Ultrasonography of the Metacarpus and Metatarsus

Ultrasonography of the Metacarpus and Metatarsus

Roger K.W. Smith1 and Eddy R.J. Cauvin2

1 The Royal Veterinary College, North Mymms, Hatfield, UK
2 AZURVET Referral Veterinary Centre, Saint-Laurent-de-Var, France

Preparation and Scanning Technique

Careful preparation of the skin is paramount, including fine clipping, scrubbing of the limb to remove dirt and cut hairs, cleansing with surgical spirit (alcohol) to degrease the skin, and liberal application of coupling gel which should ideally be given time to soak in. The area to be clipped depends on the structures being imaged but for complete evaluation of all the palmar soft tissue structures in the metacarpal/metatarsal region, the skin should be clipped from the palmar carpal region, immediately distal to the accessory carpal bone, down to the ergot of the fetlock and over the whole palmar aspect of the metacarpus from the palmaromedial to the palmarolateral aspect of the third metacarpal bone. For the hind limb, clipping should involve the metatarsal region up to the chestnut. It is possible in some thin-haired horses to perform ultrasonography without clipping, but the image quality will usually be impaired and an acceptable image may not be obtained in all cases. If scanning without clipping, it is advised to dispense with the soap scrubbing as this generates air between the hairs, and just clean the limb with surgical spirit, with or without the application of contact gel.

High-frequency (7.5–16 MHz) linear array transducers are preferred. A micro-convex transducer may be useful to image the proximal aspect of the suspensory ligament in some horses. A standoff pad is recommended to widen the ultrasound window to permit imaging of the whole width of the tendons in one picture, move the superficial structures away from the emission artifact at the top of the screen, and also has the added value in protecting the surface of the transducer. However, some operators prefer to scan the limb without the use of a pad, particularly with high-frequency transducers, to improve penetration. In this case, the contact area will be reduced so it is important to image the limb from both palmar and palmaro-abaxial approaches to obtain a comprehensive assessment of the palmar limb structures.

The scanning protocol for the metacarpal region should consist of both weightbearing and non-weightbearing images. Tendinous structures are under tension when fully weightbearing, which minimizes artifacts; however, non-weightbearing scans can be useful to assess specific structures and some lesions that are more visible when the tendons are unloaded (such as marginal lesions), the motion of each tendon in relation to the others, and for Doppler evaluation (Table 3.1).

Table 3.1 Scanning protocol for the metacarpal/metatarsal regions.

Step Loading Views
“Standard” views Weightbearing Transverse and longitudinal views from the palmar aspect
“Lesion-orientated” views Weightbearing

“Close ups” (e.g., appropriate enlargement for superficial structures)

Oblique images

Off-incidence (for the assessment of chronic tendinopathy)

Flexed Non-weightbearing

Off-incidence (proximal suspensory ligament)

Specific structures (e.g., intra-thecal tendon tears)


Tendon movement

Images should be obtained in both transverse and longitudinal orientations in sagittal and, when appropriate, frontal planes. For reference purposes, several systems have been described to determine the level of the section on the limb. None of these has gained general acceptance. The most widely-used system consists of dividing the metacarpal region into three equal tiers (proximal, middle, and distal) from the carpometacarpal joint to the proximal extent of the fetlock palmar annular ligament, termed I to III from proximal to distal. The metatarsal region is divided into four zones (I–IV), with zone I extending from the tuber calcis to the tarsometatarsal joint. Each of these zones is further divided into two halves, named a and b. The palmar fetlock area (at the level of the palmar fetlock annular ligament) is usually referred to as IIIc area in the forelimb, and IVc in the hind limb. It may be simplified further by dividing the distance from the level of the carpometacarpal joint to the distal sesamoid bones into seven regions (1–7) in both fore- and hind limbs along the length of the third metacarpal/metatarsal bone, although proximal extension of the examination over the palmar carpus and plantar calcaneus is advised for proximal lesions. This system has the advantage of not depending upon the animal’s size but is based on specific anatomic features (Figure 3.1) and therefore permits not only comparisons in the same animal, but also between individuals. An alternative technique is to measure the distance from the distal palpable border of the accessory carpal bone or from the tuber calcis to the point at which the ultrasonographic section is obtained. Although this only allows comparisons in the same individual, it may be useful when greater objectivity of position is needed, such as for research purposes.

Figure 3.1 Normal ultrasonographic anatomy of the palmar aspect of the metacarpus. (A) Transverse ultrasonographic images are presented on the right side with comparative anatomy sections in the center and diagramatic representation of the location of the transverse images on the left side for the seven standard levels (labeled 1 (Ia) to 7 (IIIc)). The structure labeled “SDFT ring” is the manica flexoria. (B) Longitudinal midline sagittal images in the three thirds of the palmar metacarpus.

Typically, the examination is performed from proximal to distal, first in transverse and then in longitudinal planes from the palmar/plantar aspect, followed by any appropriate oblique views. The authors start by examining all the palmarly/plantarly located tendons and ligaments in both planes, then the specific structures of interest by altering the settings on the machine (e.g., enlarging the image to evaluate the superficial structures such as the superficial digital flexor tendon), and then finally the branches of the suspensory ligament from the lateral and medial aspects. The limb may then be picked up for non-weightbearing views. This is most easily achieved in the fore limb by maintaining the limb in a horizontal “neutral” position (straight limb; see Chapter 2, Figure 2.40). For the hind limb, horses frequently rest one hind limb during the examination, especially when sedated (Figure 2.40), and this position provides an easier opportunity to obtain non-weightbearing views without having to hold the limb or have an assistant hold the limb raised. The limbs have to be held for flexion and extension when evaluating relative motion of the tendons. It is important to flex both the distal interphalangeal joint and fetlock joint separately to evaluate tendon movement when assessing adhesion formation. The former will move the deep digital flexor tendon independently of the superficial digital flexor tendon, while the latter will move both tendons together through the fetlock canal. Flexing the distal interphalangeal joint alone while keeping the fetlock joint in the same position may prove difficult in some heavy-set horses but with careful manipulation, even a small range of movement between the tendons can be appreciated with ultrasound.

Ultrasonographic Anatomy

The anatomy of the equine “tendon” region has been reviewed in numerous texts (see Recommended Reading at the end of the chapter). It is fairly simple, although with improving imaging, subtle anatomic details may be evaluated.

General Appearance of Tendons and Ligaments (Figure 3.2)

Figure 3.2  Ultrasonographic appearance of tendons and ligaments. (A) In transverse sectional images, the tendon parenchyma typically appears as a “granular” substance with densely packed echogenic dots. The subcutaneous tissue (sc) is hypoechogenic, while the paratenon and overlying fascia form a hyperechogenic interface (p). (B) The tendon parenchyma (SDFT) presents in the long-axis (longitudinal scans) as a series of coarse, transversely oriented hyperechogenic interfaces. These do not represent fibers as such, but rather discrete interfaces between fiber packets. They are separated by anechogenic spaces that represent endotenon (loose, vascularized connective tissue) but also non-resolvable (visible) fibers located before the next visible interface. See the key in Figure 3.1A for abbreviations.

In normal tendon, the fascicular pattern is characterized by multiple interrupted echoes in transverse images and a regular striated pattern in longitudinal scans. However, the fascicular pattern in equine tendon is complex with different sized fascicles grouped together [1]. The axial spatial resolution (i.e., the ability to differentiate two separate interfaces along a vertical line on the screen) depends mostly on the frequency of the transducer. For a 12 MHz probe, it is theoretically in the order of 0.13 mm. However, other factors, both machine and tissue related, limit the resolution, so that it is in the order of 0.5–1 mm for a broad-band (8–18 MHz), high frequency linear probe. Even when using the higher frequency ultrasound transducers available today, the ultrasonographic striations can only represent the higher-order organization of discrete interfaces between groups of fascicles and not individual bundles. Furthermore, echo formation depends on acoustic impedance differences at interfaces, so echoes may occur between many tissue types and may represent the surface of a vessel, a fiber bundle, connective tissue, or any irregularity in the parenchyma. Endotenon and the size of fiber bundles do therefore participate in the heterogeneity of tendon parenchyma and account in part for the differences in appearance between different tendons. Nevertheless, an even, regular striation aligned with the long axis of the tendon and that extends over the width of the image closely follows the underlying microstructure, and has been used to provide objective assessment of structure, so-called ultrasound tissue characterization [2]. The higher the frequency probes, the finer the assessment of fascicular structure (Figure 3.3)

Figure 3.3 The improvement in detail seen in ultrasound quality over 15 years. (A) A transverse image of the superficial digital flexor tendon obtained in 2004 and (B) a similar image obtained in 2019. Note the extra detail of the fascicular architecture visible in the more recent scan. Thus, newer equipment can provide significantly more information on the structure of the superficial digital flexor tendon.

Thoracic Limb

There are four tendons running over the palmar aspect of the metacarpus (Figure 3.1). Respectively, from palmar (superficial) to dorsal (deep), they are the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), accessory ligament of the DDFT (ALDDFT) proximally, and tendon of the third interosseous muscle, more commonly referred to as the suspensory ligament (SL). These will be reviewed in turn.

Superficial Digital Flexor Tendon (SDFT)

The SDFT arises from the SDF muscle in the distal antebrachial area, 2–6 cm proximal to the accessory carpal bone. The musculotendinous junction extends from the distal quarter of the caudal antebrachium to the proximal edge of the accessory carpal bone (ACB). Hypoechogenic muscle strands can extend more distally in young horses, often well into the carpal canal. The appearance of the junction varies between individuals and depending on breed and age. However, it is usually bilaterally symmetrical. The accessory (“radial check” or “superior check”) ligament of the SDFT (ALSDFT) lies dorsomedial to the SDFT close to the musculotendinous junction. This thick ligamentous structure derives from an atrophied radial head and runs obliquely from the caudomedial aspect of the radius just proximal to the chestnut, to join the SDFT proximal to the ACB.

Within the carpal and proximal metacarpal regions, the SDFT is oval to circular in cross-section and is located palmaromedial to the DDFT. Its dorsal aspect becomes concave as it runs distally, giving it a thick crescent shape, with thicker medial and thinner lateral borders, in zone II. It is located at the palmar aspect of the limb in this region. It becomes gradually thinner dorsopalmarly in the distal metacarpus and fetlock regions, with its dorsal surface tightly molded around the palmar contour of the DDFT. In zone IIIb (distal part of the metacarpus, within the digital flexor tendon sheath) it forms a thin membranous ring around the deep digital flexor tendon called the manica flexoria, which arises from the sharp lateral and medial edges of the SDFT. The SDFT eventually divides into two branches in the mid-pastern area, distal to the ergot, each branch blending abaxially into the middle scutum (a palmar fibrocartilaginous pad on the palmar aspect of the proximal interphalangeal joint) before inserting on the middle phalanx (P2). The SDFT is a very dense, echogenic tendon, although it is normally less echogenic than the DDFT and ALDDFT. Using high-frequency ultrasound transducers (>10 MHz), fiber fascicles can be differentiated within the parenchyma. In longitudinal sections the tendon has a regular, continuous striated pattern, characteristic of tendons.

Deep Digital Flexor Tendon

The DDFT also originates in the distal antebrachium from the reunion of three muscular heads (humeral, ulnar, accessory or radial), and its musculotendinous junction occurs at the same level as that of the SDFT. The muscle heads, along with that of the SDF, are indistinguishable ultrasonographically, although the radial head can sometimes be identified cranial to the other heads in the distal antebrachium [3]. The DDFT is initially dorsolateral to the SDFT in the carpal region, where it runs over the medial surface of the ACB. It lies dorsal to the superficial digital flexor tendon in the middle and distal thirds of the metacarpus. It is oval in shape throughout its path but flattens to form sharp medial and lateral borders within the fetlock canal. In the pastern area, it becomes slightly bilobed, with a “ski goggle” appearance on ultrasonographs. It becomes flatter in the foot over the palmar aspect of the navicular bone before inserting on the palmarodistal surface of the distal phalanx (P3).

Accessory Ligament of the Deep Digital Flexor Tendon (ALDDFT)

The ALDDFT (“inferior or distal check ligament”) originates on the palmar aspect of the carpus as a distal continuation of the palmar carpal ligament. It joins the deep digital flexor tendon at the mid-metacarpal region (zone II). The junction is very gradual from IIb to IIIa. As the fibers of the ALDDF are oblique, this creates a hypoechogenic, off-incidence artifact within the dorsal aspect of the DDFT in this area (see zone 2A (level 4) in Figure 3.1). The ALDDFT is rectangular in cross-section in zones IA and B (levels 1 and 2); see Figure 3.1, then becomes crescent shaped in zone II (3 and 4), curving mostly around the lateral aspect of the DDFT. The carpal flexor tendon sheath lining forms a hypoechoic space between the deep digital flexor tendon and the ALDDFT down to their junction, and it occasionally contains some anechogenic fluid.

Suspensory Ligament (SL)

The SL is formed by the atrophied interosseous III muscle and its tendon. It is anatomically divided into three approximately equal portions: the proximal origin (3–5 cm long), the body (main portion down to the bifurcation), and the two branches (lateral and medial). The origin and body of the suspensory ligament are examined from the palmar aspect of the limb, whereas the branches should be examined in turn from the lateral and medial aspects. Some muscle fibers remain in young horses, giving the ligament a mottled, hypoechoic appearance. These may decrease with age. Most studies have showed that the heterogenous pattern of the proximal SL in normal horses is bilaterally symmetrical at any level. The SL originates from the proximal palmar cortex of the third metacarpal bone but has an additional component extending proximally to the carpus, blending into the joint capsule of the carpometacarpal joint and palmar carpal ligament. The origin and body of the SL are rectangular in cross-section, although usually only the middle half or two-thirds of the ligament is visible within the ultrasound window in a weightbearing limb from a palmar approach. To evaluate the entire volume of the SL, it is necessary to rotate the probe from medial to lateral (It may be useful to use a convex probe in the proximal metacaparpus). The origin is divided into two lobes, sometimes visible as separated by hypoechogenic tissue. These two lobes are molded by the slightly concave surfaces on the palmar aspect of the third metacarpal bone, separated by a variably prominent, sagittal bony ridge. The body of the SL has a coarser, less echogenic appearance than the tendons. The SL divides in the distal third of the metacarpus into two branches (medial and lateral). Each inserts over the abaxial surface of the ipsilateral proximal sesamoid bone. The medial branch is slightly larger than the lateral, but both are pear- or tear-drop- shaped. As they insert they become triangular to crescent-shaped. The striated pattern is regular over the whole length of the SL, although it is not uncommon in adults and older horses to have a thinner or less marked striation at the proximal and body regions.

The SL branches are continued distodorsally by the extensor branches, which course dorsally and join in the distal pastern onto the common digital extensor tendon (CDET). The sesamoid bones are an integral part of the suspensory apparatus, as are their distal ligaments (see Chapter 1). Ideally, these should therefore be examined along with the SL, as combined injuries are fairly common.

Extensor Tendons

In the metacarpal area, the extensor tendons are easily identified over the dorsal aspect of the third metacarpal bone (Mc3). They have a less echogenic, coarser appearance than flexor tendons. They are very thin and flat in cross-section, located in the connective tissue between the skin and periosteum. The CDET is dorsolateral proximally and dorsal in the distal half of the metacarpus. The lateral digital extensor tendon (LDET) is much smaller, round in cross-section, and situated laterally in the proximal metacarpus. It runs obliquely to follow the lateral edge of the CDET in the distal third, where it becomes flatter and blends into the fetlock joint capsule.

Other Structures

The surface of the third metacarpal bone (Mc3) is round and forms a smooth, sharp, hyperechoic interface. The overlying periosteum is clearly visible as an echogenic, 2–3 mm thick, homogeneous layer overlying the bone. The splint bones (Mc2 and Mc4) can also be assessed; they are smooth and regular in long-axis scans and sharply rounded in cross-section. The interosseous ligament between the splint bones and the cannon bone forms an echogenic space between the bone interfaces, and it may become mineralized in older horses.

All tendons, except in sheathed areas, are surrounded by a thin connective tissue layer (the paratenon). This is less echogenic than the tendon parenchyma and contains numerous small vessels that are not visible in normal tendons. The carpal and digital flexor tendon sheaths are described in other sections of this text.

The neurovascular bundles are easily identified in this part of the limb (Figure 3.4). In the proximal and mid-metacarpal area, the medial palmar artery and nerve run together and are identified close to the skin surface but deep to the fascia, along the medial aspect of the DDFT, while their lateral counterpart is more deeply and dorsally located on the lateral aspect of the ALDDFT. The corresponding veins are larger and located slightly more dorsally in the looser connective tissue space dorso-abaxial to the ALDDFT. The lateral vein is more axially located. In the distal third of the metacarpus, the bundles run over the dorsoabaxial border of the DDFT/ALDDFT and then become more superficial, running over the abaxial aspect of the proximal sesamoid bones palmar to the SL branch insertions. The nerve is always palmar to the associated artery. The connecting branch between the two palmar nerves arises from the medial palmar nerve at the mid-metacarpal level and runs obliquely in a latero-distal direction over the palmar surface of the SFDT before joining onto the lateral palmar nerve 1 or 2 cm further distally. This may be felt and cause a slight elevation of the skin and probe in relation to the SDFT.

Figure 3.4 Transverse ultrasonograph from the mid-metacarpal region using a palmaromedial approach. Neurovascular structures are identified: the medial common palmar nerve (n) has coarse, grainy appearance; the associated artery (a) is round in section with a thick wall; the veins (v) are easily compressed due to the thin walls. Note the thick superficial fascia (f).

The nerves have a coarse striated pattern on long-axis scans and a coarse grainy appearance on cross-section.

The veins are anechogenic with thin, deformable walls, so that they may be easily obliterated by excessive probe pressure on the skin. Echogenic whorls of moving blood cells may be identified in the veins and larger arteries because of the slow flow in equine extremities. Venous valves can occasionally be identified and can be seen to open and close with flow. However, blood flow is often static in a horse standing still. The arteries are of much smaller caliber and have thicker walls, which gives rise to twin echoes on their superficial and deep surfaces. They are always round in cross-section and more difficult to compress. The arterial flow is laminar with poorly marked systolic surges in Doppler studies.

Pelvic Limb Differences

The overall arrangement is similar to that of the thoracic limb. It is virtually the same in the distal two-thirds of the metatarsus. Proximally, the SDFT is slightly flatter than in the fore limb, and is located plantarolateral to the DDFT as it runs over the plantar aspect of the tuber calcis of the fibular tarsal bone (calcaneus) and overlying long plantar ligament. The SDFT does not have any muscular body in the equine pelvic limb. The DDFT is formed by the fusion of two tendons: the lateral digital flexor tendon (LDFT) is the main part, running medial to the tuber calcis over the sustentaculum tali of the calcaneus and then plantar to the distal tarsal bones. The medial digital flexor tendon (MDFT) is a small, cylindrical, tendon that runs over the medial aspect of the tarsus, in a groove on the medial aspect of the tibial tarsal bone, closely associated with the medial collateral ligament of the tarsus, before joining onto the medial border of the LDFT in the proximal quarter of the metatarsus to form the DDFT. Both structures have their own tendon sheath in the tarsal area, although these sometimes communicate distally.

Contrary to what has been previously described in some anatomy textbooks, there is an ALDDFT in the hind limb. It is often rather thin and varies between individuals from a thin aponeurotic membrane to a fully developed ligament, similar but substantially smaller than that encountered in the thoracic limb. It arises from the short plantar ligament of the tarsus.

Finally, the SL has two, well differentiated heads in the hind limb, the lateral head being larger and tightly packed against the axial border of the fourth metatarsal bone (Mt4). It is more triangular to oval shaped than in the fore limb. The intercapital ridge on the plantar proximal metatarsus is rather less marked than in the fore limb. A strong, deep fascia can sometimes be identified plantar to the SL, running between the heads of the two splint bones and enclosing the proximal suspensory ligament in a canal. In the proximal part of the metatarsus, imaging the SL from a plantar approach can be limited, because of the prominent and axially concave head of the fourth metatarsus (lateral splint bone). It is therefore useful to use a plantaromedial approach, the so-called “medial window,” because the second metatarsal (medial splint) bone is considerably smaller than its lateral counterpart. In some horses, the use of a micro-convex array transducer can be very useful to image the origin of the ligament as the pie-shaped beam easily encompasses the whole cross-section of the SL from a plantaromedial to plantar entry point.

Quantitative Assessment of Flexor Tendon and SL Size

Reference measurements for the SDFT have a wide range because of a greater than two-fold variation in tendon size between normal individuals, and because of variations between breeds or type of horses. Cross-sectional areas (CSA) of the SDFT in Thoroughbreds in the United States has been given as 0.8–1.2 cm2, while a larger range (0.72–1.93 cm2) has been reported as the normal range for Thoroughbreds in Great Britain, with 0.77–1.39 cm2 in level 4 (zone IIb; mid-metacarpal region) in National Hunt horses [4]. Tendon CSA varies with breed and size, and is smaller in Arabian-type horses (0.6–0.8 cm2) and in ponies. In contrast, Smith et al. (1994) did not find a significant difference between Thoroughbreds and Irish Draughts [5]. The CSA varies with the anatomic level; it is smallest in the mid-metacarpal area and largest in the fetlock region. It may vary somewhat with age and training. The lateromedial dimension varies between 1 and 3 cm, depending on the location. The dorsopalmar thickness has been reported to be 0.7–0.8 cm proximally, decreasing to approximately 0.4 cm distally. but these measurements are less accurate for determining enlargement compared to measuring the CSA. DDFT CSAs vary from 1.35–2.2cm2 [5], with its smallest CSA immediately proximal to the insertion of the accessory ligament of the deep digital flexor tendon [5].

For the SL in racing Thoroughbreds and Standardbreds in the United States, the cross-sectional area rangеs from 1.0–1.5 сm2, with most horsеs having a сross-sесtional arеa of 1.0–1.2 cm2 for the suspensory body. It is slightly larger in the pelvic limb (1.2–1.75 cm2). The branches measure 0.6–0.8 cm2 proximally to 1–1.2 cm2 distally. Measuring the dorsopalmar/plantar thickness of the ligament is probably of greater clinical interest than for the SDFT, especially in the origin and proximal body area, as it is difficult to image the entire SL in one image necessary for CSA measurement. It is normally less than 1 cm in average-sized horses (0.8–0.9 cm in Thoroughbreds). The branches are less than 1 cm thick in a dorsopalmar direction.

Ultrasonographic Abnormalities


General Ultrasonographic Changes

Tendinitis/desmitis refers to spontaneous loss of structural integrity of the fibrous parenchyma of tendons and ligaments respectively. The complex pathogenesis of this condition has been reviewed elsewhere and will not be reviewed here [6]. Tendinopathy/desmopathy is the more appropriate term to cover all types of tendon injury, although tendinitis/desmitis remains more widely used.

Ultrasonography allows us to detect variations in gross anatomy (size and shape of the tendon or ligament) but also in the overall structure of the parenchyma (refer to the Section on Ultrasonographic Anatomy). Acute damage is associated with several changes in the ultrasonographic appearance of the tendon – an increase in size, a reduction in echogenicity with loss of the normal striated pattern, often affecting the central area of the tendon, and changes in shape, margination, and position. Immediately after the injury, the lesion fills with blood and debris, which are variably echogenic (hypo- to hyperechogenic) and heterogeneous. The lesion in the first few days may be subtle or even missed, as matrix debris and clots may have similar echogenicity to that of the normal parenchyma (Figure 3.5). The acute lesion is usually poorly defined and heterogeneous but long-axis images will confirm loss of fiber alignment (Figure 3.6).

Figure 3.5 Transverse ultrasound scan image at level 1 showing a poorly defined area within the SDFT that is grossly isoechogenic to the remaining parenchyma but with altered echotexture. More subtle lesions are easily missed at this early, acute stage.

Figure 3.6 Transverse (A) and longitudinal (B) ultrasound scan images showing a poorly defined, heterogeneous, and slightly hypoechogenic area in the transverse plane within the SDF tendon (yellow arrows). The sagittal plane image shows severe fiber disruption and decreased echogenicity. Note the increased cross-sectional area of the SDFT and peripheral swelling of the paratenon (white arrow).

Edema leads to increased water content and decreased echogenicity in the surrounding tendon, paratenon, and subcutaneous tissues (Figure 3.6). The tendon may be swollen, although this is variable initially, but peritendinous and subcutaneous edema should be detectable. Comparison with the contralateral limb (which may not be completely normal with many overstrain injuries) may help confirm increased tendon size. After a few days, an organized hematoma and early, immature granulation tissue fill in the lesion. They are hypoechogenic and provide the typical, discrete appearance of many tendinitis lesions (Figure 3.7).

Figure 3.7 Transverse (A) and longitudinal (B) ultrasound scan images of the palmar mid-metacarpal region (level 3). A well defined, hypoechogenic “core” lesion is present in the central part of the tendon. This appearance is related to early granulation tissue which appears homogeneously hypoechogenic.

The lesion may increase in size for several days after injury, because of repeat bleeding and local release of degrading enzymes by invading inflammatory cells, usually making the lesion more evident after several days. Therefore, to establish a baseline severity scan, lesions are best examined ultrasonographically at 7–10 days after injury: this is usually the stage when the lesion is largest and most obvious. If any doubt persists, the animals should remain box-rested and be re-evaluated 2 weeks later.

SDF Tendinopathy

A common manifestation of acute SDFT injury is a discrete hypoechoic lesion visible in the central region of the tendon (hence the usual term, “core lesion;” Figure 3.7). It is most commonly centered in the mid-metacarpal region (usually zone 4/IIb). Lesions can also occur more eccentrically within the tendon (Figure 3.8). Lesions occurring at the periphery of the tendon are often associated with extension of the hematoma into the paratenon or even peritendinous tissues. These may be traumatic in origin (see Section on Local Trauma). Such lesions often alter the shape of the tendon. There is also an increase in tendon cross-sectional area, which is highly variable between lesions. The size of the lesion is relative to the cross-sectional area of the tendon, but its proximodistal length should also be ascertained to aid in prognostication (see Section on Semi-Objective Assessment of Severity) and for future reference.

Figure 3.8 Transverse image obtained at level 4, showing an acute, hypoechogenic lesion with a honeycomb pattern, typical of organized hematoma. It distorts the lateral aspect of the SDFT (thick arrows). The paratenon is markedly thickened (thin arrows) around the lesion, extending around the SDFT.

Diffuse lesions are more challenging to visualize: the tendon becomes enlarged, hypoechogenic, and heterogeneous (Figure 3.9). Sagittal ultrasound scans confirm diffuse loss of striation. Enlargement and diffuse hypoechogenicity is sometimes seen in young Thoroughbreds without other clinical signs (so-called “juvenile tendinitis”). The aetiology is unclear and affected horses often recover without significantly increased risk of re-injury.

Figure 3.9 Diffuse tendinopathy similar to that seen in young horses (“juvenile tendinitis”). Transverse (A) and longitudinal (B) ultrasound scan images of the palmar mid-metacarpal region (zone 3). The SDFT is generally enlarged, hypoechogenic without a discrete lesion being visible. The striation is poorly organized and uneven on the longitudinal scan.

Tendon enlargement can be measured objectively by measuring the cross-sectional area (CSA) of the tendon on transverse images. Injured tendons have a CSA greater than normal (see Section on Quantitative Assessment of Flexor Tendon and SL Size), although this can be difficult to ascertain, given the large variation in normal cross-sectional areas. If previous scans are available, a greater than 10% increase in cross-sectional area is usually significant or, if comparing with the contralateral limb, a greater than 20% difference would be considered significant [5], although this may not be the case if both limbs are affected or there is a previous injury.

In very subtle cases, often the only finding can be enlargement and/or change in shape of the tendon. This can be accompanied by peritendinous edema, which is not specific for tendinitis and can also result from local trauma (Figure 3.10) or any other inflammatory condition in the limb. Providing there is no evidence of tendon injury and the edema disappears, work can be resumed after a short period of rest. If edema persists, however, the presence of tendinitis should be suspected and repeat examinations are warranted.

Figure 3.10 Transverse ultrasonographs from the mid-metacarpal region (level 4) showing thickening of the paratenon (solid arrow) and subcutaneous tissues (dashed arrow) associated with peritendinous edema but without any tendon enlargement (A) when compared to the contralateral limb (B). This can either be a sign of mild local trauma or early overstrain injury. The limb should be re-examined ultrasonographically if the edema does not spontaneously resolve within a few days.

There is some controversy as to the ability to detect subclinical or preclinical lesions with ultrasound. Certainly, gradual degeneration as observed biochemically or histologically is at a level that cannot be detected by the resolution of ultrasound, and recent studies have failed to identify prodromal changes ultrasonographically [4], even though some subtle heterogeneity is sometimes interpreted as signs of aging change (Figure 3.11

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Nov 6, 2022 | Posted by in EQUINE MEDICINE | Comments Off on Ultrasonography of the Metacarpus and Metatarsus
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