A Review of Lameness Attributable to the Shoulder in the Dog: Part One

Osteochondrosis

Osteochondrosis is a disturbance of the normal process of endochondral ossification. The term osteochondritis dissecans (OCD) is used if the disease progresses to the point of formation of an articular cartilage flap resulting in inflammatory changes within the joint. The pathophysiology of OCD has been well described in the literature.^1–3 In the shoulder joint, OCD is manifested as a fragment of cartilage arising from the caudocentral or caudomedial aspect of the humeral head, usually opposite the caudoventral rim of the glenoid.

Large-breed, male dogs are most commonly affected, with 27% to 68% of dogs having bilateral lesions.¹ Clinical signs generally occur between 4 and 8 months of age, but some dogs do not show clinical signs until middle age.

The lameness associated with OCD is usually unilateral and typical in dogs with osteoarthritis, in that it worsens with time and is accentuated by rest and exercise. Muscle atrophy of the supraspinatus, infraspinatus, and deltoideus muscles may be palpated if the lameness is chronic.^2–4 Pain with palpation and crepitation are uncommon, but range of motion, especially extreme extension, often elicits a pain response.^2–4

Radiographic examination reveals a subchondral bone defect in the caudal aspect of the humeral head on the lateral view [Figure 1]. If no defect is observed, a more medial lesion may be present, or inadequate radiographic positioning may obscure the lesion. The most common radiographic technical errors that preclude observing the subchondral bone defect include poor radiographic detail, oblique positioning, and failure to adequately distract the limb away from the thorax. On occasion, the flap will become detached from the subchondral bed, migrate to the caudal joint space, reattach to the synovium, and become mineralized to the point where it can be visualized radiographically.^2–4 Positive contrast arthrography allows identification of the cartilage flap if it is not mineralized.^2–4

The cartilage flap associated with OCD should be surgically removed.^2–5 Arthroscopy provides a minimally invasive way to confirm the diagnosis and treat the disease simultaneously.⁵ The flap is observed through a lateral portal, and a caudolateral instrument portal is created to remove the flap⁵ [Figure 2]. The remaining subchondral defect is débrided to bleeding subchondral bone using either a bone curette or motorized shaver. The same techniques can be performed through an arthrotomy. Several approaches are suited to removal of an OCD flap,^6,7 but arthroscopy allows the surgeon to see areas of the joint that cannot be visualized by arthrotomy. Regardless of how the OCD flap is removed, the prognosis for minimal osteoarthritic change and normal or near-normal function after treatment is good to excellent.^2–5

Incomplete Ossification of the Caudal Glenoid

Incomplete ossification of the caudal glenoid has also been reported as a cause of lameness in medium- to large-breed dogs ranging from 8 months to 10 years of age.⁸ Incomplete ossification of the caudal glenoid is usually asymptomatic, but pain may result if the caudal fragment is mobile. Mild to moderate shoulder pain and regional muscle atrophy are usually present. In all cases, the incompletely ossified caudal glenoid is easily demonstrated by a lateral radiograph [Figure 3]. Arthroscopic examination typically reveals an osteochondral fragment with variable degrees of villous synovial hyperplasia and synovial fluid turbidity. Pathological findings consistent with other diseases of the shoulder may also be present, so a thorough examination of all intraarticular structures is important. The fragment should be removed if it is mobile when manipulated with an arthroscopic probe. In the small number of reported cases, arthroscopic removal of the fragment and débridement of the adjacent edge of the glenoid resulted in resolution of the lameness.⁸

Chondrocalcinosis

Chondrocalcinosis is a deposition of hydroxyapatite in the articular cartilage. The lesion is found primarily in the plateau region of the humeral head of greyhounds, but it has also been reported in the femoral head of the German Shepherd dog.⁹ Chondrocalcinosis is also described as pseudogout. Lesions may occur as single spots or as small, multifocal areas of chondrocyte mineralization. Histologically, an articular surface lesion appears as a white, raised focus of mineralized chondrocytes surrounded by a translucent halo.¹⁰ Grossly, small pits in the cartilage surface, scarring, scoring, and cracking of the articular surface are also reported.¹⁰ The lesion is typically unilateral or, if bilateral, more pronounced in the right shoulder due to the repetitive high pressure placed on that joint during greyhound racing.¹¹ This shoulder lesion is often found incidentally and without clinical signs. As such, its clinical relevance is unknown.

Glenoid Dysplasia and Congenital Luxation

Hypoplasia or dysplasia of the glenoid can result in a grossly abnormal articulation of the shoulder joint, leading to shoulder luxation. The cause of the abnormal development is unknown. Congenital luxations are generally diagnosed in dogs between 3 and 10 months of age that are presented for recurrent lameness.¹² Breed predispositions include toy and miniature poodle, Chihuahua, Pomeranian, Shetland sheepdog, Norwegian elkhound, collie, dachshund, pug, Pekingese, miniature pinscher, Lhasa Apso, and Cavalier King Charles spaniel.¹³ Toy breeds appear to be most susceptible.¹³

With congenital luxations, often no history of a traumatic episode exists, or only mild trauma has occurred. Medial luxation occurs more frequently than lateral. Cranial and caudal luxations are rarely reported.¹³ Lateral and craniocaudal radiographs of the shoulder may reveal conformational defects such as a misshapen glenoid cavity [Figure 4] and, less commonly, a flattened humeral head. Recognizing these bony changes is important, because the surgical treatment depends on whether or not the bony anatomy is normal. Animals with medial luxations may hold the affected limb with the elbow flexed and adducted, with the distal limb abducted. Abnormal spatial differences between the acromion and greater tubercle are appreciated with palpation. If survey radiographic views demonstrate normal bone anatomy, varus and valgus stressed views may be necessary to identify soft-tissue instability.¹³

Conservative treatment of congenital shoulder luxations is usually unrewarding.¹⁴ Surgical stabilization of congenital medial shoulder luxation can be done by any of several techniques. ¹⁴ If the glenoid is dysplastic, excision arthroplasty of the glenoid or the humeral head is preferred [Figure 5].^14,15 Caution should be exercised to avoid damaging the suprascapular nerve when excising the glenoid. Physical therapy after surgery may improve functional outcome.

Arthrodesis is an alternative to glenoid or humeral head excision arthroplasty, but it is technically demanding. If the bony anatomy of the shoulder is normal and only soft-tissue instability is present, other techniques may be used to achieve joint stability. Placement of synthetic heavy suture secured to the medial aspect of the scapular neck and humeral head by combinations of screws, bone anchors, or bone tunnels [Figure 6] is probably the most biomechanically sound method; this augments the existing medial glenohumeral ligament, subscapularis tendon, and joint capsule. In one study, suture anchors and bone tunnels were utilized to specifically anchor large monofilament suture at the origins and insertions of the cranial and caudal bands of the medial glenohumeral ligament in a V-shaped manner. In nine of 10 dogs in the study, this technique provided medial stability through a range of motion with good congruency, mobility, and less alteration of normal anatomy.¹⁶

Alternatives include transposition of the bicep brachii tendon or supraspinatus tendon to the medial or lateral aspect of the shoulder, depending on the direction of luxation. ^15,17–19 Both transposition techniques result in some degree of alteration of shoulder biomechanics, incongruency of the articular surfaces, and osteoarthritis; but accurate, long-term outcomes have not been described.^20,21

Shoulder Instability/Subluxation

Shoulder instability with or without subluxation results from repetitive overuse; it exists as a separate and distinct entity, apart from true congenital or traumatic luxation. Shoulder instability or subluxation has been recently suggested by some to be one of the most common causes of shoulder lameness.^16,22–24 The pathophysiology of shoulder instability in dogs has not been described fully, but a number of different mechanisms have been proposed based on extrapolation from the human literature.²²

One mechanism of shoulder instability may be disruption of concavity compression. In this mechanism, the concavity provided by the glenoid to the humeral head provides stability much like a round ball pressed into a concavity on a table top. The ball in the concavity will have much greater resistance to translational forces than a ball placed on a flat surface. The stability is further enhanced by 1) the depth of the concavity, provided by the articular cartilage and glenoid labrum or glenohumeral ligaments; and 2) the magnitude of the compressive force holding the ball in place, provided by the muscles associated with the joint capsule.²⁵ Any disruption or abnormality of either the concavity of the glenoid or the compressive forces holding the humeral head in the glenoid will result in instability. Thus, instability as a result of concavity compression might result from an abnormally small or flat glenoid, a torn or avulsed glenohumeral ligament, or a decrease in the depth of the concavity as a result of injury or chronic repetitive wear.²⁵

Another mechanism resulting in instability may be disruption of scapulohumeral balance.²⁶ Scapulohumeral balance refers to the principle that the humeral head is balanced in the glenoid if the net joint reaction force passes through the center of the joint, the glenoid fossa. The muscles associated with the joint capsule and periscapular musculature provide a compressive force across the joint in different positions, such that net force remains within the stable arc provided by the glenoid. Furthermore, the capsuloligamentous restraints, which activate in positions that result in the joint reaction force falling outside the stable arc, also exert a force on the humeral head to normalize the joint reaction force and maintain stability. Potential causes of instability by this mechanism might include a dynamic muscle imbalance, abnormal glenoid position (version), or disruption of the capsuloligamentous restraints.^25,26

Dogs with shoulder instability often are presented with a history of chronic lameness that is often subtle and intermittent but may, on occasion, be severe and continual. These dogs are usually minimally responsive or nonresponsive to nonsteroidal anti-inflammatory drugs (NSAIDs). Physical examination findings may include mild atrophy of the shoulder muscles of the affected limb and varying degrees of pain when the scapulohumeral joint is manipulated. The biceps tendon test [Figures 7A, 7B] and the shoulder drawer test are usually positive.^22,23

Normal ranges of subluxation have not been fully established; however, in a recent study, goniometry and digital image analysis were used to measure angles of abduction in clinically affected and control dogs. Shoulder abduction angles measured under sedation were approximately 30° in control dogs and 50° in clinically affected dogs.²⁷ The authors of that study concluded that visual observation of asymmetry between normal and affected shoulders may be sufficient for preoperative diagnosis of medial shoulder instability.²⁷ The degree of abduction should be determined with the elbow and shoulder in extension.²¹ Increased instability may be elicited during sedated examination, because pain and apprehension may result in periarticular muscle tightening when the dog is awake. The direction of the instability may be craniocaudal, mediolateral, or multidirectional. The direction of instability can often be determined by comparison to the contralateral limb and verified by arthroscopy. While the majority (80%) of subluxations are medial, identifying the direction of instability is necessary since treatment differs for medial and lateral subluxations.^23,28

Radiographic examination of the shoulder, including mediolateral and craniocaudal views, may be normal or may show signs of degenerative joint disease such as osteophyte formation on the caudal margin of the humeral head on a mediolateral view. Degenerative changes in the scapulohumeral joint in the absence of osteochondrosis have been proposed to be strongly indicative of shoulder instability, ^22,23 but this suggestion remains to be conclusively demonstrated.

Arthroscopic examination of the shoulder is extremely valuable in several ways. It is useful when the degree of stability is inconclusive during a sedated orthopedic examination or when the radiographic appearance is normal. Arthroscopic examination also helps to determine the extent and type of degenerative changes within the joint. In a retrospective study of 45 dogs, arthroscopic findings were described.²² Superficial erosion of the caudal articular cartilage of the humeral head was the most common finding, followed by erosion of the medial ridge of the glenoid, distension of the medial glenohumeral ligament, and synovitis. ²² Although currently limited, the application of arthroscopic methods of joint stabilization may someday be viable as well.

Treatment of glenohumeral joint instability involves definitive surgical stabilization, because the unstable shoulder is generally unresponsive to conservative therapy. Delays in treatment may result in chronic lameness and more severe degenerative joint disease. Tendon transfer procedures and collateral ligament augmentation/ reconstruction, as previously described for the management of shoulder luxations, have traditionally been used to treat the unstable shoulder.^16,20,21,29

Recently, radiofrequency-induced thermal modification or thermal capsulorrhaphy via arthroscopy^30,31 was proposed as a method of tightening the stretched joint capsule to achieve shrinkage and stability. Although these techniques initially enjoyed great popularity in the treatment of shoulder instability in humans, the nuances of what constituted effective treatment and, more importantly, long-lasting effectiveness were ill-defined. Because long-term success was limited, capsular shrinkage techniques have been largely abandoned in humans. Collagen, which is the component of the joint that shrinks with heat, tends to stretch back to its original length with time and use. Furthermore, the best results in humans after thermal treatment of joint collagen seem to occur when the joint is protected from movement and stretching for a protracted period of time.³² A recent retrospective investigation reported improved clinical function in 93% of dogs treated with radiofrequency-induced thermal capsulorrhaphy, but the authors did not unequivocally recommend the procedure.³² Protecting a joint from weight bearing and motion for the protracted period of time necessary to allow the treated joint capsule to fully heal seems somewhat unrealistic, given the nature of dogs and their quadruped stance. Considering these limitations, capsular shrinkage techniques are not likely to be effective for shoulder instability.

Traumatic Luxation

Traumatic luxations of the scapulohumeral joint are uncommon. As with congenital luxation, luxation resulting from trauma occurs most frequently in a medial direction. Lateral, cranial, and caudal luxations are reported more rarely.¹³ Physical examination findings in cases of traumatic medial luxations are identical to those of congenital medial luxations. Trauma in large-breed dogs often results in lateral luxation.^15,17 Lateral luxations present similarly, except the distal limb is held in adduction. Again, distortion of the spatial relationship between the acromion and greater tubercle may be palpated if not precluded by the swelling associated with the trauma. Lateral and craniocaudal survey radiographic views are usually sufficient to document the luxation.¹³

Conservative treatment of traumatic medial humeral joint luxations is usually successful if the luxation is reduced and stabilized with a Velpeau sling soon after injury.¹³ If reduction cannot be accomplished or the joint is grossly unstable, reconstruction or augmentation of the collateral ligament(s) is the best course of action. As for congenital luxation, medial or lateral transposition of the biceps tendon (with medial, lateral, or cranial luxations) or transposition of the supraspinatus tendon (with medial luxation) can be performed.^15,17–19 Again, joint incongruency and degenerative joint disease may make these techniques less desirable than suture augmentation techniques. Salvage procedures such as arthrodesis and excision arthroplasty could be considered in situations where gross instability or extensive osteoarthritis is present.^13,15 The prognosis for shoulder luxation, if treated promptly without the need for glenoid excision or arthrodesis, is generally good. Dogs that undergo glenoid excision or arthrodesis should be expected to have some minor degree of gait alteration and limited range of motion.