Elasticity of the supraspinatus tendon-muscle unit is preserved after acute tendon tearing in the rabbit

Supraspinatus tendon tearing is one of the most common causes of the shoulder pain and dysfunction, which often requires a surgical repair. In this situation, proximal tendon stump is usually retracted medially from its original insertion. For successful reduction of the retracted tendon stump to its original insertion, the elasticity of the tendon-muscle unit should be preserved by the time of surgery. The purpose of the present study was to clarify the chronological changes in the elasticity of the supraspinatus tendon-muscle unit after acute tendon tearing to determine the optimal timing for the surgery. Right supraspinatus tendon was detached (detached side) in 40 male Japanese white rabbits, with left shoulders served as controls (control side). Eight animals were euthanized at 3 days and 1, 2, 4, or 8 weeks after surgery. Tissue sound speed that closely correlates to its elasticity was measured with a scanning acoustic microscope. In the supraspinatus tendon, tissue sound speed at 3 days after surgery was 1691.1 m/s, compared to 1714.3 m/s at the control side, but the difference was not statistically significant at any postoperative time period up to 8 weeks. In the supraspinatus muscle, tissue sound speed was not affected at all by the detachment of the tendon. The present study indicated that the elasticity of the supraspinatus tendon-muscle unit was well preserved for 8 weeks after the detachment. In the clinical practice, the retracted supraspinatus tendon stump could be repaired without excessive tension by 8 weeks from the acute tendon tearing.