Brian S. Beale, DVM, DACVS
Elbow dysplasia is the most common cause of forelimb lameness in dogs. Elbow dysplasia includes an array of abnormalities including fragmented medial coronoid process (FCP), osteochondritis dissecans (OCD), joint incongruity and ununited anconeal process (UAP). The severity of clinical signs, pathologic change and outcome vary greatly amongst patients. Early diagnosis and prompt treatment gives patients the best chance of avoiding debilitating osteoarthritis.
Dogs affected by elbow dysplasia generally begin to show clinical signs prior to reaching two years of age. Clinical signs include intermittent or persistent lameness, elbow swelling, decreased range of motion and joint pain. Plain film radiography including cranio-caudal, medio-lateral, and flexed lateral views should enable the differentiation of ununited anconeal process (UAP), fragmented coronoid process (FCP), and osteochondrosis dissecans (OCD). Additional oblique views have been advocated by some authors and may aid in the diagnosis of FCP. The value of computed tomography (CT) in the diagnosis of FCP has been well documented. We have found CT to be unnecessary in the diagnosis of FCP based on a high specificity of plain radiography; however, we perform CT on more confusing and milder arthroscopy patients in order to more accurately define the FCP lesion and assess for incongruity. Step lesions associated with short radius syndrome are clearly seen using CT. Although in the vast majority of cases the CT and arthroscopy findings concur, this has not been uniformly the case. In numerous cases, fragments have been demonstrated by CT that were not visible or palpable when directly visualized arthroscopically. This is most likely due to incomplete fragmentation of the coronoid. Other patients have been found to have small arthroscopic fragments that were not evident by CT. Nuclear scintigraphy (bone scan) has also been found to be helpful in diagnosing early or subtle FCP lesions.
Ununited Anconeal Process
Ununited anconeal process (UAP) occurs most commonly in German shepherd dogs, but can be seen in many dog breeds. The anconeal process should fuse to the metaphyseal region of the proximal ulna at 4–5 months of age. If the process does not fuse, elbow stability is compromised and osteoarthritis ensues. Most patients having UAP have a short ulna relative to the length of the radius. The hypothesis is that as growth progresses the anconeal process of affected dogs presses against the humeral trochlea. This creates a shear separating the anconeal process from the ulnar metaphysis. In cases of UAP the site of non-union will be clearly visible as an area of cartilage loss with exposed dense subchondral bone and possibly fibrocartilage. Cartilage erosion is also commonly seen in these patients. Surgical treatment is recommended. Arthroscopy allows assessment of the integrity of the anconeal fragment, the degree of fragment stability and the extent of osteoarthritis. The medial coronoid process should also be examined as many patients having UAP also have FCP.
Incongruity and Short Radius Syndrome
Elbow dysplasia may be associated with a shortened radius relative to the length of the ulna. Incongruity of the joint surfaces occurs. This results in concentration of weightbearing forces on the medial aspect of the joint, leading to cartilage wear and fragmentation of the medial coronoid process and sclerosis and cartilage wear of the medial humeral condyle. The amount of shortening may vary from severe to subtle. CT evaluation is likely the best way of documenting short radius syndrome incases of subtle shortening. When shortening of the radius becomes more marked, radiographic and arthroscopic examination can lead to a diagnosis.
Osteochondritis dissecans (OCD) affects the medial humeral condyle. OCD lesions are more difficult to view than FCP lesions. OCD can occur in combination with FCP. The OCD lesion may extend relatively far caudally and medially along the medial humeral condyle. Osteochondrosis dissecans will usually appear on the humeral trochlea as a thickened flap of cartilage overlying a relatively deep (1–2 mm) subchondral bone defect. This is in contrast to subchondral bone exposure found with osteoarthritis, which is more polished and follows the regular contours of the joint surface. Osteoarthritic lesions of varying degree may also accompany the OCD on either the adjacent humeral trochlea or on the opposing ulnar surface.
Fragmented Medial Coronoid Process
Changes associated with FCP are extremely varied. The area of fragmentation is most commonly on the craniolateral aspect of the medial coronoid adjacent to the radial head. The bone of fragments will most often be dead and yellow in appearance in contrast to the well-vascularized red colored bone of the remainder of the joint. Fully migrated fragments may have revascularized from attachments to the synovium. Additional changes associated with FCP will include varying degrees of osteoarthritic lesions on the remainder of the coronoid, trochlear notch and humeral trochlea. Changes seen in the radial head may include rounding off of the cranial and medial borders.
Canine Elbow Dysplasia
Canine elbow dysplasia (ED) is a commonly reported thoracic limb disorder. ED signifies an abnormal development of the elbow joint coupled with characteristic pathological changes of the medial compartment. Pathologic changes are associated with the coronoid process and humeral condyle. Pathology of the medial coronoid is typified by subchondral bone micro-cracks and fragmentation as well as cartilage erosion secondary to incongruence as seen. Many hypotheses have been formulated about the etiopathogenesis of the pathologic changes including radio-ulnar incongruence. The prevailing belief is that radio-ulnar incongruence is secondary to improper growth of the radius/ulna during maturation. The result is mal-alignment of the articular surfaces where the medial coronoid is subject to high mechanical loads and microfracture or fragmentation. There is no question that malalignment and incongruence occurs. However, fragmentation and incongruence secondary to radius/ulna growth abnormality would best explain abnormalities found in younger patients or mature patients with chronic recurring clinical problems. On the other hand, abnormal growth and incongruence may not explain medial coronoid pathology seen cases, which exhibit lameness but no other abnormal physical or radiographic findings. Arthroscopy confirms fragmentation of the medial coronoid adjacent to the radial head without the presence of visible cartilage erosion. In these cases, fragmentation/microfracture of the medial coronoid may be secondary to mechanical overload associated with contraction of the biceps brachii/brachialis muscle complex. The histologic and ultrastructural appearance of FCP is consistent with mechanical failure and subsequent unsuccessful fibrous repair.
The biceps/brachialis muscles constitute a large muscular complex. The anatomic origin and insertion of the biceps and brachialis muscles are such that the muscular complex exerts considerable force on the medial compartment of the elbow. The force exerted by the biceps is continuous since it is a pennate muscle with central tendon. More importantly, because the insertion of the biceps/brachialis complex is at the ulnar tuberosity, a large polar (rotational) moment is exerted at the cranial segment of the medial coronoid. The magnitude of the polar moment is a product of the moment arm (distance from the ulnar tuberosity to the tip of the coronoid) multiplied by the force created by the biceps/brachialis muscular complex. The polar moment rotates and compresses the craniolateral segment of the medial coronoid against the radial head. The compressive force is medial to lateral transverse to the long axis of the coronoid. A compressive force generates internal shear stress at an oblique angle to the applied compressive force. In this situation, maximal internal shear stress would be oblique to the long axis of the coronoid. Under the right circumstances, the polar moment and resultant compressive force produced by the biceps/brachialis complex may produce sufficient internal shear stress to exceed the material strength of the cancellous bone in the craniolateral segment of the medial coronoid. The result would be microfracture/fragmentation adjacent to the radial head at an oblique angle to the long axis of the medial coronoid. Interestingly, microfracture/fragmentation of the coronoid seen clinically is in the craniolateral segment of the medial coronoid adjacent to the radial head. This location corresponds to the plane of maximal shear stress generated by the compressive force exerted by the polar moment produced by contracture of the biceps/brachialis complex.
Elbow Dysplasia in Cats
Cats are commonly affected by osteoarthritis (OA) of the elbow. A common cause of elbow OA is believed to elbow dysplasia. The etiology is unknown and the syndrome has been described infrequently. The diagnosis of elbow dysplasia in the cat is made based on the similarity of arthroscopic and pathologic changes seen in the dog.
Clinical signs appear to be insidious, often becoming visible in the middle or latter stage of life. Cats may have an intermittent or persistent weight-bearing lameness. Owners often report that the cat has become reclusive and is reluctant to perform tasks (jumping, playing, etc.) they normally were anxious to do. They typically have decreased activity and owners comment that they seem to feeling the effects of advancing age.
A stiff gait with a shortened stride is typically seen. Palpation of the elbow may invoke pain. Crepitus and firm soft tissue swelling may be palpated. Extensive joint effusion is not typically present. A reduced range of elbow motion (particularly flexion) may be seen. Radiographic changes are similar to the dog with elbow dysplasia except for the presence of mineral deposits may be seen in the periarticular soft tissues. The mineralization can be quite extensive at times. Periarticular osteophytes and subchondral bone sclerosis are often seen in the region of the trochlear notch, medial humeral condyle and medial coronoid process. Severity of osteoarthritic radiographic signs varies from very mild to severe (see figures to right).
Arthroscopic treatment of elbow dysplasia is recommended due to very low morbidity, improved visibility and more precise treatment of the condition. Elbow dysplasia in cats can be treated in similar fashion to dogs. Arthroscopic examination reveals characteristic changes seen in canine patients with elbow dysplasia. The changes that have been seen on the cat include cartilage erosion of the trochlear notch, radial head, medial humeral condyle and medial coronoid process. The medial compartment of the elbow is primarily involved. Fragments from the medial coronoid process may also be found.
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