How to Ultrasound the Carpal Canal and Caudal Antebrachium

1. Introduction

Lameness associated with carpal sheath effusion results from a variety of underlying factors. Pathologic changes associated with the carpal canal include septic tenosynovitis, superficial digital flexor (SDF) tendinopathies, and desmopathies of the accessory ligament of the superficial (AL-SDFT) and deep digital flexor tendons.^1–8  A recent report identified several cases with intrathecal tears of the accessory ligament of the superficial digital flexor tendon with associated hemorrhage in the carpal sheath.⁹  Although many equine practitioners exhibit proficiency with the ultrasonographic evaluation of the metacarpal structures, the more proximal carpal canal and antebrachium can be a source of frustration and intimidation. Previous reports have described the ultrasonographic anatomy and technique for evaluation of the AL-SDFT.^2,10,11  This paper expands on these studies with a thorough anatomic review. The proposed technique describes the evaluation of each structure individually in transverse and longitudinal planes, in contrast with the more historic zone approach.¹¹  By improving ultrasonographic proficiency and anatomic competency, the practitioner will be better equipped to diagnose various conditions of the equine carpal canal. 

2. Materials and Methods

Anatomic Study

Three forelimbs from horses euthanized for unrelated reasons were obtained for the anatomic portion of this study. Ultrasound examination and magnetic resonance images (MRI^a) of the limbs from the level of mid radius to mid metacarpus were performed postmortem on two of the limbs. Proton density (PD), proton dense fat-saturated, and T1- weighted gradient echo images were obtained in the transverse and sagittal planes. One of these limbs was frozen, and transverse, 2-mm cut sections were made to correspond with previously acquired ultrasound and MRIs. Figure 1 is used as a reference for corresponding anatomic locations of the transverse sections used in the subsequent figures. In a third cadaver limb, used solely to demonstrate the margins of the carpal canal, noniodinated contrast medium (30 mL iohexol^b [350 mgI/mL], diluted to a total volume of 60 mL) was injected into the carpal canal and lateral and dorsolateral to palmaromedial oblique radiographs were obtained (Fig. 2).
 

Ultrasound Study

The caudomedial aspect of the limb is clipped with a No. 40 to No. 50 blade from 4 cm proximal to the chestnut to the mid metacarpus. The limb is washed with warm water and soap, dried, and ultrasound-coupling gel is applied. A high-frequency (8–18 mHz) linear transducer^c is used.

A total of 10 veterinary students and veterinarians were recruited to perform the technique as described on a standing horse. There were a total of seven veterinary students with minimal ultrasound experience and three veterinarians with novice to moderate ultrasound knowledge and skills and with minimal to no experience imaging the carpal canal. The written technique was provided to participants and followed in a step-by-step manner.

Carpal Sheath

The carpal canal contains the superficial and digital flexor tendons. Its proximomedial extent is bordered by the AL-SDFT or superior check ligament.¹² The distal margin is bordered by the accessory ligament of the deep digital flexor tendon (AL-DDFT). The palmar carpal ligament borders the dorsal aspect of the carpal sheath and the palmar and medial margins are delineated by the palmar carpal retinaculum.¹² In Fig. 2, intrathecal contrast medium shows the proximal extent (distal third of the radius) and distal extent of the carpal sheath, which extends to the mid-metacarpal region in most horses. The flexor retinaculum forms a band on the caudal aspect of the carpus, and spans from the accessory carpal bone to the medial collateral ligament and proximal aspect of the second metacarpal bone (Fig. 3).¹³ The tension of the flexor retinaculum results in restriction of the fluid within the carpal sheath at this level, which can be seen in Fig. 2 at the level of the accessory carpal bone. A larger accumulation of contrast is visible proximal to the flexor retinaculum.

Ultrasound examination of the metacarpal structures can identify effusion contained within the carpal canal, which is usually seen between the deep digital flexor tendon (DDFT) and the AL-DDFT. With more severe effusion, it is visible surrounding, or lateral and medial to the SDFT and DDFT. Proximal to the carpus, the largest accumulation of fluid is visible laterally between the ulnaris lateralis and the lateral digital extensor and medially between the flexor carpi ulnaris and flexor carpi radialis.¹⁴ In the authors’ experience, inadvertent introduction of gas can be present in the carpal canal after a low 4-point diagnostic block is performed. Many veterinary students and practitioners may be surprised by the distal extent of the carpal canal. [...]