Elbow dysplasia is a collective term for the developmental orthopedic diseases of the canine elbow. Traditionally, it refers
to ununited anconeal process (UAP), osteochondrosis of the humeral condyle (OCD), and fragmentation of the medial coronoid
process (FCP). Recently, some authors have included elbow joint incongruity and diseases such as ununited medial epicondyle
in this group as well. While all of these diseases are grouped under the same syndrome, and multiple may be diagnosed simultaneously
in a single joint, it is likely that the etiopathogenesis of each is distinct from the other. It is our lack of knowledge
of the precise causes of developmental elbow diseases that allow them to be grouped as one syndrome.
The inciting events for the development of elbow dysplasia are unclear. General agreement exists that some form of joint incongruity
or instability is a predisposing cause. The most popular incongruity theories relate to "ulnar notch/humeral condylar incongruity"
and "radioulnar incongruity". Ulnar notch incongruity has been subjectively visualized on clinical radiographs, and theorized
to concentrate forces at the anconeal process and coronoid process of the ulna, thus potentially predisposing to both UAP
and FCP. Breed variability of the radiographic shape of the ulnar notch has been described, and some degree of physiologic
incongruity has been demonstrated to be normal. It is unclear how much of a role ulnar notch incongruity plays in elbow dysplasia.
Radioulnar incongruity is characterized by either a relatively short radius or short ulna. A short radius has been experimentally
shown to cause force concentration on the medial coronoid process, which may lead to FCP. In contrast, a shortened ulna could
cause increased pressure on the anconeal process, resulting in UAP. Both a shortened radius and shortened ulna are seen clinically,
and may lead to FCP or UAP respectively. However, neither appears to be necessary for development of FCP and UAP. Further, neither defect would be sufficient to explain the development of both FCP and UAP in the same joint.
In an attempt to overcome the limitations of the linear model of radioulnar incongruity, a three-dimensional model of incongruity
recently has been proposed. The "angular vector model" describes potentially complex three-dimensional adjustments that may
occur in the growing animal as a result of either a relatively short radius or short ulna. While it currently lacks objective
supporting data, this theory is appealing, as it attempts to move beyond a two-dimensional model for elbow dysplasia. A three-dimensional
understanding of the joint is also an important concept for evaluating the effectiveness of corrective osteotomies in addressing
elbow joint incongruity.