Abnormal development of the elbow joint. Three bones of the joint – humerus, radius and ulna do not fit together properly, which leads to abnormal high contact pressure and the elbow does not work as effectively as it should. Secondary osteoarthritis can follow, and the elbow will become swollen and painful. Elbow Dysplasia is primarily genetic, however, environmental factors such as obesity as a puppy, breed predisposition, over exercising a young dog/excessive use of stairs, injury or trauma to the joint when the dog is still young.