Injury to the infraspinatus muscle is a relatively rare condition that is most commonly seen in highly athletic dogs, usually after vigorous activity such as bird hunting. It may be associated with lack of prior athletic conditioning. Prevention is achieved by keeping dogs in good athletic condition and warming them up to full activity gradually, just like the recommendations for human athletes. Once an injury to the muscle occurs, damage is minimized by treating muscle inflammation and appropriately rehabilitating the injury with physical therapy and ancillary modalities (ex. laser therapy). Occasionally, muscle damage is bad enough to form scar tissue that limits extension of the shoulder joint (i.e., contracture). This condition is not painful, but can create a disability. Surgery for this contracture is relatively straight forward and very successful. Dogs can return to full activities after surgery and rarely experience long-term complications.
When a dog is running, he/she extends the shoulder fully and then contracts the infraspinatus muscle and other shoulder muscles to pull or flex the shoulder back and move the body forward. During very vigorous or long duration running, the infraspinatus muscle is very sensitive to “over doing it” and the muscle fibers get torn, stretched or otherwise damaged. This condition is commonly seen at the beginning of bird-hunting season and may be a classic example of the Weekend Warrior—poorly conditioned dogs with very eager personalities overdoing it!
A common human athlete example similar to the original insult that occurs in dogs is a “hamstring pull”—a muscle is torn during activity. When muscle fibers are damaged, the area becomes inflamed. This inflammation hurts; the area is warm, swollen, and tender to the touch. Moving the muscle, usually in the stretch direction (shoulder extension), hurts as well. Dogs will hold the leg in a manner that limits extension of the shoulder; they may simply carry the leg off the ground and go on three legs, or they will dramatically shorten their stride resulting in a noticeable limp.
A significant thing about muscle healing is that it only heals by fibrosis (scar tissue formation); severely damaged muscle cells do not repair themselves. Over a 1–2 month period of healing after the original injury, the damaged muscle heals. First the pain goes away as the inflammation subsides; the dog may start using the leg more comfortably. But as the healing progresses, more and more scar tissue is formed in the muscle. Normal muscle is flexible and stretchy; scar tissue is not. As the scar tissue matures, it tightens, contracts, and gets firmer. Now the dog transitions to a lameness that is persistent but appears like it is not painful. Some dogs will even have their foot up off the ground when they are sitting. This stage of the disability is related to scarring of the infraspinatus muscle such that the shoulder joint can no longer extend like it should; it is permanently flexed but well healed and pain-free. Because of this, the leg is slightly shorter and the dog cannot extend the leg forward to walk or run normally.
The diagnosis of infraspinatus muscle contracture is usually based on physical and orthopedic examinations. Your primary care veterinarian may recommend x-rays to be certain there are no changes to the underlying bones or joints. Additionally, more advanced diagnostics such as musculoskeletal ultrasound or MRI may be recommended to specifically assess the muscle or tendinous insertion.
The surgical procedure can be straightforward and involves minimal tissue disruption; patients experience moderate post-surgical pain for a few days that is well managed with common pain-control techniques. By two weeks post-operatively, patients appear quite comfortable at their surgical site.
One minor complication following this surgical procedure is an incisional seroma. This is a collection of fluid under the skin at the surgery site that results from joint fluid leakage or excess movement of the tissues in the surgical site. Usually this very minor problem will go away by itself, with perhaps some help in the form of warm compresses. Rarely, these fluid pockets get too big to reabsorb on their own, and they need to be drained with a needle. Seromas can be minimized with careful surgical technique and strict post-operative rest for four weeks.
After the surgery site is well healed, these dogs may return to their normal activity routines with no restrictions. Care should be taken to gradually condition these patients prior to any new and vigorous activities so that the opposite shoulder does not develop the same problem in the future.