Proximal sesamoid bones: Abaxial fractures

Abaxial fractures of the proximal sesamoid bones are seen only in race horses. Because only a part of the suspensory branch inserts on the fragment, they are usually not displaced.

Dorsopalmar radiographic projection depicting an abaxial fracture of the medial proximal sesamoid bone.

Introduction

Fractures of the proximal sesamoid bone occur primarily in racing breeds.
Occurrence of the fracture is usually associated with fast strenuous exercise, but has been reported in horses on paddock exercise and in young foals running on large pastures.

Fractures of the proximal sesamoid bone are more commonly diagnosed in the forelimb and the medial proximal sesamoid bone is the most frequently affected structure.

Clinical signs

Typically the patient is presented with an acute non-weight-bearing lameness and minimal soft-tissue swelling associated with or without joint effusion.

Overview of fracture types

The following fracture types are diagnosed:

I. Apex fractures
II. Midbody fractures
III. Base fractures
IV. Abaxial fractures
V. Axial fractures
VI. Comminuted fractures

In this chapter the management of the different fracture types will be discussed in sequence as they are listed here, which corresponds with ascending severity and descending prognosis as listed in the human fracture classification. Axial fractures almost exclusively occur in conjunction with break-down injuries, or a displaced condylar fracture, which will be discussed under Metacarpo-/metatarsophalangeal arthrodesis.

Example of an abaxial fracture

Example of an axial fracture combined with a lateral condylar fracture

Imaging

Standard radiographic views of the metacarpo/metatarsophalangeal (fetlock) region including dorsopalmar/plantar, lateromedial and two oblique views commonly reveal the fracture on more than one view. In selected cases additional radiographic views may be needed to confirm the diagnosis.

Scintigraphy, computed tomography (CT) (see image), and magnetic resonance (MR) imaging are valuable aids for the diagnosis, especially in the assessment of associated soft tissues. CT and MRI are especially useful to assess articular fractures and to identify displacement of the fragments.