The effect of muscle paralysis using Botox on the healing of tendon to bone in a rat model

Hypothesis: Despite good clinical results after rotator cuff repair, follow-up studies show significant rates of failed healing. This may be because of excessive tension on the repaired tendon due to shoulder motion. We hypothesized that botulinum toxin A injections would result in improved attachment strength and collagen organization at the tendon-bone interface at early time points but may result in decreased mechanical properties at later time points because of the negative effects of stress deprivation. Materials and methods: We performed division and repair of the supraspinatus tendon in 132 rats: 66 underwent repair alone and 66 received injections of botulinum toxin into the muscle before repair. Rats were killed at 4, 8, and 24 weeks and were evaluated by use of histologic, biomechanical, and micro-computed tomography analyses. Results: At 4 and 24 weeks, there was no significant difference in load to failure between groups. At 8 weeks, the botulinum group had a significantly lower load to failure compared with controls (27.7 N vs 46.7 N, P < .01). The weight of the supraspinatus muscle was significantly decreased at 4 and 8 weeks in the botulinum group, but it recovered by 24 weeks. Micro-computed tomography analysis showed the botulinum group to have significantly less bone volume, total mineral content, and total mineral density at 8 weeks. Histologic analysis showed formation of a more normal tidemark and increased collagen fiber organization in the botulinum specimens at 4 weeks. Discussion: Botulinum toxin A-treated specimens had increased collagen fiber organization at 4 weeks and decreased mechanical properties at later time points. The rapid healing of the rat rotator cuff likely makes it difficult to realize benefits from reduction in strain.