Introduction

Carpal lesions are a frequent cause of lameness in horses. An accurate diagnosis can help prevent and treat lesions at an early stage, and limit the risk of disease deterioration. 
Conventional radiography and ultrasonography are primarily used for assessing osteoarticular lesions in horses. However, the carpal joint is complex: the numerous bony superimpositions may mask a pathological process, and deep ligamentous or tendinous structures may be poorly identified. Computed tomography (CT) and arthrography are becoming increasingly accessible techniques for completing these osteoarticular assessments, with multiplanar reconstructions providing a more accurate assessment of lesions. The possibility of performing these examinations on a standing horse under certain conditions is an additional argument in favor of these exams. 
In this module, we describe the normal anatomy of the horse's carpus on CT, in the three main planes.

Material and methods

A CT scan of the thoracic limb of a 2-year-old French Trotter was performed by Dr Mickaël Robert, Dipl. ECVS, CHVE de Livet (14140 Livarot-Pays-d'Auge, France).

Images were reconstructed in the three main planes (transverse, sagittal and dorsal) and supplemented by 3D images of the carpal bones. All images were labeled by Stephan Mahler, DVM, MSc, PhD (Veterinary Anatomist - IMAIOS), according to the Nomina Anatomica Veterinaria (NAV). 
Anatomical structures have been grouped into different themes:

• Body parts
• Regions
• Bones
• Joints
• Ligaments
• Muscles
• Tendons
• Fascias
• Tendon sheaths and bursae
• Arteries
• Veins
• Nerves

Results and discussion

In the horse, the ulna is clearly visible in its proximal part, but can no longer be distinguished from the radius in its distal part. As a result, the NAV attaches the two styloid processes to the radius, naming them the medial and lateral styloid processes. However, as in other species, the lateral styloid process develops from the ulna. The term ulnar styloid process is therefore also possible, and both have been used in the module.

The accessory carpal bone has two articular surfaces in carnivores and horses: 

• a main articular surface with the ulnar carpal bone
• a second articular surface, particularly developed in horses, with the styloid process of the ulna. As the styloid process of the ulna usually fuses entirely with the radius in horses, some authors consider that the accessory carpal bone articulates with the radius in this species.

We have taken the view that this second joint belongs to the antebrachiocarpal joint and that the articulation of the accessory carpal bone is limited to the joint between the accessory carpal bone and the ulnar carpal bone: it is therefore equivalent to the pisiform joint in humans (pisotriquetral joint).

The horse used for this examination, as seen in this module, had a contralateral thoracic limb lameness. However, incidental lesions were also found on the apparently healthy limb: the dorsal margins of the radial and carpal III bones are moderately sclerosed, with small areas of lysis, with no visible fragmentation.

A few linear artifacts are visible on the lateral surface of the carpus, adjacent to the distal end of the radius, which can interfere with interpretation, particularly when identifying the lateral collateral ligament of the carpus.