Shoulder instability due to peri-articular soft tissue pathology is a commonly diagnosed problem in humans. A shoulder instability syndrome has been described in dogs. Although lateral and multidirectional shoulder instabilities have been described, medial shoulder instability (MSI) appears to be the predominant type of shoulder instability seen in dogs. Dogs diagnosed with MSI are most commonly middle-aged, large-breed dogs with a history of chronic unilateral forelimb lameness. Dogs with MSI typically have an intended purpose of work, performance or very active pet-level function.

The aetiology of MSI is not known, but available data suggest that this problem occurs as a result of chronic overuse injury with episodes of 'micro' trauma to the shoulder occurring during the development of disease. The pathology associated with MSI includes lameness, shoulder joint laxity, pain on manipulation of the shoulder, and abnormalities of the medial joint capsule, subscapularis tendon and medial glenohumeral ligament observed via arthroscopy.

Signalment, complete history, complete orthopaedic and neurological examination, blood work, radiographs and ultrasonography are integral to the comprehensive diagnostic approach. Ultrasonographic evaluation is recommended for all potential MSI patients to rule out primary or concurrent musculotendinous pathology. Definitive diagnosis of MSI is based primarily on palpation and exploratory arthroscopy. Palpation under sedation is a key component of preoperative diagnosis of MSI. Range of motion in flexion, extension and rotation, the 'shoulder drawer sign' and the shoulder abduction test should be assessed in each limb. Shoulder abduction angles measured under sedation have been reported to provide objective data for diagnosis of MSI in dogs. Shoulders with clinical and arthroscopic evidence of MSI have significantly higher angles (53.7 ± 4.7 degrees) compared to shoulders considered normal (32.6 ± 2.0 degrees). Currently, arthroscopic exploration provides the most definitive diagnosis of MSI and allows assessment of cartilage damage and other key structures in the shoulder joint. Arthroscopic identification of tearing and laxity of the subscapularis tendon, medial glenohumeral ligament and joint capsule are consistent findings in reported cases of MSI.

Treatment recommendations are based on extent and severity of instability, concurrent shoulder pathology, concurrent orthopaedic problems, the overall health of the patient, the intended use of the patient and the commitment of the owner. Several case series have been published regarding outcomes of single treatments for medial or lateral shoulder instability in dogs. However, to our knowledge, only one study has compared outcomes of various treatments for medial, lateral and multidirectional instability. The study is a multicentre, retrospective cohort study of 130 canine patients diagnosed with shoulder instability. The dogs were categorised into diagnosis cohorts of medial (MSI), lateral (LSI) or multidirectional (MDI) instability, and then subcategorised by treatment type. Treatment cohorts were defined as non-surgical management, radiofrequency-induced thermal capsulorrhaphy (RITC) or shoulder reconstruction. Outcomes based upon re-evaluation by the primary clinician and owner input greater than 1 year after diagnosis were used to determine success, failure and complication rates.

The majority of patients were diagnosed with MSI but 23% had LSI or MDI. Patients with MSI treated by reconstruction were more likely than patients treated without surgery to have a successful outcome (odds ratio = 3.0; p = 0.01). Similarly, treatment of MDI with reconstruction was more likely to be associated with a successful outcome than non-surgical management (odds ratio = 5.0; p < 0.01). Success with surgical and non-surgical management was equivocal between LSI treatment cohorts, but the number of patients with LSI was small. Complication rates for all treatment cohorts were less than 10%.

These data show that LSI and MDI comprise a substantial portion of canine patients diagnosed with shoulder instability. Surgical reconstruction for treatment of MSI and MDI appears to produce a higher likelihood of achieving successful outcomes than non-surgical management, and surgical complication rates were low. So, treatment of shoulder instability is realistic and can be associated with full return to function in working dogs as well as pets. However, further study and high evidentiary value is needed before strong recommendations can be made regarding indications for treatment, treatment type and safety and efficacy of various treatments.