TWO: Ultrasonography of the Fetlock

Ultrasonography of the Fetlock

Eddy R.J. Cauvin1 and Roger K.W. Smith2

1AZURVET Referral Veterinary Centre, Cagnes sur Mer, France

2The Royal Veterinary College, North Mymms, Hatfield, UK

The Fetlock Joint

Preparation and Scanning Technique

High-frequency (7.5–16 MHz) linear array transducers provide optimal information in this area, although a micro-convex probe may be useful to image the distal aspect of the proximal 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, which may thus be overlooked.

Images obtained in a longitudinal (parasagittal) plane, i.e. in a direction perpendicular to the joint space, are easier to interpret, although both longitudinal and transverse planes should be used in combination (Figure 2.1). The joint is examined with the horse weightbearing. Examination starts on the dorsal aspect, then both abaxial aspects are assessed, including the collateral ligaments, abaxial aspect of the sesamoid bones, and the palmar/plantar joint pouch. The intersesamoidean ligament is assessed from a palmar approach through the flexor tendons and proximal scutum (fibrocartilage covering the palmar aspect of the sesamoid bones). The limb is eventually picked up and flexed to evaluate the distal metacarpal/metatarsal joint surfaces.

Figure 2.1    Schematic views showing the position of the transducer to examine the four quadrants of the fetlock. Each time the probe is rotated 90° to obtain both longitudinal (A and B) and transverse plane (C) images. In between, oblique positions are often necessary to better visualize the various structures. The examination starts on the dorsal aspect, in a sagittal plane (A and B), bringing the sagittal ridge into view. The probe is then slid sideways into a parasagittal plane (A) to image the medial and lateral condyles. The same examination is performed in transverse planes (C). The abaxial aspects are imaged longitudinally in a frontal plane (A), although 20–30° clockwise and anticlockwise rotation is necessary to align the beam with the ligament branches. In between, oblique positions may be useful to assess all the surfaces. Finally the palmar aspect is imaged in both transverse and longitudinal planes (C).

Ultrasonographic Anatomy of the Normal Fetlock Joint

The ultrasonographic anatomy of the fetlock has been described by Denoix (see Recommended Reading at the end of the chapter). The fetlock is grossly similar in the thoracic and pelvic limbs. “Metacarpus” and “palmar” will therefore be used for both “metatarsus” and “metacarpus”, and both “palmar” and “plantar” in the following text.

The structures at the dorsal aspect of the joint are schematically reviewed in Figure 2.2. The distal metacarpus forms a smooth, cylindrical surface, horizontally 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.

Figure 2.2    Sagittal section of the dorsal aspect of the fetlock. The schematic shows the organization of the soft tissue components. The smooth sagittal ridge (sr) is lined by hyaline cartilage (c); the proximal dorsal aspect of P1 (p), capsule lined with synovial membrane (s), and common (or long) digital extensor tendon (e) are highlighted by the synovial cavities (in black). Note the synovial reflection in the proximal aspect of joint (sp). The subtendinous bursa (b) is not visible in normal horses, little or no fluid is usually visible in the normal joint space.
Figure 2.3    (A) Sagittal ultrasound scan over the sagittal ridge, showing the smooth sagittal ridge (sr) with overlying hyaline cartilage (c), the proximal dorsal aspect of P1 (p), synovial membrane (s), and common (or long) digital extensor tendon (e). Note the synovial pad reflection in the proximal aspect of joint (sp). The subtendinous bursa (b) is not visible in normal horses, little or no fluid is usually visible in the joint space. fc: fibrous capsule; sc: subcutaneous tissue. Note the hypoechogenic area within the synovial membrane proximal to P1 (arrow). This is an artifact due to the use of a linear array transducer. (B) A parasagittal image obtained medial to the sagittal ridge shows the round medial metacarpal condyle (mc) and dorsomedial proximal eminence of P1 (me). The cartilage is thinner than on the sagittal ridge. The triangular, dorsal space between the joint surfaces (arrow) is filled by synovial membrane which forms a pointy transverse ridge. (C) Transverse image over the dorsal aspect of the fetlock. The sagittal ridge (sr) and condyles (lateral [lc] and medial [mc]) are smooth and even. The cartilage is clearly visible except over the sides of the sagittal ridge where it is off-incidence. The synovial membrane fills the space on either side of the ridge (thick arrow). A small amount of fluid may be seen on the side of the sagittal ridge, a thin interface is seen between the anechogenic cartilage and the fluid (thin arrow).

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).

Figure 2.4    Sagittal view of the dorsoproximal aspect of the sagittal ridge, showing a common variation in the normal bony contour, proximal to the sagittal ridge (SR) the cartilage thins out gradually at its edge. The bone surface proximal to it is lined by synovial membrane (arrow). The bone there may be irregular. Although this may be associated with joint disease, it is often encountered in clinically normal horses.

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 edge of the condyles and distally on the dorsoproximal aspect of P1, approximately 2 cm distal to the proximal edge. 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, 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 (CDET) 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 extends from the metacarpal epicondyle down to the abaxial border of P1, 2 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. The transversely oriented collateral ligaments of the proximal sesamoid bones blend into the deep branches of the collateral ligament.

Figure 2.5    (A) Dissected specimen showing the structure of the collateral ligaments (db: deep branch; sb: superficial branch). (B) Positioning of the transducer, along the axis of the metacarpus to image the sb (1) and after an approximate 30° rotation to image the db (2). (C) Ultrasonographic image obtained in a frontal plane (probe aligned with the metacarpus) showing the superficial branch of the collateral ligament (thin arrows) extending from proximal to the epicondyle (ep) to the abaxial aspect of P1 (p). The joint space is indicated by the thick arrow. (D) Rotation of the probe will bring the deep branch into view (yellow arrows). Its extends from the underside of the epicondyle to the proximal edge of P1.

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.

Figure 2.6    Imaging of the palmar aspect. (A) Position of the transducer. (B) Ultrasonographic image with the transducer as in (A). The scutum proximale (sp) forms a thick fibrocartilage, which encloses the sesamoid bones (sb). The intersesamoidean ligament (il) should be homogeneous and regular with a transverse fiber pattern. Only a small width of the sagittal ridge (sr) is visible between the two sesamoid bones. (C) The palmar recess of the metacarpophalangeal joint is imaged through a palmaroabaxial approach. (D) The palmar recess is located in a triangle between the metacarpus (mc3), proximal sesamoid bone, and branch of the suspensory ligament (susp). It is filled with synovial villi (s) with little fluid normally visible.

The palmar 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 synovial folds and contains little fluid. It can however fill up with a moderate amount of fluid in clinically normal horses.

Ultrasonographic Abnormalities of the Fetlock


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 race horses. 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”) 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.

Figure 2.7    Acute synovitis. (A) Dorsal aspect, transverse plane: the dorsal pouch is filled with anechogenic fluid displacing the capsule and extensor tendon dorsally. The cartilage interface is visible (arrows). (B) Dorsal aspect, sagittal plane: the synovial membrane (s) is hypoechogenic, the transverse synovial ridge remains sharp and triangular (arrow). (C) Dorsomedial aspect, parasagittal plane: in severe cases, very enlarged capsular vessels are seen (arrows), edema causes the synovium to appear heterogeneous (s).
Figure 2.8    Within hours of injury, the proximal synovial pad thickens up and takes on a clubbed appearance (calipers), though remaining hypoechogenic (dorsomedial aspect, parasagittal plane).

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).

Figure 2.9    Hemarthrosis (dorsomedial aspect, parasagittal plane, distal to the left). The joint fluid is abnormally echogenic and grainy. This may be indistinguishable ultrasonographically from purulent septic fluid.
Figure 2.10    In the subacute stage, blood is removed but fibrin pannus or strands (arrows) may be visible (A: dorsal aspect transverse oblique plane), and an organizing hematoma (H) may become visible in the redundant palmar pouch (B: palmar lateral aspect, transverse plane).

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).

Figure 2.11    In chronic synovitis, the synovial membrane becomes more echogenic and rounded (thin arrow). Note the cartilage interface (calipers) and thickened proximal synovial reflection (thick arrow) (A: dorsal aspect, sagittal plane). The dorsoproximal synovial pad becomes clubbed, forming a dense, mass-like structure most prominent abaxially (arrow) (B: dorsal aspect, transverse plane).

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. (See Recommended Reading for further information.)

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 2.12). 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. Similar hyperplastic masses are also common in the palmar pouch. 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.

Figure 2.12    Chronic hypertrophic synovitis can produce firm synovial masses (between calipers) filling the joint recesses and displacing the extensor tendons (A: dorsal aspect, sagittal plane). These are space occupying and can eventually cause pressure remodeling of the underlying bone (arrow) (B: dorsal aspect, sagittal plane). mc3: third metacarpal bone; sr: sagittal ridge.

Enthesophytes, characterized by an irregular capsule-to-bone interface and the presence of hyperechogenic spurs at the capsule insertions, often occur at the collateral ligament insertions and at the capsule and extensor tendon attachments on the dorsoproximal aspect of P1.

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.

Figure 2.13    Trauma or concussion to the joint surface can cause focal destruction of the cartilage and erosion or chipping of the underlying subchondral bone. (A) A focal defect is seen on the sagittal ridge (thin arrows), with loss of cartilage and direct contact between the synovium and subchondral bone (dorsal aspect, sagittal plane). Subacute synovitis is present with synovial edema and effusion (thicker arrow). (B) Parasagittal view over the distal aspect of the metacarpus (lateral condyle) with the fetlock fully flexed, showing focal loss of cartilage and an irregular defect in the subchondral bone (arrows). mc: medial condyle; P1: proximal phalanx; sr: sagittal ridge.

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.

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 (Figure 2.14). In severe cases, there may be barely any visible cartilage left, the capsule coming into contact with the subchondral bone. 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.

Figure 2.14    Flexed parasagittal view over the dorsomedial aspect. The subchondral bone is smooth and even but there is diffuse thinning of the cartilage (arrows), which appears slightly hyperechogenic.
Figure 2.15    There is small chip fragment at the dorsomedial proximal border of the proximal phalanx (thick arrow). A discrete defect is noted in the cartilage and subchondral bone on the medial condyle, at the area of contact of the fragment during hyperextension (“kissing lesion”, thin arrow). The rest of the cartilage is slightly irregular.

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.

Figure 2.16    Various locations and appearances of osteophytes (A) Spur-like exostosis at the dorso-abaxial proximal border of P1, with remodeling of P1 and of the proximal edge of the metacarpal condyle. (B) Irregular new bone at the dorsoproximal aspect of the sagittal ridge (thin arrows), with focal erosions and areas of hyperechogenicity in the sagittal ridge cartilage (thick arrows). (C) Lateral abaxial articular edge of P1 (arrow), underneath the insertion of the collateral ligament (LCL). mc3: third metacarpus; sr: sagittal ridge.

Enthesophytes are bony new bone productions within the insertion of capsules, ligaments, or tendons. They may secondary to chronic inflammation, OA, primary capsulitis, or collateral desmitis. 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, 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). Lesions may also occur immediately lateral and/or medial to the sagittal ridge dorsoproximally. 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. Cartilage dissection (osteochondrosis dissecans or OCD) may be identified by the formation of an interface between the cartilage and bone defect in some cases (Figure 2.18). Hyperechogenic areas of mineralization may be seen within affected cartilage and loose cartilage or osteochondral fragments (“joint mice”) may be identified within the joint space (Figure 2.19).

Figure 2.17    (A) Osteochondrosis lesion of the sagittal ridge, with an abnormally flat to slightly concave shape of the subchondral bone and focal thickening of the overlying cartilage (calipers). The latter is regular and anechogenic, there is no sign of dissection. This is usually an incidental finding. (B) More severe OC lesion on the dorsal aspect of the medial condyle, immediately medial to the sagittal ridge. The subchondral bone is irregular and concave, the cartilage is irregular and heterogeneous (arrows). Thickened synovium adheres to the lesion. Note the chronic synovitis.
Figure 2.18    Osteochondrosis of the dorsal sagittal ridge. The subchondral bone is irregular proximally with thickened, echogenic, and heterogeneous cartilage (between large arrows). A linear interface (thin arrow) is noted between the deep portion of the cartilage and a shallow subchondral bone defect, characteristic of cartilage dissection (OCD). The cartilage flap is thinned and heterogeneous.
Figure 2.19

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Jun 8, 2017 | Posted by in EQUINE MEDICINE | Comments Off on TWO: Ultrasonography of the Fetlock

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