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Canine Medial Shoulder Instability (MSI)

Canine foreleg lameness is often a surprisingly frustrating diagnostic challenge even amongst board certified veterinary surgeons such as the doctors at Colorado Canine Orthopedics (CCO). In years past, unless the lameness etiology was something obvious, (e.g., trauma or neoplasia), common diagnoses included carpal arthritis, elbow arthritis/dysplasia, and some form of soft tissue injury. If the shoulder was thought to be the lameness origin, biciptal tenosynovitis was a typical diagnosis given. But overall, the shoulder has not been pursued as aggressively as other aspects of the front limb for the cause of lameness. Compared to our human counterparts, veterinary surgeons are behind the curve regarding shoulder disorders due to the commonality of rotator cuff problems in humans. Dogs do not have a rotator cuff as we think of in people, but they do have active and passive stabilizers of the shoulder. Of these, the medial collateral ligament (MCL) and the subscapularis muscle are critical components for medial stability of the shoulder joint.

Most cases of medial shoulder instability (MSI) involve outdoorsy, active, medium to large breed dogs. Interestingly, it is not uncommon for owners to describe episodes of severe lameness with later periods of mild lameness. Rest and NSAIDs may improve the lameness somewhat, but usually once the dog becomes active again the lameness returns and progresses. 

Diagnosing MSI includes a thorough orthopedic exam, starting from the paw and progressing proximally should be performed. Once the shoulder is reached, there are a few important points to evaluate for MSI. Dogs with MSI exhibit discomfort and/or pain during shoulder hyperextension (not all dogs but most).

In dogs with MSI, pain can also be elicited during abduction of the shoulder joint. In addition to evaluating for pain during abduction, this “stress test” is critical to identify medial instability. By performing this test to both front limbs and obtaining a comparison, it is easier to make a tentative diagnosis of MSI.

Radiographs are commonly non-diagnostic or may only confirm shoulder arthritis in most cases of MSI. Shoulder radiographs are important to rule out other problems such as trauma and neoplasia. Shoulder MRI and /or CT scan is useful for evaluating the shoulder region, especially if there is ever concern for other causes of lameness such as a brachial plexus tumor. That being said, the shoulder joint can be difficult to scan and the scan interpretation can be highly dependent on the radiologists’ experience and comfort level with the shoulder joint. Thus, a shoulder scan can have false negatives.

Shoulder arthroscopy has become not just a surgical repair tool, but an important diagnostic tool for dogs with MSI. Arthroscopy allows visualization and evaluation of the important shoulder joint stabilizers (fig 1). Arthroscopy is critical in obtaining the information needed for the surgeon and owner to develop an appropriate treatment plan.

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