Ultrasonography of the Equine Tarsus - Home Page - International

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Ultrasonography of the Equine Tarsus Mary Beth Whitcomb, DVM Ultrasound of the tarsus, like other joints in the horse, provides valuable information to document and characterize soft tissue and osseous injury. The collateral ligaments, superficial digital flexor tendon, long digital extensor tendon, gastrocnemius tendon, and peroneus tertius are most commonly affected, but injuries to all tendons and ligaments can be seen. The frequency of wounds makes the tarsus a prime target for septic arthritis, tenosynovitis, and bursitis. Ultrasound can be used to show involvement of synovial structures in many cases of sepsis. Author’s address: Department of Surgical & Radiological Sciences, School of Veterinary Medicine, University of California at Davis, Davis, CA 95616; e-mail: [email protected]. © 2006 AAEP. 1. Introduction Ultrasonographic imaging of equine joints is gener- ally well accepted as a valuable diagnostic modality for tendon, ligament, and other soft tissue injuries. Compared with other joints such as the shoulder and stifle, tarsal ultrasound may easily be consid- ered the most challenging. The complex anatomy of the tarsus, including several tendons and liga- ments coursing in multiple directions and numerous synovial structures, contributes to this difficulty. A solid understanding of tarsal anatomy is critical to performing a successful ultrasound examination. Radiography has traditionally been the primary im- aging modality for hock disease, but it provides little to no information on tendon or ligament injury or the appearance of synovial structures. Ultrasound remains the most cost effective imaging modality to diagnose soft tissue injuries and can be performed by motivated practitioners with basic ultrasound skills and equipment. Similar to other regions in the horse, the information gained from tarsal ultra- sound often provides a diagnosis when physical- exam findings and radiographic evaluation have failed to reveal the source of lameness and/or clinical signs. Although computed tomography and mag- netic resonance imaging of the tarsus have been described, the use of these imaging modalities is not yet practical in most cases. 1,2 Common indications for tarsal ultrasound include distention of synovial structures, hock swelling, and/or wounds and lacerations. Ultrasound is very useful to elucidate the cause of tarsal swelling by differentiating between cellulitis, effusion of syno- vial structure(s), and/or tendon or ligament injury. The equine tarsus is a common site for puncture wounds and lacerations that often involve synovial structures, tendons, and/or ligaments. Communi- cation with these structures is often difficult to de- termine by palpation alone. Contrast radiography can be helpful for this purpose, but it gives little information on associated soft tissue injury. Ultra- sound is well suited to document wound communi- cation with synovial structures and/or bone and to assess the degree of soft tissue injury. Ultrasound can also help to differentiate between septic and non-septic synovitis/tenosynovitis and can identify the best location to obtain synovial fluid samples for AAEP PROCEEDINGS Vol. 52 2006 13 IN-DEPTH: HOCKS NOTES

Transcript of Ultrasonography of the Equine Tarsus - Home Page - International

Page 1: Ultrasonography of the Equine Tarsus - Home Page - International

Ultrasonography of the Equine Tarsus

Mary Beth Whitcomb, DVM

Ultrasound of the tarsus, like other joints in the horse, provides valuable information to documentand characterize soft tissue and osseous injury. The collateral ligaments, superficial digital flexortendon, long digital extensor tendon, gastrocnemius tendon, and peroneus tertius are most commonlyaffected, but injuries to all tendons and ligaments can be seen. The frequency of wounds makes thetarsus a prime target for septic arthritis, tenosynovitis, and bursitis. Ultrasound can be used toshow involvement of synovial structures in many cases of sepsis. Author’s address: Department ofSurgical & Radiological Sciences, School of Veterinary Medicine, University of California at Davis,Davis, CA 95616; e-mail: [email protected]. © 2006 AAEP.

1. Introduction

Ultrasonographic imaging of equine joints is gener-ally well accepted as a valuable diagnostic modalityfor tendon, ligament, and other soft tissue injuries.Compared with other joints such as the shoulderand stifle, tarsal ultrasound may easily be consid-ered the most challenging. The complex anatomyof the tarsus, including several tendons and liga-ments coursing in multiple directions and numeroussynovial structures, contributes to this difficulty.A solid understanding of tarsal anatomy is critical toperforming a successful ultrasound examination.Radiography has traditionally been the primary im-aging modality for hock disease, but it provides littleto no information on tendon or ligament injury orthe appearance of synovial structures. Ultrasoundremains the most cost effective imaging modality todiagnose soft tissue injuries and can be performedby motivated practitioners with basic ultrasoundskills and equipment. Similar to other regions inthe horse, the information gained from tarsal ultra-sound often provides a diagnosis when physical-exam findings and radiographic evaluation have

failed to reveal the source of lameness and/or clinicalsigns. Although computed tomography and mag-netic resonance imaging of the tarsus have beendescribed, the use of these imaging modalities is notyet practical in most cases.1,2

Common indications for tarsal ultrasound includedistention of synovial structures, hock swelling,and/or wounds and lacerations. Ultrasound is veryuseful to elucidate the cause of tarsal swelling bydifferentiating between cellulitis, effusion of syno-vial structure(s), and/or tendon or ligament injury.The equine tarsus is a common site for puncturewounds and lacerations that often involve synovialstructures, tendons, and/or ligaments. Communi-cation with these structures is often difficult to de-termine by palpation alone. Contrast radiographycan be helpful for this purpose, but it gives littleinformation on associated soft tissue injury. Ultra-sound is well suited to document wound communi-cation with synovial structures and/or bone and toassess the degree of soft tissue injury. Ultrasoundcan also help to differentiate between septic andnon-septic synovitis/tenosynovitis and can identifythe best location to obtain synovial fluid samples for

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analysis. Ultrasound should also be performed inhorses with radiographic evidence of fracture(s) sus-picious for collateral ligament avulsion and inhorses with evidence of osteomyelitis in close prox-imity to synovial structures. Less common indica-tions include nuclear scintigraphic findings ofincreased radiopharmaceutical uptake in the hockregion and lameness localized to the tarsus.

The purpose of this paper is to review the sono-graphic anatomy of the tarsus, describe techniquesused to image the dorsal, medial, lateral, and plan-tar structures of the tarsus, and review the distri-bution of tendon, ligament, bone, and synovialinjuries during the 6.5-yr study period (1999–2005)from the Large Animal Ultrasound Service at theUniversity of California, Davis, Veterinary MedicalTeaching Hospital (UCD-VMTH). Although the ul-trasonographic appearance and clinical findings ofmany soft tissue injuries of the tarsus have beenpreviously described in the veterinary literature,3–9

the distribution of injuries from a large patient pop-ulation has not been reported.

2. Ultrasonographic Technique

Ultrasonographic evaluation of the equine tarsus isbest divided into four regions (dorsal, medial, lat-eral, and plantar) because of the large number oftendons, ligaments, and synovial structures.Structures evaluated from the dorsum include theperoneus tertius (PT), cranial tibial tendons of at-tachment (primarily cunean tendon), long digitalextensor (long DE) tendon/tendon sheath, and tibio-tarsal (TT) joint capsule. The joint capsules of theproximal intertarsal (PIT), distal intertarsal (DIT),and tarsometatarsal (TMT) joints can also be eval-uated from the dorsum but are not typically seen inmost horses. Medial structures include the super-ficial and deep components of the medial collateralligament (MCL). The cunean tendon is also imagedmedially. Lateral structures include the lateraldigital extensor (lat DE), tendon/tendon sheath, andsuperficial and deep components of the lateral col-lateral ligament (LCL). Structures evaluated fromthe plantar aspect of the tarsus include the longplantar ligament (LPL), superficial digital flexortendon (SDFT) and its retinacular calcaneal attach-ments, gastrocnemius (GN) tendon, deep digitalflexor tendon (DDFT), and small medial DDFT.Plantar synovial structures include the subcutane-ous bursa, calcaneal bursa, GN bursa, tarsal sheath,and plantar pouches of the TT joint.

Horses should be sedated with either detomidineHCla (0.005–0.01 mg/kg IV) or xylazine HClb (0.3–0.4 mg/kg IV). Butorphanol tartratec (0.01–0.02mg/kg IV) should be used with intractable horses orthose with severe pain or lameness. When avail-able, horses should be restrained in stocks for equip-ment and operator safety. As usual, the bestimages are obtained by clipping the hair of the hockregion with #40 blades, washing the skin with soapand water, and applying ultrasound coupling gel.

Alcohol saturation can be used, but horses can be-come irritated by alcohol running down their legs.Evaluation of all structures can be accomplishedusing standard ultrasound equipment available inmost equine practices. A high-frequency (7–14MHz) linear transducer used for most musculoskel-etal imaging is also well suited for tarsal ultrasound.A tendon-format transducer (commonly known as a“T” probe) is ideal; however, a rectal-format trans-ducer will also produce diagnostic images. A scan-ning depth of 4–6 cm is appropriate for moststructures. Whenever possible, a program de-signed for musculoskeletal imaging should be se-lected. A standoff pad is not necessary but may beuseful when evaluating bony prominences.

3. Dorsal Structures

Peroneus TertiusThe PT can be followed from its origin at the exten-sor fossa of the distal femur throughout the length ofthe tibia to its insertions on the distal tarsal bonesand proximal metatarsus (Fig. 1). The tendon isquite large in the proximal and midtibia regionswhere it is located deep to the long DE muscle belly.The PT becomes smaller and more superficially lo-cated in the distal tibia region (Fig. 2) and bifurcatesinto two primary tendons of insertion at the level ofthe talus. The tendons of insertion are often chal-

Fig. 1. Anatomy specimen showing location and orientation ofdorsal tarsal structures. Note the relative lateral location of thelong digital extensor tendon (Lo) adjacent to the peroneus tertius(PT), the close association of the dorsal tendons of attachment ofthe PT and cranial tibial (CT) muscle, and the medial location andorientation of the cunean tendon (Cu). LA, lateral digital exten-sor tendon.

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lenging to differentiate from the dorsal tendon ofinsertion of the cranial tibial muscle. PT abnor-malities are primarily limited to ruptures that canoccur secondary to falls, limb entrapment, or full-limb casting. Ruptures are most common in themid and distal tendon regions but can also occur atthe origin.10 The affected tendon is usually mark-edly enlarged with a diffusely hypoechoic and mot-tled appearance in the region of the rupture with anabsence of linear fibers on longitudinal views (Fig.3). Relaxation artifact of the proximal portion ofthe tendon may also be present in cases of rupture.

Cranial Tibial/Cunean TendonThe cranial tibial muscle belly is located deep to thePT tendon in the mid/distal tibia region and has twotendons of attachment. These include the cuneantendon (medial tendon of attachment) and the dorsaltendon that inserts onto the dorsal surfaces of the

third tarsal and third metatarsal bones. Comparedwith the dorsal tendon, the cunean tendon is readilyimaged. The tendon begins to form in the distaltibia/proximal talus region deep to and between thePT lobulations that form immediately proximal tothe PT bifurcation (Fig. 4, A and B). From thislocation, the cunean tendon quickly courses medi-ally in a transverse direction toward its insertiononto the fused first and second tarsal bones. Linearfibers are easily seen deep to the skin surface onlongitudinal views (Fig. 4C). The cunean bursa islocated deep to the tendon in the midtendon regionand is usually not visible in normal horses. Inju-ries to the cunean tendon and bursa are uncommonbut can occur secondary to trauma or lacerations.8

Long DE Tendon/SheathThe long DE tendon is dorsolaterally locatedthroughout the tarsal region. The long DE muscle

Fig. 2. Transverse (left) and longitudinal (right)ultrasound image of the normal PT in the distaltibia region, a common location for PT rupture.The PT (arrows) has a relatively thin shape be-tween the long DE and CT muscle belli-es. (Technique: 10-MHz linear, 5-cm scanningdepth.)

Fig. 3. Ultrasound image of a peroneus tertiusrupture (arrows) in a 12-yr-old Quarter Horsegelding with a gait characteristic for the disor-der. Cause of injury was unknown. An en-larged tendon with a mottled echogenicity is seenon transverse views (left). Complete disruptionis evident on longitudinal views (right). Theproximal end of the tendon can be seen in thiscase (open arrows). (Technique: 10-MHz lin-ear, 5.5-cm scanning depth.)

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is found superficial to the PT in the proximal andmidtibia regions and can be followed to its musculo-tendinous junction in the mid/distal tibia region.At the level of the talus, the long DE tendon islocated immediately lateral to the PT and shows anoval echogenic shape with a linear fiber pattern (Fig.5). The tendon is easily followed throughout thetarsus and metatarsus where it becomes more dor-sally located. Injury to the long DE tendon occursfrequently in horses with dorsal lacerations,11 butinjuries can occur without external trauma and mayresult in distention of the long DE tendon sheath.The tendon sheath is typically not visible in normalhorses, although some horses will show a small ane-choic rim of fluid surrounding or partially surround-ing the tendon. Although severe tenosynovitis of

the long DE tendon sheath can be differentiatedfrom other synovial structures by location and pal-pation, ultrasound should be performed to rule outtendon damage as a cause of distention (Fig. 6).

Tibiotarsal Joint Capsule

Distention of the dorsal TT joint capsule is a com-mon physical exam finding. Ultrasound can deter-mine the relative degree of effusion versus synovialthickening that contributes to the external appear-ance of distention, especially in severe cases. Thedorsal joint capsule is easily imaged in most horsesat the dorsomedial aspect of the tarsus between thePT and cranial tibial tendons of insertion and theMCL. Synovial fluid is typically anechoic (Fig. 7).

Fig. 4. Sequential ultrasound images of Cu from dorsal (A and B) to medial (C). (A) Transverse image showing formation of PTtendons of insertion at the level of the distal tibia/talus. The Cu (arrows) appears “off beam” deep to and between the PT tendons.(B) Same image as A, but Cu is now “on beam” and the PT tendons are “off beam.” (C) Longitudinal image of Cu (arrows) as it extendsmedially toward its insertion. (Technique: 10-MHz linear, 3.1-cm scanning depth.)

Fig. 5. Transverse (left) and longitudinal(right) ultrasound image of a normal long digitalextensor tendon (arrows) at the level of the dis-tal tibia. Note the flattened oval shape deep tothe skin surface with a clear linear fiber patternon longitudinal views. (Technique: 10-MHzlinear, 3.1-cm scanning depth.)

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Hypoechoic- or cellular-appearing fluid may be seenin cases of sepsis or hemarthrosis. Horses withseptic arthritis often show severe synovial thicken-ing without significant effusion (Fig. 8); however,synovial thickening/proliferation is also a commonfeature in horses with non-septic TT synovitis (Fig.9). Arthrocentesis should be performed in suspectcases to rule out a septic component. The plantaro-medial and plantarolateral joint capsules will beaddressed later with other plantar structures.

4. Lateral Structures

Lateral Digital Extensor Tendon/SheathThe lat DE tendon is located on the lateral aspect of thelimb from the midtibia through the tarsal region (Fig.10). The tendon becomes dorsolaterally located in theproximal metatarsal region as it nears its junction withthe long DE tendon. The lat DE tendon shows asmaller but similar appearance to the long DE tendon(Fig. 11). Similar to the long DE tendon, lacerations

Fig. 6. Ultrasound image of severe chronic longDE tenosynovitis with associated long-DE ten-don injury (arrows). Prominent synovial thick-ening with anechoic effusion is apparentsuperficial to the tendon on transverse (left) andlongitudinal views (right). The large surface de-fect of the long DE tendon (arrowhead) was con-sidered the source of tenosynovitis. Endoscopicdebridement of the tendon sheath and torn longDE fibers was successfully performed in thishorse. (Technique: 13-MHz linear, 5.2-cmscanning depth.)

Fig. 7. Longitudinal ultrasound image of severe non-septic effu-sion of the dorsal TT joint capsule (arrowheads) in a horse withrecent injury of the MCL. Minimal synovial thickening ispresent despite severe effusion. (Technique: 10-MHz linear,4-cm scanning depth.)

Fig. 8. Longitudinal ultrasound image of septic arthritis of thedorsal TT joint capsule (arrowheads) in a 9-yr-old Thoroughbredgelding with severe lameness. The entire joint capsule is filledwith echogenic material because of severe synovial thickeningand fibrin deposition. Ultrasound revealed direct communica-tion of a small lateral tarsal wound to the plantarolateral TT jointcapsule. Minimal fluid accumulation was present within thejoint capsule. Ultrasound assisted with fluid collection, and cy-tologic evaluation confirmed sepsis. (Technique: 13-MHz lin-ear, 5.2-cm scanning depth.)

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may involve the lat DE tendon.11 Injury and associateddistention of the lat DE tendon sheath can occur withouttrauma. Lat DE abnormalities seem to be less frequentthan long DE injuries (Fig. 12).

Lateral Collateral LigamentsThe anatomy of the collateral ligaments of the tar-sus is well described by Updike,12 including detailed

descriptions of its four components (one long andthree short) and their respective origins and inser-tions. The focus of this paper will be on those com-ponents, origins, and insertions most easilyidentified with ultrasound. The superficial or longcomponent of the LCL originates from the caudola-teral aspect of the distal tibia/lateral malleolus.Its primary insertion onto the distolateral aspect ofthe calcaneus is easily seen on ultrasound. Addi-tional fibers extend to the fourth tarsal and fourthmetatarsal bones; however, these are less readilyvisible (Fig. 10, B-D). The superficial (long) LCL islocated plantar and adjacent to the lat DE tendon.Similar to collateral ligaments of other joints, theLCL is easiest to identify on longitudinal views (Fig.13). After identification of its densely packed lin-ear fibers, the LCL should be followed from its originto insertion to confirm imaging of the LCL versusthe adjacent lat DE. When identified on longitudi-nal views, the transducer can be rotated 90° fortransverse imaging. Injuries of the superficial LCLcan occur at any location along the length of theligament. Similar to other ligament injuries, theinjured LCL is typically enlarged with hypoechoicareas and a disrupted fiber pattern (Fig. 14). Pre-vious reports on LCL injuries have focused on horseswith lateral malleolar fractures13 or with evidence ofenthesiopathy,14 but LCL injuries can occur withouteither clinical finding. Compared with the superfi-cial LCL, the deep (short) component of the LCL issomewhat challenging to evaluate, because it curvesover the bony prominences of the talus. Its primaryorigin is located dorsal to the superficial LCL origin onthe distal tibia. The deep LCL then courses in a

Fig. 9. Longitudinal ultrasound image of non-septic effusionand synovitis of the dorsal TT joint capsule (arrowheads) in an8-yr-old Selle Frances gelding with a recent history of lamenessafter kicking in his stall. Prominent synovial thickening is pre-sent. Hypoechoic strands throughout the joint capsule are con-sistent with synovial inflammation and/or fibrin accumulationcaused by trauma. Ultrasound revealed moderate injury to themedial and lateral CLs in this horse. (Technique: 13-MHz lin-ear, 5.2-cm scanning depth.)

Fig. 10. Anatomy of lateral tarsus at multiple levels of dissection. (A) Skin and SC tissues removed. Note close association oflateral digital extensor tendon (LA) and superficial (long) LCL. (B) Extensor tendons removed to reveal association of superficial(long) and deep (short) LCL components. (C) Window removed from superficial (long) LCL to reveal the nearly transverse orientationof the deep (short) LCLs. (D) Bone specimen showing origins and insertions of superficial (long) and deep (short) LCL components.S, superficial digital flexor tendon; SL, lateral attachment of SDFT onto calcaneus; P, long plantar ligament; Cs, superficial (long)component of lateral collateral ligament; CD, deep (short) components of lateral collateral ligament; Lo, long digital extensor tendon;GN, gastrocnemius tendon.

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nearly transverse direction to its insertion onto themid portion of the calcaneus (Fig 10, C-D). Injuries ofthe deep LCL are not commonly seen.

5. Medial Structures: Medial Collateral Ligaments

Similar to the LCL, the MCL also has four compo-nents, briefly described as one long and three shortligaments (Fig. 15).12 Although the superficial lig-ament can easily be described as long, the primaryshort component is also quite long compared with itslateral counterpart and is probably better describedas a deep long component (Fig. 15C). The otherdeep (short) MCLs are challenging to visualize onultrasound because of their small size and will notbe addressed. The superficial (long) MCL origi-nates from the mid portion of the medial malleolusand has several areas of insertion (Fig. 16). Itsprimary insertion onto the distal tuberosity of thetalus is well visualized with ultrasound. Additionalfibers continue distally toward their insertions onto

the central tarsal, third tarsal, and third metatarsalbones. The deep (long) MCL originates slightly cra-nial and distal to the superficial MCL origin on themedial malleolus and courses in a slightly transverseorientation toward its primary insertion onto the dis-tomedial surface of the sustentaculum tali of the cal-caneus (Fig. 17). Similar to the LCL, imaging of thetwo MCLs cannot be performed simultaneously be-cause of their differing orientations. Imaging of eachMCL is most easily accomplished by first locating theirlinear fibers on longitudinal axis and then followingeach ligament from its origin to insertion before trans-verse imaging. Both ligaments show a dense linearfiber pattern with the exception of the origin of thesuperficial MCL where a slightly irregular fiber pat-tern is commonly seen. Injuries can involve one orboth components. Disruption of linear fiber patternis easiest to identify in affected horses (Fig. 18). Hy-poechoic areas on transverse views with enlargementof the ligament are also present. Although infre-

Fig. 11. Transverse (left) and longitudinal(right) ultrasound image of the normal lateraldigital extensor tendon (arrows) at the distal tibia.The tendon shows a flattened oval shape ontransverse views with a linear fiber pattern onlongitudinal images. (Technique: 10-MHz lin-ear, 3.1-cm scanning depth.)

Fig. 12. Ultrasound image of lateraldigital extensor tendonitis (arrows)in a 2-yr-old Thoroughbred with adorsal laceration. The tendon is en-larged with a large hypoechoic areaon transverse views (left) and largeareas of diffusely irregular fiber pat-tern on longitudinal views (right).Although the laceration was located atthe proximal metatarsus, tendon dam-age extended proximally to the level ofthe distal tibia. (Technique: 10-MHzlinear, 3.5-cm scanning depth.)

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quently reported, MCL injuries do occur.4,15 Similarto LCL injuries, affected horses may not have associ-ated avulsion fractures or enthesiopathy.4

6. Plantar Structures

SDFTThe SDFT is the most plantar/superficial structure inthe tarsal region; however, it is located deep to the GNin the proximal and midtibia regions. From this deeplocation, the SDFT rotates medially around the GNtoward its superficial location in the distal tibia andtarsal region. The tendon has a flattened, crescentshape as it approaches the calcaneal tuber where reti-

nacular attachments secure the tendon onto the me-dial and lateral surfaces of the calcaneus. Thetendon then continues distally through the tarsus andmetatarsus along plantar midline. Lacerations to theplantar tarsus frequently involve the SDFT. Injurycan also occur without external trauma. Affectedhorses may show localized swelling that can be misin-terpreted as “curb.” Ultrasound is necessary in casesof “curb” to differentiate between injuries to the SDFTor LPL, calcaneal bursitis, or SC swelling.5,16

Disruption of either SDFT retinacular attachment tothe calcaneus can occur in horses with or without skinlaceration.5,9 The resultant luxation of the SDFT

Fig. 13. Longitudinal ultrasound image of the normal superficial (long) LCL (arrows) showing its origin on the lateral malleolus ofthe distal tibia and its primary insertion onto the distolateral aspect of the calcaneus. Note the dense linear fiber pattern of the LCL.The deep (short) LCL components cannot be visualized in this image because of their nearly transverse orientation to the superficial(long) LCL component. (Technique: 10-MHz linear, 4-cm scanning depth.)

Fig. 14. Ultrasound image of severe desmitis ofthe superficial (long) component of the LCL (ar-rows) in a 3-yr-old Thoroughbred gelding withsevere lameness. The LCL is enlarged on trans-verse views (left image) with a large hypoechoicarea along its plantar border. Disruption in fi-ber pattern is readily apparent distally on thelongitudinal view (right image). Periligamen-tous swelling is also present superficial to theLCL. A small lateral wound indicated previoustrauma and communicated with an osteomyeliticlesion of the fourth tarsal bone (not visible in thisimage). (Technique: 13-MHz linear, 5.2-cmscanning depth.)

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may be obscured in acute cases by the severe swellingthat usually accompanies this injury.17 Ultrasoundcan easily be used to document this condition. Medialretinacular disruption with lateral SDFT displace-ment is most common, but lateral disruption with me-dial luxation can occur.9

Gastrocnemius TendonAs stated above, the GN is located superficial tothe SDFT in the midtibia region. The GN rotateslaterally around the SDFT toward its locationdeep to the SDFT in the distal tibia region. TheGN tendon becomes quite large at this level as it

Fig. 15. Anatomy of medial tarsus at multiple levels of dissection. (A) Skin and SC tissues removed. Note the plantaromediallocation of the DDFT through the tarsal canal and the small medial DDFT located dorsal to the DDFT. (B) Cu and medial DDFTremoved to reveal association of superficial (long) and deep (short) MCL components. (C) Superficial (long) MCL removed to revealthe relatively transverse orientation of the primary deep MCL. (D) Bone specimen showing origins and insertions of superficial (long)and deep MCL components. Cu, cunean tendon; CS, superficial component of medial collateral ligament; CD, deep component ofmedial collateral ligament; D, deep digital flexor tendon; DM, medial tendon of deep digital flexor tendon; S, superficial digital flexortendon; SM, medial attachment of SDFT onto calcaneus.

Fig. 16. Composite longitudinal ultrasound image of a normal superficial (long) component of the MCL showing its origin on the distaltibia at the medial malleolus and its primary insertion onto the distal talus. Fibers continue distally to insert on the dorsal surfacesof the central tarsal (CT) and third tarsal (T3) bones. The deep MCL component is not visible because of its more transverseorientation compared with the superficial component. TT JT, tibiotarsal joint; JC, joint capsule of TT JT; PIT JT, proximal intertarsaljoint; DIP JT, distal intertarsal joint. (Technique: 10-MHz linear, 3.5-cm scanning depth.)

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nears its insertion onto the calcaneal tuber. Thetendon can measure up to 3– 4 cm2 in normalhorses and often shows hypoechoic areas of vary-ing number and distribution (Fig. 19). Longitu-dinal imaging may reveal relaxation artifact of thetendon proximal to its insertion in normal horsesand should not be mistaken for injury. For thesereasons, comparison to the contralateral limb isoften required in suspect cases. Similar to othertarsal structures, plantar lacerations may involvethe GN tendon (Fig. 20). GN tendonitis has also

been reported without evidence of externaltrauma.3,8,9,18

DDFT

The DDFT is located plantaromedially throughoutthe tarsal region as it courses through the tarsalsheath medial to the calcaneus. The tendon has alarge oval shape with a homogeneously hypoechoicappearance. Injuries of the DDFT within the tarsalsheath are not commonly reported.5,16 However, theDDFT should always be evaluated in horses with tar-sal sheath effusion. The DDFT is often difficult toevaluate at the level of the sustentaculum tali wheresurface fibrillations may easily be missed on ultra-sound. Endoscopy can reveal the presence of surfacetears in such cases and should be considered in horseswith persistent tarsal sheath effusion and associatedlameness.

The medial DDFT is located dorsal to the pri-mary DDFT and is not located within the tarsalsheath. The medial DDFT is surrounded by aseparate tendon sheath that sometimes communi-cates with the tarsal sheath. The medial tendoninserts into the primary DDFT at the proximal/mid metatarsus and should not be mistaken forthe inferior check-ligament remnant. Injuries tothis tendon are uncommon.

Long Plantar Ligament

The LPL is located along the plantarolateral aspectof the tarsus. The ligament originates from theplantar surface of the proximal calcaneus and in-serts onto the fourth tarsal and fourth metatarsalbones. The proximal LPL is seen deep to the SDFT.Two portions of the LPL are often visualized ontransverse views in the midligament region. The

Fig. 18. Longitudinal ultrasound image showing severe tearingwith rupture of the superficial (long) component of the MCL in a14-yr-old Friesian stallion with severe tarsal swelling and lame-ness (grade 4 of 5) after sustaining a kick to the lateral tarsusduring breeding. Although some linear fibers are present in thecenter of the image, no fibers are visible in the origin and mid-ligament region (arrows). (Technique: 8.5-MHz linear, 5.2-cmscanning depth.)

Fig. 17. Longitudinal ultrasound image showing the origin of the deep component of the MCL (right image) and its insertion onto thedistomedial surface of the sustentaculum tali of the calcaneus (left image). This linear fiber pattern can only be obtained by placingthe transducer in a slightly transverse orientation as shown in Fig 15C. The midportion of the ligament is not visible in this image.(Technique: 10-MHz linear, 3.5-cm scanning depth.)

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insertions of the LPL are best evaluated on longitu-dinal views. Injuries to the LPL create plantarol-ateral swelling that is commonly referred to as“curb.” As stated above, other injuries may alsocreate this appearance, and ultrasound is requiredto define the source of swelling.5,16 LPL injuriesseem to be more common in the Standardbred race-horse, but they may be seen in other breeds anduses.4,16

7. Plantar Synovial Structures

Multiple synovial structures are located in close prox-imity in the plantar tarsal region, including the plan-tar pouches of the TT joint, tarsal sheath, GN bursa,and calcaneal bursa (from dorsal to plantar; Fig. 21).The anatomy of these structures is best viewed by

contrast radiography (Fig. 21). Medial or lateral dis-tention can occur in the distal tibia/proximal tarsalregion within any of these structures. Distinguishingbetween synovial structures is often challenging ifswelling or cellulitis is present. Ultrasound can beused to identify the distended structure and establishthe relative degree of synovitis versus effusion. Thefrequency of plantar tarsal wounds in the horse cre-ates the high potential for extension into one or moresynovial structures.19 Injection of radiographiccontrast agents into wounds or draining tracts mayreveal communication but may not be successful ifthe wound has sealed off. Ultrasound can be usedin such cases to document communication, aid in syno-vial-fluid collection, and assist in planning for surgicalintervention.

Fig. 19. Transverse (left) and longitudinal(right) ultrasound image of the normal SDFT andGN tendons in the distal tibia region immedi-ately proximal to the GN insertion onto the cal-caneal tuber. Note the scattered hypoechoicareas within the GN tendon on transverse viewsand the slight relaxation artifact (arrows) on lon-gitudinal views. (Technique: 10-MHz linear,4-cm scanning depth.)

Fig. 20. Ultrasound image of a severe tear of the GN tendon (arrows) in the mid tibia region (16 cm proximal to the point of the hock)that occurred secondary to a plantar laceration. The GN tendon has a large anechoic area on transverse views (left) and large areasof disrupted fiber pattern on longitudinal views (right). The lateral location of the GN tendon and medial location of the SDFT isnormal at this level. (Technique: 10-MHz linear, 4-cm scanning depth.)

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Subcutaneous BursaEffusion within the SC bursa is commonly referredto as capped hock. This “bursa” is located betweenthe skin and the SDFT at the calcaneal tuber; how-ever, the presence of a true bursa is somewhat de-batable. Distention is often secondary to creationof a false bursa from trauma. Horses with acuteswelling at the point of the hock and associatedlameness should be closely evaluated for the pres-ence of wounds that extend into the bursa. Sepsismay also be present without evidence of a wound.19

Ultrasound usually reveals thickening of the bursacapsule with a large accumulation of fluid that mayappear cellular. In horses with chronic non-septicbursitis, evaluation may be difficult because of se-vere thickening of the skin. Fluid is typically ane-choic, and horses are generally asymptomatic.

Calcaneal BursaThe calcaneal bursa is located between the SDF andGN tendons in the distal tibia region. The bursacontinues deep to the SDFT to its distal extent in themidtarsal region (Fig. 21C). The calcaneal bursafrequently communicates with the GN bursa.19

Bursal distention is best recognized medial and lat-eral to the SDF and GN tendons proximal to thecalcaneal tuber. Distal distention is usually lessapparent on physical exam. Moderate to severenon-septic bursitis can occur secondary to trauma,SDF/GN tendon injury, or osseous lesions of thecalcaneus.9,17,20 Ultrasound will reveal anechoicfluid bulging medial and lateral to the SDF and GNtendons proximal to the calcaneal tuber. Distal tothe calcaneal tuber, fluid is seen medial and lateralto the SDFT (Fig. 22). Septic calcaneal bursitis is a

common sequela of plantar tarsal wounds.19

Affected horses often show echogenic fluid withinthe bursa with increased synovial thickening (Fig.23). Distention of the bursa is infrequent in normalhorses, although a small amount of anechoic fluidmay be seen.

Gastrocnemius BursaThe GN bursa is a relatively small bursa locateddorsal to the GN tendon at the level of its insertion

Fig. 21. Lateral contrast radiographic studies of plantar tarsal synovial structures. (A) TT joint capsule. Note the large size of thesuperimposed medial and lateral plantar pouches (arrow). Communication with the proximal intertarsal joint can also be seen(arrowhead). (B) Tarsal sheath. Distention of the large proximal reflection (arrow) is most apparent in horses with tarsal-sheathtenosynovitis. Distinction between the plantar pouches of the tibiotarsal joint can be difficult because of their similar locations (asshown in A). The relatively long proximal-distal extent (arrowhead) of the sheath should not be underestimated, especially in horseswith wounds. (C) Calcaneal bursa. Similar to the tarsal sheath, distention is usually apparent medial and lateral to the SDFT andGN proximal to the point of the hock (large arrow). Fluid may also be apparent distally (small arrow). The superficial location ofthe bursa makes it highly susceptible to wound penetration. Communication with the small GN bursa is evident in this horse(arrowhead).

Fig. 22. Transverse ultrasound image of mild non-septic effu-sion within the calcaneal bursa (CB) distal to the point of thehock. The bursa is located between the SDFT and the calcaneus(arrowheads) at this level. The anechoic fluid is located withinthe medial reflection of the bursa and was seen lateral to theSDFT in other images. Synovial thickening was not apparent.(Technique: 10-MHz linear, 4.5-cm scanning depth).

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onto the calcaneus. The GN bursa often communi-cates with the calcaneal bursa (Fig. 21C).19

Distention of the bursa is not commonly identified asa sole entity and is often secondary to calcanealbursitis (Fig. 24).8

Tarsal SheathThe tarsal sheath surrounds the DDFT from thedistal tibia region through the tarsal canal and intothe proximal metatarsus (Fig. 21B). Tarsal sheathdistention is commonly referred to as “thoroughpin,”although distention of other synovial structures may

be mistaken for thoroughpin. Proximal distentionis most apparent on physical exam in affected horsesand may extend medially and laterally. Distentionof the distal tarsal sheath is less apparent and maybe recognized as generalized fullness in the region ofthe chestnut. Most reports of tarsal sheath teno-synovitis have focused on abnormalities of the sus-tentaculum tali,21–23 but tenosynovitis may also bepresent in horses without osseous abnormali-ties.5,24,25 In cases of moderate to severe effusion,ultrasound should be performed to rule out associ-ated DDFT injury. Sonographically, tarsal sheatheffusion is easiest to identify in the proximal meta-tarsus where anechoic fluid is seen surrounding theDDFT (Fig. 25). In the distal tibia region, out-pouching of fluid can be seen adjacent to the DDFT

Fig. 23. Composite transverse ultrasound image of septic calca-neal bursitis in a 6-mo-old Quarter Horse colt with a small woundto the point of the hock after kicking his stall. The CB is se-verely distended with echogenic fluid medial and lateral to theSDFT and GN. The wound was located in close proximity to thebursa, but a definite tract could not be identified. Cytologicanalysis of bursa fluid confirmed sepsis (total protein concentra-tion � 5.7 g/dl, nucleated cell count � 230,900 with 99% neutro-phils). Culture yielded growth of S. aureus. (Technique: 10-MHz linear, 4-cm scanning depth.)

Fig. 24. Ultrasound image of gastrocnemiusbursitis in a horse with extension of a plantarlaceration into the calcaneal bursa. The trans-verse (left) image shows a normal appearance ofthe SDFT and GN at 4 cm proximal to the pointof the hock. The GN bursa (arrows) is seen onthe longitudinal (right) image deep to the GNtendon and immediately proximal to its insertiononto the calcaneus (arrowheads). (Technique:8.5-MHz linear, 5.2-cm scanning depth.)

Fig. 25. Transverse ultrasound image of moderate non-septictarsal sheath effusion in the proximal metatarsus (16-cmDPOH). The anechoic fluid surrounds the medial DDFT (DM)and partially surrounds the DDFT. (Technique: 13-MHz lin-ear, 5.2-cm scanning depth.)

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(Fig. 26). Septic tenosynovitis should be consideredin horses with wounds along the length of the sheath.Similar to other septic synovial structures, hypoechoiccellular fluid can be seen in addition to pronouncedthickening of synovial membranes (Fig. 27).

Plantar Pouches of the Tibiotarsal JointDistention of the medial and/or lateral plantarpouch of the TT joint is usually seen in conjunctionwith distention of the dorsal pouch (Fig. 21A). Ul-trasound of the distended plantar pouch is readilyperformed by placing the transducer directly on thejoint capsule. The plantar pouches seem to showmore synovial thickening than the dorsal pouch,especially in cases of sepsis. In horses without dor-sal distention, plantar-pouch distention can be eas-ily confused with tarsal-sheath effusion because oftheir close association. In such cases, confirmationcan be obtained by digital compression of the dorsalpouch while simultaneously scanning the distendedplantar structure and watching for fluid and/or sy-novial movement.

8. Results

Records were reviewed of all horses presenting tothe UCD-VMTH Large Animal Ultrasound Servicefrom July 1999 through December 2005 (a 6.5-yrstudy period). Ultrasonographic evaluation of thetarsal region was performed in 128 horses for a total

of 168 examinations, including 133 primary examsand 35 recheck exams. Both limbs were evaluatedin three horses. Two horses presented for a newproblem in the same tarsus. Right (69) and left (64)tarsal exams were evenly distributed.

SignalmentAges ranged from 2 mo-27 yr with a mean age of 8.7yr. Foals constituted a small percentage of casesand included three foals under 6 mo and 7 yearlings.There was no apparent sex predilection. Breed dis-tribution was similar to our hospital population andincluded Thoroughbred (36), Quarter Horse breeds(33), Warmblood (29), Arabian (12), Morgan (4),mule (3), draft breeds (3), Tennessee Walking Horse(2), American Saddlebred (1), Peruvian Paso Fino(1), Standardbred (1), Mustang (1), Friesian (1), andMissouri Fox Trotter (1). As expected from thebreed distribution, a variety of uses were repre-sented, including English show (39), pleasure/trail(26), Western performance (14), race (10), breeding(5), and endurance (4). Use was not recorded in 30horses.

Clinical DataLameness was present in the affected limb in 83% ofcases. Mean lameness grade was 2.8 using theAmerican Association of Equine Practitioners(AAEP) grading scale of 0–5. Lameness was acutein onset in 81 horses and chronic in 31 horses.Lameness was not present in 18 horses. Swellingwas present in 77 horses (58%) and was consideredcellulitic in an additional 7 horses. Thirty-fourhorses (26%) had wounds involving the plantar, 19had wounds involving the dorsal, 8 had wounds in-volving the medial, and three had wounds involving

Fig. 27. Transverse (left) and longitudinal (right) ultrasound im-age of septic tarsal sheath tenosynovitis at the level of the proximalmetatarsus (16-cm DPOH). The tarsal-sheath synovium is in-flamed (arrowheads), and echogenic fluid is visible within the sheathsurrounding the DDFT (arrows). Cytologic analysis of synovialfluid was consistent with sepsis (total protein concentration � 6.3g/dl, nucleated cell count � 296,400 with 98% neutrophils).

Fig. 26. Longitudinal ultrasound image of the lateral reflectionof the tarsal sheath (arrowheads) in the distal tibia region of a5-yr-old Thoroughbred racehorse with septic tenosynovitis causedby hematogeneous spread from a cranial tibial venous thrombusin the contralateral limb. Severe cellular effusion with multiplefibrin strands and synovial thickening were seen throughout thetarsal sheath. The location of effusion adjacent to the DDFTassists in identifying this synovial structure as the tarsal sheath.Severe overlying SC edema is also present.

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the lateral surfaces of the tarsus. Horses withwounds presented to the VMTH an average of 24.9days post-injury (range � 0–120 days). Fifty per-cent of cases presented within 2 wk of injury. Dis-tention of synovial structures was present in 43horses and was localized to the TT joint (25), tarsalsheath (9), long DE tendon sheath (6), and calcanealbursa (3). Synovial distention was likely present inadditional horses, but severe tarsal swelling pre-cluded identification of the affected joint capsule,tendon sheath, or bursa on physical exam in manycases.

Radiographic Findings

A total of 110 radiographic studies were performedin 107 horses. Degenerative joint disease was themost common finding in 65% of tarsi (72 of 110),affecting the TMT (67), DIT (57), PIT (17), or TTjoints (3). Fractures were seen in 27 horses (25%),including single fractures in 19 horses and multiplefractures in 8 horses. Fractures were locatedthroughout the tarsus without any predilection site.Additional radiographic findings included osteitis(14), osteomyelitis (7), suspect osteomyelitis (3), os-teochondrosis lesions (6), soft-tissue mineralization(4), and suspect septic arthritis (3). Abnormalitiesof the sustentaculum tali (osteitis) were only seen intwo horses.

Ultrasonographic Findings

Ultrasonographic abnormalities were detected inthe majority of studies performed (96%, 128 of 133primary exams). Tendon and/or ligament injurieswere most common and were diagnosed in 67 of 133tarsi (50%). A summary of the distribution andseverity of tendon and ligament injuries and their

association with wounds and fractures is shown inTable 1.

CL injuries were identified in 23 horses. A totalof 18 MCL injuries and 8 LCL injuries were identi-fied. Three horses sustained medial and lateral in-juries. Injuries to the superficial (long) MCL weremost common in 17 horses and included two horseswith concurrent injury to the deep MCL. Injurywas associated with fracture in only three horses.Two horses with medial malleolar fractures had in-jury to both MCL components. Another horse witha fracture of the distal tuberosity of the talus hadassociated insertional injury of the superficial (long)MCL. The only horse with a sole injury to the deepMCL was graded as mild. Of the eight horses withLCL injuries, the superficial and deep componentswere injured in six and two horses, respectively.Concurrent injury to both LCL components was notseen. Injuries were associated with a lateral mal-leolar fracture and a suspect fourth metatarsal frac-ture in one horse each. Collateral ligament injurywas associated with an overlying wound in only onehorse.

Long DE tendonitis was the most common dorsaltendon abnormality. Tendonitis was associatedwith moderate to severe tenosynovitis in 6 of 10horses. Peroneus tertius injuries (6) were the sec-ond most common dorsal abnormality and includedfive ruptures. The site of rupture was located inthe mid to distal tibia region in all cases. Rupturewas caused by falls (2), slipping (1), full-limb cast(1), and unknown injury (1). Mild calcific tendon-itis was seen in the remaining horse. Cunean ten-don and bursa abnormalities were rare but were theprimary source of lameness in one horse with septicbursitis post-intra-articular injection of the distal

Table 1. Distribution and Severity of Tendon and Ligament Injuries in the Tarsal Region, Including Association With Fractures and Wounds(n � 133 exams)

Total Mild Mod Sev (Rupture) Fracture Wound

Dorsal structuresPeroneus tertius 6 1 0 5 (5) 0 0Cunean tendon 2 0 1 1 1 1Long digital extensor tendon 10 2 4 4 (3) 0 2Lateral digital extensor tendon 4 2 1 1 (1) 0 2

Lateral collateral ligament 8Superficial (long) component 6 1 2 3 1 1Deep (short) component 2 1 1 0 1 0

Medial collateral ligament 18Superficial (long) component 17 6 8 3 (1) 3 0Deep (long) component 3 1 0 2 (1) 2 0

Plantar structuresSuperficial digital flexor tendon 12 5 0 7 (5) 0 8SDF retinacular attachment 4 0 0 4 0 1Gastrocnemius tendon 8 1 5 2 0 2Long plantar ligament 4 1 1 2 1 0Deep digital flexor tendon 4 1 2 1 0 1

Mod, moderate; Sev, severe.

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intertarsal joint. Additional abnormalities of dor-sal structures not listed in Table 1 include dorsal-intertarsal ligament desmitis (1) and cranial-tibialmuscle tearing (1).

The SDFT was the most frequently injured plan-tar tendinous or ligamentous structure, and it sus-tained the most severe injuries in 7 of 12 horses,including 5 horses with lacerations that severed thetendon. Four horses had retinacular tears that re-sulted in SDFT luxation. Lateral luxation causedby medial retinacular rupture was seen in threehorses, and medial luxation caused by lateral-reti-nacular rupture was found in one horse. GN ten-donitis was secondary to extension of lacerationsthrough the SDFT in two horses with severe injury.The remaining six cases were not associated withwounds. Injuries to the LPL were only seen in fourhorses. DDFT injuries in the tarsal region wereinfrequently identified (4 horses) and included onlyone horse with associated bony remodeling of thesustentaculum tali. Severe DDFT injury wascaused by extension of a laceration through theSDFT and tarsal sheath in one horse. Three horseswere diagnosed with moderate proximal suspensoryligament desmitis.

A summary of ultrasonographic abnormalities oftarsal synovial structures is shown in Table 2.Effusion and synovitis of the TT joint was the mostcommon abnormality in 60 tarsi followed by thetarsal sheath (31), calcaneal bursa (22), long DEtendon sheath (11), GN bursa (5), SC bursa (5), latDE tendon sheath (3), and cunean bursa (2) Twen-ty-five synovial structures showed an appearanceconsistent with sepsis, 14 of which showed definitecommunication with skin wounds. Ultrasono-graphic parameters for sepsis included thickening ofsynovial membranes with a lacey edematous ap-pearance, echogenic synovial fluid accumulation,and/or direct wound communication. Severe effu-sion was not a common finding in cases of sepsis.An additional 17 synovial structures were suspi-cious for sepsis on ultrasound. Wounds were lo-cated in close proximity to septic or suspect septicsynovial structures in 13 horses. One horse withseptic arthritis of the TT joint cause by wound con-

tamination also had involvement of the PIT, DIT,and TMT joints. Two horses had septic arthritis ofthe TMT joint, one caused by an overlying woundand one secondary to recent intra-articular injec-tion. Cases with tarsal sheath tenosynovitis in-cluded two horses with synovial herniations andthree horses with severe intrasynovial hemorrhage.Tarsal sheath tenosynovitis was associated with ab-normalities of the sustentaculum tali in only onehorse. Two horses with TT hemarthrosis were alsoseen. Calcaneal bursitis was seen in 8 horses withSDFT or GN injuries.

Wounds were noted to extend directly to bone inseven horses. Evidence of osteomyelitis was seenin all seven cases, including proliferative bone withoverlying fluid and/or granulation tissue. Addi-tional findings included primary cellulitis (12), SCabscesses (4), complete septic thrombophlebitis ofthe cranial tibial vein (1), and septic lymphangitis(1).

SynoviocentesisSynoviocentesis was performed in 38 horses for atotal of 43 samples obtained from the TT joint (23),tarsal sheath (11), SC bursa (4), calcaneal bursa (2),cunean bursa (1), TMT joint (1), and unknown loca-tion (1). Cytologic analysis was consistent withsepsis in 10 horses, suspicious for sepsis in 5 horses,and non-septic in 28 horses. Of the 26 samplessubmitted for culture, only 4 produced bacterialgrowth, including Staphylococcus aureus from theTT joint, tarsal sheath, and calcaneal bursa in onehorse each and Streptococcus equisimilis from theSC bursa in another horse.

9. Discussion

The results of this study show the value of tarsalultrasound to diagnose a wide variety of soft tissueinjuries in horses presenting with tarsal lameness.The distribution of injuries is reflected by the largenumber of structures in the hock region. Collateralligament injuries were the most common tendon/ligament injury but were seen in only 17% (23 of133) of horses. MCL injuries were more commonthan LCL injuries (18 of 133 versus 8 of 133 horses,

Table 2. Ultrasonographic Abnormalities of Tarsal Synovial Structures (n � 133 exams)

Grade

TibiotarsalTarsal SheathTenosynovitis

CalcanealBursitis

GNBursitis

SQBursitis

CuneanBursitis

Long DETenosynovitis

Lat DETenosynovitisEffusion Synovitis

Severe 18 22 13 8 1 4 2 2 1Moderate 26 24 6 10 3 0 0 7 2Mild 11 10 12 4 1 1 0 2 0Total 60 (45%) 31 (23%) 22 (16%) 5 (4%) 5 (4%) 2 (1%) 11 (8%) 3 (2%)Septic 8 4 7 1 3 2 0 0Suspect sepsis 8 3 3 1 0 0 0 2Wound extends to 3 3 5 0 1 2 0 0Wound near 3 3 3 2 0 0 0 2

GN, gastrocnemius; SQ, subcutaneous; DE, digital extensor; Lat, lateral.

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respectively) and involved the superficial (long)MCL in 74% of horses with collateral injury. Theseresults suggest that MCL injury is more frequentthan has been reported in the veterinary litera-ture.4,15 Other reports have focused on the associ-ation of fracture and/or enthesitis with LCLinjury.13,14 Osseous abnormalities were infre-quently identified in association with injury to ei-ther collateral ligament in our study. Therefore,the absence of localizing osseous abnormalities onradiographs should not deter ultrasonographicevaluation.

Severe tendon or ligament injuries were seen in arelatively large percentage of horses (40%, 35 of 86)and included 16 complete ruptures. Although se-vere injuries were distributed throughout the tar-sus, ruptures most commonly involved the PT andthe SDFT. As expected, none of the PT ruptureswere caused by lacerations, but all SDFT ruptureswere caused by lacerations.

The use of ultrasound was very important to doc-ument and characterize abnormalities of tarsal sy-novial structures. Diffuse tarsal swelling withlameness was a common presenting complaint in thestudy population. Ultrasound was often requestedto determine the presence of synovial abnormalitiesand if present, to determine the best location forsynoviocentesis to avoid cellulitic regions whereverpossible. The large number of horses with effusionand synovitis of the TT joint (45%) was not surpris-ing. Tarsal sheath tenosynovitis was also rela-tively frequent among our population, butassociated abnormalities of the sustentaculum taliwere only seen in 1 of 30 horses. This is in contrastto prior reports of tarsal sheath tenosynovitis wherethe majority of horses had sustentaculum tali abnor-malities.21–24 The occurrence of calcaneal bursitisin our study is similar to previous studies.19 Non-septic bursitis was often caused by SDFT or GNtendonitis, and septic bursitis was commonly causedby plantar wounds.

Wounds extended to synovial structures, bone,tendons, or ligaments in 23 of 34 (67%) horses withwounds. Septic synovial structures were identifiedin another three cases where direct communicationwas not apparent. Wounds extended to more thanone synovial structure in six horses. In manycases, communication was not able to be determinedon physical exam. Ultrasonographic evidence ofcommunication provided an early diagnosis at thetime of contamination before deep-seated infectioncould be established. Appropriate treatment withsystemic antimicrobials and/or lavage of synovialstructures could be instituted to increase the likeli-hood of a successful outcome and prevent extensioninto previously normal structures.26 Synovialstructures were also determined to be septic in eighthorses without wounds. Sepsis was due to recentintra-articular injection in two horses, but the re-maining horses had no history of recent intra-artic-ular injection to suggest sepsis. Ultrasound helped

to differentiate between primary cellulitis and septicarthritis in these horses that typically presentedwith similar clinical signs of severe lameness, heat,and swelling.

Not surprisingly, lacerations or wounds to theplantar aspect of the tarsus occurred frequently inour study population. Although many plantarwounds are treated without sequelae in the ambu-latory setting, practitioners should consider ultra-sound and/or referral in horses with worseningclinical signs or if leakage of synovial fluid ispresent. Unfortunately, the proximal to distal ex-tents of synovial structures are often underesti-mated. This creates a dangerous situation wherepractitioners can falsely believe that a wound lacksthe potential for extension into a synovial structure.Undiagnosed cases may go unnoticed until extensiveinfection causes more severe clinical signs. Ultra-sound may also be useful in cases with tendon orligament defects visible or palpable through lacera-tions. Injury is often more extensive than is appar-ent on physical exam and may extend into otherstructures. This was illustrated by several cases inour study where SDFT injuries were visible throughthe wound, but significant DDFT or GN injurieswere not apparent until ultrasound was performed.

In summary, tarsal ultrasound should be per-formed in horses with tarsal wounds, distention oftarsal synovial structures, or tarsal swelling thatcannot be explained by radiographic findings.Although tarsal ultrasound can be challenging, itsimportance should not be overlooked, because soft-tissue injuries are a common source of lameness inthe horse. Armed with a strong knowledge of anat-omy, including relationships between structures,tarsal ultrasound can be successfully performed.

References and Footnotes1. Tomlinson JE, Redding WR, Berry C, et al. Computed to-

mography of the equine tarsus. Vet Radiol 2003;4:174–178.2. Blaik MA, Hanson RR, Kincaid SA, et al. Low-field mag-

netic resonance imaging of the equine tarsus: normal anat-omy. Vet Radiol 2000;41:131–141.

3. Dik KJ. Ultrasonographic of the equine crus. Vet Radiol1993;34:28–34.

4. Dik KJ. Ultrasonography of the equine tarsus. Vet Radiol1993;34:36–43.

5. Dik KJ, Leitch M. Soft tissue injuries of the tarsus. VetClin North Am [Equine Pract] 1995;11:235–247.

6. Denoix JM. Joints and miscellaneous tendons (Miscellaneoustendons and ligaments). In: Rantanen NW, McKinnon AO,eds. Equine diagnostic ultrasonography. Baltimore: Wil-liams & Wilkins, 1998;493–500.

7. Redding WR. Sonographic examination of synovial struc-tures in the horse. In: Rantanen NW, McKinnon AO, eds.Equine diagnostic ultrasonography. Baltimore: Williams& Wilkins, 1998;523–538.

8. Reef VB. Musculoskeletal ultrasonography. In: Reef VB,ed. Equine diagnostic ultrasound. Philadelphia: W.B.Saunders, 1998;39–186.

9. Dyson SJ. Other soft tissue injuries. In: Dyson SJ, RossMW, eds. Diagnosis and management of lameness in thehorse. Philadelphia: W.B. Saunders, 2003;708–711.

10. Koenig J, Cruz A, Genovese R, et al. Rupture of the pero-neus tertius tendon in 27 horses. Can Vet J 2005;46:503–506.

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11. Belkap JK, Baxter GM, Nickels FA. Extensor tendon lacer-ations in horses: 50 cases (1982–1988). J Am Vet MedAssoc 1993;203:428–431.

12. Updike SJ. Functional anatomy of the equine tarsocruralcollateral ligaments. Am J Vet Res 1984;45:867–874.

13. Wright IM. Fractures of the lateral malleolus of the tibia in16 horses. Equine Vet J 1992;24:424–429.

14. Boero MJ, Kneller SK, Baker GJ, et al. Clinical, radio-graphic, and scintigraphic findings associated with enthesitisof the lateral collateral ligaments of the tarsocrural joint inStandardbred racehorses. Equine Vet J 1988;6(Suppl):53–59.

15. Rose PL, Moore I. Imaging diagnosis—avulsion of the me-dial collateral ligament of the tarsus in a horse. Vet Radiol2003;44:657–659.

16. Ross MW, Genovese RL, Reef VB. Curb: a collection ofplantar tarsal soft tissue injuries, in Proceedings. 48th An-nual American Association of Equine Practitioners Conven-tion 2002;337–342.

17. Dik KH, Keg PR. The efficacy of contrast radiography todemonstrate ‘false thoroughpins’ in five horses. Equine VetJ 1990;22:223–225.

18. Dyson SJ, Kidd L. Five cases of gastrocnemius tendonitis inthe horse. Equine Vet J 1992;24:351–356.

19. Post EM, Singer ER, Clegg PD, et al. Retrospective study of24 cases of septic calcaneal bursitis in the horse. Equine VetJ 2003;35:662–668.

20. Bassage LH, Garcia-Lopez J, Currid EM. Osteolytic lesionsof the tuber calcanei in two horses. J Am Vet Med Assoc2000;217:710–716.

21. Edwards GB. Changes in the sustentaculum tali associatedwith distension of the tarsal sheath (thoroughpin). EquineVet J 1978;10:97–102.

22. Santschi EM, Adams SB, Fessler JR, et al. Treatment ofbacterial tarsal tenosynovitis and osteitis of the sustentacu-lum tali of the calcaneus in five horses. Equine Vet J 1997;29:244–247.

23. Hand DR, Watkins JP, Honnas CM, et al. Osteomyelitis ofthe sustentaculum tali in horses: 10 cases (1992–1998) J Am Vet Med Assoc 2001;219:341–345.

24. Dik KJ, Merkens HW. Unilateral distension of the tarsalsheath in the horse: a report of 11 cases. Equine Vet J1987;19:307–313.

25. Cauvin E. Tarsal sheath. In: Dyson SJ, Ross MW, eds.Diagnosis and management of lameness in the horse. Phila-delphia: W.B. Saunders, 2003;687–692.

26. Wright IM, Smith MRW, Humphrey DJ, et al. Endoscopicsurgery in the treatment of contaminated and infected syno-vial cavities. Equine Vet J 2003;35:613–619.

aDormosedan, Orion Corporation, Espoo, Finland FI-02101.bTranquiVed, Vedco, Inc., St. Joseph, MO 64507.cEquanol, Vedco, Inc., St. Joseph, MO 64507.

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