Impact of Pulley Radius on Tendon Durability Under Cyclic Loading in a Rabbit Model

Factors influencing damage to tendons routed about curved implant surfaces ("pulleys") remain poorly understood. This study thus investigated the effect of pulley radius on tendon durability under cyclic loading. Rabbit extensor digitorum communis tendon bundles were wrapped 160° about titanium pulleys of radius 2 mm, 3 mm, or 4.5 mm and subjected to 100,000 (n = 21) or 500,000 (n = 12) cycles of low-magnitude tensile loading over a fixed range of forces. Pulley radius had no significant effects on loading-induced changes in effective stiffness in the tests to 100,000 cycles. The bundles exhibited large, rapid initial increases in effective stiffness, consistent with increasing gliding resistance, with short- and longer-term exponential rate constants of approximately 1200 and 22,500 cycles. For all pulley radii, tendon bundle effective stiffness showed continued, slight linear increases, averaging 1.3 µN/mm/cycle, from 150,000 cycles onward in the 500,000-cycle tests. No tendon failures were observed, but there were visible tendon bundle damage and consistent tendon-pulley adhesion for all pulley radii. The findings suggest that, within the simulated postoperative period and range of radii considered, pulley radius has a negligible influence on tendon durability. Clinical Significance: It appears the implant radius of curvature can vary over a large range without notably increasing the risk of tendon failure, but also without notably reducing the adverse effects of tendon-implant contact. Damage countermeasures other than manipulating the radius of curvature of implant surfaces may be needed to achieve clinically acceptable durability of tendons that interact with implants.