Achilles Tendon Injury Alters Lymphatic and Venous Clearance in Rats

The Achilles tendon is a frequently injured component of the musculoskeletal system. Like other musculoskeletal tissues, its homeostasis depends in part on microcirculatory pathways, including the lymphatic system. While previous studies have shown that Achilles tendon interstitial fluid content increases in clinical disease states, microcirculatory clearance pathways in tendon remain poorly understood. We characterized lymphatic and venous clearance pathways in naïve and surgically ruptured female Sprague-Dawley rat Achilles tendons using size-specific near-infrared tracers. We hypothesized that Achilles tendon rupture would impair microcirculatory transport, as measured by altered size-specific tracer clearance rates. In naïve tendons, clearance was tracer size-dependent, with venous tracers clearing significantly faster than lymphatic tracers. Tendon rupture significantly increased lymphatic clearance rates at 2-weeks post-injury but decreased lymphatic clearance rates at 8 weeks compared to both uninjured contralateral and naïve controls. Venous clearance rates were significantly elevated in injured tendons compared to uninjured contralateral limbs at 4- and 8-weeks post-injury. Qualitative histology showed disruption of both the tendon sheath and core structure after injury. Elevated pain and decreased function were quantified post-injury by measuring tactile allodynia (Von Frey), limb loading (Dynamic Weight Bearing), and tendon mechanical properties (tensile testing). Limb loading returned to baseline by 4 weeks, while tactile allodynia did so at 8 weeks. Injured tendon stiffness returned to contralateral levels by 8 weeks. Together, these data demonstrate that microcirculatory function is significantly altered following surgical tendon rupture. Strategies targeting tendon microcirculatory function and fluid accumulation may represent a promising treatment approach for injured and diseased tendons.