ultrasonography of the dorsal and lateral aspects of the equine carpus: technique and normal...

ULTRASONOGRAPHY OF THE DORSAL AND LATERAL ASPECTS OF THE EQUINE CARPUS: TECHNIQUE AND NORMAL APPEARANCE

MOHAMED TNIBAR, DMV, BARBARA KASER-HOTZ, DR. MED. VET., JOERG A. AUER, DR. MED. VET., MS

The normal ultrasonographic appearance of the dorsal and lateral soft tissue structures and anatomic landmarks of the equine carpus useful in clinical imaging are described.

Both limbs of 5 cadavers and 5 clinically sound adult horses were imaged using a 7.5 MHz sector transducer. At the dorsal aspect of the carpus and distal radius, the extensor carpi radialis- and the common digital extensor tendon and their tendon sheaths were easily identified. These two tendons are the most prominent structures in this region. Smaller and more difficult to identify are the tendon and tendon sheath of the extensor carpi obliquus-, the lateral digital extensor- and the ulnaris lateralis muscle. The ultrasonographic appearance and course through the carpal region of the tendons and tendon sheaths mentioned are described. Other soft tissue structures examined include the lateral collateral ligament, the carpal joint capsule and the distal articular cartilage of the radius. Ultrasono- graphic findings correlated well with gross anatomy in the cadavers limbs. Veterinary Radiology & Ultrasound, Vol. 34, No. 6, 1993, p p 413425.

Key words: ultrasonography, horse, carpus, dorsal and lateral aspects.

Introduction

ANY HORSES are presented with soft tissue swelling or M injuries in the carpal region. There is little informa- tion in the literature concerning diagnostic methods to evaluate the soft tissue structures of the equine carpus. While conventional radiography is reliable to detect bone injury, it is much less helpful for soft tissue examination. Negative and positive contrast techniques provide improved visualization of the tendon margins, tendons sheaths and carpal joints. These techniques, however, are invasive. Over the past few years, diagnostic ultrasound has provided the clinician with an accurate, non invasive and reliable means of evaluating, equine musculoskeletal problems.P-' Ultraso- nography appears to be ideal to evaluate the soft tissue structures surrounding articular regions, e.g., the equine carpus.

Therefore, the purposes of this paper of this paper are to describe the normal ultrasonographic appearance of the dorsal and lateral aspects of the equine carpus and to define anatomic landmarks useful in clinical imaging.

From the Veterinar-Chirurgische (Tnibar, Auer) und Veterinar- Medizinische (Kaser-Hotz) Klinik der Universitat Zurich, Winterthurerstr. 260, CH-8057, Zurich, Switzerland.

Address reprint requests and correspondence to B. Kaser-Hotz, Veter- in&-Medizinische Klinik der Universitat Zurich, Winterthurerstrasse 260, CH-8057 Zurich, Switzerland.

Received October 19, 1992; accepted for publication December 15, 1992.

Anatomic Review (Fig. 1)

Equine carpal anatomy has been described in detail.12 The extensor carpi radialis tendon is the largest structure on the dorsal aspect of the carpus. It inserts at the metacarpal tuberosity and is covered by a synovial sheath. The common digital extensor tendon is smaller and located lateral to the extensor carpi radialis tendon. There is a branch of the common digital extensor tendon, which may join the lateral digital extensor tendon or may pursue a separate course to the fetlock. A common synovial sheath surrounds both, the common digital extensor tendon and its branch. The abduc- tor digitus longus, also named the extensor carpi obliquus, muscle curves from proximolateral to mediodistal over the distal part of the radius. Its tendon runs in the oblique groove of the distal radius and attaches to the proximal aspect of the second metacarpal bone. The tendon is provided with a synovial sheath which covers it dorsally between the extensor carpi radialis tendon and the medial collateral ligament and palmarly along most of its length.I3 All these tendons are covered by extensor retinaculum.

On the lateral carpal aspect, the lateral collateral ligament extends distally from the lateral (ulnar) styloid process to the fourth and third metacarpal bones. Between its long and superficial, and short and deep, parts a canal provides pas- sage to the lateral digital extensor tendon, the smallest of the extensor tendons. In the carpal region this is surrounded by a synovial sheath. Distal to the carpus, the tendon runs dorsally and accepts a strong band (ALB) from the acces-

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FIG. 1. Dorsal (A) and lateral (B) views of the equine carpus, the zones used in the sonographic examination are superimposed. ECR, Extensor> carpi radialis; CDE, Common digital extensor; LDE, Lateral digital extensor; ECO, Extensor carpi obliquus; LCL, Carpal lateral collateral ligament; MCL, Carpal medial collateral ligament; TP, Branch of the common digital extensor tendon; UL, Ulnaris lateralis; L, Long tendon of the ulnaris lateralis; S, Short tendon of the ulnaris lateralis; ALB, Accessorio- Lateral digital extensor band; DDF, Deep digital flexor muscle; R, Radius; Ca, Accessory carpal bone; LSP, Lateral styloid process; MSP, Medial styloid process; McIII, Third metacarpal bone; McIV, Fourth metacarpal bone.

sory carpal bone, and occasionally the tendinous branch of the common digital extensor tendon. Distally, the ulnaris lateralis (UL) muscle has 2 tendons of insertion. The short one (S) is associated with the flexor carpi ulnaris tendon and attaches to the accessory carpal bone. The long tendon arises in the proximal carpal region and runs distodorsally through a groove on the accessory carpal bone to insert on the proximal part of the fourth metacarpal bone. A synovial sheath covers the long tendon throughout its course. Prox- imally, the palmarolateral pouch of the antebrachiocarpal joint is interposed between the long tendon and the ulnar styloid process.

Materials and Methods Both forelimbs of five equine cadavers and both fore-

limbs of five live adult horses were used to study the normal ultrasonographic anatomy of the dorsal and lateral carpal regions. The horses were of various ages and breeds (6 Warmbloods, 3 Thoroughbreds, one Drafthorse, 3- 18 years old). All carpi were normal based on the history, physical and radiographic findings. The cadaver limbs were disar- ticulated at the shoulder to maintain soft tissues tautness around the carpus.

Ultrasonography was performed with a real-time B-mode portable sector scanner using a 7.5 MHz transducer with a

built-in stand-off pad.* The carpal area was clipped and shaved prior to the examination and an ultrasonographic coupling gel was applied. t The tendons and tendon sheaths of the extensor carpi radialis, common digital extensor, extensor carpi obliquus, lateral digital extensor, and the tendons of the ulnaris lateralis were studied. The lateral collateral ligament and the distal articular cartilage of the radius were also examined. To facilitate interpretation of the images, a zoning system was developed (Fig. 1). The medial (radial) styloid process was used as the reference point as it can be palpated easily. Zones A and B are located proximally, and zones C l , C2, and D distally to the medial (radial) styloid process. Each zone is approximately 5 cm long. To document the examination level, a centimeter ruler was attached to the medial aspect of the carpus with the O-cm mark located 10 cm proximal to the medial (radial) styloid process, the proximal most aspect of zone A.

Examination of each structure included both sagittal and transverse scans, which were recorded on a video tape.$ The diameters of the various structures were measured at one centimeter intervals and the shape and appearance of these structures were described. Hard copies of all images were archived using a thermal paper printer.$

After the ultrasonographic examination, the limbs from cadavers were dissected to provide gross anatomic compar- ison with the ultrasound images. Two cadavers limbs were frozen and sliced in the carpal area, one transversally, the other longitudinally. One-centimeter thick slices were obtained, photographed and correlated with the sonographic findings.

On two additional cadavers limbs, the antebrachiocarpal joint, the middle carpal joint and the tendon sheaths of the extensor carpi radialis and common digital extensor were injected with 20 to 40 ml of water to further determine sonographically the boundaries of these structures.

Results

The tendons of the extensor carpi radialis, common digital extensor, extensor carpi obliquus, lateral digital extensor, the tendons of the ulnaris lateralis, the lateral collateral ligament and the distal articular cartilage of the radius were imaged and compared with gross anatomy.

Extensor Carpi Radialis Tendon

Transverse Images

In the proximal part of zone A, the tendon is visualized as an oval-to-elliptical shaped echogenic structure with a pal-

*Ausonics Micro Imager 2000, Ausonics AG, 1700, CH-Freiburg. ?Aquasonic 100, Polymed GmbH, 8152, CH-Glattbmgg. $Panasonic, Lay John Electronics AG, 6014 CH-Littau. OSony UP-811, Sony (Schweiz) AG, 8952 CH-Schlieren.

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FIG. 2. Anatomic cross section (A) and transverse image (B) with corresponding diagram (C) of the extensor carpi radialis tendon (ECR) at the level of the distal aspect of zone B. The transducer (TR) is positioned transversely over the tendon. ECO, Extensor carpi obliquus tendon; R, Radius.

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FIG. 3. Anatomic cross section (A) and transverse image (B) with corresponding diagram (C) of the extensor carpi radialis tendon (ECR) at the level of the distal aspect of zone C1. The transducer (TR) is placed transversally over the tendon. JC, Carpal joint capsule; F, Fat pad; FT, Fibrous tissue; TS, Tendon sheath; ER, Extensor retinaculum; Cr, Radial carpal bone; Ci, Intermediate carpal bone.

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FIG. 4. Transverse image (A) and corresponding diagram (B) of the extensor carpi radialis tendon (ECR) at the identical level as Fig. 3. The extensor carpi radialis tendon sheath (TS) and the antebrachiocarpal joint space (JS) were injected with water to visualize the carpal joint capsule (JC) with its fibrous tissue (FT) and fat pads (F). Cr, Radial carpal bone; Ci, Intermediate carpal bone; RE, Extensor retinaculum.

mar hypoechoic part representing the remaining part of the muscle. The tendon is in close proximity to the radius and has a dorsopalmar thickness of 6-7 mm and width of 16-19 mm. In the distal part of zone A, the tendon becomes ho- mogeneously echogenic and shape flattens and widens. Lat- erally beneath the tendon, the hypoechoic extensor carpi

obliquus muscle can be imaged. In zone B, the tendon flattens further, is 4-6 mm thick and 19-21 mm wide. Its downward course is interrupted dorsally by the isoechoic extensor carpi obliquus tendon (Fig. 2). The extensor carpi radialis tendon then passes through the middle groove of the distal radius extremity.

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FIG. 5 . Anatomic cross section (A) and transverse image (B) with corresponding diagram (C) of the extensor carpi radialis tendon (ECR) in zone C2 The transducer (TR) is placed transversally to the tendon. ER, Extensor retinaculum; C3, Third carpal bone.

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As it proceeds through zone C 1, the tendon is elliptical in shape and crosses the antebranchiocarpal joint. The joint capsule of the antebrachiocarpal joint is echogenic. Within this joint capsule, there are two relatively less echogenic fat pads (Fig. 3). When water was injected into the antebrachiocarpal and middle carpal joints and the extensor carpi radialis tendon sheath, these fat areas contained in the joint capsule could be localized. One was associated with the antebrachiocarpal joint and the other with the tendon sheath of the extensor carpi radialis (Fig. 4). The tendon continues its course over the junctional area between the radial carpal bone and intermediate carpal bone. At the end of zone C1, the tendon widens to 23 to 26 mm. In zone C2, the tendon flattens slightly and has an asymmetric elliptic cross section, with the larger portion situated laterally. It runs over the dorsal aspect of the third carpal bone (Fig. 5) and continues to widen to reach a width of 28 to 32 mm at the level

of the carpometacarpal joint. Subsequently the tendon acquires a crescent shape and expands before it inserts on the metacarpal tuberosity. The tendon sheath was seen in all horses as a small anechoic space surrounding the tendon. Injection of water into the sheath of two cadavers limbs demonstrated the sheath beginning at the proximal part of zone A and terminating dorsally at the level of the antebrachiocarpal joint and palmarly at the level of the middle carpal joint. The extensor retinaculum was also imaged over zones C1 and C2 as an echogenic line dorsal to the tendons (Figs. 3-5).

Sagittal Images

The tendon has a parallel linear ultrasonographic appearance (Fig. 6). Because we used a sector scanner, the middle part of the scanned portion of the tendon appeared more echogenic than the proximal and distal parts, as the ultra-

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FIG. 6. Anatomic longitudinal section (A) and longitudinal image (B) with corresponding diagram (C) of the extensor carpi radialis tendon (ECR). The transducer (TR) is placed parallel to the tendon over zones C1 and C2. TS, Tendon sheath; JC, Carpal joint capsule; F, Fat pad; ET, Fibrous tissue; JS, Joint space; Cr, Radial carpal bone; C3, Third carpal bone; R, Radius; ER, Extensor retinaculum.

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FIG. 7 . Longitudinal image (A) and corresponding diagram (B) of the extensor carpi radialis tendon (ECR) over zones C1 and C2. The extensor carpi radialis sheath (TS) and the antebrachiocarpal joint space (JS) were injected with water. Note the dorsal and palmar extensions of the tendon sheath, the joint capsule (JC) with its fibrous tissue (FT) and the fat pads (F). Ci, Intermediate carpal bone; C3, Third carpal bone; R, Radius.

sound beam is only perpendicular to the tendon fibers in the center area. Over zone C1 and the proximal part of zone C2, the proximal row of carpal bones can be identified. Medi- ally the radial carpal bone surface, as it appears in figure 6 , and laterally the intermediate carpal bone surface were imaged. Between these two bones an echogenic interface rep-

resents the radiointermediate ligament. The dorsal surface of the third carpal bone was imaged at the level of the distal row of carpal bones. With water injected into the extensor carpi radialis tendon sheath and the antebrachiocarpal joint, the boundaries and the extension of the extensor carpi radialis tendon sheath were demonstrated (Fig. 7). A com-

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FIG. 8. Anatomic cross section (A) and transverse image (B) with corresponding diagram (C) of the common digital extensor tendon (CDE) at the distal aspect of zone B. The transducer (TR) is positioned transversally to the tendon. TS, Tendon sheath; ER, Extensor retinaculum; R, Radius.

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plete sagittal scan requires the transducer to be moved from side to side to visualize the medial and lateral aspects of the tendon.

Common Digital Extensor Tendon

Transverse Scans At the top of zone A, the common digital extensor tendon

arises from the muscle and is visualized first as a triangular and then as an oval-shaped echogenic structure. The tendon is superimposed on a hypoechoic structure which represents the extensor carpi obliquus muscle. As the tendon ap- proaches zone B, the extensor carpi obliquus muscle is no longer visible in this plane. In both zones, the tendon is 4-5 mm thick and 1 6 1 8 mm wide. In zone B, the tendon flattens slightly, is located directly cranial to the radius and passes through a groove on the distal end of the radius (Fig. 8). As the tendon enters zone C1, it narrows slightly and its cross section acquires first a concave shape dorsally, then becomes elliptical. The tendon runs mainly over the intermediate carpal bone and partially over the ulnar carpal bone (Fig. 9). In zone C2, the tendon accepts a more flattened cross section, appears fusiform in shape, and courses over the interface between the third carpal bone and the fourth

A

carpal bone (Fig. lo). In zone D, the tendon gradually becomes elliptical in shape, inclines medially over the third metacarpal bone, and appears less echogenic. It is 3 to 4 mm thick and 12 to 14 mm wide. In zone C2, the delicate branch of the common digital extensor tendon was identified in 11 limbs as an echogenic, round, small structure accompanying the common digital extensor tendon laterally. Distally, it emerges laterally from the common digital extensor tendon, becomes flatter and is visible in zone D as an echogenic band which fused with the lateral digital extensor tendon in 17 limbs. The synovial sheath enveloping the two tendons was identified as an anechoic line in all horses examined.

Sagittal Images

Palpation of the common digital extensor tendon as it passes through its groove on the distal end of the radius facilitates longitudinal scanning. The common digital extensor tendon has the same parallel alignment as the extensor carpi radialis tendon. The tendon sheath, the carpal joint capsule and the carpal joints could also be assessed (Fig. 11). On cadaver limbs, where the common digital extensor tendon sheath was injected with water, the sheath could be

Dors Med @ Lot

Palm

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FIG. 9. Anatomic cross section (A) and transverse image (B) with corresponding diagram (C) of the common digital extensor tendon (CDE) at the level of the distal aspect of zone Cl. The transducer (TR) is placed transversally to the tendon. TS, Tendon sheath; ER, Extensor retinaculum; Ci, Intermediate carpal bone; Cu, Ulnar carpal bone.

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FIG. 10. Anatomic cross section (A) and transverse image (B) with corresponding diagram (C) of the common digital extensor tendon (CDE) at the level of zone C2. JC, Carpal joint capsule; ER, Extensor retinaculum; C3, Third carpal bone; C4, Fourth carpal bone.

seen as a fine anechoic line which begins in the middle of zone A. Distally, the sheath expanded to a wide anechoic space, then tapered down at the proximal end of the metacarpus in zone D (Fig. 12).

Extensor Carpi Obliquus Tendon

Transverse Images

The outline of this tendon was difficult to distinguish because of its small size and its superficial course but it could be imaged in all limbs. At the end of zone B, the tendon crosses the extensor carpi radialis tendon and appears as a flat and echogenic structure which is approximately 3 mm thick and 8 to 10 mm wide. As it enters zones C1 and C2, the tendon becomes elliptical and narrows slightly (Fig. 13 a-b).

Sagittal Images

To localize the tendon, palpation is useful. The extensor carpi obliquus tendon is then scanned first in a transverse plane where it is superimposed on the extensor carpi radialis tendon in zone B. To obtain the longitudinal plane, the transducer should to be placed at an approximately 40 de- gree angle to the long axis of the limb from proximolateral to distomedial. The tendon is imaged as an echogenic structure which courses along the extensor carpi radialis tendon,

the radius (Fig. 13 c-d), the radial carpal bone and the medial collateral ligament. When it crosses the medial collateral ligament, the cross-section flattens slightly and the echogenicity decreases. The tendon sheath was visualized in all horses as two echolucent lines demarcated by two echoic lines, of which the latter represents the interface of the sheath and surrounding tissues.

Lateral Digital Extensor Tendon

Transverse Images

In the proximal part of zone A, the muscle terminates and the emerging tendon is visualized as a low-level echogenic elliptic structure with a poorly defined border. The lateral digital extensor tendon is located at the lateral aspect of the radius and is 3 to 4 mm thick and 9 to 10 mm wide. In zone B, it flattens and increases slightly in echogenicity . It passes over the antebrachiocarpal synovial pouch, towards the radius and runs distally through its groove on the LSP (Fig. 14 a-b). The tendon could not be identified through the two parts of the lateral collateral ligament. Distal to the carpus, the tendon runs from a lateral to a dorsal location and continues distal to the carpus. Proximally, the lateral collateral ligament is flat. Distally, it gradually becomes oval and is 3 to 4 mm thick and 10 to 12 mm wide. In zone D, the tendon receives a strong band from the accessory carpal bone and,

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FIG. 11. Longitudinal image (A) and corresponding diagram (B) of the common digital extensor tendon (CDE) over zones C1 and C2. JC, carpal joint capsule; TS, Tendon sheath; JS, Joint space; ER, Extensor retinaculum; Ci, Intermediate carpal bone; C4, Fourth carpal bone; R, Radius.

in 17t horses, the branch of the common digital extensor tendon. Due to its small size, the tendon sheath was not well identified on sonograms.

Sagittal Images

In zones A, B, and D, palpation of the tendon during scanning facilitates the visualization of this small echogenic structure (Fig. 14 c-d). Only by using this technique could the tendon be imaged throughout during its course through the lateral collateral ligament. The two parts of the lateral collateral ligament are much more echogenic compared to the lateral digital extensor tendon. This greater echogenicity helps differentiate the two structures.

Lateral Collateral Ligament

Transverse Images

This ligament is located in zones C1 and C2 over the distal part of the radius, the ulnar carpal bone and the fourth carpal bone and has an echogenic texture. The outline of its two parts was poorly differentiated from the surrounding tissues. Just distal to its origin, the ligament is approximately 5 mm thick from the skinsurface inward and 27 to 29 mm wide. It narrows over the ulnar carpal bone to 19 to 21

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FIG. 12. Longitudinal image (A) and corresponding diagram (B) of the common digital extensor tendon (CDE) over zones C1 and C2. The CDE tendon sheath (TS), the antebrachiocarpal joint and the middle carpal joint were each injected with water. The Tendon sheath with its distal expansion and the joint spaces (JS) are apparent. JC, Carpal joint capsule; Ci, Inter- mediate carpal bone; C4, Fourth carpal bone; R, Radius.

mm and thickens slightly. The deep part migrates dorsally and the superficial one palmarly . Distally, the ligament widens and thickens progressively over the fourth carpal bone (Fig. 15 a-b), becomes crescent shaped, and is 8 to 10 mm thick, 25 to 29 mm wide, and expands to its insertion.

Sagittal Images

With the transducer positioned in the distopalmar direc- tion over the ligament, the two echogenic parts of the ligament were imaged. The lateral digital extensor tendon can be seen as a linear structure between the two parts of the ligament (Fig. 15 c-d).

Ulnaris Lateralis Tendons

Ultrasonography of the Long Tendon

Transverse Images

To help identify this tendon, it should be palpated where it passes over the lateral surface of the flexed carpus. The tendon courses from a palmaroproximal to dorsodistal di- rection along its groove on the lateral surface of the acces-

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FIG. 13. Transverse image (A) with corresponding diagram (B) and longitudinal image (C) with corresponding diagram (D) of the extensor carpi obliquus tendon (ECO) at the level of zone C1. JC, Carpal joint capsule; ER, Extensor retinaculum; R, Radius; Cr, Radial carpal bone.

FIG. 14. Transverse image (A) with corresponding diagram (B) and longitudinal image (C) with corresponding diagram (D) of the lateral digital extensor tendon (LDE) at the level of zone B. P, Lateropalmar pouch of the antebrachiocarpal joint; DDF, Deep digital flexor muscle; R, Ra- dius.

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FIG. 15. Transverse image (A) with corresponding diagram (B) at the level of zone C2 and longitudinal image (C) with corresponding diagram (D) of the carpal lateral collateral ligament (LCL). SP, Superficial part of the LCL; DP, Deep part of the LCL; LDE, Lateral digital extensor tendon; Cu, Ulna carpal bone; C4, Fourth carpal bone.

sory carpal bone, over the ulnar carpal bone and finally passes underneath the lateral collateral ligament to insert on the fourth metacarpal bone. With the transducer placed in zone C1, the tendon is imaged as an echogenic round structure 7 to 9 mm wide and approximately 5 mm thick from the skinsurface inward (Fig. 16 a-b). Towards its insertion, it flattens and narrows slightly, and has poorly defined irreg- ular outlines. A synovial sheath was visualized in all horses.

Sagittal Images

These images show the parallel linear texture of the tendon which crosses the bony landmarks mentioned above, and runs distally towards its insertion (Fig. 16 c-d). The short tendon of the ulnaris lateralis was visualized in the distal part of zone B as an small, echogenic, elliptical structure.

Articular Cartilage

The only articular cartilage that can be evaluated ultra- sonographically is located at the distal radius. To achieve this, the carpus had to be held in the fully flexed position and the transducer applied in a transverse position. The articular cartilage appeared as a smooth, anechoic layer surrounded by two echogenic lines representing the soft tissue/ cartilage and the cartilagehadial bone interface respectively (Fig. 17). In a weight-bearing position, it is not possible to outline any carpal cartilage.

Discussion

Clinical evaluation combined with contrast radiography is currently the accepted diagnostic method for carpal soft tissue abnormalities. 1-8 Whereas positive and negative contrast studies are invasive techniques, ultrasonography is a non invasive, non irritating, fast and relatively inexpensive imaging modality.

In this study, the ultrasonographic appearance of the soft tissue structures at the dorsal and lateral aspects of the equine carpus is described. Visualization of the extensor carpi radialis- and the common digital extensor tendon is far less difficult compared to the extensor carpi obliquus-, lateral digital extensor-, ulnaris lateralis tendon as well as the lateral collateral ligament. A thorough knowledge of the anatomy and simultaneous palpation of these tendinous structures during scanning facilitates their identification on sonograms .

In contrast to the ultrasonographic technique in the metacarpal or metatarsal area, where all the structures can be imaged from a single plane, in the carpal region each tendon has to be scanned separately and the transducer has to be repositioned for each structure. In the transverse scans, all tendons have a homogeneous echogenic texture. Parallel linear echoes are seen in longitudinal scans of these tendons.

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FIG. 17. Transverse image (A) and corresponding diagram (B) representing the distal articular cartilage of the radius (R). The image was made with a fully flexed position of the antebrachiocarpal joint. The arrow points at the interface (echogenic line) of the joint fluid with the articular cartilage (sonolucent space). The extensor carpi radialis tendon (ECR), the carpal joint capsule (JC) and the joint space (JS) are apparent.

FIG. 16. Transverse image (A) with corresponding diagram (B) at the level of zone C1 and longitudinal image (C) with corresponding diagram (D) of the long tendon (L) of the ulnaris lateralis muscle (UL). TS, Tendon sheath; R, Radius; Ca, Accessory carpal bone; Cu, Ulnar carpal bone. Note the long tendon is passing through its groove on the accessory carpal bone in Fig. 16-a.

The largest and most medially located structure is the extensor carpi radialis tendon. The common digital extensor tendon is located lateral to the extensor carpi radialis tendon and can be easily imaged over the distal radius, the carpus and the proximal metacarpus. The ulnaris lateralis long tendon and the extensor carpi obliquus tendon require some practice to be scanned, but their visualization is satisfac- tory. Owing to the small size of the ulnaris lateralis short tendon, its sonographic appearance is of no great impor- tance. The lateral digital extensor tendon and the lateral collateral ligament are harder to image. The only articular cartilage which can be scanned is located at the distal radius and can be seen, on transverse scans, with the carpus in a fully flexed position. Ultrasonic image differentiated cartilage from subchondral bone, which may be useful to detect early or small cartilage lesions of the cranial aspect of the distal radius.

The carpal joint capsule with its two fat areas at the level of the antebrachiocarpal joint and the proximal row of carpal bones was identified on sonograms.

Water injection studies of this joint and the overlying synovial sheath of the extensor carpi radialis tendon local-

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ized the joint capsule with its fat areas as described in a xeroradiographic contrast study. l4 The tendon sheaths of the extensor carpi radialis, common digital extensor, extensor carpi obliquus, and the ulnaris lateralis long tendon were visualized. The lateral digital extensor tendon sheath was not well identified. The boundaries of the tendon sheaths of the extensor carpi radialis- and the common digital extensor tendon were also visualized following water injection in 2 cadavers limbs and the results obtained confirm those reported in a contrast radiographic study.15 The carpal joint spaces were well distinguished on scans which may help in localizing abnormal communications between joints and tendon sheaths reported in the

Because of the small sample size and the different breeds studied, measurements of the various structures were pre-

sented only with the intention to help the clinician in imaging these structures. The small values were usually obtained from thoroughbreds. Similar sonographic appearance was found between left and right carpi of all horses, as well as between carpus specimens and live horses. The gross anatomy correlated well with ultrasonographic images.

Based on our findings in normal horses, ultrasonography appears to be very helpful to further characterize lesions in horses with carpal soft tissue abnormalities. Fortunately, the more clinically significant structure of the examined areas, the extensor carpi radialis- and the common digital extensor tendon, were the most easily visualized structures.

Future investigation should focus on the clinical useful- ness of ultrasonography for the diagnosis and prognosis of specific carpal soft tissue injuries.

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ultrasonography of the dorsal and lateral aspects of the equine carpus: technique and normal...

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