Clinic for Small Animal Surgery, Department of Small Animals, Vetsuisse-Faculty, University of Zürich, Zürich, Switzerland
Shoulder instability is a reported cause of chronic forelimb lameness in dogs. Franklin et al. reported a relative distribution of instability with 78% of the cases presenting with medial, 8% with lateral and 15% showing a multidirectional instability. Medial instability has been related to be traumatic in 49% of the dogs. Traumatic shoulder instability is more common in active younger dogs. Its presentation is typically acute and is usually associated with a severe lameness. Shoulder instability as a result of repetitive overuse and attenuation of stabilizing structures is described in human literature and has been implicated for shoulder instability in dogs, especially in older dogs. In the more chronic type of instability, seen in geriatric patients, the grade of lameness is less severe, but persistent. The dog presents with lameness that worsens after exercise and only partially responds to anti-inflammatory treatment. One frequent challenge to diagnose the exact source of lameness in the older dog is that other joints may be affected (i.e., elbow) by osteoarthritis. Thus, the clinician should try to isolate the joint before manipulation, because of the risk of flexing elbow and shoulder at the same time, making difficult defining the exact cause of pain. In addition, other differential diagnosis such as cervical disc and nerve sheath tumors should be considered.
The most common injuries associated with medial shoulder instability are partial or complete ruptures of the medial glenohumeral ligament (MGHL) and tears of the subscapularis tendon. In addition, articular cartilage erosions on the medial glenoid rim and caudal humeral head are frequent arthroscopic findings suggesting chronic instability. These findings highlight the importance of the MGHL and the subscapularis tendon and their role as shoulder stabilizers. The biceps tendon can also be injured or inflamed, further complicating decision-making. Surgical stabilization of medial shoulder instability is recommended when conservative treatment failed. They can be divided into biological repair and prosthetic reconstruction. Biological repair aims at repairing the anatomic structure contributing to shoulder instability. For medial shoulder instability, biological repair includes imbrication of the subscapularis tendon or radiofrequency-induced thermal capsulorrhaphy. Both techniques are described with good outcome. Although radiofrequency-induced thermal capsulorrhaphy is described for treatment of shoulder instability in people, it is discussed controversially in human literature and should be used with caution to avoid cartilage injury.
Another common biological repair technique for medial shoulder instability (MSI) is the transposition of the biceps tendon. Such a transposition increases shoulder stability, but causes incongruity of the articulating joint surfaces and alteration of normal joint rotation. Furthermore, transposition techniques do not consider the significant role of the MGHL and subscapularis tendon in maintaining medial shoulder stability. Placement of prosthetic devices is a surgical reconstruction technique with the attempt to preserve the biomechanical function of a given anatomic structure. MGHL repair using bone tunnels or suture anchors is described for medial shoulder instability with good to excellent clinical outcome. In a cadaveric study, MGHL reconstruction prevented medial subluxation and preserved normal joint motion compared to the transposition of the biceps tendon. Most surgical techniques used for the treatment of medial shoulder instability in dogs are performed through an open approach. An arthroscopic treatment of MSI would offer the advantages of less tissue dissection compared to open techniques and a fast concurrent treatment during diagnostic shoulder arthroscopy, if necessary. For medial shoulder instability, arthroscopic treatment is only described as radiofrequency-induced thermal capsulorrhaphy and recently Cook and Schultz developed an arthroscopic-assisted technique for medial shoulder stabilization using TightRope®.
Arthroscopic management of lateral glenohumeral ligament rupture was described from Pettitt et al. for two dogs. Based on our clinical experience we believe that an all-arthroscopic treatment of medial shoulder instability is feasible and favored compared to open repair techniques. Our goal has been to develop an arthroscopic technique for repairs and imbrications of the MGHL and the subscapularis tendon using knotless anchors. The insertion of an anchor in the cranial and caudal glenoid is challenging regarding safety and accuracy of insertion because of limited bone stock and exposure during arthroscopy. Based on the clinical and biomechanical data from rotator cuff repair in people, the insertion angle of the anchor is another important parameter that needs to be considered for the strength of the repair. We performed a study to test the safety and accuracy of the anchors placed into the cranial and caudal medial glenoid rim for MGHL repair and into the humerus for subscapularis tendon repair. We found that the anchor in the humerus is safer (greater safety angle and bone stock) than anchors in the caudal glenoid. However, all anchors could be placed safely in most shoulders. Anchors inserted through cranio-medial and caudo-medial arthroscopy ports are inserted at the ligament and tendon footprints and in an insertion angle of 45° to 90° relative to the articular surface as recommended in human shoulder repair.