The elbow joint in dogs is formed by three bones: the radius, ulna and the humerus. These bones rotate around a cylinder of bone attached to the humerus called the humeral condyle giving a “hinge” type of arrangement.
The cylinder of the humeral condyle develops from two separate parts which “fuse” usually between 8 and 12 weeks of age. This process is called ossification. However, in some pups this process fails to take place leaving an area of cartilage in the middle of the condyle which weakens this region significantly (so called incomplete ossification of the humeral condyle or IOHC).
However, recent work suggests that in some dogs, a stress fracture occurs through previously solid bone of the condyle. This situation is more common in mature dogs over 18 months old. Spaniel breeds, especially Springers, are particularly prone to IOHC/fissure fracture and the condition is likely to have a strong hereditary basis.