CHAPTER 124 Ultrasound of the Distal Portion of the Limb
Examination of the soft tissue structures of the distal part of the limb is becoming increasingly important as a diagnostic technique in horses with lameness or injury. Radiography, which has been the most used diagnostic procedure, is not very useful in evaluating soft tissue structures in the distal limb segments. Radiographs can reveal changes in the insertion or origin of ligaments and tendons and thus give information about the attachment sites of tendons and ligaments, but the tendon or ligament structure itself is not well imaged by radiography. Although magnetic resonance imaging (MRI) seems to be the diagnostic tool of the near future, it is not yet available to most practices. Also, the costs associated with using MRI for imaging the distal limb and its soft tissues may limit its use, especially for regular follow-up during rehabilitation.
Ultrasound, however, can provide good-quality images of the tendon and ligament structures of the extremities. Ultrasound is less costly to perform and is the tool of choice to evaluate an injury, especially for follow-up examinations.
For optimal image quality and high resolution, a 12- to 15-MHz transducer is needed, as well as a microconvex 5- to 8-MHz transducer for imaging the most distal part of the limb. Linear sector probes with at least a 7- to 10-MHz transducer in a tendon format (not too large, about 2 to 4 cm) can be useful but the resolution will be lower. When using the lower-frequency settings, a stand-off device is necessary to image superficial structures. With frequencies in the 12- to 15-MHz range, image quality of the near field is often perfect without a stand-off device. Absence of the stand-off device facilitates manipulation of the transducer and allows better access to positions that are difficult to reach when a stand-off pad is used.
PREPARATION OF THE HORSE AND LIMB
Many minor changes in the soft tissues of the limb can be seen with ultrasonography, but little is known about the clinical significance of these isolated findings. For this reason, ultrasonography of the distal limb should not be performed without previous diagnostic evaluation, at least to identify the region of the distal limb suspected to be the cause of lameness. Concurrent examination of the opposite limb with split-screen settings can be used to compare images between the left and right limbs and can yield valuable information about the relevance of any ultrasonographic finding.
The limb should be clipped from fetlock to coronet with a no. 40 blade electrical clipper. After clipping, moisturizing of skin in the clipped area greatly improves skin conductivity. We avoid the use of alcohol because it irritates the skin and can cause hyperemia, which can interfere with Doppler evaluation of blood vessel activity. Bandaging the limb with a wet wrap for 10 to 15 minutes is a good way to improve conductivity of the skin for ultrasound waves. Also a combination of water and contact gel can be used. For horses that tolerate it, placing the distal part of the limb in a bucket of lukewarm water is even better, and the entire ultrasonographic examination can be done in the bucket, with the probe beneath the surface of the water. This technique is especially useful when performing ultrasonographic examination of the navicular area through the pastern region, between the bulbs of the foot. Optimal conductivity and contact between skin and the microconvex probe can be obtained when performing the examination under water.
It is standard procedure to sedate every horse for ultrasonographic examination of the distal portion of the limb. Only when the horse is extremely cooperative or when sedation is contraindicated (for example, in severe neurologic disease) is it examined without sedation. A quiet, standing horse that does not lift its limbs poses little danger to the transducer and examiner and greatly contributes to proper image quality. For proper preoperative imaging before ultrasonographically guided injection of medication into a lesion in a ligament or tendon, local anesthesia or even general anesthesia may be required. However, when diagnostic anesthetic blocks are performed, it is useful to realize that some injections can interfere with the quality of the ultrasonographic image, for instance, when air is injected or is aspirated into the tissue by negative pressure. Also, vascular activity patterns can be altered by local anesthesia, especially when a combination of an anesthetic and a vasoconstrictive agent is used. When ultrasonographic evaluation is performed shortly after a nerve block has been placed, even fluid accumulation may be observed, making differentiation between pathologic changes and iatrogenic artifact difficult.
STRUCTURES OF THE DISTAL PORTION OF THE LIMB
The intersesamoidean ligament is attached to the axial surfaces of both proximal sesamoid bones in the fetlock region. A transverse view of the fetlock, with the transducer on midline at the palmar aspect of the fetlock just proximal to the annular ligament, is the best position to image and evaluate this ligament. The ligament is dense in echogenicity; injuries result in a mottled appearance of the ligament along the axial margins of the sesamoid bones. The presence of severe injury or even partial rupture of the ligament, divergence of the apices of the sesamoid bones can be radiographically visible. Generalized desmitis manifests radiographically as an irregular axial border of the sesamoid bone and a less radiodense structure of the axial part of the sesamoid bone.
Annular Ligaments and Proximal Annular Ligament
The distal annular ligaments in the pastern region are difficult to image with ultrasonography and are rarely a cause of lameness. The proximal annular ligament is, however, a more common cause of lameness, especially when there is enlargement of the sesamoidean tendon sheath and possibly the deep digital flexor tendon (DDFT) and superficial digital flexor tendon (SDFT). Because of stress on the annular ligament when tendon sheath and involved tendon structures are enlarged, the annular ligament enlarges.
In clinically normal horses, differentiation between annular ligament and outer layer of the tendon sheath is possible only with high-resolution ultrasonography (i.e., 12 to 15 MHz). A stand-off pad is necessary to bring the annular ligament into view when the frequency of the transducer is less than 12 to 15 MHz because of the proximity of the ligament to the skin (just 2 to 3 mm deep). When enlargement is present, more then 5 mm of thickness in the annular ligament is considered to be of clinical significance.
Proper evaluation of the structures in the tendon sheath is advisable whenever enlargement of the proximal annular ligament is present because the cause of enlargement seldom originates with the annular ligament itself. Detection of synovitis within the tendon sheath is common, along with primary tendon lesions in the SDFT or DDFT. Only with ultrasound equipment of high quality (high resolution and frequencies in the 10- to 15-MHz range) can these primary tendon lesions be seen. Tenoscopy often reveals the extent of the lesions in the tendon structures within the tendon sheath as well as the changes in the tendon sheath itself.