Ultrasonography of the Foot and Pastern
1University of Pretoria, Onderstepoort, South Africa
2The Royal Veterinary College, North Mymms, Hatfield, UK
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.
Ultrasonography Proximal to the Coronary Band
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 it 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 curvilinear probe) 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 “T” ligament, and the navicular bursa down to the level of the proximal border of the navicular bone (Figure 1.2). 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.
For the dorsal, medial, and lateral aspects, the transducer is positioned both transversely adjacent to the coronary band and longitudinally overlying the coronary band (Figure 1.3) 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 1.4) can be imaged immediately proximal to the coronary band. The collateral ligaments traverse the coronary band and so only the more proximal parts 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 1.5). Further caudally lie the collateral cartilages, which are hypoechoic but can show areas of ossification (and therefore acoustic shadowing).
Via the palmar window, only chronic DDFT pathology, where there is retained echogenicity and/or mineralization within the off-incidence hypoechoic DDFT, can usually be visualized (Figure 1.6), limiting this view for comprehensive evaluation of the DDFT in this region of the foot. In some cases, DDFT pathology will extend sufficiently proximally to be visible in standard views within the distal pastern (see Pastern, later in this chapter).
Abnormalities of the distal interphalangeal joint can result in changes to the dorsal pouch which are visible ultrasonographically – both distension and synovial thickening (Figure 1.7) as well as osteophytosis in cases of osteoarthritis (Figure 1.8). Where ultrasonography of this region carries the most useful imaging is for collateral ligament desmitis, especially when there is palpable swelling in the region of the ligament (dorsomedially or dorsolaterally) at the level of the coronary band. Ultrasonographic abnormalities vary between enlargement and complete rupture (Figure 1.9).
Transsolar and Transcuneal Ultrasonography
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 pared smooth. The foot should then be soaked in bandages/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 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 stand-off medium. A handler 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 upwards.
For the navicular bone and associated structures (Figure 1.10), the area over the frog is scanned in both transverse and sagittal planes. 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 1.11). 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, can be seen fanning out to implant on the 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 distal sesamoidean impar ligament (DSBIL) fibers can be seen extending distally to implant on P3 proximal to the facies flexoria. 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 (Figures 1.12, 1.13, 1.14, 1.15).