Medial shoulder instability (MSI) is a term used to describe a disease process affecting the medial (closer to the body) aspect of the shoulder joint and it’s soft tissue connections. The glenohumeral ligaments are paired ligaments on both the medial and lateral (outside, away from the body) aspect of the glenohumeral (shoulder) joint that play a part in passive shoulder stability. Medial shoulder instability or subluxation of the shoulder joint occurs when the medial glenohumeral ligament, the subscapularis tendon or the shoulder joint capsule get inflamed and become progressively more lax and frayed. This condition affects both large and small breed dogs. In larger breed dogs, it is generally thought to be secondary to repetitive microtrauma (overuse), whereas congenital laxity is considered more likely in small/toy breeds. The condition is uncommon in cats but demonstrates a similar pathology (disease process).
Dogs and cats with shoulder joint instability often present with a history of chronic lameness, which is often subtle and intermittent but can be severe and continuous. These patients tend to respond poorly to therapy with nonsteroidal anti-inflammatories alone. Physical signs of atrophy (wastage) of the shoulder muscles of the affected limb, as well as pain at shoulder joint manipulation are common.
Medial shoulder instability is diagnosed through a combination of examination and tests. Findings of shoulder pain, muscle atrophy and instability at physical examination aid in a diagnosis but definitive diagnosis is obtained via some or all of the following:
Treatment of medial shoulder instability is contentious and can prove challenging.
Surgery can be used to repair any instability present and prevent progression of the ensuing degenerative joint disease. Surgical options include transposition of the tendon of origin of the biceps brachii muscle or of the tendon of the supraspinatus muscle and placement of synthetic sutures to augment existing collateral stability.
Radiofrequency-induced thermal modification (RITM- applying thermal energy to the joint capsule to shrink and tighten collagen fibres) is advocated by some surgeons although there is conflicting evidence as to whether this is a suitable technique.
Arthroscopy (keyhole surgery through a rigid camera with an instrument portal), whilst valuable for diagnosis, is limited in terms of surgical treatment of MSI. Some arthroscopic repair techniques are being developed.
The treatment chosen for each patient is individual, based on the areas causing the instability, surgeon preference, patient signalment (age, weight, breed, etc.), as well as the severity of the condition and any co-existing degenerative joint disease. Shoulder arthrodesis (joint fusion) is an option in some severe cases.
The prognosis following treatment of medial shoulder instability is unpredictable. Results are generally good to excellent in the long term provided owner and patient compliance are good, although ongoing lameness can be seen in some cases.
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