Treadmill running initiation times and bone-tendon interface repair in a murine rotator cuff repair model

Postoperative exercise has been demonstrated to be beneficial for bone-tendon interface (BTI) healing, yet the debate regarding the optimal time to initiate exercise after tendon enthesis repair is ongoing. This study aimed to evaluate the initiation times for exercise after enthesis repair. A total of 192 C57BL/6 mice underwent acute supraspinatus tendon injury repair. The animals were then randomly assigned to four groups: free cage activity after repair (control group); treadmill running started on postoperative day 2 (2-day delayed group); treadmill running started on postoperative day 7 (7-day delayed group), and treadmill running started on postoperative day 14 (14-day delayed group). Mice were euthanized at 4 and 8 weeks postoperatively, and histological, biomechanical, and bone morphometric tests were performed. Higher failure loads and bone volume fractions were found for the 7-day delayed group and the 14-day delayed group at 4 weeks postoperatively. The 7-day delayed group had better biomechanical properties and higher bone volume fractions than the 2-day delayed group at 4 weeks postoperatively. Histologically, the 7-day delayed group exhibited lower modified tendon-to-bone maturity scores than the control group and the 2-day delayed group at 4 and 8 weeks postoperatively. Quantitative reverse-transcription polymerase chain reaction results showed that the 7-day delayed group had higher expressions of chondrogenic- and osteogenic-related genes. Statement of clinical significance: Postoperative treadmill running initiated on postoperative day 7 had a more prominent effect on BTI healing than other treatment regimens in this study and could accelerate BTI healing and rotator cuff repair.