There are three indications for tendon ultrasound in the horse: screening for historic injuries, monitoring progress of an injury, and evaluating tendons and ligaments because of a lameness localized to that limb. For the most part, this means that either during the examination process, or at a previous examination, you will have used a lameness exam and other modalities to localize the site of your examination.
Often times, it may be desirable to ultrasound the corresponding sites on the opposite limb as compensatory lameness can occur due to excess stress on the tendon.
To prepare the patient, the limb should be shaved with a 40 blade, washed with a mild soap, thoroughly rinsed, and then prepped with alcohol. Following this, ultrasound gel should be liberally applied and given a brief period to settle and de-gas. If shaving is not a viable option, as with show horses or in winter-time, a thorough cleaning with mild soap and liberal application of 70% Isopropyl alcohol can facilitate imaging as well. Sometimes patients may require sedation to facilitate preparation and examination. This should be evaluated on a case-by-case basis.
To prepare the ultrasound, the frequency should be no less than 6.5 MHz, though the higher the frequency, the better the detail. Superficial structures can be assessed at 5 cm of depth or lower. A stand-off pad or device may aid the visualization of some structures and should be 5-10 mm in thickness. I prefer to scan structures both with and without the stand-off, as the stand-off does allow visualization of more superficial structures (the DDFT and SDFT), while the probe without the stand-off will allow for better visualization of the deeper structures like the check ligament and suspensory ligament. From the level of the fetlock down to the hoof, some practitioners prefer a micro-convex probe, as the probe head will more easily adapt to the irregular shapes encountered here. Others still prefer the linear t-probe, but in this region, it is critical to ensure adequate contact. It is beneficial to decrease your gain, as adequate preparation of the limb should ensure a good image with appropriate contrast. With the bony structures and tissue interfaces present, the increased gain can result in worsening artefacts and decreased contrast. A lower frame rate can straighten your edges and make the contrast between structures a little more visible.
Starting from the level of the base of the carpus, a transverse image should be taken at 90 degrees to the DDFT. At this level, in order of depth, the SDFT, the DDFT, the check ligament, and the suspensory ligament are seen. As you scan distally, approximately a third of the way down the leg, the check ligament will start to join the DDFT, and will be completely joined by the mid-shaft of the cannon. Over this same region, the DDFT and SDFT, which started as ovoid structures, will change from very similar in appearance to the more superficial structure (the SDFT) becoming thinner and wrapping around the DDFT, which becomes almost round by the mid-shaft of the cannon, and will become a narrow oval by the time the proximal sesamoids are reached. The suspensory ligament and check ligament will appear as ovoid structures as well, and the suspensory will flatten out and at the mid-shaft, take on a peanut-like shape. After this, the suspensory divides into its medial and lateral branches, which are circular in cross-section, and will wrap around the front of the fetlock. Below this point, the SDFT will become extremely thin, and then divide and insert on the sides of P2. They will appear to be thick commas. It is important at the level of the sesamoids not to miss the SDFT and mistake the DDFT as its more superficial neighbor, and the straight sesamoidean ligament as the DDFT. Below the fetlock, the DDFT will become the most superficial structure, as the SDFT branches off to the sides. At this point it also takes on a peanut-like shape until it inserts on the back of P3.
I prefer to scan in transverse, followed by a scan in longitudinal with a stand-off along the length of the cannon bone. Following this, I prefer to scan the area between the fetlock and hoof in transverse (longitudinal is of limited usefulness due to difficulty in obtaining adequate contrast. After scanning in the stand-off, I prefer to scan the cannon region in the same way without a standoff. As the structures below the fetlock are extremely superficial, I only rescan at the level of the heel bulb with a micro-convex if available. Rolling up and down along the cannon bone during the scan can help to verify that subtle hypoechoic areas are in fact, changes in texture and not artefacts.
The primary focus of an ultrasound of these structures is to note changes in texture, diameter, and brightness. Typically, these structures should appear close-textured, moderately hyperechoic, and uniform in thickness, with homogenous stippling in transverse and homogenous linear striation in longitudinal. More echogenic regions may be indicative of scar tissue, while less echogenic regions are usually an indication of edema. Abrupt changes in diameter also suggest alteration in the normal structure of the ligaments. In sub-acute injuries, as edema decreases, the texture of the fibers within a tendon or ligament will return, and these can be better evaluated at that time. Often, these fibers will be irregular following injury.
Other ligaments may be examined similarly, including the dorsal aspects of the suspensory branches, the common digital extensor, and the medial and middle patellar ligaments. Due to the superficial nature of most of these, I prefer to view these with a stand-off when possible. In the case of the patellar ligament, the stifle may also be examined. See future ruminations on intra-articular ultrasound. Dr. Ruffin Hutchison, DVM