Preparation and Scanning Technique

High-frequency (7.5–18 MHz) linear array transducers provide optimal information in this area, although a micro-convex probe may occasionally be useful to image the distal aspect of the sesamoid bones. A standoff pad may be used to improve probe-to-skin contact and structure alignment. However, with copious amounts of coupling gel, placing the probe directly on the skin provides finer control of the pressure exerted on the skin and underlying structures. Excessive pressure may cause pain, alter the shape of structures examined, and displace an effusion or a mobile fragment, which may thus be overlooked.

The technique is fairly simple as most structures are close the skin surface and relatively perpendicular to the sound beam when the probe rests on the skin. Images obtained in a longitudinal plane, i.e., in a direction perpendicular to the joint space and parallel to the long axis of the limb, are easier to interpret, although both longitudinal and transverse planes should be used in combination. A standard examination protocol is presented in Figure 2.1. The joint is first examined with the horse weightbearing on the limb examined. Examination starts on the dorsal aspect of the joint in the sagittal plane, to visualize the sagittal ridge of the metacarpal/metatarsal condyle (MC3) (Figure 2.1A). By convention, the proximal aspect of the limb is placed to the left of the image. The probe is then moved laterally or medially, in parasagittal planes, in order to scan the whole width of the condylar surfaces and the dorsoproximal border of the proximal phalanx (P1) (Figure 2.1B). It is important that the probe remains perpendicular to the joint surface, i.e., in planes parallel to the sagittal plane, to obtain the brightest (most echogenic) and sharpest image of the subchondral bone interface. This applies to most structures to be evaluated. Once the whole dorsal articular surface has been examined in longitudinal views, the transducer is placed over the sagittal ridge and rotated 90 degrees so that the lateral aspect of the limb is to the left of the image (Figure 2.1C). The joint is then scanned in transverse planes from proximal to distal and from lateral to medial, including the entire joint, i.e., from the distal one-third of the metacarpus/metatarsus to the proximal third of P1.

Both abaxial aspects (i.e., lateral and medial aspects) are subsequently assessed, including the abaxial surface of the sesamoid bones. The examination is performed in both longitudinal (i.e., frontal) (Figure 2.1D) and transverse planes (Figure 2.1E), to image the collateral ligaments, epicondyles, joint space, abaxial aspect of the sesamoid bones, and the palmar/plantar joint pouch. Complete evaluation of the collateral ligaments will require slight rotation of the probe in the long axis, in order to bring the superficial and deep branch fibers in alignment with the scan plane (i.e., align the ligament fibers across the image) (Figure 2.1F). The superficial branch is more or less parallel to the long axis of the metacarpus with the limb weightbearing. The deep branch crosses its superficial counterpart, so that the probe must be tilted counterclockwise laterally and clockwise medially (i.e., the proximal end of the probe is rotated dorsally and the distal end toward the palmar aspect of the limb).

Finally, the examination is continued over the palmar/plantar aspect of the fetlock to image the tendon sheath structures (see below) but also the sesamoid bone surfaces, inter-sesamoidean ligament, and distal sesamoidean ligaments for a comprehensive assessment of the fetlock area (Figures 2.1G and 2.1H). It may be useful to use a convex array transducer to image the space between the base of the sesamoid bones and the proximal palmar border of P1 and the short distal sesamoidean ligaments (Figure 2.1I). A small-footprint linear transducer may also be used. This will be described in the second part of this section.

The limb is eventually picked up and flexed as much as the horse will tolerate in order to evaluate the distal metacarpal/metatarsal joint surfaces (Figure 2.1J). This may be performed in both longitudinal and transverse planes.

Ultrasonographic Anatomy of the Normal Fetlock Joint

The gross morphology of the fetlock has been described in textbooks and the reader is encouraged to review the anatomy in detail in order to improve both the technique and ability to detect subtle anomalies. The main anatomical features to bear in mind will be reviewed here before addressing the normal ultrasonographic features.

The fetlock may be considered a typical condylar diarthrodial joint (Figure 2.2). As for all diarthodial articulations, the fetlock comprises the bone surfaces, the overlying hyaline cartilage, the joint capsule with an inner synovial membrane, and outer fibrous capsule (ligaments are local fibrous reinforcements of the fibrous capsule).

The pathology will therefore reflect this structure and many ultrasonographic alterations encountered in each joint tissue will be the same in the same tissue type for all diarthrodial joints.

The distal articular surface of the third metacarpus (MC3) or metatarsus (MT3) (referred to as metacarpal or metatarsal head in the Nomina Anatomica Veterinaria) forms a transversely arranged, cylindrical surface, separated into two condyles by a sagittal ridge. These parts of the metacarpal/metatarsal head fit into reciprocal depressions on the proximal surface of the proximal phalanx (P1). This arrangement only allows for flexion and extension movements in the sagittal plane of the limb. The joint is encased in a single joint capsule, although this is functionally separated into dorsal and palmar/plantar compartments by the collateral ligaments medially and laterally. These are tightly pushed against the abaxial surfaces of the MC3 or MT3 head, although synovial membrane fills the space between the concave supracondylar fossa of the MC3/MT3 and the ligament, leaving only a very thin, slit-like passage for the synovial fluid between the two compartments. Fluid distension can therefore only occur dorsal or palmar/plantar to the collateral ligaments, unless these have ruptured. Dorsally, the capsule is quite redundant to allow for extension. It forms a wide and thin synovial fold dorsoproximally and a fibroelastic transverse ridge which fills the triangular space between the capsule, metacarpal head, and proximal P1. The digital extensor tendons are closely associated with the capsule, although there is a synovial bursa between these two structures dorsoproximally.

The proximal sesamoid bones are ossified structures developing within the palmar/plantar joint capsule. They are joined by a short and strong, transversely arranged intersesamoidean ligament. They are covered on the dorsal, articular surface by hyaline cartilage and on the palmar/plantar surface by fibrocartilage to sustain the pressure from the deep digital flexor tendon. This modified palmar/plantar capsule is refered to as proximal scutum (scutum proximale). It attaches to the palmar/plantar aspect of the proximal and middle phalanges by a series of strong distal sesamoidean ligaments, reinforcing the capsule (see below). Finally, each sesamoid bone has an ipsilateral collateral sesamoidean ligament whose transverse fibers blend into the capsule and superficial branch of the collateral ligaments.

The ultrasonographic anatomy of the fetlock has been described by Denoix (1996) [1]. The fetlock is grossly similar in the thoracic and pelvic limbs. “Metacarpus” and “palmar” will therefore be used for both “metacarpus”/“metatarsus,” and “palmar”/“plantar” in the following text. The basic organization of the joint is schematically reviewed in Figure 2.2.

The distal metacarpus forms a smooth, cylindrical surface, transversally oriented and separated into two condyles (lateral and medial) by the sagittal ridge. The latter is perfectly round and smooth in longitudinal section and triangular in cross-section (Figure 2.3). The cartilage is anechogenic and regular in thickness. It is thickest over the sagittal ridge (usually 1–1.2 mm) and thinner over the condyles, typically less than 0.7 mm. The mineralized part of the cartilage and underlying subchondral bone cannot be differentiated; they form a smooth, hyperechogenic interface, producing shadowing and reverberation.

Proximal to the condyles and sagittal ridge, the cartilage is interrupted and there is often a small step between the edge of the cartilage and bare bone proximal to it. In this location, bone is actually covered by synovium up to the proximal insertion of the joint capsule. This area may be variable in shape, slightly irregular, or concave (Figure 2.4).

The dorsoproximal edge of the proximal phalanx (P1) is rounded and smooth and its proximal articular surface is not visible (Figure 2.3). The dorsomedial and dorsolateral eminences of P1 are slightly convex, the medial one being slightly more prominent. In normal joints, the capsule and synovium are tightly applied against the bone surfaces. With severe effusion, the capsule may be displaced away from the bone surfaces.

The joint capsule on the dorsal aspect of the joint is thick, fibrous (isoechogenic to the extensor tendons), and elastic (Figures 2.2 and 2.3). It inserts proximally on the metacarpus 3–4 cm proximal to the proximal limit of the condyles and distally on the dorsoproximal aspect of P1, approximately 2 cm distal to the proximal border. There, the capsule adheres to the common/long digital extensor tendon. The lateral digital extensor tendon blends broadly into the fibrous capsule.

Dorsally, the synovial membrane forms a transverse ridge, triangular in longitudinal section, which fills the space between the metacarpal condyles and P1. Proximally, the membrane reflects back to form a thin, flat, and transversely oriented fold or pad (also called the synovial plica), normally not visible unless there is marked thickening and/or effusion. No fluid is normally visible.

The thin (2–4 mm) common (or long) digital extensor tendon (CDE) is separated from the joint capsule by a small subtendinous bursa. The latter is virtual and rarely visible in normal horses (Figure 2.3).

Abaxially (laterally and medially), the medial (MCL) and lateral (LCL) collateral ligaments are mere focal thickening of the joint capsule (Figure 2.5). Their borders are therefore difficult to differentiate from the rest of the fibrous capsule in cross-sections. In longitudinal (frontal) planes, parallel striation similar to that of tendons is clearly identified. Each ligament is made out of two separated branches: a long, superficial part and a shorter, deep part. The superficial branch originates from the metacarpal epicondyle and blends into the periosteum some 3 cm proximal to it. It inserts distally on the abaxial border of P1, 2–3 cm distal to the joint space. The deep branch originates dorsodistal to the epicondyle, crosses in a distopalmar direction under the superficial branch, and inserts on the edge of P1 close to the joint space. The collateral ligaments are therefore X-shaped and each branch must be examined separately (Figure 2.1). The transversely oriented collateral ligaments of the proximal sesamoid bones blend into the ipsilateral deep branch of the collateral ligament.

The palmar aspect of the joint is largely hidden by the proximal sesamoid bones (Figure 2.6). The sagittal ridge of the distal metacarpus is visible from the palmar aspect of the limb between them, through the transversely oriented intersesamoidean ligament and hypoechogenic proximal scutum.

The palmar synovial recess of the fetlock joint is a large synovial pouch located between the metacarpus, the apex of the sesamoid bones, the branches of the suspensory ligament, and the distal extremity (“button”) of the splint bones (second and fourth metacarpal bones). In normal horses, it is mostly filled by thick, highly vascularized synovial folds and contains little fluid. It can, however, fill up with a moderate amount of fluid in clinically normal horses.

Synovitis/Arthritis

Synovitis refers to inflammation of the synovial lining of the joint. It is a feature of most joint diseases but may occur as a primary entity, particularly in sports and racehorses. It should not be mistaken for a non-inflammatory joint effusion (“cold effusion”), where abnormal amounts of anechogenic fluid are present without any thickening of the synovium.

Acute synovitis is characterized by mild to severe thickening of the synovial layer of the capsule (this layer is not normally visible) because of edema, vascular hyperemia, and distension of the synovial pouches by joint fluid (Figure 2.7). The inflammation is most obvious dorsoproximally. The dorsal fold (“synovial pad or plica”) is displaced away from the articular surface, and becomes hypoechogenic, mildly thickened, and clubbed (Figure 2.8). The palmar pouch contains large synovial villi that rapidly become hypertrophied and fill up the pouch within a matter of days. These form dense, amorphous, and highly vascularized masses.

The fluid is normally anechogenic. In early, acute stages, mildly echogenic, “cellular” and grainy fluid may be present (Figure 2.9). This is typical of hemarthrosis, although this appearance is very similar to that of septic exudate. Bleeding probably occurs through trauma and tearing of the capsule and synovium. The fluid rapidly becomes anechogenic unless recurrent bleeding or sepsis is present, although fibrin clots and pannus may remain, adhering to cartilage for several days to weeks (Figure 2.10).

Chronic synovitis may develop as a complication of nearly any fetlock joint pathology that has been left untreated for too long or has not responded to treatment. The echogenicity of the synovial membrane increases, mostly because of a cellular infiltrate and secondary fibroplasia. It becomes more homogeneous and isoechogenic relative to the fibrous capsule. Fibrosis and retraction lead to rounding of the synovial pads and ridges (Figure 2.11).

Assessment of the synovial fold on the dorsal aspect of the joint is usually used as a reference point to confirm chronic inflammatory changes. There currently is a lack of quantitative data regarding what tissue thickness should be regarded as abnormal. Denoix suggests that thickening of the dorsoproximal synovial fold over 2 mm in thickness is a sign of inflammation, while other authors use a cut-off thickness of 4 mm [1].

In the palmar pouch, hypertrophic villi may be clubbed or rounded or form localized, dense masses, which can eventually undergo fibrocartilaginous metaplasia or even become mineralized (thus producing hyperechoic acoustic shadowing artifacts).

Synovial hypertrophy and hyperplasia can form space-occupying lesions (Figure 2.12A). Initially referred to as villonodular synovitis, they are most frequently recognized in the dorsal proximal pouch, where a pressure-induced, smooth, and rounded defect is often observed on the dorsodistal aspect of the metacarpus on radiographs as on ultrasound images (Figure 2.12B). Similar hyperplastic masses are also common in the palmar pouch (Figure 2.12C). This condition is purely inflammatory in horses and should therefore be referred to as chronic hypertrophic synovitis. In-between stages, spanning from mild proliferation to very large mass formation, are encountered.

Traumatic Cartilage/Subchondral Bone Injury

Traumatic arthritis of the fetlock can include bone contusion and various degrees of bone surface damage (subchondral bone or cartilage injuries, fragmentation, etc.). Although radiography, using specific oblique views, can show the presence of fragmentation or defects, radiographs are often non-diagnostic. Scintigraphy and magnetic resonance imaging (MRI) are the most sensitive techniques to confirm such lesions. Ultrasonography will often confirm the presence of subchondral bone defects or cartilage erosions (Figure 2.13). Lesions located on the palmar part of the metacarpal condyles are hidden from view by the sesamoid bones. Large lesions may, however, be identified with the joint fully flexed. Proximal P1 subchondral injuries are not amenable to ultrasonographic diagnosis. Further investigations may be indicated if there is ultrasonographic evidence of severe synovitis without specific lesions identified on either radiography or ultrasonography.

Osteoarthritis (OA)

Fetlock OA is extremely common. Radiographic signs are often subtle and occur late in the disease process, at a stage where cartilage damage has become irreversible. Ultrasonography provides much earlier signs of joint disease and permits identification of very subtle lesions or new bone production (Figure 2.14).

Typically, the earliest signs of OA include mild to severe synovitis and subtle cartilage changes, with focal or diffuse irregularity and discrete hyperechogenic foci. Thinning is most obvious on the sagittal ridge (although care should be taken not to misinterpret the incidental irregularity frequently seen at the dorsoproximal end of this ridge). Erosions may be visible on the distal aspect of the metacarpal condyles. In severe cases, there may be barely any visible cartilage left, the capsule coming into contact with the subchondral bone (Figure 2.14). Focal defects may result from contact (“kissing”) lesions induced by osteochondral fragments (Figure 2.15). High-frequency probes (12–18 MHz) may be required to provide details of cartilage integrity.

Osteophytes are bony proliferations at the ends of the subchondral bone plate, where the synovial membrane replaces cartilage. True osteophytes form irregular to spiky, hyperechogenic interfaces that protrude into the joint (Figure 2.16). They are typically in alignment with the joint surface. They are most commonly encountered at the dorsoproximal border of P1, along the proximo-abaxial edges of P1 and the disto-abaxial edges of the metacarpal condyles, along the dorsoproximal margin of the cartilage-covered condyles and at the apex of the sesamoid bones. There may also be marked irregularity of the proximal extremity of the sagittal ridge.

Enthesophytes are bony new bone productions within the insertion of capsules (Figure 2.16C), ligaments, or tendons. They may be secondary to chronic inflammation, OA, primary capsulitis, or collateral desmitis (see below). They are encountered within the dorso-abaxial insertion of the capsule, and at the collateral ligament attachments on P1 and the metacarpus.

Chronic hypertrophic synovial hyperplasia is usually present in OA (Figure 2.14C), although repeat intra-articular medication with steroid drugs may prevent chronic inflammatory changes, especially in young racehorses, making the diagnosis more difficult to confirm on ultrasound examination.

Osteochondrosis and Cyst-Like Lesions

Subtle erosions or defects in the subchondral bone outline are more readily identified ultrasonographically than radiographically. The extent of the lesion in both longitudinal and transverse planes can be accurately established. Associated soft tissue inflammatory changes, cartilage thinning, and adhesions can also be detected. Depending on the stage of the lesion, it may be difficult to differentiate osteochondrosis from osseous cyst-like lesions, subchondral bone trauma, and erosions secondary to OA.

Osteochondrosis (OC) is common in the fetlock. The most common site is the dorsal aspect of the sagittal ridge where it can span from mild, usually clinically silent flattening of the ridge, to various degrees of subchondral bone defects (Figure 2.17). The thickness and echogenicity of the cartilage are both increased in OC, although thinning of the cartilage will eventually occur as a result of fragmentation and fibrillation of the damaged cartilage, and secondary OA. Ultrasonography provides useful information on cartilage dissection or fragmentation and on the extent and location of the lesions (Figure 2.18). Cartilage dissection (osteochondritis dissecans or OCD) may be identified by the formation of an interface between the cartilage and bone defect in some cases (Figure 2.18A). Hyperechogenic areas of mineralization may be seen within affected cartilage (Figures 2.18B, 2.18C, and 2,18D) and loose cartilage or osteochondral fragments (“joint mice”) may be identified within the joint space. Large fragments, usually lenticular (i.e., thin and oval shaped) may be encountered within, or attached to the dorsoproximal synovial fold (Figure 2.18E

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