The major concern of any fracture repair is to maintain adequate stability for fracture healing to occur. The stability provided by open reduction and internal fixation (ORIF) is difficult to achieve with other methods of fracture repair. However, ORIF invades the fracture site, can lead to further disruption of vasculature and soft tissue and may provide a mechanism for infection to develop or persist. Alternative mechanisms such as external coaptation (primarily casts) and external skeletal fixation (ESF) to provide stability without invasion of the fracture site should be considered. In some instances, internal fixation can be combined with external coaptation or ESF to provide adequate stability with minimal implants at the fracture site.

Transfixation casts, a form of external fixation, are not new and reports in the literature date back to the 1950's.¹ However, improvements in casting materials, transfixation pins, and a greater understanding of the mechanics of the transfixation casts have improved the stability provided by transfixation casts and have reduced the risk of complications.

The transfixation cast effectively transfers the forces of weightbearing from the bony column to the fiberglass cast.² The transfixation cast utilizes pins placed through bones in the limb proximal to the fracture and the cast suspends the limb distal to the pins. The transfixation cast can be used as the sole method for support or in combination with methods of internal fixation such as lag screws.

The transfixation cast is technically easy to construct and consists of readily available components. Equipment required to construct a transfixation cast is limited and an extensive inventory of hardware is not required. Proper pin placement is the most important step in constructing a transfixation cast that will provide long term weightbearing. I prefer to use IMEX® Large Animal centrally threaded transfixation pins^a . This ¼ inch positive profile transfixation pin has a tap to thread the bone following predrilling with a 6.2 mm bit. The three step pin placement method reduces thermal necrosis of bone that can occur with self tapping transfixation pins, and prolongs the stability of the bone-pin interface.³ Pins should be placed as far from the top of the cast as possible so the cast does not act as a lever at the bone-pin interface. This decreases the likelihood of fracture through the bone-pin interface. For most adult horses, 2 or 3 transfixation pins are adequate. I often use 2 pins; however I have seen horses with 3 pins wear the cast well for 6 weeks. For adult horses, I use the ¼ inch pin described above; the 3/16 inch centrally threaded transfixation pin can adequately support foals. Pins should be placed through releasing incisions of adequate length so that the skin will lie smoothly around the pin. During drilling and tapping, the bit and tap should be irrigated with saline to lubricate and dissipate the heat. Placing pins divergent from the frontal plane appears to decrease the likelihood of secondary fracture through the bone-pin interface.⁴ It is important to insure that you do not impinge on the caudal cortex as this will significantly weaken the bone.

For phalangeal injuries, a short limb cast to the proximal third metacarpal or metatarsal (MC/MT) bone with pins through the distal MC/MT is adequate. When the transfixation pins need to be placed through the mid or proximal MC/MT or distal radius, a full limb cast is indicated. I place the cast as I would for any normal full or short limb cast. I use a double layer of stockinette, felt at the top and heel-bulb region, and a thin layer of cast padding. An assistant is needed to cut the casting tape, parallel to the roll, as you roll over the pins. Cutting the material allows it to lie smoothly against the limb. When an adequate amount of fiberglass is applied, cut the pins nearly flush with the cast. The ends of the pins can be covered with either another roll of fiberglass cast material or with acrylic (Techovit)^b.