifies changes in subtendon load sharing 6-12 months following Achilles tendon rupture when patients are cleared by their surgeon to fully return to physical activities. We combined transverse plane ultrasound imaging with neuromuscular electrical stimulation of individual triceps surae muscles and applied a Kanade-Lucas-Tomasi point tracking algorithm to characterize subtendon behavior. We developed a surrogate measure of subtendon load sharing by quantifying differences in point displacement trajectory angles between select regions within the tendon cross section. In patients recovering from rupture injuries (n = 19), subtendon load sharing significantly increased in the injured tendon compared to the contralateral uninjured side during lateral gastrocnemius (p = 0.0094), medial gastrocnemius (p = 0.021), and soleus stimulations (p = 0.048). These differences were not present between right and left legs in the uninjured cohort (n = 17). Linear regression analysis also revealed that the presence of tendon injury was significantly associated with subtendon load sharing, with injured tendons showing up to a 44% decrease in subtendon independence compared to the contralateral uninjured tendon during gastrocnemius stimulations. Statement of Clinical Significance: These results propose a novel biomarker of tendon health and suggest muscle-dependent changes in subtendon function following Achilles tendon rupture.