Written by Suzan Oakley | DVM, Dipl ACVSMR, Dipl ABVP (Equine), Certified Member, ISELP
Wellington Equine Sports Medicine

Effusion in the carpal sheath is an indication for ultrasound examination of the carpal canal. Septic tenosynovitis, superficial digital flexor (SDF) and deep digital flexor (DDF) tendinopathies, desmopathy of the accessory ligament of the superficial digital flexor tendon (AL-SDFT), and distal radial osteochondromas can easily be diagnosed with ultrasound. Ultrasound of this area is not done frequently, but understanding the anatomy makes this a very straightforward procedure. This article will illustrate the anatomy, ultrasound examination technique, and normal ultrasound appearance of the structure in the carpal canal. Note that medical and proximal are to the left in all ultrasound images.

Anatomy of the Carpal Canal

The carpal canal is formed by the proximal aspect of the ALSDFT medially, the accessory carpal bone and associated ligaments laterally, the palmar ligament dorsally, and the palmar carpal retinaculum and palmar carpal fascia on the palmar surface. The carpal sheath is contained within the carpal canal and extends distally to the AL-DDFT in the proximal metacarpus. Structures within the carpal sheath include the SDF, AL-SDFT, DDFT, flexor carpi radialis (FCR), and the median artery and nerve. The cephalic vein, medial vein, and distal radial vessels are superficial to the palmar carpal retinaculum.

Superficial Digital Flexor Tendon and Muscle (SDF)

The SDF muscle originates on the medial epicondyle of the humerus, the olecranon, and the caudal surface of the radius. The musculotendinous junction is near the level of the chestnut. Distal to the accessory carpal bone, the SDF becomes palmar in orientation and continues to its insertions on the medial and lateral aspect of P2. When examining the SDF in the proximal metacarpal area it is important to continue proximally to evaluate the portion in the carpal canal.

Accessory Ligament of the Superficial Digital Flexor Tendon (AL-SDFT)

The AL-SDFT (also known as the superior check ligament) originates from the caudomedial aspect of the radius near the level of the chestnut. It is trapezoidal in shape and thins before it fuses with the SDF tendon. It travels in a proximodorsal to distopalmar direction.

Deep Digital Flexor Tendon (DDF)

The DDF muscle originates on the medial epicondyle of the humerus, the olecranon, and the caudal surface of the radius. The radial head of the DDF muscle is deep, just caudal to the radius. It is more echogenic than the adjacent muscle and has fewer variations in echogenicity. The ulnar and humeral muscle bellies of the DDF are superficial to the radial head and are more lateral.

Flexor Carpi Radials (FCR)

The FCR originates on the medial epicondyle of the humerus and inserts on the second metacarpal bone. The muscle belly is in the antebrachium, and its tendon passes just medial to the carpal sheath. It is rarely injured but is a useful landmark when imaging the area with ultrasound.

Median Artery, Vein, and Nerve

The median artery and nerve are within the carpal canal. The median artery is easily visible on ultrasound due to its thick wall. The median vein is superficial to the carpal retinaculum.

Ultrasound Technique

Image quality is proportional to patient preparation. The best images will be obtained when the horse is clipped (if the hair is long) and washed with soap before scanning. A high-frequency ultrasound transducer (7 to 15 MHz) at 4-6 cm depth is appropriate for the more superficial structures in most horses, but a convex probe of lower frequency will allow a more complete evaluation of the distal radius and deeper structures.

The SDFT, AL-DDFT, and DDFT should be systematically evaluated in both the long and short axis and the caudal aspect of the radius. It is important to remember the orientation of the ligaments when scanning and align your probe parallel or perpendicular to the long axis of the structure you are examining. (The AL-DDFT runs at an oblique angle to the ground).

The starting point of the examination is often on the medial side of the limb at the level of the chestnut. For ultrasonographers who do not regularly examine this area, it may be easier to orient by starting on the palmar aspect of the limb in the proximal metacarpus and following the SDF proximally.

Begin in the proximal metacarpus and follow the SDF proximally, moving medially around the accessory carpal bone. Continue proximally until the insertion of the AL-DDFT is seen separating from the SDFT. To follow the AL-DDFT to its origin, you will have to angle the probe perpendicular (or parallel) to the ligament since it is not parallel to the SDF and DDF. There are often linear artifacts in the short axis view from the median artery. The FCR is a good landmark to track your progress. As you progress up the leg, the AL-SDFT will move on the image from its position deep to the median artery to be deep to the FCR just before its origin on the radius. All structures should be examined on the short and long axis. Normal reference images are shown in figure 3.

Ultrasound Tips

• The level of the musculotendinous junction in the SDF can vary slightly from horse to horse. Be careful not to confuse the junction with a lesion in the SDF.
• In a horse with no swelling, it is not always possible to image the origin of the SCL with a tendon probe due to lack of contact with the probe.
• When imaging the superior check ligament (AL-SDFT) in the short axis, be sure to have your probe perpendicular to the ligament, not the ground.
• To examine the caudal radius, it is helpful to flex the limb and use a convex probe.

References

1. Carstens, A. Ultrasonography of the Carpus. In: Kidd, Lu and Frazer, eds. Atlas of Equine Ultrasonography. Sussex, UK: Wiley and Sons, Ltd., 2014; 110-112.
2. Cauvin ERJ, Munroe GA, Boswell J, et al. Gross and ultrasonographic anatomy of the carpal flexor tendon sheath in horses. Vet Rec 1997;141:489–495.
3. Denoix JM, Essentials of the Equine Locomotor System. Boca Raton, FL, USA: CRC Press, 2019;40-47.
4. Denoix JM, Busoni V. Ultrasonographic anatomy of the accessory ligament of the superficial digital flexor tendon in horses. Equine Vet J 1999;31:186–191.
5. Jorgensen JS, Stewart AA, et al. Ultrasonographic examination of the caudal structures of the distal antebrachium in the horse. Equine Vet Educ 2010;22:146–155.
6. Probst A, Macher R, Hinterhofer C, et al. Anatomical features of the carpal flexor retinaculum of the horse. Anat Histol Embryol 2008;37:415–417.
7. Redding WR. Carpal Sheath. In: Stashak TS, ed. Adams and Stashaks’ Lameness in Horses. 6th ed. Sussex, UK: Blackwell Publishing, 2011;545–546.
8. Smith LJ, Mair TS. Rupture of the superficial flexor tendon in the forelimb in aged horses: A report of nine cases. Equine Vet Educ 2007;19:183–186.
9. Vaughan B, Whitcomb MB, Galuppo L, et al. Spontaneous rupture of the proximal superficial digital flexor tendon: A clinical syndrome in aged equids, in Proceedings. Am Assoc Equine Pract 2014;60:257.
10. Wright IM, Minshall GJ. Clinical, radiological and ultrasonographic features, treatment and outcome in 22 horses with caudal distal radial osteochondromata. Equine Vet J 2012;44:319–324.

This post was written by Jillian Sinclair.