Development of a Rabbit Chronic-Like Rotator Cuff Injury Model for Study of Fibrosis and Muscular Fatty Degeneration

This protocol demonstrates a complex surgical technique for inducing a chronic-like rotator cuff injury in rabbits, and is useful in studying rotator cuff pathogenesis and developing regenerative therapies. Rabbits are mid to large animal models that are easy to house logistically, manipulate surgically, and show high levels of rotator cuff muscle fatty degeneration when injured, similar to humans. The subscapularis injury model shown here is anatomically analogous to the supraspinatus bone tendon muscle complex, one of the most frequently injured rotator cuff units.

To begin, prepare the surgical window by shaving the intended incision site of an anesthetized rabbit. Using cotton swabs, clean the area with three alternating applications of Betadine and 70%alcohol in circular motions. Administer 20 milligrams per kilogram of cephalexin intramuscularly as a prophylactic anti-infective agent.

Make a three to four centimeter skin incision inferior to the clavicle, split the deltopectoral interval using a scissor, and retract to gain access to the shoulder. To locate the subscapularis muscle tendon unit, first identify the coracobrachialis muscle. Then identify the subscapularis tendon and insert a right angle clamp to expose the entire tendon at its insertion on the lesser tuberosity of the humerus.

Isolate the subscapularis muscle tendon. After administering pre-surgery analgesic locally near the transection site, wrap the subscapularis muscle tendon unit in silicone-based penrose tubing to prevent undesired attachment to the surrounding tissues and aid subsequent tissue retrieval. To induce injury, create a full thickness transection at the muscle tendon junction using a surgical number 11 scalpel.

Where necessary, stop bleeding by applying pressure with a piece of gauze and use saline to irrigate the wound as needed. To close the wound, use a 4-0 polyglycolic acid suture to reapproximate the deltoid muscle tissue and a 4-0 nylon or silk suture to close the skin wound, followed by wound cleaning. Administer 0.03 milligrams per kilogram of buprenorphine subcutaneously as an analgesic once immediately after surgery, and twice daily for the next 48 hours.

The histological analysis of the chronic-like rotator cuff injury model at four weeks is shown in this figure. The H&E staining confirmed the loss of muscle cellularity and organization, which was replaced with large numbers of adipocytes in injured subscapularis muscles relative to the control group. The quantification of the injured muscle fat accumulation percentage is presented in this figure.

Masson's trichrome staining also confirmed muscle atrophy and disorganized collagen fiber arrangements in injured subscapularis muscles relative to the control group. The quantification of the proportion of muscle and fibrotic tissue are shown in this figure. The assessment showed a reduction in muscle cellularity for injured subscapularis muscles relative to the control group.

A high degree of fibrosis was also observed in injured subscapularis muscles. Identifying the subscapularis tendon and wrapping the subscapularis muscle tendon unit in penrose tubing are the most important steps of this procedure. Following this procedure, a therapeutic intervention may be applied.

In this case, the analyses described here, along with additional myography or gait analysis, may be used to assess intervention outcome. This technique is crucial for aiding researchers in the study of rotator cuff pathophysiology, as well as facilitating the development of novel therapeutics for muscle tendon repair and regeneration.