ONE: Ultrasonography of the Foot and Pastern

Ultrasonography of the Foot and Pastern

Ann Carstens1 and Roger K.W. Smith2

1University of Pretoria, Onderstepoort, South Africa

2The Royal Veterinary College, North Mymms, Hatfield, UK

The Foot

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.

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.

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).

Figure 1.3    Linear transducer positioning to evaluate the dorsal and dorsolateral/dorsomedial aspects of the foot: (A) transverse, (B) longitudinal. Note the transducer spanning the coronary band in the longitudinal orientation.
Figure 1.4    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 1.5    Hypoechoic region within the collateral ligament in a transverse image. There is no accompanying enlargement to the ligament and so such isolated hypoechoic areas should be interpreted with caution, as they can be generated artifactually by slight off-incidence orientations of the transducer.

Ultrasonographic Abnormalities

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).

Figure 1.6    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 1.7) 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).

Figure 1.7    (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 1.8    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 1.9    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. Images (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.

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.

Scanning Procedure

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).

Figure 1.10    Anatomical structures in a sagittal section of the distal digit. CL: collateral ligament of the DSB; DC: digital cushion; DDFT: deep digital flexor tendon; DSB: distal sesamoid bone; ff: facies flexoria; NB: navicular bursa; P2: second phalanx; P3: third/distal phalanx; * distal recess of the NB.
Figure 1.11    Diagrammatic illustration of the position of the DSB and P3 on a solar view of the foot. DSB: distal sesamoid bone; P3: third/distal phalanx.
Figure 1.12    Sagittal transcuneal ultrasound image of the hyperechoic flexor cortical surface of the DSB and the solar surface of P3. DC: digital cushion; DDFT: deep digital flexor tendon; DSB: distal sesamoid bone; DSBIL: distal sesamoidean impar ligament; ff = facies flexoria = DDFT insertion on P3; NB: navicular bursa; P3: third/distal phalanx. 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 probe position.
Figure 1.13    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. DC: digital cushion; DDFT: deep digital flexor tendon; DSB: distal sesamoid bone; NB: navicular bursa. Proximal is to the left and the solar surface is at the top of the image.
Figure 1.14    Transverse transcuneal ultrasound image of the hyperechoic flexor cortical surface mid DSB. DC: digital cushion; DDFT: deep digital flexor tendon; DSB: distal sesamoid bone; NB: navicular bursa. The solar surface is at the top of the image.
Figure 1.15

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Jun 8, 2017 | Posted by in EQUINE MEDICINE | Comments Off on ONE: Ultrasonography of the Foot and Pastern

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