Ann Carstens1 and Roger K.W. Smith2 1 University of Pretoria, Onderstepoort, South Africa Lameness associated with the foot is common and routinely evaluated using radiography. However, many causes of lameness are associated with soft tissue pathology where there are no or minimal radiographic changes. While magnetic resonance imaging (MRI) has become the imaging modality of choice for identifying such soft tissue causes, MRI is costly and not always available. Therefore, ultrasonography is a logical imaging modality to consider but its use is compromised by the presence of the hoof capsule, which precludes imaging through it. However, there are three ultrasonographic windows where images can be obtained of structures of the foot – proximal to the coronary band palmarly and dorsally, and transcuneally/transsolarly. A number of structures within the foot extend proximal to the coronary band and so lend themselves to ultrasonographic examination. The hair should be clipped and cleaned as for other ultrasound examinations. Gel should be rubbed onto the area and left for a few minutes to improve contact as this is often limiting. For the palmar aspect of the foot a small footprint transducer (ideally a microconvex transducer) can be placed longitudinally between the bulbs of the heel, with the foot placed on a wooden wedge, as used for foot radiography (Figure 1.1), so as to have the fetlock partially flexed and the foot extended. This allows the assessment of the deep digital flexor tendon (DDFT), the palmar pouch of the distal interphalangeal (DIP) joint, the collateral sesamoidean ligament (“T” ligament), and the navicular bursa distally to the level of the proximal border of the navicular bone (Figure 1.2). The small footprint of a microconvex transducer can also enable transverse images in this location. However, the DDFT is off-incidence to the ultrasound beam and hence is hypoechoic, and the imaging window incorporates only the middle portion of the DDFT, making identification of DDFT tears, most commonly present in the lobes, difficult unless there is fibrosis which results in retained echogenicity within an anechoic tendon. Figure 1.1 Positioning of either a curvilinear (A) or linear (B) transducer between the bulbs of the heels to image the palmar aspect of the foot. Figure 1.2 Normal sagittal ultrasonographic anatomy of the palmar foot. Proximal is to the right. A microconvex transducer can also be used in an oblique orientation to guide a spinal needle into the navicular bursa without going through the DDFT (Figures 1.2 and 1.3). Figure 1.3 Ultrasound guided tendon sparing approach to the navicular bone [1]. (A) shows the orientation of the microconvex transducer between the bulbs of the heel. (B) shows the alignment of the spinal needle (introduced just proximal to the ipsi-axial collateral cartilage) to the microconvex transducer. (C) shows a representative image, which is annotated in (D). (E) shows the spinal needle being guided into the navicular bone deep to, and avoiding penetrating, the deep digital flexor tendon. (Images courtesy of Dr Michael Schramme). For the dorsal, medial, and lateral aspects, the transducer is positioned both transversely and longitudinally adjacent and directly over the coronary band where the ultrasonographic transmission properties are improved (Figure 1.4) and moved from the dorsal aspect to the dorsomedial and dorsolateral aspects where the dorsal joint capsule and collateral ligaments of the DIP joint (Figure 1.5) can be imaged immediately proximal to the coronary band. The collateral ligaments traverse the coronary band and so only the proximal third of the ligament are visible ultrasonographically. Care should be taken to ensure the transducer is on-incidence to the collateral ligament as it is easy to generate off-incidence artifacts in the ligaments that can resemble pathology (Figure 1.6). Further caudally lie the collateral cartilages, which are hypoechoic but can show areas of ossification (and therefore acoustic shadowing). Figure 1.4 Linear transducer positioning to evaluate the dorsal and dorsolateral/dorsomedial aspects of the foot. Note the transducer spanning the coronary band in the longitudinal orientation. Figure 1.5 Normal ultrasonographic appearance of the distal interphalangeal joint collateral ligaments. (A) Transverse image: note the oval-shaped collateral ligament (arrow) lying in a depression in the underlying bony surface of the second phalanx. (B) Longitudinal image (proximal to the left): the longitudinal striations of the ligament are visible (arrow). Note the acoustic shadowing over the hoof capsule. Figure 1.6 Ultrasound-guided injection of the distal interphalangeal joint collateral ligaments. (A) A 1.5- or 2-inch needle is inserted vertically downward in the standing limb approximately 2 cm proximal to the coronary band. Its position over the collateral ligament is identified using a transversely orientated view at the level of the coronary band (B). The needle is then advanced distally, entering the collateral ligament as superficially as possible, with in-plane targeting (C and A). The end of the needle is no longer visible once it disappears under the hoof capsule but it is advanced until in contact with bone. A radiograph can be obtained to determine the needle’s accurate location in the fossa of insertion of the ligament onto the distal phalanx (D) but there is still a high risk of entering the distal interphalangeal joint [2]. Injuries to the collateral ligaments are often associated with a guarded prognosis and hence there has been enthusiasm to perform intraligamentar injections of affected cases. Both radiographic, MRI, and, most recently, ultrasonographic guided injection techniques have been published. As most collateral ligament injuries involve the distal portions of the ligament, longitudinally guided injections are essential. The ultrasound technique involves inserting a 1.5-inch needle vertically downward approximately 2 cm proximal to the coronary band over the affected ligament in a location determined from a transverse ultrasound examination. Once the needle has been inserted through the skin, the transducer is swivelled 90 degrees into a longitudinal orientation over the ligament. The needle is then advanced under ultrasound guidance ensuring that the needle enters the ligament as abaxially as possible within the ultrasound window. The needle is then advanced until it hits bone. Concurrent radiographic assessment of its distal position can be helpful but it must be appreciated that even when using this guided technique there is a high risk of injecting the joint accidentally. Via the palmar window, distension of the palmar/plantar pouch of the distal interphalangeal joint and/or the proximal pouch of the navicular bursa, and chronic DDFT pathology, where there is retained echogenicity and/or mineralization within the off-incidence hypoechoic DDFT, can be visualized ultrasonographically (Figure 1.7). In some cases, DDFT pathology within the foot will extend sufficiently proximally to be visible in standard views within the distal pastern (see Chapter 2). Figure 1.7 Transverse ultrasound scan obtained with a linear transucer directed distally from the distal palmar ultrasound “window” to the foot. There is retained echogenicity in an off-incidence transverse image of the distal deep digital flexor tendon consistent with chronic tendinopathy and/or mineralization. Abnormalities of the distal interphalangeal joint can result in changes to the dorsal pouch which are visible ultrasonographically – both distension and synovial thickening (Figure 1.8) as well as osteophytosis in cases of osteoarthritis (Figure 1.9). Where ultrasonography of this region carries the most useful imaging is for collateral ligament desmitis, although only when the proximal third of the ligament is involved. This is only in a minority of cases, as most pathology is distal within the foot and hence not identifiable ultrasonographically. The most appropriate cases to scan are those with palpable swelling in the region of the ligament (dorsomedially or dorsolaterally) at the level of the coronary band. Ultrasonographic abnormalities vary between enlargement to complete rupture characterized by an anechoic ligament (Figure 1.10). Careful comparison with the contra-axial and contralateral ligaments is essential to be confidant of a diagnosis. Figure 1.8 (A) Longitudinal image adjacent to the dorsal coronary band showing the extensor process of the distal phalanx (solid arrow), the digital extensor tendon (dotted arrow), and hypertrophied synovium (dashed arrow) together with a distended distal interphalangeal joint in a horse with distal interphalangeal joint sepsis (proximal to the right). (B) shows the corresponding transverse image and (C) the transverse image with Doppler imaging, showing the marked hyperemia of the joint capsule. Figure 1.9 Distal interphalangeal joint osteoarthritis. Dorsal longitudinal ultrasonographic image (A) – proximal to the right), showing irregular new bone on the dorsal surface of the second phalanx, as seen radiographically (B). Figure 1.10 Ruptured collateral ligament of the distal interphalangeal joint. (A) and (B) show the transverse (A) and longitudinal (B) images of the normal contralateral medial collateral ligament. (C) and (D) show the corresponding ultrasonographic images of the ruptured ligament. Note the absence of any organized echogenic ligament tissue where the ligament should be. (E) and (F) show the “regeneration” of a new ligament after 2 months in a distal limb cast, indicating that these injuries, although seemingly severe, can heal satisfactorily when the joint is immobilized adequately. The third phalanx (P3), distal sesamoid bone (navicular bone) (DSB), navicular bursa (NB), implantation of the deep digital flexor tendon (DDFT), distal sesamoid impar ligament (DSBIL), and other suprasolar structures can be evaluated ultrasonographically transcuneally (through the frog) and transsolarly (through the sole). A 7.5 MHz transducer, preferably curvilinear, should be used, although lower multi-frequency transducers such as a 3.5 MHz transducer used at 6 MHz can also give adequate images. Since the frog and sole are relatively impenetrable to ultrasound waves, and loose solar and frog keratin can trap air, it is important to prepare them to optimize the image. The sole and frog should be trimmed to get rid of loose scaly keratin and then pared smooth. The foot should then be soaked in bandages/a poultice for at least an hour in water. This may need to be prolonged to overnight soaking, if there is initially minimal softening of the sole or frog. Application of acoustic coupling gel (ACG) for 10–15 minutes prior to and during scanning also helps image visibility. Copious amounts of ACG that fill the collateral and central sulci of the frog also help to establish a clearer image with fewer artifacts and serve as a standoff medium. An assistant can hold the leg in a position so that the ultrasonographer can access the sole or the ultrasonographer may elect to hold the pastern between his/her knees with the sole facing upward. For the navicular bone and associated structures (Figure 1.11), the area over the frog is scanned in both transverse and sagittal planes; oblique parasagittal planes can also be evaluated. The proximal aspect of the DSB is approximately at the middle of the frog and the insertion of the DDFT on P3 is just proximal to the apex of the frog (Figure 1.12). Using this technique, the hyperechoic collateral ligament of the DSB can be seen indistinctly on the proximal aspect of the DSB. The DSB flexor surface and distal aspect can be seen as a hyperechoic surface. Palmar/plantar to this is the hypoechoic DSB fibrocartilage, then the navicular bursa followed by the fibers of the DDFT, which, when followed distally and dorsally, can be seen fanning out to implant on the semi-lunar facies flexoria of P3. Palmar/plantar and proximal to the DDFT and closely associated to it, is the very thin (hardly visible) distal digital annular ligament. From the distal aspect of the DSB, the 3–4 mm thick distal sesamoidean impar ligament (DSBIL) fibers can be seen extending distally to implant on P3 approximately halfway between the DDFT implantation (facies flexoria) and the palmar articulation surface of P3. The echogenicity of the DSBIL may vary, even in normal sound horses, but is usually slightly more echogenic than that of the DDFT. This also depends on the angle of insonation. Figure 1.11 Anatomical structures in a sagittal section of the distal digit. P2: second phalanx; P3: third/distal phalanx; DSB: distal sesamoid bone; DDFT: deep digital flexor tendon; CL: collateral ligament of the DSB; NB: navicular bursa; DC: digital cushion; ff: facies flexoria; * distal recess of the NB. Figure 1.12 Diagrammatic illustration of the position of the DSB and P3 on a solar view of the foot. P3: third/distal phalanx; DSB: distal sesamoid bone. Between the DSBIL and the distal DSB is the hypo- to anechoic distal palmar recess of the distal interphalangeal joint (DIPJ). Between the DSBIL and the implantation of the DDFT is the distal recess of the navicular bursa, usually only a potential space. Between the DDFT and the sole is the inhomogeneously hyperechoic digital cushion, the most dorsal portion of which is hyperechoic and fibrous (Figures 1.13, 1.14, 1.15, and 1.16). Figure 1.13 Sagittal transcuneal ultrasound image of the hyperechoic flexor cortical surface of the DSB and the solar surface of P3. DSB: distal sesamoid bone; P3: third/distal phalanx; DDFT: deep digital flexor tendon; DSBIL: distal sesamoidean impar ligament; NB: navicular bursa; ff = facies flexoria = DDFT insertion on P3; DC: digital cushion. Proximal is to the left and the solar surface is at the top of the image. The stippled red box in this, and all subsequent images in this chapter, indicates the transducer position. Figure 1.14 Sagittal transcuneal ultrasound image of the hyperechoic flexor cortical surface of the proximal DSB. Note the loss of image proximal to the DSB where transducer contact is poor and the DDFT is obliquely oriented to the angle of incidence of the ultrasound beam. DSB: distal sesamoid bone; DDFT: deep digital flexor tendon; NB: navicular bursa; DC: digital cushion. Proximal is to the left and the solar surface is at the top of the image. Figure 1.15 Transverse transcuneal ultrasound image of the hyperechoic flexor cortical surface mid-DSB. DSB: distal sesamoid bone; DDFT: deep digital flexor tendon; NB: navicular bursa; DC: digital cushion. The solar surface is at the top of the image. Figure 1.16 Sagittal (right) and transverse (left) transcuneal ultrasound images. The transverse image is taken immediately distal to the DSB. DSB: distal sesamoid bone; DDFT: deep digital flexor tendon; P3: third/distal phalanx; CD: digital cushion. The solar surface is at the top of both images and proximal is to the left on the sagittal image.
1
Ultrasonography of the Foot and Pastern
2 The Royal Veterinary College, North Mymms, Hatfield, UK
The Foot
Ultrasonography Proximal to the Coronary Band
Preparation
Technique
Ultrasound Guided Injections of the Collateral Ligaments of the Distal Interphalangeal Joint
Ultrasonographic Abnormalities
Transsolar and Transcuneal Ultrasonography
Preparation
Scanning Procedure
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