Complete removal of load is detrimental to rotator cuff healing

Hypothesis: This study evaluated the effect of the mechanical environment on the healing rotator cuff by paralyzing the supraspinatus muscle in the operative shoulder of a rat model of rotator cuff injury and repair. Methods: Unilateral shoulders, of, rats underwent a supraspinatus injury and repair. Botulinum toxin A was used to paralyze the muscle after repair. Postoperatively, 1 group was immobilized and 1 group was allowed free range of motion. Saline-injected, casted rats were used as the control group. Repairs were evaluated histologically, geometrically, and biomechanically. Results: Specimens from the saline-injected rats had greater scar volume and cross-sectional area of the repair compared with the paralyzed groups. Structural properties were increased in the saline group compared with the paralyzed groups. Free range of motion (ie, uncasted group) resulted in modest improvements in biomechanical properties but did not obviate the effect of paralysis. Conclusions: Complete removal of load was detrimental to rotator cuff healing, especially when combined with immobilization. Level of evidence: Basic science study. (C) 2009 Journal of Shoulder and Elbow Surgery Board of Trustees.