Ultrasonography of the Metacarpus and Metatarsus
1The Royal Veterinary College, North Mymms, Hatfield, UK
2Centre d’Imagerie, Cagnes sur Mer, France
Preparation and Scanning Technique
It is important that the animal be examined fully weightbearing, so that tendinous structures are under tension, but it is still useful to examine the non-weightbearing limb, to assess the motion of each tendon in relation to the others and for Doppler evaluation. Careful preparation of the skin is paramount, including fine clipping, cleansing with scrub and alcohol, and liberal application of coupling gel. The area to be clipped depends on the structure 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, to distal 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. As regards 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 always be impaired and an acceptable image may not be obtained in all cases.
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 improve probe-to-skin contact and permit imaging of the whole width of the tendons in one picture. However, some operators prefer to scan the limb without the use of a pad, particularly with high-frequency transducers. In this case it is important to image the limb from both palmar and palmaro-abaxial approaches to obtain a comprehensive assessment of the palmar limb structures.
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 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 fore limb, 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 the fore limb, and nine in the hind limb. This system has the advantage of not depending upon the animal’s size but is based on specific anatomic features (Figure 3.1). One alternative is to measure the distance from the distal palpable border of the accessory carpal bone or from the tuber calcis to the ultrasonographic section examined. Although this only allows comparisons in the same individual, it may be useful for follow-up.
Typically the examination is performed from proximal to distal, first in transverse and then in longitudinal planes, followed by any appropriate oblique views. The authors start by examining the superficially located flexor tendons in both planes, then the deeper structures by altering the settings on the machine, and then finally the branches of the suspensory ligament. Off-incidence views are routinely obtained by tilting the transducer by about 10 degrees as an assessment of tissue organisation post injury. The limb may eventually be picked up and flexed to evaluate relative motion of the tendons and for Doppler imaging of pathology.
Ultrasonographic Anatomy
The anatomy of the equine “tendon” region has been reviewed in numerous texts (see Recommended Reading at the end of the chapter). The anatomy in this region is fairly simple, although with improving imaging, subtle anatomic details may be evaluated.
General Appearance of Tendons and Ligaments (Figure 3.2)
In normal tendon, the striation imaged in sagittal or frontal plane scans does not correspond to fiber fascicles as such but rather to discrete interfaces between groups of fascicles. 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 recently to provide objective assessment of structure, so-called ultrasound tissue characterization (van Schie et al., 2003). The use of higher-frequency probes (>12 MHz) permits finer assessment of intraparenchymal structure. Tendon is highly vascularized but the endotenon (loose connective tissue separating the collagenous fascicles) and associated vessels are usually of a caliber well below the resolution of diagnostic ultrasound (0.1–0.5 mm). Endotenon and the size of fiber bundles do participate in the heterogeneity of tendon parenchyma and account in part for the differences in appearance between different tendons.
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 DDF muscle (ALDDF), 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
The SDFT arises from the SDF muscle in the distal antebrachial area, 2–6 cm proximal to the accessory carpal bone. The musculotendinous junction is short but progressive, extending from the distal quarter of the caudal antebrachium to the proximal edge of the accessory carpal bone (ACB). Hypoechogenic muscle strands extend more distally in young horses, often well into the carpal canal. The accessory (“radial check” or “superior check”) ligament of the SDFT (ALSDFT) lies dorsomedial to the SDFT. This thick ligamentous structure derives from an atrophied SDFT radial head and runs obliquely from the caudomedial aspect of the radius at the level of the chestnut to join the SDFT proximal to the ACB.
Within the carpal region, the SDFT is oval to circular in cross-section. In the proximal metacarpal region, it becomes oval in cross-section and is located palmaromedial to the DDFT. Its dorsal aspect becomes concave as it runs distally, giving it an asymmetric crescentic shape with a thicker medial border and a tapered lateral margin 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) 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 proximal pastern area, distal to the ergot, each branch blending into the middle scutum (fibrocartilaginous palmar capsule of the proximal interphalangeal joint) before inserting on 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 (14 MHz and over), fiber fascicles can be differentiated within the parenchyma. In longitudinal sections the tendon has a regular, continuous striated pattern, and the striation is similar to that of the DDFT.
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 SDFT, are indistinguishable ultrasonographically. The DDF tendon 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. In the pastern area, it becomes 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.
Accessory Ligament of the Deep Digital Flexor Tendon
The ALDDFT (“inferior 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 Ia to IIb. As the fibers of the ALDDFT are oblique, this creates a hypoechogenic, off-incidence artifact within the dorsal aspect of the DDFT in this area (see level 4 or zone 2b in Figure 3.1). The ALDDFT is rectangular in cross-section in zone I (levels 1 and 2), then becomes crescent shaped in zone II (levels 3 and 4), curving mostly around the lateral aspect of the DDFT. The carpal flexor tendon sheath 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
The SL is formed by the atrophied interosseous III muscle and its tendon. It is anatomically divided into three 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 decrease with age. Most studies have showed that the SL in normal horses is always bilaterally symmetrical at any level. The SL originates from the proximal palmar cortex of the third metacarpal bone. It blends 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 the origin is divided into two distinct lobes separated by hypoechogenic tissue. These two lobes are molded on slightly concave surfaces on the palmar aspect of the third metacarpal bone, separated by a variably prominent bony ridge. The body of the SL has a coarser, less echogenic appearance than the tendons. The SL divides in the distal third 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 initially oval and then become “tear-drop”’ in shape more distally. 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 origin and proximal body.
The SL branches are continued distodorsally by the extensor branches which course dorsally and join distally in the pastern onto the common digital extensor tendon. The sesamoid bones are an integral part of the suspensory apparatus, as are their distal ligaments (see Chapter 1); these should therefore ideally be examined along with the SL, as combined injuries can occur.
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. The common digital extensor tendon (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 of interest include the surface of MC3, which is round and smooth, and the overlying periosteum which is clearly visible as an echogenic, 2–3 mm thick, homogeneous layer over the hyperechogenic bone interface. 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 third metacarpal 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 well identified in this part of the limb (Figure 3.3). In the proximal and mid-metacarpal area, the medial palmar artery and nerve are identified close to the skin surface on the dorsomedial aspect of the DDFT, while their lateral counterpart is more deeply located. The corresponding veins are large and located in the connective tissue space dorsal to the ALDDFT/DDFT and palmar to the SL; the lateral vein is more axially located. In the distal third of the metacarpus, the bundle becomes more superficial and follows the abaxial borders of the DDFT before running over the abaxial aspect of the proximal sesamoid bones palmar to the SL branch insertions.
The nerves have a coarse striated pattern on long-axis scans, and the veins are anechogenic with thin, deformable walls, although echogenic whorls of moving blood cells may be identified in the veins and larger arteries. Venous valves are usually visible and can be seen to open and close. The arteries are of much smaller caliber and have thicker walls. They are always round in cross-section. The arterial flow is fairly regular with indistinct 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. This tendon has a poorly developed or no muscular body in the hind 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 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 within the medial collateral ligament, before joining onto the medial border of the LDFT in the proximal quarter of the metatarsus to form the DDFT sensu stricto.
Contrary to what has long been described in many anatomy textbooks, there is an ALDDFT in the hind limb also. It is often rather thin but varies between individuals from an aponeurotic membrane to a fully developed ligament, similar to that encountered in the thoracic limb. It arises from the short plantar ligament of the tarsus.
Finally, the SL in the hind limb has a large lateral head that fills the concave space over the medial aspect of the 4th metatarsal bone. 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 is sometimes identified plantar to the SL, running between the heads of the two splint bones. In the proximal part of the metatarsus, it may be difficult to image the SL from a plantar approach, because of the prominent and axially concave head of the fourth metatarsus (lateral splint bone). It may therefore be useful to use a plantaromedial approach. 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 are difficult to establish because of a greater than two-fold variation in tendon size between normal individuals and depending on breed or type of horses. A review by Reef (see Recommended Reading) yields cross-sectional areas (CSA) of 0.8–1.2 cm2 in Thoroughbreds in the United States, while a larger range (0.72–1.93 cm2) has been reported as being normal for Thoroughbreds in Great Britain, with 7.7–13.9 cm2 in zone 4 (IIb) in National hunt horses. Tendon CSA varies with breed and size, and is smaller in Arabian-type horses (0.6–0.8 cm2) and in ponies. Smith and co-workers (1994) did not find a significant difference between Thoroughbreds and heavier horses. The cross-sectional area 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 although these are less sensitive measurements.
Dimensions for the SL have also been reviewed by Reef (1998). For racing Thoroughbreds and Standardbreds in the United States, the cross-sectional area ranges from 1.0–1.5 cm2, most horses having a cross-sectional area of 1.0–1.2 cm2 in the body part. 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
Tendinopathy/Desmopathy
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 (Smith, 2010) and will not be reviewed here. As inflammation is not always a major pathophysiological component of the condition, tendinopathy/desmopathy is more appropriate. However, the terms tendinitis/desmitis remain 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 Ultrasonographic Anatomy section). Acute damage is associated with an increase in size, and a reduction in echogenicity with loss of the normal striated pattern, often affecting the central area of the tendon. Immediately after the injury, the lesion fills up 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 3.4). The acute lesion is usually poorly defined and heterogeneous but long-axis images will confirm loss of fiber alignment (Figure 3.5).
Edema leads to increased water content and decreased echogenicity in the surrounding tendon, paratenon, and subcutaneous tissues (Figure 3.5). The tendon may be swollen, although this is variable initially, but one should detect peritendinous and subcutaneous tissue thickening. Comparison with the contralateral limb (which may not be completely normal with many overstrain injuries) may help confirm increased size. After a few days, organized hematoma and early, immature granulation tissue fill the lesion. They are hypoechogenic and provide the typical, discrete appearance of many tendinitis lesions (Figure 3.6).
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. It may become more evident after several days. In fact, to establish a baseline severity scan, all 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 3.6). It is most commonly located in the mid-metacarpal region. Lesions can also occur more eccentrically within the tendon (Figure 3.7). 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 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 relative to the cross-sectional area of the tendon, and also its proximodistal length, should be ascertained to aid in prognostication (see Semi-Objective Assessment of Severity) and for future reference.
Diffuse lesions are more challenging to visualize: the tendon becomes enlarged, hypoechogenic, and very heterogeneous (Figure 3.8). Sagittal ultrasound scans confirm diffuse loss of striation.
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 Quantitative Assessment of Flexor Tendon and SL Size). A greater than 20% difference between limbs is considered significant, although this may not be the case if both limbs are affected or may be due to 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 3.9). 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.
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, even though some subtle heterogeneity is sometimes interpreted as signs of aging change (Figure 3.10). However, the detection of previous injury, which may have been missed clinically, is a recognized risk for re-injury. Therefore, with increasing use of ultrasonography as a preventive means, the identification of chronic pathology or very mild injuries will increase. Subclinical tears or degeneration are represented by slightly hypoechogenic foci or diffuse areas, without overt signs of tendinitis (Figure 3.11).
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