Normograde and Retrograde Pinning of the Distal Fragment in Feline Humeral Fractures

Abstract

To determine if normograde and retrograde pinning of the distal humeral fragment, performed to maximize pin purchase in this fragment, would damage vital structures in and around the elbow and shoulder joints in cats. Anatomic study. Cadaveric cats (n = 12; 24 thoracic limbs). Twelve thoracic limb pairs were harvested from domestic short-haired cats and 1 limb from each pair was allocated to 1 of 3 groups. A transverse osteotomy was created at the junction of the middle and distal thirds in the diaphyseal osteotomy group (n = 8) and proximal to the supracondylar foramen in the metaphyseal osteotomy group (n = 8). Humeri in the normograde group (n = 8) were left intact. Retrograde pinning of the distal fragment in both osteotomy groups was performed with the elbow flexed. Pins were driven into the medial epicondyle until they exited the skin caudal to the elbow and dissection of the soft tissues around the exit tract of the pin was performed. The fracture was reduced and the pin was advanced until it exited the proximal humeral fragment. In the specimens in normograde group, pinning was initiated on the distal aspect of the medial epicondyle. A 1.0 mm guide hole was drilled from the medial epicondyle to the center of the medullary cavity of the distal humeral metaphysis. A bevelled 1.5 mm IM pin was driven proximally through the guide hole until it exited the proximal humerus. Pins exiting the distal aspect of the medial epicondyle passed through muscle origins in 5 specimens in the diaphyseal osteotomy group and in all 8 specimens in the metaphyseal group. The ulnar nerve was entrapped in 1 specimen in both the metaphyseal osteotomy and diaphyseal osteotomy groups. The articular cartilage of the trochlea was damaged in 5 specimens in the diaphyseal osteotomy group and in 1 specimen in the metaphyseal osteotomy group. There was no damage to articular or periarticular structures by pins exiting the proximal humerus. Retrograde pinning of the distal fragment in humeral fractures in the cat may damage the articular cartilage and cannot be recommended. Normograde pinning is safe and maximizes pin purchase in the distal fragment.