The effect of rhPTH on the healing of tendon to bone in a rat model.

Successful rotator cuff tendon repair depends on secure tendon-to-bone healing. Recombinant human parathyroid hormone (rhPTH) has been shown in multiple studies to accelerate bone healing. Recent studies have also shown that rhPTH is chondrogenic by increasing chondrocyte recruitment and differentiation. We hypothesized that rhPTH would improve tendon-to-bone healing in a rat rotator cuff repair model. One hundred and fourteen Sprague Dawley rats underwent division and repair of the supraspinatus tendon. Fifty seven rats received daily subcutaneous injections of 10 µg/kg of rhPTH. Rats were sacrificed at 3, 7, 14, 28, and 56 days for histologic and immunohistochemical analysis. In addition, rats in each group were sacrificed at 14, 28, and 56 days for biomechanical testing and micro CT analysis. At 2 weeks the controls had a significantly higher load to failure than the rhPTH group. At 28 and 56 days there were no differences in load to failure. rhPTH specimens had significantly higher stiffness at 56 days. MicroCT analysis showed that the rhPTH group had significantly greater total mineral content at all time points, as well as significantly higher bone volume (BV) at 14 and 28 days. Histologically, the rhPTH specimens had more fibrocartilage, osteoblasts, and blood vessels at all timepoints, with significantly better collagen fiber orientation at 28 and 56 days. Although treatment with rhPTH resulted in an increase in bone and mineralized fibrocartilage formation, as well as better collagen fiber organization, this did not translate into improved biomechanical properties.