Tenoligamentous lesions causing shoulder instability are a common finding in canine medicine but are difficult to contextualise in terms of both their classification and therapy. The advent of arthroscopy in veterinary medicine has brought considerable benefits in the diagnosis of these disorders, finally allowing a more precise classification and identification, however many doubts are still present concerning the therapeutic approaches used which are largely, if not entirely, taken from human medicine where shoulder biomechanics is apparently different. Shoulder instability was accurately described for the first time by J.F. Bardet in 1998 as a “common traumatic or microtraumatic orthopaedic disorder affecting adult dogs, characterised by altered joint biomechanics and concomitant ligament, cartilage and capsulo-synovial lesions”. Moreover, some authors have suggested that shoulder instability should be considered as one of the most common causes of shoulder lameness in adult dogs.
AETIOPATHOGENESIS
Because the glenoid cavity of the scapula articulates with only a third of the humeral head, joint stability mainly depends on a complex interaction between active and passive stabilisers acting in unison. The components responsible for passive stabilisation are the ligamentous compartment, meaning the medial (MGHL) and lateral (LGHL) glenohumeral ligaments, the joint capsule, joint conformation and the glenoid labrum (when present). The perfectcongruity between the humeral head and the glenoid cavity allows for an increased stability of the joint; under load, axial compressive forces are generated which hamper joint dislocation. The mechanism of adhesion and cohesion, which explains why two wet and superimposed microscope slides are difficult to separatedespite presenting sliding movements, is based on the principle that between two wet, smooth and perfectly congruent surfaces a phenomenon of direct adhesion is developed. This mechanism is dependent on the reduced volume of synovial fluid contained within the joint.
Active stabilisation is under the responsibility of the rotator cuff, made up of the muscles biceps brachii, subscapularis, infraspinatus, supraspinatus and teres minor. This group of muscles has several stabilising functions: the first is an activity of control, as it allows the modulation of the tension of the collateral ligaments and of the joint capsule, which are surrounded by the above mentioned tendons and muscles which act as "active ligaments"; secondly they compress the humeral head into the glenoid cavity through the simultaneous contraction of the muscle cuff. This complex stabilising system works at 100% only if the active and passive mechanisms act synergistically. A necessary condition for joint instability is that the mechanisms of passive stabilisation and possibly also those of active stabilisation are compromised. Single traumatic events or repeated micro-traumas (overuse injury), typical but not exclusive of athletic subjects, are the underlying cause of the development of this condition. Multiple moderate traumatic episodes, individually not capable of altering the anatomical structures, can lead to a progressive mechanical failure of the support structures due to a cumulative effect over time; or they may be the cause of inflammatory processes which, by secondarily involving these means of stabilization, may alter their function. In small or toy breeds the aetiopathogenesis is instead different: in these subjects the frequent anatomical alterations of the humeral head and/or of the glenoid cavity, consequent to congenital shoulder dysplasia, are the predisposing factors for the development of this condition.
EPIDEMIOLOGY
Shoulder instability can affect subjects of any age and breed; this said, adult dogs of medium-large size, athletes, working animals or in any case active subjects are the most likely candidates. This is not a breed predisposition as various factors play an important role in the determinism of shoulder instability, among which the subject’s physical activity and temperament.
CLINICAL SIGNS
At physical examination patients with shoulder instability typically exhibit chronic front limb lameness which worsens with physical activity and is only initially responsive to rest and anti-inflammatory treatment. That said, the onset and consistency of the clinical signs vary considerably according to the type of instability. The orthopaedic examination shows atrophy of shoulder muscles, caused by the chronicity of the lesion, and pain during passive joint movements especially in extension. These characteristic signs cannot be attributed to shoulder instability alone, as they are present in all shoulder orthopaedic conditions. The degree and type of lameness are therefore not characteristic and the orthopaedic examination of the nonsedated patient only allows the localisation of the site of pain.
Patient examination under heavy sedation allows veterinary surgeons to perform the shoulder drawer and abduction test. The “shoulder drawer test”, similarly to the “drawer test” used to detect cruciate ligament rupture, is based on the detection of a sign of joint subluxation. The correct manual procedure requires holdingthe scapula with one hand, with the thumb on the acromion and the forefinger around the craniomedial aspect of the neck of the scapula, while the other handgrasps the humerus, with the thumb at the level of the caudolateral aspect of the proximal metaphysis of the humerus and the forefinger on the medial side of the greater tubercle. If the size of the subject allows it, the entire scapula can beheld with one hand while the forefinger and thumb of the other hand can be respectively placed on the lateral and medial aspect of the proximal humerus (Fig. 1). The application of forces in cranial, caudal, medial and lateral direction reveals pathological translational movements. Tenoligamentous lesions typically affect the medial compartment of the joint, with a consequent medial instability of the shoulder and a positive drawer test in lateromedial direction. Eventual shoulder crepitus may be detected while performing the test as a consequence of joint instability.
The abduction test, or more precisely measuring the abduction angle, is used to assess the medial instability. The test is carried out with the animal in lateral recumbency, with the healthy limb in contact with the table; the operator, while maintaining the dog’s elbow in full extension and the shoulder in neutral position, so as to align arm and forearm, perpendicular to the trunk, exerts a compressive force at the level of the acromion in order to maintain the scapula steady while simultaneously abducting the forearm and arm of the subject under examination (Fig. 2). A goniometer is used to measure the angle, with the scapulohumeral joint as the centre of rotation, with an arm held parallel to the spine of the scapula and the free arm placed along the humerus. The angle formed between these lines, when no force is applied, corresponds to 0 degrees. The abduction angle is measured after forcing the humerus in abduction. This procedure must also be repeated on the healthy contralateral limb, in order to compare the angles measured: for the difference to be significant it must be > 20°. The physical examination therefore allows to suspect a specific diagnosis which must then be confirmed with instrumental investigations.
DIAGNOSTIC APPROACH
The diagnosis of shoulder instability is not made on the basis of plain or contrast radiography. In the presence of acute shoulder instability radiography is not clinically relevant (Fig. 3); in the presence of chronic conditions only the indirect signs caused by arthrosis are detected, and will vary depending on the chronicity of the disorder (Figs. 4 and 5). Tenomuscular ultrasonography provides useful information on the status of the biceps brachii tendon and of peri-articular muscles, but in the presence of suspected shoulder instability it cannot be considered as a specific examination. Magnetic resonance imaging may provide important information on joint status and on the tenomuscular component. Tenoligamentous lesions are easily identified with MRI, even if a tendency to underdiagnose the severity of the damage observed has been described. The use of MRI in veterinary orthopaedics for suspected shoulder instability is however quite recent, unlike in human medicine where this examinations is regularly performed.
Arthroscopy, which is both diagnostic and therapeutic, is the gold standard for the diagnosis of all tenoligamentous conditions: it allows the inspection of the entire joint while assessing both the status of the joint cartilage and the integrity of the intra-articular tenoligamentous structures (Figs. 6, 7a and 7b, 8). Arthroscopy has the disadvantage of being totally subjective, especially in view of the fact that there is no univocally accepted and validated scale to classify the tendon and ligament alterations observed. It is intuitive that minor lesions of the tenoligamentous compartment, not associated with cartilagineous alterations, are unlikely to be clinically relevant (Fig. 9). However, such findings could be predictive of more severe lesions which could be the cause of real instability following excessive physical activity or major trauma or, on the other hand, remain asymptomatic in companion animals with limited physical activity.
THERAPEUTIC APPROACH
Arthroscopy has allowed considerable progress with regard to the diagnostic approach, however some doubts still remain when having to decide on the best therapeutic approach. Treatment options must be based on the extent of clinical symptoms, on the severity of lesions being detected and on the type of activity of the dog. Minor lesions can be clinically relevant in sporty subjects after hours of activity, whereas they can be completely asymptomatic in companion animals.
Veterinary publications report both conservative management and surgical stabilisation techniques. Conservative treatment envisages a reduction in the subject’s body weight, when indicated, the use of anti-inflammatory drugs and the use of a brace or bandaging preventing the abduction of the front limbs, to be maintained forapproximately 60 days (Figs. 10 and 11). The brace or bandaging allows limb use,thus avoiding the severe hypomiotrophies caused by non-weight bearing containments, and at the same time protect the medial joint compartment. Associated physiotherapy is also necessary for limb muscles strengthening. Restoring a good muscle tone can allow a satisfactory clinical recovery, compensating for poorly functional tenoligamentous structures. This approach is taken from human medicine and allows a good functional recovery in the majority of subjects with mild lesions.
The first surgical techniques reported in veterinary publications consisted in the transposition of the biceps tendon and of the supraspinatus tendon; however, both these techniques determine an alteration of joint biomechanics, joint incongruence and osteoarthritis. For this reason, especially in medium-large subjects, these techniques are no longer considered as a valid therapeutic option. Severe impairments of tendons and ligaments can be treated surgically with ligament implants, imbrication, capsulorrhaphies and with a combination of these techniques. Ligament implants, which can be medial or lateral depending on the type of instability present, use different types of suture materials which are implanted by means of open or arthroscopic surgery. The implants can be inserted into bone tunnels created in the isometric points, using either the traditional open approach or the arthroscopic technique with bone anchors placed as close as possible to the origin or insertion of the injured ligament. The joint capsule imbrication procedure aims at creating a capsular overlap in order to improve joint stability; this is usually achieved with a U-shaped suture, with absorbable stitches. This technique can be associated witharthroscopic radiofrequency-induced thermal capsulorrhaphy, which has become very popular in veterinary medicine in the last ten years. Imbrication has been reported also for the tendon of the subscapularis muscle; it has been used in 5 cases with resolution of the lameness in 3.
Radiofrequency-induced thermal therapy causes structural alterations in the collagen structure and determines a shortening and strengthening of the joint capsule and ligaments, resulting in restoration or improvement of joint stability. A key component of this procedure is the protection of the shoulder after the capsulorrhaphy; tissues treated with radiofrequency become weaker, making the use of a protective, usually non-weight bearing bandaging for 3 weeks necessary, followed by cage rest or controlled physical activity for at least another 3 weeks. The use of the previously mentioned brace or bandaging is another option for the postoperative management of these patients. This technique has been enthusiastically adopted as it is an easily implemented, minimally invasive solution, which does not preclude the subsequent possible use of surgical techniques with implants in case of failure. Severe lesions characterised by marked lameness, recurrent dislocation/subluxation, severe osteoarthritis, in which stabilisation attempts have not resulted in a good functional recovery may be treated with shoulder arthrodesis.
PROGNOSIS
The prognosis of shoulder instability is dependent on the severity of the existing lesion and on the type of activity performed by the dog. The prognosis is good for companion animals and poor for the possibility ofreturning to sport activities, unless in the presence of minor lesions which are treated conservatively.
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