The Veterinary Journal 193 (2012) 129–134

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The Veterinary Journal

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Ultrasonographic appearance of bony abnormalities at the dorsal aspectof the fetlock joint in geriatric cadaver horses

K. Vanderperren ⇑, I. Gielen, A. Van Caelenberg, E. Van der Vekens, E.V. Raes, S. Hauspie,H. van Bree, J.H. SaundersDepartment of Veterinary Medical Imaging and Small Animal Orthopaedics, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium

a r t i c l e i n f o a b s t r a c t

Article history:Accepted 24 September 2011

Keywords:UltrasonographyHorseFetlock jointBoneComputed tomography

1090-0233/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.tvjl.2011.09.018

⇑ Corresponding author. Tel.: +32 9 2647650.E-mail address: Katrien.Vanderperren@UGent.be (

This article describes the ultrasonographic (US) appearance of bony abnormalities on the dorsal aspect ofthe third metacarpal/metatarsal bone of the equine fetlock in cadavers with radiographic signs of osteo-arthrosis. After US, computed tomography was undertaken to better characterise the lesions. Twelve fet-lock joints were collected and all had more than one bone abnormality on US. Normal subchondral boneappeared on US as a well-defined and regular hyperechoic line with distal acoustic shadowing.

Bone abnormalities detected on US included (1) gaps in the proximal subchondral bone filled withmaterial of heterogeneous echogenicity, (2) bone fragments represented as small straight smoothlydelineated hyperechoic lines with distal shadowing located superficial to the surface of the adjacentbone, (3) proximal new bone formation visible as mild to severe cortical protrusions, (4) marginal oste-ophytoses seen as an elevation of the hyperechoic surface of the subchondral bone at the edges of thejoint surfaces, (5) indentations in subchondral bone seen as a concave deviation of the hyperechoic linewithout interruption, (6) focal or diffuse irregularities of the subchondral bone seen as disruptions of thenormal smooth bony contours, and (7) focal hyperechoic spikes originating from the subchondral plateand invading the articular cartilage. These findings are discussed.

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Introduction

The fetlock (metacarpo/metatarsophalangeal) joint of the horseis a high motion joint which, of all the joints in the horse, is themost susceptible to traumatic and degenerative lesions (Pool andMeagher, 1990). Traditionally, radiography is the first diagnosticimaging technique used to evaluate most bone lesions that causelameness in horses (Vanderperren and Saunders, 2009) whileultrasonography (US) is widely used to detect lesions in soft tis-sues, such as tendons and ligaments, and to assess the fetlock joint(Denoix, 1996, 1998; Redding, 2001; Smith and Smith, 2008).

Evaluation of bone lesions using US is limited by the inability ofthe sound beam to penetrate bony tissue, which means that UScannot be used to evaluate the internal structure of the bone (Keenand Conaghan, 2009). However, the high reflectivity of sound at thebone–soft tissue interface makes US ideal for evaluating bone con-tours (Cho et al., 2004). Indeed in human medicine, US hasemerged as a promising alternative to radiography for the diagno-sis of fractures, osteoporosis and osteoarthrosis (Hans and Krieg,2008; Keen et al., 2008). Very often, changes such as osteoarthrosis,erosion and periosteal reactions can easily be seen with US before

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K. Vanderperren).

they can be seen on radiographs (Reef et al., 2004; Rasera and Mac-oris, 2007; Keen et al., 2008; Wittoek et al., 2010). However, the lit-erature on ultrasonographic assessment of bone in horses is scarce.

This article describes the bony abnormalities in the equine fet-lock joint detected using US at the dorsal aspect of the third meta-carpal/metatarsal bone in cadavers with osteoarthrosis. Computedtomography (CT) was then undertaken to better characterise thelesions found with US, as CT is an excellent imaging modality forthe evaluation of bone structures of the equine fetlock joint (Vand-erperren et al., 2008).

Materials and methods

Specimens

Twenty fetlock joints, transected at the carpometacarpal or the tarsocrural joint,of five cadaver horses (four Warmblood and one Andalusian; mean age 19.5 years(range 15–24 years)) euthanased for reasons unrelated to this study, were collected.A complete radiographic examination (Mobilux, X-ray Equipment Verachtert) ofeach joint was made and included five projections: lateromedial, flexed lateromedi-al, dorso-palmar (plantar), dorso 45� lateral-palmaro (plantaro) medial oblique anddorso 45� medial-palmaro (plantaro) lateral oblique.

The radiographs were evaluated for the presence of bone lesions compatiblewith osteoarthrosis. Twelve of the joints showed osteoarthrotic changes and wereused for the present study. All specimens were stored at �20 �C until, just priorto examination, when the limbs were thawed to room temperature. No clinical his-tories of the horses were available.

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Ultrasonography

The limbs were clipped, washed and coated with US coupling gel. Ultrasonogra-phy was undertaken using a grey-scale US machine (Mylab 30, Esaote) with a 6–9 MHz and a 10–15 MHz linear transducer. A thorough examination of the dorsalaspect of the fetlock joint, including at maximum flexion, was performed with spe-cial attention to the third metacarpal/metatarsal bone, and appropriate scanningplanes and images/films were recorded for subsequent evaluation.

Computed tomography

CT scans were made using a multi-slice helical scanner (GE Lightspeed QX/i,General Electric). Prior to the CT study, the joints were cut open and the metacar-pal/metatarsal bones were removed. The metacarpal/metatarsal bones were posi-tioned in lateral recumbency to obtain transverse slices with the long axis of thebones parallel to the CT table. A scout image was taken to check for symmetryand to ensure that the entire region to be examined was included in the image.Acquisition variables were: 100 kV and 140 mA, a 9.1 cm field of view, and a matrixsize of 512 � 512. The bones were scanned in a distal-to-proximal direction, fromthe most distal aspect of the trochlea of the third metacarpal/metatarsal bone tothe attachment of the articular capsule.

Transverse CT scans of 1.3 mm thickness with an overlap of 0.6 mm were ob-tained using a bone algorithm, and 2D multi-planar reconstructed images weremade in a sagittal plane. The DICOM studies were retrieved and analysed on theeFilm Workstation PACS software (Merge eFilm, Merge eMed).

Image analysis

The first author evaluated all of the ultrasonographic and CT images. The dorsalaspect of the fetlock joint was scanned from proximal to distal, starting at the distalbody of the third metacarpal/metatarsal bone. The trochlea of the third metacarpal/metatarsal bone was divided into the lateral condyle, the sagittal ridge and themedial condyle. Any bone abnormality (i.e. deviation from normal subchondralbone, described as a well-defined and regular hyperechoic line with distal acousticshadowing) at the dorsal aspect was recorded. The description of the lesions wasbased on their ultrasonographic appearance.

The US evaluation and the CT evaluation were performed separately. After-wards, the US images showing bone abnormalities were matched with the corre-sponding CT images. The results are presented in parallel with CT images forbetter understanding of the lesions.

Results

On US, all joints presented a variable degree of osteoarthrosischaracterised by marginal osteophytosis or new bone formationand abnormalities of the subchondral bone plate varying from asmall indentation to severe irregularities. More than one abnor-

Fig. 1. Transverse ultrasound image (lateral is left on the image) of the dorsomedialaspect of the fetlock joint ((1) sagittal ridge of the third metacarpal bone; (2) medialcondyle of the third metacarpal bone). The reference ultrasonographic image ofnormal subchondral bone demonstrated a well-defined and regular hyperechoicline with distal acoustic shadowing. The subchondral bone on the border of thesagittal ridge is hypoechoic compared to the top of the sagittal ridge and the medialcondyle, because the transducer is not perpendicular to the bone. The homogeneousanechoic band (�) between the articular capsule and the subchondral bone is thecartilage. (3) Articular capsule; (4) extensor digital communis tendon.

mality of the subchondral bone was detected in all joints. Fig. 1shows normal subchondral bone as seen on US with a well-definedand regular hyperechoic line with distal acoustic shadowing.

In all cases, the subchondral bone of the proximal aspect of thefetlock joint, at the level of the transition between the body of thethird metacarpal/metatarsal bone and the trochlea, showed smallinterruptions of the hyperechoic line, representing focal breaks inthe subchondral bone. The bottoms of the focal breaks were cov-ered with a hyperechoic line parallel to the subchondral bone(Fig. 2). At that level, these interruptions are a normal finding.

The proximal subchondral bone, just proximal to the trochlea,had a normal contour (n = 7 joints) or a mild to severe irregularcontour (n = 5). The irregular contour was visible as mild to severecortical protrusion on US, representing new bone formation(Fig. 3). In all joints, the most proximal aspect of the trochleashowed mild to severe irregularities in the contour of the subchon-dral bone.

In two joints, a gap in the subchondral bone was seen at theproximal lateral condyle (4 � 4 mm) (Fig. 4) and at the proximalmedial condyle (4 � 3 mm). The whole contour of the bone defectwas visible in one joint, whereas in the other joint only the distalborder of the bone defect was seen as a distal grey line at the bot-tom of the gap. Material of heterogeneous echogenicity could beseen within the defect. In another joint, a bone fragment couldbe seen as a small straight smoothly delineated hyperechoic linewith distal acoustic shadowing, located at the proximal medialcondyle superficial to the surface of the adjacent bone. This frag-ment was present in the medial synovial pad (Fig. 5).

Marginal osteophytosis, overgrowth of bone tissue located atthe edges of the joint surfaces, was seen at the lateral (n = 8) andmedial (n = 7) aspects of the joint. These osteophytes at the bonemargin appeared as an elevation of the hyperechoic surface ofthe subchondral bone (Fig. 6).

An indentation in the subchondral bone was detected at themedial condyle in two joints and at the sagittal ridge in one joint.This indentation was seen as a concave deviation of the hyperech-oic line/bone that did not interrupt the line (Fig. 7). In another se-ven joints, an indentation of the subchondral bone was seen justmedially to a marginal osteophyte on transverse US scans.

Irregularities of the subchondral bone were seen as a disruptionof the smooth bone contour. Focal irregularities or irregularitiesaffecting the entire length of the subchondral bone of the condylesand the sagittal ridge on the transverse scan were detected in alljoints (Fig. 8). These irregularities were detected at the sagittalridge (n = 9), the lateral condyle (n = 8) and the medial condyle(n = 10).

During the ultrasonographic examination of the joints, 11 focalhyperechoic spikes (white spots), originating from the subchondralplate and invading the articular cartilage, were detected in fivejoints (Fig. 9). In these five joints, the spikes were seen at the lateralcondyle (n = 8), sagittal ridge (n = 1) and medial condyle (n = 2).Three of the 11 spots were seen with the joint in flexion and in2/5 joints, multiple spikes were detected.

Discussion

This study describes the US appearance of bony abnormalitiesthat can be detected in osteoarthrotic fetlock joints, and demon-strates the ability of US to visualise a wide range of bony abnor-malities in the equine fetlock.

Any disruption in the linearity of the subchondral bone mayrepresent a lesion (Relave et al., 2009) and US allows easy detec-tion of abnormalities, such as erosions, irregularities and osteo-phytes (Denoix et al., 1996; Keen and Conaghan, 2009). In allcases in the present study, the subchondral bone of the proximal

Fig. 2. (A) Tranverse ultrasonographic image (lateral is on the left of the image; position of the transducer: dotted area in B). Proximal aspect of the joint shows smallinterruptions in the subchondral bone surface. Note the hyperechoic lines parallel to the subchondral bone corresponding to the bottom of the defects on CT (arrows). (B)Corresponding transverse CT image.

Fig. 3. (A) Transverse ultrasound image (lateral is on the left; position of the transducer: dotted area in B) of the dorsoproximal aspect of the fetlock joint with severe newbone formation. The proximal subchondral bone appears rough and irregular. (B) Corresponding transverse CT image.

Fig. 4. Ultrasonographic images in (A) transverse (lateral is on the left) and (B) longitudinal (proximal is left) plane of the dorsolateral condyle (position of the transducer:dotted area in C and D), and corresponding CT images in (C) transverse and (D) sagittal planes. Ultrasonography of the dorsolateral condyle demonstrates an interruption ofthe bone margin with a step down contour defect (circle). Note the small distal grey line at the bottom of the gap (arrow) representing the distal border of the bone defect onCT.

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aspect of the fetlock joint at the level of the transition between thebody of the third metacarpal/metatarsal bone and the trochlea,showed focal breaks. This is a normal ultrasonographic findingdue to vascular channels or foramina present at that level, and sothe focal breaks should not be misinterpreted as fractures (Denoixet al., 1996; Cho et al., 2004). The bottoms of these defects were

visible on US as hyperechoic lines parallel to the subchondral bone.The sides of the vascular channels were not visible on US becausethe beam was parallel to them.

Mildly to severely irregular subchondral bone was seen in alljoints at the most proximal aspect of the trochlea of the third meta-carpal/metatarsal bone. The literature reports that on radiography,

Fig. 5. (A) Transverse (lateral is on the left) and (B) longitudinal (proximal is left) ultrasound images (position of the transducer: dotted area in C and D) of a fragment at theproximomedial aspect of the fetlock joint. The fragment is seen as a hyperechoic line with a distal acoustic shadowing and lying in a more superficial position on both thetransverse and longitudinal plane compared to the bone surface. The fragment is lying in the medial synovial pad. Corresponding CT images in the transverse (C) and sagittal(D) plane where the fragment is seen as an oval hyperattenuating structure lying in the soft tissues dorsally to the medial condyle. (1) Sagittal ridge of the third metacarpalbone; (2) medial condyle of the third metacarpal bone.

Fig. 6. (A) Transverse ultrasonographic image (lateral is on the left; position of the transducer: dotted area in B) of a large smoothly delineated marginal osteophyte, visible asan elevation of the margin profile present at the border of the lateral condyle. (B) Corresponding transverse CT image.

Fig. 7. (A) Longitudinal ultrasonographic image (proximal is on the left; position of the transducer: dotted area in B) of the fetlock joint showing the sagittal ridge (1). A smallindentation (arrow) in the subchondral bone of the sagittal ridge is present. Note that the cartilage (�) is still intact. (B) Corresponding CT image in the sagittal plane.

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the dorsoproximal aspect of the sagittal ridge may show someradiographic variation (irregularity, lucency, indentation, notch-

ing) in young horses without clinical problems (Hauspie et al.,2010). In our study, one small fragment was also detected by US

Fig. 8. (A) Transverse ultrasonographic image (lateral is on the left; position of the transducer: dotted area in B) of the medial part of the sagittal ridge and the medial condyle.Both show an irregular outline over the complete length. (C) Longitudinal ultrasonographic image (proximal is on the left; position of the transducer: dotted area in D) of thesagittal ridge demonstrating a focal irregular area in the middle part of the ridge (arrow). (B) Corresponding transverse and (D) sagittal CT images showing the irregularities ofthe subchondral bone.

Fig. 9. (A) Transverse ultrasound image (lateral is on the left; position of the transducer: dotted area in B) of a white spot visible within the cartilage and originating from thesubchondral bone at the lateral condyle (arrow). (B) Corresponding transverse CT image.

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in the medial synovial pad. This fragment, superimposed on thesagittal ridge, was suspected on the lateromedial radiographicprojection.

Ultrasonography has been shown to be superior to radiographyfor detecting and characterising dorsal bone fragments (Vander-perren et al., 2009). Because most fragments appear as hyperechoiclines with distal acoustic shadowing, the underlying bone cannotbe seen and, therefore, cannot be evaluated. In two joints, a largedefect was seen on US at the proximal aspect of the fetlock joint.A recent study has demonstrated that it is feasible to use diagnosticUS imaging techniques to assess sub-millimetre bone defects inreal time and with high accuracy and precision (Parmar et al.,2010).

Marginal osteophytosis, at the periphery of the osteochondraljunction, represents a well-known component of osteoarthrosisin horses and was easily visualised in our study. Osteophytes aredefined on US as ‘single or multiple characteristic irregularities ofthe bone profile, located at the edges of the joint surfaces’ (DelleSedie et al., 2008) and as ‘cortical protrusions seen in two planes’

(Keen et al., 2008). An indentation of the subchondral bone wasmost often seen at the most lateral/medial surface of the boneand often in combination with the presence of marginal osteophy-tosis. Irregularity of the subchondral bone plate of the trochlea onUS could be focal or it could affect the entire length of the subchon-dral bone during transverse scanning.

During the US examination of the joints, multiple focal hyper-echoic spikes (called white spots in this study) were detected orig-inating from the subchondral plate and invading the articularcartilage. These findings correspond with the central osteophytesin the fetlock of the horse that have recently been described (Oliveet al., 2009). However, we did not perform any histological exam-ination. Central osteophytes are found typically in joints with mar-ginal osteophytes and in advanced osteoarthrosis (McCauley et al.,2001).

A major advantage of US includes visualisation of multiple scan-ning planes in real time. Although it is possible to save images ofthe examination, it can be difficult for an observer who has not per-formed the examination to interpret these images confidently (Rel-

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ave et al., 2009). US is non-invasive, does not require the use ofradiation and gives information on both soft tissue and bone andon their interaction. An abnormality should be scanned in the lon-gitudinal and transverse plane.

Another advantage of ultrasound is that the examination is dy-namic, which provides the opportunity to scan in various planes inorder to better define the extent of the pathology and its relation tothe surrounding anatomy (Denoix, 1998; Redding, 2001). Theexamination of the flexed fetlock joint considerably increases theextent of the exposed bone surface allowing a more completeassessment of the subchondral bone lesions (Denoix, 2009). How-ever, in contrast to the dorsal aspect of the fetlock joint, the prox-imal articular surface of the proximal phalanx and the palmar/plantar articular surface of the metacarpal/metatarsal trochleacannot be accessed by the ultrasound probe and, therefore, cannotbe imaged (Denoix and Audigie, 2001; Denoix, 2009).

A major disadvantage of US is that only contour interruptionsare visible due to the total reflection and absorption of the ultra-sound waves by the bone surface. Therefore, US is not a substitutefor radiography but should be used in combination with radiogra-phy. Another disadvantage is that US is an operator-dependenttechnique (Relave et al., 2009).

In the present study, we used CT as a reference method becauseof its ability to demonstrate bone details (Vanderperren et al.,2008). The CT images were shown in parallel to better understandthe bony lesions detected by US.

The present study had some potential limitations. As this studywas performed on cadavers rather than on live horses, the radio-graphs were taken ex vivo in a simulated standing position thatdid not adequately reflect joint loading. However, in our opinion,this had no influence on the detection of osteoarthrotic changeson radiography. Each lesion detected in this study occurred natu-rally and was not produced artificially. Therefore, the bony abnor-malities detected by US in our study should also be detectable inlive horses. The mean age of the horses in the study was high(19.5 years), so the morphological changes may be associated withage and not necessarily with pathology. The lack of clinical infor-mation on these selected cases is also a limitation of this study.Further studies detailing the sensitivity and, even more impor-tantly, the specificity of US to diagnose or rule out fetlock pathol-ogy should still be conducted.

Conclusions

Ultrasound is a cost-effective and good technique for the evalu-ation of the subchondral bone plate of the metacarpal/metatarsalcondyles. In this study, an ultrasonographic study of cadavers ofgeriatric horses with fetlock osteoarthrosis enabled the visualisa-tion of several bony changes including new bone formation, mar-ginal osteophytes, subchondral bone irregularities, indentations,bone fragments, gaps in the subchondral bone and focal hyperech-oic spikes originating from the subchondral bone plate and invad-ing the articular cartilage at the dorsal aspect of the equinemetacarpal/metatarsal bone.

Conflict of interest statement

None of the authors of this paper has a financial or personalrelationship with other people or organisations that could inappro-priately influence or bias the content of the paper.

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