The effect of mechanical load on tendon-to-bone healing in a rat model.

Background: Joint motion is commonly prescribed after tendon repair surgeries such as rotator cuff repairs; however, the ideal rehabilitation program to optimize tendon-to-bone healing is unknown. Hypotheses: (1) Delayed loading would result in a mechanically stronger and better organized tendon-to-bone interface compared with prolonged immobilization or immediate loading. (2) Low-magnitude load would lead to superior healing compared with high-magnitude load. Study Design: Controlled laboratory study. Methods: A total of 192 rats underwent unilateral patellar tendon detachment and repair followed by placement of a custom external fixator. Rats were assigned to immobilization, immediate postoperative loading, or delayed-onset loading (4- or 10-day delay). Loading was controlled using a specially designed motorized device to apply constant strain until 3 N (low load) or 6 N (high load) of axial tensile force was reached through the healing bone-tendon complex for 50 cycles per day. Rats were sacrificed at 4, 10, 21, or 28 days postoperatively for histomorphometric, immunohistochemical, radiographic, molecular, and biomechanical analyses. Results: The load to failure was significantly higher in the immobilized group compared with the immediate and delayed loading groups (P < .05). Compared with loaded specimens, the immobilized specimens had significantly less fibrocartilage (at 4, 10, and 28 days), significantly better collagen fiber organization (at 4, 10, and 21 days), decreased expression of matrix metalloproteinase-13 (at 10, 21, and 28 days), and significantly fewer apoptotic cells (at 21 and 28 days). Micro-computed tomographic analyses showed that the 3-N immediate load group had significantly less total volume (P = .012), bone volume (P = .012), and bone mineral density (P = .023) for cortical bone, and the immobilized group had significantly more specimens with new bone formation at the enthesis (100%; P = .001). Conclusion: Immobilization results in a stronger tendon-bone complex, with less scar tissue and a more organized tendon-bone interface compared with all loading regimens in this study. Clinical Relevance: Given the relatively high rate of failure after rotator cuff and other tendon-to-bone repairs, identification of optimal rehabilitation programs postoperatively is an important research goal.