Mangagement of Small Metacarpal / Metatarsal Bone Fractures: Surgical Approach

Introduction
Fractures of the vestigial Metacarpal and Metatarsal II and IV (the splint bones) are relatively common in horses and may be caused by direct or internal trauma. Although vestigial in nature, the small metacarpal and metatarsal bones are important and integral parts of the supporting and stabilizing structures of the equine limb.

Anatomical considerations
Anatomy textbooks generally focus on the bones and joints, but the soft tissues that stabilize the splint bones are not described in detail. However, detailed knowledge of the soft tissues is not only helpful for the treatment (decision making and surgical anatomy) of splint bone fractures but also to understand the pathogenesis of fractures sustained without external trauma. Jackson et al. (2005) examined the soft tissue structures related to the splint bones of the fore- and hindlimb macro- and microscopically. Unfortunately, their paper is written in German, but there is a summary in English and 13 figures in colour nicely illustrating the anatomy of the relevant soft tissue structures (Fig 1). Their findings were that the connection of the splint bone to the cannon bone established through the metacarpal/metatarsal interosseus ligament is very variable between horses, between limbs and within limbs ranging from osseous union to a proper ligamentous structure. They also identified a ligament-like structure originating from the distal end of the splint bone and inserting in a fan shaped manner to the dorsolateral and dorsomedial aspect of the condyles of distal metacarpus (MC) III and metatarsus (MT) III, respectively. But possibly more relevant in regard to the treatment of splint bone fractures is the palmar metacarpal/plantar metatarsal fascia that runs from the medial to the lateral splint bone embracing the flexor tendons and the suspensory ligament at the palmar and plantar aspect of MC III and MT III, respectively. In the hindlimb the fascia emanates from the tarsal fascia and is particularly strong in the proximal third of the splint bones (2 mm). Thus, forces acting on the MT IV are not only transmitted by the long lateral collateral ligament and the long plantar ligament, but also by the other soft tissue structures described above.