B. Van Ryssen; J. Saunders
Medial coronoid disease (MCD) refers to different types of lesions of the medial coronoid process of the ulna. Until recently the term ‘fragmented coronoid process’ was utilized but with the growing knowledge about this disorder, largely thanks to CT and arthroscopy, this term became insufficient to cover the different abnormalities that can be found at the level of the medial coronoid process. The term ‘medial compartment disease’ has also been used to name medial coronoid lesions, but this may lead to confusion because of the broader anatomic significance including the medial part of the humeral condyle.
Medial coronoid disease is the most important elbow disorder in large breed dogs and is part of the elbow dysplasia complex together with osteochondritis dissecans of the humeral condyle (OCD) and an ununited anconeal process (UAP). Initially, MCD was considered as a developmental problem affecting young large breed dogs. In the last decade, it has also been reported in adult and old dogs. In those cases, imaging and arthroscopic findings are similar with the lesions seen in young dogs, but the severity and the distribution of the different types of lesions is different. In older dogs two groups of lesions can be diagnosed: the chronic lesions with clear clinical, radiographic and arthroscopic pathology and the more recent lesions with discrete changes.
Medial coronoid disease has also been reported as a rare cause of front leg lameness in small breed dogs. Often small breeds are affected at an adult age. The lesions are similar with those of large breed dogs, but the diagnosis may be challenging because of the small size of the joints. Besides MCD several other disorders should be included in the differential diagnosis of elbow pain. Elbow incongruity is characterized by a short radius, a short ulna or an abnormal shape of the trochlear notch. It is considered as one of the causes of elbow dysplasia and is therefore often seen in combination with the disorders belonging to the elbow dysplasia complex. Osteochondritis dissecans of the medial aspect of the humeral condyle is characterized by a cartilage flap or defect and is often combined with medial coronoid disease. Lesions may be discrete and radiographic diagnosis may be missed in case of small lesions or improper positioning of the joint. An ununited anconeal process can easily be diagnosed because of the obvious fragmentation of the anconeal process. Since MCD may occur simultaneously with an ununited anconeal process, the joint should be examined carefully. Medial compartment erosion can be seen in a varying severity and is diagnosed under different circumstances: the erosions may be present in chronic cases of MCD in joints with a large coronoid fragment (kissing lesions), after surgical treatment of medial coronoid lesions or without any evident underlying cause. In the last case, the lesions should be considered as a primary form of medial compartment erosion, representing a differential diagnosis for MCD because of similar radiographic findings. Flexor enthesopathy is a recently recognized cause of elbow pain originating from the flexor muscles and their attachment to the medial humeral epicondyle. Radiographic findings vary form discrete sclerosis and spur formation at the medial epicondyle to obvious calcified bodies within the flexor muscles. The main challenge is to differentiate primary flexor enthesopathy as the sole lesion from the concomitant form combining flexor pathology and MCD. A rare disorder is an incomplete ossification of the humeral condyle (IOHC): besides being the cause of complex elbow fractures induced by a limited trauma, this developmental disorder may cause elbow pain in young and young adult dogs. The intercondylar defect is most clearly seen on a craniocaudal projection on the condition that the radiographic beam follows the direction of the fissure.
A diagnosis of MCD is suspected based on the history (mostly young, large breed dogs, non-traumatic cause of insidious or acute lameness), and the clinical examination demonstrating muscle atrophy, distension, a decreased range of motion and pain. The clinical findings may be obvious but also discrete or even absent. In most cases diagnosis can be confirmed by the typical radiographic findings: an unclear delineation or abnormal shape the medial coronoid process and in some cases a bony fragment cranial to the affected medial coronoid process. Concomitant findings are subtrochlear bone sclerosis, incongruity and secondary osteoarthritis. Radiographic diagnosis of MCD may be challenging because of unclear or discrete pathology, particularly in the absence of clear clinical findings. In those obscure cases of MCD, scintigraphy may be used to localize the problem.
Computed tomography is the preferred non-invasive tool for further diagnostic workup of the elbow following inconclusive radiographs. Findings of the medial coronoid process vary from one or more loose fragments, a nondisplaced fragment or a discrete fissure. Early secondary osteoarthritis, joint incongruity and subtrochlear bone sclerosis can be accurately determined. Other elbow disorders, such as OCD, IOHC and flexor enthesopathy can be demonstrated or excluded.
Arthroscopy allows the direct inspection of the joint surfaces and remains the golden standard for the evaluation of cartilage lesions. However, some lesions may be missed: subchondral bone cracks may not show as a cartilage lesion and large displaced fragments may not be visible. The combination of CT and arthroscopy increases the diagnostic accuracy in many cases.
Because of the small size of the joint, ultrasonography (limited acoustic window) and MRI (limited bone detail and lack of distinction between the cartilage of the humerus, ulna and radius) are less useful in the diagnosis of MCD.
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