Canine elbow dysplasia- Part 1
The elbow joint is a stable compound ginglymus, or hinge joint, formed by the humerus, the radius and the ulna, all of which must articulate accurately for satisfactory function. The most important movements of the elbow are flexion and extension from approximately 35-160 degrees (Figure 1). The additional movements are pronation and supination, of approximately 60 degrees, as a result of the articulation between radius and ulna (1). Canine elbow dysplasia (CED) is a general term used to identify an inherited disease of the elbow in medium to large breed dogs. Four main conditions make up CED and can occur independently or in conjunction with one another.
These conditions include:
• Ununited anconeal process (UAP): failure of the union between the anconeus and the main part of the ulna beyond 20 weeks of age (2).
• Osteochondritis dissecans (OCD) of the medial humeral condyle: a primary disturbance or failure of endochondral ossification resulting in increased articular cartilage thickness, promoting a separation between the calcified and noncalcified cartilage layers in response to normal joint motion and loading (2).
• Medial coronoid disease (MCD): pathological change in the region of the medial coronoid process, starting as microscopic damage in the subchondral tissue and progressing to cartilage malacia, fibrillation, fissuring, and erosion, in addition to subchondral bone microfissuring and fragmentation (3,4). The latter condition is usually known as Fragmented coronoid process (FCP). Note that OCD and MCD have recently been grouped together under the term medial compartment disease.
• Elbow incongruity (EI): where there is poor alignment of the bones forming the joint so that the joint space is not parallel. Two forms are recognized:
- The first form is caused by unequal growth between the radius and ulna (short ulna/short radius). This condition is basically subdivided into discrete and severe forms: the distinction is made on radiographic examination of the “step” between the two bones, which may be lesser or greater than 2 mm (Figures 2 and 3). The severe form is encountered less commonly than the discrete one.
-The second form is seen where the trochlear notch of the ulna develops an elliptical shape. Both conditions can cause increased local pressure within the joint, resulting in loose fragments at different locations so that UAP, FCP and/or OCD may develop (5).
Clinical evaluation is critical for diagnosis as affected dogs can remain surprisingly mobile or behaviorally active despite severe elbow pathology (3) and the lameness may be quite subtle. Furthermore patients are often bilaterally affected, making recognition of lameness difficult. Physical examination is helpful in identifying the anatomical source of the lameness but clinical signs are not pathognomonic as to which condition is present.
The first indications of CED are usually noticed at 4-5 months of age, although they have been reported in younger animals, and indeed clinical signs associated with secondary osteoarthritis (OA) may become apparent at any age (2,6). The lameness is usually subtle at first, especially if both legs are affected. Gait examination may demonstrate varying degrees of lameness, adduction of the elbow and external rotation of the foot. An early sign may be outward rotation of the paw, with the elbow held close to the body, giving the dog a “duck-footed” appearance (1). The lameness is usually worse following rest or heavy exercise.
Orthopedic examination may reveal forelimb muscle atrophy and swelling of the elbow joint with increased fluid, fibrosis, or bone production. The range of elbow movement may be reduced, with pain and sometimes crepitus on flexion and extension. Evaluation of discomfort caused by elbow manipulation is a strong indicator of CED. In particular the most reproducible discomfort for medial compartment disease (OCD, MCD) can be elicited on firm supination (external rotation) of the antebrachium while the elbow is held in moderate flexion (Figure 4). Response to deep digital pressure in the region of the insertion of the biceps brachii muscle over the medial aspect of the coronoid process is also a valuable indicator of potential disease. Positive responses to these tests, without any other identified source of lameness or pain, creates a high index of suspicion and diagnostic imaging should be directed toward identification of medial compartment disease. If results of non-invasive imaging (radiology and computer tomography) are equivocal, direct observation by arthroscopy or arthrotomy may be justified (3).
For many years radiography has been the standard imaging method for diagnosis, grading, and registry of CED. For accurate and complete radiographic assessment of CED, four projections should be evaluated: flexed mediolateral, standing-angle (also termed neutral or extended) mediolateral, craniocaudal, and cranio-lateral-15°-caudomedial oblique (7,8). Whereas UAP and OCD are typically diagnosed definitively by comprehensive radiographic assessment, the presence and severity of MCD and EI can be difficult to diagnose with certainty using radiography alone (7-12). MCD is often considered an “elimination” diagnosis made when there is radiographic evidence of elbow osteoarthritis without definitive signs of UAP, OCD, trauma, or EI (7,13). Note radiographic detection of the coronoid fragment is uncommon because of the small size and location of the fragment - the highest sensitivity reported for definitive identification of a fragmented coronoid process using radiography is only 62% (14).
•The flexed mediolateral projection avoids superimposition of the medial epicondyle on the olecranon and is particularly useful to identify and diagnose UAP. Normally the anconeal process develops as part of the ulnar diaphysis, but in several breeds, such as the German shepherd dog, it develops as a separate center of ossification. The physis associated with the anconeal ossification center is seen radiographically until 20-22 weeks of age (8). If the physis remains visible beyond this time, the anconeal process is considered ununited (Figure 5). Co-existence of UAP and MCD has been reported with an incidence of 16% (15). In the presence of UAP, it is usually not possible to clearly identify MCD via radiography, therefore advanced diagnostic techniques (computer tomography (CT) or magnetic resonance imaging (MRI)) and/or direct inspection of the medial compartment of the joint via arthrotomy or arthroscopy, are suggested in order to diagnose and treat MCD. The flexed mediolateral projection also allows easier identification of osteophytes along the medial epicondyle and the anconeal process.
• The standing-angle mediolateral projection is useful to identify subchondral sclerosis of the semi-lunar notch of the ulna, proximal radial and proximal anconeal osteophytosis, and an irregular aspect to the medial coronoid process. These signs are usually secondary changes associated with MCD (Figure 6). The normal medial coronoid process is observed radiographically as a sharply marginated triangularshaped area of subchondral bone with its silhouette superimposed over the radial head and the joint surface (Figure 7). The standingangle mediolateral projection is also useful to evaluate elbow congruency (Figures 2 and 3). The main radiographic features of EI are a ‘step’ between radius and ulna, an elliptical shape to the trochlear notch, an increased joint space, and cranial displacement of the humeral head. Despite use of a variety of in vitro and in vivo studies, radiographic determination of EI remains challenging (10-12,16,17). One study on the use of radiography for detection of the severe form of EI (step >2 mm) demonstrated a high sensitivity, regardless of beam angle (16); however, other studies report lower sensitivity because of the complex anatomy of the elbow and limitations associated with identifying a 3-dimensional structure on a 2-dimensional image (8).
•The craniocaudal and craniolateral-15°-caudomedial oblique projections give roughly the same information, but the latter is more sensitive to detect medial compartment disease (14).
Elbow OCD lesions occur almost exclusively on the weight-bearing surface of the medial part of the humeral condyle (7,8,18). OCD is observed as a radiolucency, an irregularity, a flattening, or a defect in the subchondral bone of the articular margins of the humeral condyle (Figure 8). Usually there is associated sclerosis of the subchondral bone surrounding the radiolucency. Occasionally the cartilage flap may ossify and can be seen (8).
‘‘Kissing lesions’’, thought to result from erosive changes in articular cartilage and subchondral bone associated with MCD, also occur on the medial part of the humeral condyle and appear as subchondral sclerosis with or without associated lucency or concavity of the articular margin of the condyle (Figure 9). In some cases, kissing lesions can be distinguished from OCD lesions by the presence of subchondral lucency or sclerosis of the surface of the radius or ulna adjacent to a humeral condylar OCD lesion (9).
In a normal elbow, these views allow the medialcoronoid process to be seen as a distinct, triangular process extending from the proximomedial aspect of the ulna. In patients affected by MCD, radiographic changes involving the process can include an ill-defined margin and/or flattening, rounding, proliferation or distinct fragmentation of the process. Finally, secondary changes due to osteoarthritis may be seen, including osteophytes along the medial ulna, and osteophytes and enthesiophytes along the medial humeral condyle and epicondyle (8).
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This article was previously published in 2011