33rd Annual Scientific Meeting proceedings

Learning Objectives

• Definition of shoulder instability in people and dogs
• Current diagnostic approach to shoulder instability in dogs
• Potential diagnostic pitfalls leading to overdiagnosis

Introduction
The shoulder is a ball and socket joint, characterized by a less constrained geometry than the hip joint because of a relatively shallow socket (glenoid). While the bony geometry is the major contributor to hip stability, soft tissues play a primary role in shoulder stability. The glenoid labrum, which is less developed in canine than human shoulders, increases the scapular articular surface to better accommodate the humeral head. The other soft tissue stabilizers are joint capsule, gleno-humeral ligaments and muscle-tendons crossing the joint. The exact role of each soft tissue stabilizer has not been thoroughly investigated in veterinary medicine, although the muscles are probably playing a major role.

Ball and socket joints are typically described as 3-degrees of freedom joints, although the human shoulder joint has been described as a 6-degree joint. Based on a veterinary study investigating shoulder kinematics with biplanar fluoroscopy, flexion-extension, internal-external rotation and abduction-adduction occurred during the gait cycle, suggesting that the canine joint is primarily characterized by rotational motions. This should be accounted in diagnostic and treatment decision making.

The prevalence of shoulder musculo-skeletal injuries in dogs is unknown but epidemiological studies are available for specific population such as dogs competing in agility. From a survey of 217 handlers and 431 dogs, shoulder injuries occurred in 16% of the cases. The accuracy of this data is unclear because the approach to diagnose “shoulder injuries” was variable. One of the challenges in interpreting veterinary literature is the inconsistent definition of shoulder injuries in dogs. This often comes from variable disease definition and diagnostic approaches. Furthermore, the treatment recommendations become anecdotal or based on short case series.  

Shoulder instability syndrome or isolated injuries? Acute traumatic or chronic degenerative? Instability or laxity?
In people there is a clear definition of shoulder instability. In human orthopaedics, shoulder instability is defined as a disease where pain or discomfort are associated with excessive translation of the humeral head during active shoulder motion. Contrariwise, in veterinary orthopaedics, there is a lack of a standardized definition for shoulder instability. In most of the studies, shoulder instability is defined as an increased laxity of the shoulder joint characterized by an abnormal humeral head translation, which causes discomfort and dysfunction of the shoulder. However, this definition is rather unspecific and can be misleading especially because animals cannot speak about the feeling of “joint popping” described by human patients with shoulder instability.

Shoulder instability is not clearly defined in dogs due to 1) lack of agreement among definitions in literature, 2) unclear understanding of the disease processes and 3) inaccurate and inconsistent diagnostic approach. A common pitfall is diagnosing isolated injuries in cases where a more complex process is affecting multiple structures. The opposite may also happen, with a general diagnosis of “shoulder instability” in cases where there is a clear isolated injury (i.e. subscapularis tendon rupture). A structured approach to definition of shoulder instability might be similar to fracture classification:

• Side of the limb (right, left)
• Direction (medial, lateral, multidirectional)
• Etiology (traumatic vs degenerative)
• Anatomical location of the injury (i.e. subscapularis tendon and MGHL rupture)

An example would be “medial, acquired, traumatic SI with grade 2, subscapularis tendon tear and grade 2 MGHL. This approach will help defining the disease process and the options for treatment.

The next step requires a good knowledge of the shoulder stabilizing structures and how they may fail. Acute isolated tears occur in the biceps, infraspinatus and subscapularis muscle-tendons. Acute tears of the infraspinatus can lead to contracture. Tendinitis can affect the biceps, supraspinatus and subscapularis tendons. Instability is typically diagnosed in association with major trauma and acute tears. Chronic (intermittent?) instability is poorly defined and understood in dogs. In any of these pathologic processes multiple structures may be affected, suggesting that the term “rotator cuff pathology” should be used in case of tendinopathies.

Unfortunately, the lack of understanding of the disease process can lead to misdiagnosis. For example, “shoulder instability” is often diagnosed based on an increased shoulder abduction angle and concurrent shoulder pain. However, an increased abduction angle may be associated with severe muscle atrophy without any structural tear. In these cases, there may be laxity without any pathologic instability.

We should consider a 360 degree approach to shoulder diagnosis. The initial evaluation of the patient, including a thorough history, biomechanical assessment and orthopedic examination, might be sufficient to suspect a tear, a tendinitis or an instability. Similarly to people with shoulder injuries, high field MRI is an invaluable tool because it allows a global assessment of all shoulder structures, including bone marrow and cartilage (with some limitations). CT has limited value for the diagnosis of rotator cuff disease. Ultrasound is more widespread than any other imaging technique for the diagnosis or muscle-tendon diseases in dogs. With ultrasound, both acquisition and interpretation of the images depends on the skills of the clinician. For MRI, the greatest variability is noted in the interpretation of the images. Arthroscopy is the only modality that allow to evaluate with high accuracy the cartilage and, importantly, can detect typical lesions associated with chronic instability.

References

1. Zachary Y Kerr, Sarah Fields, R Dawn Comstock. Epidemiology of injury among handlers and dogs competing in the sport of agility. J Phys Act Health, 2014 Jul;11(5):1032-40.
2. Devitt CM, Neely MR, Vanvechten BJ. Relationship of physical examination test of shoulder instability to arthroscopic findings in dogs. Vet Surg. 2007 Oct;36(7):661-8.
3. Murphy SE, Ballegeer EA, Forrest LJ, Schaefer SL. Magnetic resonance imaging findings in dogs with confirmed shoulder pathology. Vet Surg. 2008 Oct;37(7):631-8.
4. Franklin SP, Devitt CM, Ogawa J, Ridge P, Cook JL. Outcomes associated with treatments for medial, lateral, and multidirectional shoulder instability in dogs. Vet Surg. 2013 May;42(4):361-4.
5. Morten Devor, R Sørby. Fibrotic contracture of the canine infraspinatus muscle: pathophysiology and prevention by early surgical intervention. Vet Comp Orthop Traumatol 2006;19(2):117-21.