Katherine S. Garrett Rood and Riddle Equine Hospital, Lexington, KY, USA Ultrasonography of the tarsus can be challenging, but it is an important part of a complete diagnostic evaluation of tarsal disease. As with other body regions, a thorough understanding of normal anatomy is essential. Comparison with magnetic resonance, computed tomographic, and/or radiographic images, and dissected specimens can be helpful in understanding the anatomic relationships and the precise locations and paths of the tendinous and ligamentous structures. Excellent reviews of scanning techniques for the tarsal region have been published. Regardless of the particular approach chosen, a systematic method for evaluation of this complex structure is helpful. The structures to be examined include the tendons and ligaments, the synovial structures, the bony surfaces, and the subcutis. Comparison with the opposite limb can be extremely helpful in determining if an unusual abnormality is present or in cases of mild disease. A linear transducer (8–12 MHz) is generally most useful, but a micro-convex transducer (7–10 MHz) can be helpful in some situations. Sedation is often necessary to ensure patient compliance and sonographer safety. If the hair in the area can be clipped, image quality will be improved. Scanning the contralateral or unaffected hock can be useful for comparison. The tarsal region contains many tendons and ligaments, some of which have complex attachments or insertions distant from the tarsus itself. The lateral and medial collateral ligaments both have two major components, a long superficial component and a short deeper component. The long portion of the lateral collateral ligament originates on the caudal portion of the lateral malleolus of the distal tibia and has insertions on the calcaneus, fourth tarsal bone, and third and fourth metatarsal bone. The tripartite short portion originates on the cranial portion of the lateral malleolus and extends in a nearly horizontal plane to the lateral aspect of the talus and calcaneus. The lateral collateral ligament is less distinct than the medial collateral ligament. The long medial collateral ligament originates on the medial malleolus of the distal tibia and extends distally, inserting on the distal talus, central, fused first and second, and third tarsal bones, and second and third metatarsal bones. The short portion of the ligament originates on the medial malleolus and has three subsections that travel in a more horizontal plane than the long portion and insert on the proximal medial talus and the sustentaculum tali. During ultrasonographic examination, the lateral and medial collateral ligaments can be located most easily in the longitudinal plane. As the short components lie in a more transverse plane than the long components, an image in the transverse plane can then be obtained by rotating the transducer by 90 degrees. In addition, some of the short portions are in partial relaxation when the horse is weightbearing, so imaging these ligaments while the horse is not fully weightbearing (and the ligament is more taut) may aid in evaluation of the ligament. Desmitis of any of the collateral ligaments can occur, but is more commonly seen in the long medial collateral ligament. Ultrasonographic signs of desmitis are similar to those in any ligament and include increased size, decreased echogenicity, and abnormal fiber pattern (Figures 6.1 and 6.2). If the insertion of the ligament is involved, bony irregularity or avulsion fragments may be imaged (Figure 6.3). Horses with collateral ligament desmitis typically also have synovial effusion and synovial membrane thickening due to an intra-articular component. Figure 6.1 (A) Longitudinal image of a normal long medial collateral ligament (LMCL) and short medial collateral ligament (SMCL). TCJ: tarsocrural joint. Proximal is to the right of the image. Transverse (B) and longitudinal (C) plane images of an abnormal LMCL. The ligament is enlarged with a focal region of marked hypoechogenicity and fiber disruption (arrowheads). Synovial effusion is present in the TCJ. Dorsal and proximal are to the right of the images. (D) Transverse plane magnetic resonance image of the same horse. Note the marked synovial effusion in the TCJ and focal region of increased signal (arrowhead) in the LMCL. Dorsal is to the top of the image, medial is to the left of the image. Figure 6.2 Transverse (A) and longitudinal (B) images of the long portion of the lateral collateral ligament. Dorsal (A) and proximal (B) are to the right of the images. The ligament is enlarged with decreased echogenicity diffusely (arrowheads). There is thickening of the subcutaneous tissues as well (arrow). Figure 6.3 Longitudinal plane images of the proximal insertion on the medial malleolus of a normal (A) and abnormal (B) medial collateral ligament (arrows). In (B) there are areas of hypoechogenicity within the ligament as well as small avulsion fragments (arrowhead) and thickening of the subcutaneous space. Proximal is to the right of the images. The short lateral collateral ligament is invariably involved in fractures of the lateral malleolus of the distal tibia, while the long lateral collateral ligament is less commonly affected (Figure 6.4). These fractures are usually caused by external trauma. Ultrasonography can be useful to assess the degree of involvement of the collateral ligaments and potential for instability of the joint. Figure 6.4 (A) Longitudinal plane image of a normal short lateral collateral ligament (SLCL); (arrows) origin on the lateral malleolus. Proximal is to the right of the image. (B) Dorsoplantar radiographic image of a horse with a lateral malleolus fracture (arrow). Note the distal displacement of the fragment. Lateral is to the right of the image. (C) Longitudinal plane image of the lateral malleolus fragment (arrowheads) and involvement of the SLCL (arrows) of the horse in (B). The fragment has displaced distally and plantarly along the course of the SLCL to lie deep to the long lateral collateral ligament (LLCL). Short collateral ligament association with the fragment was confirmed arthroscopically. Thickening of the subcutaneous tissue is evident. Proximal is to the right of the image. The common calcaneal tendon/gastrocnemius tendon, superficial digital flexor tendon, and long plantar ligament are found on the plantar aspect of the tarsus. The gastrocnemius tendon inserts on the proximal surface of the tuber calcanei. The long plantar ligament originates on the plantar surface of the proximal tuber calcanei and inserts on the fourth tarsal and fourth metatarsal bones. The superficial digital flexor tendon largely passes over the surface of the tuber calcanei, but the medial and lateral aspects insert on the proximal aspect of this bone via a retinaculum. Soft tissue swelling of the plantar aspect of the tarsus (“curb”) may be caused by injury to the long plantar ligament or superficial digital flexor tendon. Subcutaneous edema or thickening may give a similar external appearance and seems to be more common than tendon or ligament injuries. Desmitis of the long plantar ligament is characterized by enlargement and hypoechogenicity of the ligament (Figure 6.5 ). Figure 6.5 (A) Transverse plane image of a normal long plantar ligament (LPL). CAL: calcaneus; SDFT: superficial digital flexor tendon. Lateral is to the right of the image. (B) Longitudinal plane image of the LPL distal insertion on the fourth tarsal bone (T IV) and fourth metatarsal bone (MT IV). Proximal is to the right of the image. (C) Transverse (left image) and longitudinal (right image) images of a horse with LPL desmitis. Hypoechogenicity of the ligament is apparent (arrowheads and arrows). SDFT: superficial digital flexor tendon. Lateral and proximal are to the right of the images. (D) Longitudinal plane image of an LPL with focal mineralization within the ligament (arrow). Proximal is to the right of the image. The lateral deep digital flexor tendon is located on the plantaromedial aspect of the tarsus, passing over the sustentaculum tali within the tarsal sheath. The smaller medial deep digital flexor tendon is contained within its own synovial sheath located dorsomedial to the lateral deep digital flexor tendon. The medial tendon joins the lateral tendon in the proximal metatarsal region. While scanning the tarsal sheath, the transducer should be positioned in a caudomedial/plantaromedial location in the distal crus and tarsal regions and rotate toward the plantar aspect of the limb as it is moved distally. This region of the tarsus has irregular contours in the normal horse, which can make maintaining good transducer contact challenging. As a result, images of the lateral deep digital flexor tendon can often be more easily obtained by using a standoff pad with a linear probe or by using a micro-convex probe due to its smaller footprint. When soft tissue swelling is present in this area, it tends to smooth the surface contours of the limb and facilitate good transducer contact. Ultrasonographic examination of the lateral deep digital flexor tendon within the tarsal sheath is complicated by multiple factors. The musculotendinous junction generally extends distally to within the proximal aspect of the tarsal sheath and its heterogeneous pattern can be confused with tendonitis. The normal curvature of the tendon as it passes over the sustentaculum tali can also introduce hypoechogenic artifacts (Figure 6.6). Although tendonitis of the deep digital flexor tendon is uncommon, it can be a source of lameness. Ultrasonographic signs are similar to those seen with tendonitis in general (hypoechogenicity, irregular fiber pattern, and/or enlargement), along with an effusion of the tarsal sheath. Figure 6.6 Images of the normal tarsal sheath. The anatomic locations of these images are indicated by the horizontal lines on the limb of the horse in (A). Transverse (B) and longitudinal (C) images of the lateral deep digital flexor tendon in the proximal tarsal sheath at the musculotendinous junction (level 1 in (A)). The normal heterogeneous pattern of the muscle is apparent. (D to G): Transverse (D and F) and longitudinal (E and G) images at the level of the sustentaculum tali (level 2 in (A)). In (F) and (G), a standoff pad is used to improve image quality, but artifactual areas of hypoechogenicity remain in all four images (arrows) and the margin of the DDFT artifactually appears irregular. These artifacts are due to small differences in tendon fiber orientation as the tendon follows a curved path over the sustentaculum tali. Slight changes in transducer position and angle can be used to assess these different fiber orientations in real time. (H to K) Transverse (H and J) and longitudinal (I and K) images at the level of the proximal metatarsus (level 3 in (A)). In (J) and (K), a standoff pad is used to improve image quality. (L and M) Transverse (L) and longitudinal (M) images at the level of the distal tarsal sheath (level 4 in (A)). At this level, the smaller medial deep digital flexor tendon has joined the larger lateral deep digital flexor tendon. Proximal or lateral is to the right of the images. DDFT: lateral deep digital flexor tendon; MDDFT : medial deep digital flexor tendon; MTIII: third metatarsal bone; SDFT: superficial digital flexor tendon; SL: suspensory ligament; ST: sustentaculum tali. The peroneus tertius and the tendons of the cranial tibial muscle have complex insertions on the dorsal aspect of the distal tarsal region. The distal peroneus tertius forms a tunnel through which the distal cranial tibial tendon emerges. The peroneus tertius is the most echogenic structure on the dorsal tibia, making it easier to identify. The dorsal tendon of the peroneus tertius then inserts on the central and third tarsal and third metatarsal bones. The lateral tendon inserts on the fourth tarsal bone and the distolateral calcaneus and talus. The dorsal tendon of insertion of the cranial tibial muscle inserts on the third tarsal and third metatarsal bones. The cunean tendon is the medial tendon of insertion of the cranial tibial muscle. It passes medially across the central tarsal bone, inserting on the fused first and second tarsal bone, central tarsal bone, and second metatarsal bone. The tendons of insertion of the cranial tibial and peroneus tertius can be identified by following each of the tendons individually from their origins in the distal tibia.
6
Ultrasonography of the Hock
Introduction
Tendons and Ligaments
Collateral Ligaments
Plantar Aspect
Plantaromedial Aspect
Dorsal Aspect
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
