Ultrasonography of the Fetlock

Ultrasonography of the Fetlock

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

1 AZURVET Referral Veterinary Centre, Centre d’Imagerie, Saint-Laurent-du-Var, France
2 The Royal Veterinary College, North Mymms, Hatfield, UK

The Fetlock Joint

The fetlock region is one of the most common seats of conditions causing lameness in horses of all types and uses. It is a relatively “simple” diarthodial articulation and is probably the easiest starting point for anyone willing to learn to scan joints: most structures are directly accessible through the skin and can be imaged with standard equipment. Many ultrasonographic changes encountered in the fetlock joint may be extrapolated to any other joint or synovial structure throughout the equine body.

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.

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 degrees 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. (A) Sagittal plane scan images over the sagittal ridge of MC3. The probe is placed over the dorsal-most aspect of the joint and moved proximally and distally from distal metacarpus to proximal third of P1. (B) The probe is then moved abaxially to assess both medial and lateral MC3 condyles, the sound beam remaining parallel to the sagittal plane in order to remain perpendicular to the joint surfaces. (C) The examination of the dorsal joint region is repeated in a transverse plane. This time the probe plane must follow the curvature of the MC3 condyle and bone surfaces. This is achieved by constantly looking for the brightest bone interface. The probe must be moved both from proximal to distal and lateral to medial in order to scan the whole volume of the dorsal aspect of the joint. (D) and (E) both abaxial (i.e., lateral and medial) aspects of the joint are evaluated both in longitudinal and transverse planes in the same manner. In the transverse plane, the metacarpo-sesamoidean ligament (collateral ligament of the sesamoid) can be seen as a thin, striated structure caudal to the collateral ligament. Special attention is devoted to visualizing the collateral ligaments. To do so, the probe must be aligned to the ligament fibers by gentle rotation of the probe until the required image is obtained (F). The superficial branch is close to the frontal plane of the metacarpus, while the deeper branch is obliquely oriented, the metacarpal origin being dorsal to the long branch and the phalangeal insertion being palmar. (G) and (H) Finally the palmar aspect of the joint is visualized along with the structures of the digital flexor tendon sheath, abaxial surfaces of the proximal sesamoid bones and suspensory ligament structures (see below). Both transverse (H) and longitudinal (G) plane images must be obtained over the whole extent of the digital sheath from proximal to distal and from lateral to medial, including the abaxial surface of the sesamoid bones and P1. In order to image the oblique and short distal sesamoidean ligaments, the probe must be angled in order to align the beam with the ligament fibers. This requires thorough knowledge of the anatomy and some practice. (I) To image the short (deep) distal sesamoidean ligament, a microconvex transducer must be used and placed distoabaxial to the base of the sesamoid bones, in the space between the latter and the palmar processes of P1. It is angled upward and axially. (J) The examination is completed by flexing the joint and holding it off the ground or with the toe resting on a block, in order to expose the distal aspect of the metacarpal condyle. The examination is performed over this area as in (A) to (C).

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).

Figure 2.2 Schematic organization of the fetlock joint – Sagittal (A) and frontal (B) sections. MC3: third metacarpal/metatarsal bone; P1: proximal phalanx; SR: sagittal ridge of the metacarpal condyle; 1: subchondral bone; 2: hyaline cartilage; 3: joint capsule (fibrous with inner synovial lining); 4: dorsal proximal synovial fold; 5: joint fluid; 6: transverse fold; 7: enthesis (capsular/ligamentous insertions); 8: periosteum; 9: palmar joint pouch with villous synovial folds; 10: proximal scutum/intersesamoidean ligament; 11: common/long digital extensor tendon; 12: short distal sesamoidean ligament; 13: straight distal sesamoidean ligament; 14: deep digital flexor tendon; 15: superficial digital flexor tendon; 16: collateral ligament (superficial branch, continuous with periosteum); 17: collateral ligament (deep branch, obliquely transsected); 18: metacarpal epicondyle; 19: epicondylar fossa; 20: joint space.

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.

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 (dorsal synovial fold) in the proximal aspect of joint (sf). Little or no fluid is usually visible in the joint space. Sc: subcutaneous tissue; fc: fibrous capsule. The triangular, dorsal space between the joint surfaces (yellow arrow) is filled by a rigid synovial membrane extension, which forms a pointy transverse fold. (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 transverse synovial fold may be surrounded by a small amount of joint fluid (arrow). (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, and 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) where 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 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.

Figure 2.5 (A) Dissected specimen showing the structure of the collateral ligaments (sb: superficial branch; db: deep branch). (B) Positioning of the transducer, along the axis of the metacarpus to image the sb (1) and after an approximate 30-degree rotation to image the db (2). (C) Dissected specimen and ultrasonographic image obtained in a frontal plane (probe aligned with the metacarpus in position 1 showing the superficial branch of the collateral ligament (red arrows) extending from proximal to the epicondyle (ep) to the abaxial aspect of P1 (p1). The superficial branch blends into the periosteum (yellow arrow). The joint space is indicated by the white arrow. ef epicondylar fossa; mc metacarpal condyle. (D) Rotation of the probe according to position 2 will bring the deep branch into view (yellow arrows). Its extends from the distal aspect of the epicondyle (ep) to the proximal edge of P1, spanning the epicondylar fossa (ef). The space between the bone and ligament is filled with hypoechogenic synovial membrane (s). The white arrow points to the joint space.

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 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.

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 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.

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 (double red arrow). The cartilage interface is visible (yellow arrows). (B) Dorsal aspect, sagittal plane: the synovial membrane (s) is hypoechogenic, and the transverse synovial ridge remains sharp and triangular (arrow). (C) Dorsomedial aspect, parasagittal plane: in severe cases, very enlarged capsular vessels are seen (arrows), and 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 capsule is thickened, heterogeneously hypoechogenic, and displaced by the synovial fluid effusion (e). MC3: distal metacarpus, c = joint capsule.

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). The joint fluid (e) is abnormally echogenic and grainy. This may be indistinguishable ultrasonographically from purulent septic fluid. C: joint capsule.

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). SR: sagittal ridge of MC3; SL: lateral suspensory ligament branch.

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 (A) Dorsal aspect, sagittal plane. In chronic synovitis, the synovial membrane becomes more echogenic and rounded (yellow arrow). Note the cartilage interface (calipers) over the sagittal ridge (sr) and clubbed, thickened proximal synovial fold (red arrow). (B) Dorsal aspect, transverse plane: The dorsoproximal synovial pad becomes rounded to club-shaped, forming a dense, mass-like structure most prominent abaxial to the sagittal ridge, immediately proximal to the medial and/or lateral condyles (arrow).

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.

Figure 2.12 (A) Dorsal aspect of the fetlock, sagittal plane. Chronic hypertrophic synovitis can produce firm synovial masses (between calipers) filling the joint recesses and displacing the fibrous capsule (c) and extensor tendons, cde: common digital extensor tendon. (B) Same plane in a different animal. These synovial masses are space occupying and can eventually cause pressure remodeling of the underlying bone (double red arrow), especially prominent dorsoproximally, proximal to the sagittal ridge (SR) and condyles. (C) Transverse image over the palmar joint pouch showing a tissue mass (double yellow arrow) filling the space between the medial suspensory ligament branch (SL) and the palmar aspect of MC3. SR: sagittal ridge; MC3: third metacarpal bone; e: proximal capsular enthesis; p: periosteum.

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 subchondral bone of the sagittal ridge (yellow arrows), with loss of cartilage and direct contact between the synovium and exposed bone (dorsal aspect, sagittal plane, horse weightbearing). Subacute synovitis is present with synovial edema and effusion (red arrow). Similar images may be obtained with “kissing lesions.” (B) Parasagittal view over the distal aspect of the metacarpus (lateral condyle) with the fetlock fully flexed (probe position as on Figure 2.1j), showing focal loss of cartilage, exposing the underlying subchondral bone (yellow arrows) which appears slightly irregular. The synovial membrane fills the defect (red arrow). mc: medial condyle; P1: proximal phalanx; SR: sagittal ridge; ca: hyaline cartilage; c: capsule; cde: common digital extensor tendon.

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).

Figure 2.14 Standing, medial parasagittal image over the medial metacarpal condyle (MC). The cartilage is abnormally thin and very irregular in thickness (calipers). It is heterogeneously hyperechogenic (yellow arrows) and a focal chondral erosion is visible (red arrow).

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.

Figure 2.15 There is small, mineralized “chip” fragment at the dorsomedial proximal border of the proximal phalanx (red arrows). (A) A discrete defect (yellow arrows) is noted in the cartilage and subchondral bone on the medial condyle, at the area of contact of the fragment during hyperextension (“kissing lesion”). The cartilage proximal to the impact lesion is markedly thinned and heterogeneous (white arrow heads). Note an early osteophyte at the proximal extent of the subchondral bone interface (blue arrow). (B) Flexed parasagittal view over the dorsomedial aspect of the joint. The subchondral bone of the medial metacarpal condyle is smooth and even but there is diffuse thinning of the cartilage (arrows), which appears slightly hyperechogenic and heterogeneous. (C) Partially flexed, medial parasagittal view over the medial metacarpal condyle. There is severe, chronic synovitis with the capsule displacing the common digital extensor (CDE) tendon. The cartilage is very irregular and heterogeneously echogenic (yellow arrows) and there is widespread loss of cartilage, the synovial membrane coming into contact with the underlying bone interface (red arrows). Note the osteophytic bone spur on P1 (white arrow). C: capsule.

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 (yellow arrow) at the dorso-medio-proximal border of P1 (medial dorsal eminence) (dorsal parasagittal image). Note the echogenic thickening of the capsule (c) and entheseous new bone remodeling (red arrow) at the capsular insertion on P1 (e). (B) More diffuse irregular new bone (yellow arrows) at the dorsoproximal aspect of the sagittal ridge (SR), with focal erosions and areas of hyperechogenicity of the hyaline cartilage (red arrows). (C) Spur-like new bone production at the lateral articular border of P1 (red arrow). Note this osteophytic bony spur is continuous with the articular surface of P1 and deep to the insertion of the collateral ligament (LCL). This must be differentiated from entheseophytes at the insertion of the deep branch of the ligament (yellow arrow). More diffuse, irregular osteophytic new bone is visible at the abaxial border of the metacarpal condyle (white arrow-head). (D) Long-standing, severe OA can lead to exuberant new bone productions (yellow arrows) at the articular margins, as here on P1 (transverse view over the dorsal aspect of the proximal P1).

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

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