Spontaneous healing process of a supraspinatus tendon tear in rabbits

Introduction The rotator cuff has a characteristic structure, in that one surface faces articular cartilage and another faces bursa. This structure may produce differences in the healing process between the rotator cuff and other tendons. We investigated the spontaneous healing process of a surgically created supraspinatus tendon tear in rabbits.Materials and methods A transverse, full-thickness tear of the supraspinatus tendon was created and its healing examined.Results A tear of 12 mm was not repaired within 3 weeks. With a tear of 5 mm, reparative tissue gradually encroached into the defect from the bursal side, and the tear united from the bursal side to the articular side by 12 weeks. The healing rates (width of reparative tissue/width of the tendon×100%) were 32.2%, 52.4%, 58.0%, 88.9%, and 93.8% at 1, 2, 3, 6, and 12 weeks, respectively. The reparative tissue had continuity to the epitenon of the bursal side. Immunohistochemical study showed that at week 1, type III collagen was detected in the reparative tissue and the cutting ends, and the expression gradually decreased. On the other hand, the expression of type I collagen in the reparative tissue was weak at week 1 and increased until week 3. PCNA-positive cells were observed in the reparative tissue.Conclusion These results show that the origin of the reparative tissue is the epitenon, and from the bursal side rather than the articular side. This model is very useful for the investigation of the remodeling process of an acute rotator cuff tear.