Elbow Arthrosis: Basic Principles
Robert Taylor United States
The elbow of the dog is a frequent cause of lameness and disability. The term elbow dysplasia has been used to describe the presence of either osteochondrosis, ununited anconeal process or fragmentation of the medial coronoid process.(1) The International Elbow Working Group has described elbow dysplasia as a group of developmental diseases of the elbow which may produce lameness and progressive degenerative joint disease.(2) Elbow dysplasia has been shown to be a multifactorial polygenic trait and is prevalent in the German Shepherd, Rottweiler, Bernese Mountain Dog and other large working breeds. Padgett and Ubbink (3,4) have shown that the manifestations of elbow dysplasia, osteochondrosis, ununited anconeal process and fragmentation of the medial coronoid are inherited independently on polygenic traits.
The term I prefer to describe elbow dysplasia is elbow incongruency. United anconeal process and fragmentation of the medial coronoid process are caused by abnormal weight-bearing stresses in the dysplasic elbow. In a normal elbow the proximal radial articular surface provides 75%–80% of the weight-bearing surface on the joint, (5) and the ulnar part of the joint provides joint stability via the anconeal process, coronoid process and trochear notch. The elbow or cubital joint is a classic hinge joint and, with full extension of the joint (170-180°), the anconeal process fits into the olecranon fossa of the humerus to prevent lateral or rotational instability.
Failure of coordinated and congruent development of the three bones that constitute the joint is responsible for the pathological changes that occur. Prior to, and at, skeletal maturity, the humeral articular surface (trochlea and capitulum), radial head, and ulnar surface (coronoid processes and anconeal process) must exist as a smooth seamless cuticular surface. Since ulnar physeal ossification occurs at about 20–24 weeks, (6) incongruent growth of either of these three bones can have profound effects on the future health of the cubital joint at an early age. If the radius grows faster than the ulna and becomes relatively larger than the ulna, it can exert a proximally directed pressure on the humeral condyle. The pressure transferred through the trochlea of the humerus causes distortion of the anconeal process and failure of ossification of the physis. Further evidence to support this premise was developed by Gilson (7), who showed improvement in dogs with humero-ulnar subluxation following ulnar osteotomy. Ulnar osteotomy has been shown to allow ossification of the anconeal process in dogs with ununited anconeal process.(6) The exact locale of the ulnar osteotomy varies with surgeon preference, some choosing a proximal ulnar osteotomy with an intramedullary pin to maintain ulnar axis stability, and others preferring a proximal ulnar osteotomy without a pin, and still others, a midshaft ulna osteotomy. Judging from the location and strength of the interosseous ligament, it would seem that a proximal ulnar osteotomy is better. The configuration of the coronoid process is such that with a shortened radius, either due to premature radial physeal closure due to trauma or a genetically determined shorter radius, the medial ulna is subjected to excessive weight bearing. Depending on the degree of incongruency, this may produce fragmentation and/or various grades of osteochondral damage between the humero ulnar joint surfaces. Historically the fragmented coronoid process resembles a fracture but recent (8,9) work showing the presence of collagen type X often associated with disorders of endochondral ossification suggests a different pathogenesis of the disease. In addition to asynchronous growth of the radius and ulna, Slocum has suggested limb alignment abnormalities process abnormal weight bearing in the humero-radio-ulnar articulation.
Osteochondrosis is a disturbance of endochondral ossification and often occurs in the elbow of the dog. While osteochondrosis has been reported on other joints of the dog, when present in the elbow, it is part of the elbow dysplasia or incongruency triad. It has been suggested that the existing incongruency creates abnormal stresses on the humero-ulnar joint causing the osteochondritic region to fragment.
The cubital or elbow joint is a hinge joint made by the articulation of the radial head, humeral capitulum (articulates with the radial head), humeral trochea (articulates with the ulna), and the ulnar articulation surface (coronoid, semi ulnar notch and coronoid). Normal range of motion is from 30°–40° in flexion to 170°–180° in extension. Lack of a synostosis between the radius and ulna allows the radial head to rotate, producing distal pronation and supination. The joint is supported laterally by the lateral collateral ligament, which has cranial and caudal crura. The cranial crura is uniform in shape from the humerus to the radius. The caudal crura fans out distally. The caudal crura contacts the annular ligament and its fibers are mixed.(10) The medial collateral ligament originates from beneath the flexor carpi radialis and continues distally as two crura. The caudal crural is larger and more uniform and extends into the interosseus space and attaches to the caudal aspect of the radius.
Clinical signs usually manifest in the first six months of life. The Labrador, Rottweiler, Burmese Mountain Dog, German Shepherd, Golden Retriever, and Newfoundland breeds have an increased incidence. In one study examining 1,018 Labradors, 17.8% of dogs had radigraphic indices of elbow dysplasia.(11) Rarely do all three problems of the elbow triad occur in the same elbow, but fragmentation of the medial coronoid and osteochondrosis may both be present and/or ununited anconeal process and osteochondrosis may occur together. Dogs with elbow dyspasia usually have a mild to moderate (grade II–III) weight-bearing lameness. There may be palpable articular synovial effusion, pain upon flexion of the elbow, and/or altered stance and gait. Lameness is often worse after manipulation and palpation of the elbow.
Diagnosis of Elbow Dysplasia
Radiography is the mainstay for confirmation of diagnosis and assessment of the cubital joint. Carpenter compared imaging modalities for the diagnosis of elbow dysplasia. His results are summarized: (9,12)
Important Radiographic Findings in Elbow Dysplasia
Incongruency of umero-ulnar joint.
Abnormal contour or lack of visualization of the medial coronoid process.
Periarticular osteophytes associated with medial coronoid process.
Periarticular osteophytes associated with coronoid process.
Calcified body adjacent to medial coronoid process.
Flattened irregular subchondral bone.
Calcific cartilage flap.
Ununited anconeal process.
Overlapping humeral cordyles with true lateral film.
Abnormal humeroradial joint space (crumo cordal view).
This concludes Part I of elbow arthrosis. Part II covers treatment and aftercare for surgical correction or treatment of elbow arthrosis.
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12. Carpenter L G, et al: Comparison of radiologic imaging techniques for diagnosis of fragmented medial coronoid process of the cubital joint of the dog. JAVMA 230: 1: 78-83. 1993.
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