Atlas of Equine Ultrasonography

Jessica A. Kidd, Kristina G. Lu, Michele L. Frazer, Jessica A. Kidd, Kristina G. Lu, Michele L. Frazer

• Publisher: Wiley-Blackwell
• Year: 2014
• ISBN:
  9781118798133
• Edition: 1
• Topic: Medicine
• Subtopic: Equine Veterinary Science

The only visual guide to equine ultrasonography based on digital ultrasound technology. Atlas of Equine Ultrasonography provides comprehensive coverage of both musculoskeletal and non-musculoskeletal areas of the horse. Ideal for practitioners in first opinion or referral practices, each chapter features normal images for anatomical reference followed by abnormal images covering a broad range of recognised pathologies. The book is divided into musculoskeletal, reproductive and internal medicine sections and includes positioning diagrams demonstrating how to capture optimal images. With contributions from experts around the world, this book is the go-to reference for equine clinical ultrasonography.

Key features include:

• Pictorially based with a wealth of digital ultrasound images covering both musculoskeletal and non-musculoskeletal areas and their associated pathologies.
• Each chapter begins with a discussion of normal anatomy and demonstrates how to obtain and interpret the images presented.
• A video library of over 50ultrasound examinations is available for streaming or download and viewing on-the-go. Access details are provided in the book.

SECTION 1
Musculoskeletal

CHAPTER ONE
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.

Preparation

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.

Technique

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),...