IN-DEPTH: ULTRASONOGRAPHY IN LAMENESS DIAGNOSIS


nicity of the ligament in this region due to the vari- able presence of hypoechoic muscle tissue and the bilobed shape of the ligament at its insertion on the proximal metacarpus. The distal branches are more easily scanned, and pathology will often run down to the sesamoid attachment. Cases with in- juries close to or involving the insertion onto sesa- moids benefit from a radiographic series of the fetlock to assess the level of sesamoiditis and rule out the presence of a sesamoid fracture. Alter- nately, injuries of the proximal suspensory insertion on the cannon bone also benefit from radiographic evaluation to rule out avulsion fracture or bone injury. The proximal suspensory ligament region and its insertion on the palmar region of the metacarpus is one of the more common sites of lameness in the author’s Thoroughbred racetrack practice where horses are trained on a variety of tracks, including sand, dirt, poly, and turf. The majority of these cases appear to be a transient inflammation of the periligamentous tissues without any obvious struc- tural damage to the suspensory ligament itself. This inflammation, within the anatomical confines of the proximal suspensory region, causes the clini- cal signs typical of proximal suspensory lameness, including a transient mild to moderate lameness (grade 1–2/5), with very subtle swelling and heat in the proximal suspensory region (proximal third of the palmar metacarpus when assessed in a standing position). Firm palpation when non-weightbearing with the carpus flexed generally elicits a sharp pain response, but there is no obvious palpable thicken- ing of the suspensory ligament itself when compared with the contralateral unaffected limb and using the caudal border of the splint bones as a reference. Both fetlock and carpal flexion tests will often elicit a worsening of the lameness, as will firm palpation of the proximal suspensory ligament, but responses can be variable with all three manipulations. The horses can also show increased lameness on the turn at the walk or when decelerating in transition from trot to walk. The surface they are trotted on can also influence the lameness as can the presence of a rider. Ultimately, local anesthesia may be re- quired to determine the proximal suspensory re- gion as the site of lameness. Ultrasonography is generally unnecessary in these cases, as the ma- jority respond quickly to phenylbutazone, ice, rest, and reduced workload with for 3–5 days, with resolution of lameness, swelling, and pain. Alter- nately, cases can be treated with a regional infil- tration of corticosteroids as well. Ultrasound is indicated for cases that fail to respond to the above treatment approach. Injuries suspected as having a desmitis or an insertional bone injury of the proximal suspensory ligament are indicators for immediate imaging, starting with ultrasound (linear 7.5–14 Mz) of the proximal suspensory region. The exam should be performed both weightbearing and non-weight-


bearing, including using an off-incidence tech- nique, as mild injuries will be missed if relying on weightbearing sonography alone. In addition, ra- diographs of the proximal cannon bone can assist in ruling out obvious insertional bone injury but has a low sensitivity for bone lesion in this area. These cases with either a true desmitis or inser- tional bone injury tend to respond poorly to anti- inflammatory therapy with persistent swelling and variable lameness. On presentation, they tend to be more lame (grade 3–4/5) and have a more focal pain response on palpation compared with cases with only periligamentous inflamma- tion. The suspensory ligament will palpate obvi- ously thickened when assessed with the carpus flexed in a non-weightbearing position, often with swelling more palmar than the palmar border of the splint bones. Similar management and reha- bilitation to flexor tendon overstrains can be used, with large suspensory body core lesions requiring a similar 9–12 months. Suspensory branch inju- ries and proximal suspensory insertional injuries can often be rehabilitated more rapidly, with year- lings and 2-year-old horses often able to be reha- bilitated based on ultrasonographic healing with some suitable to return to training as early as 3–4 months.


Digital Sheath Soft Tissue Injuries


Soft tissue injuries involving the digital flexor ten- don sheath (DFTS) typically cause lameness, swell- ing, and enlargement of the structure damaged within the tendon sheath, along with varying de- grees of DFTS effusion in the acute phase. The most common injuries in Thoroughbred racehorses in this region are overstrain injuries of the SDFT or sesamoidean ligaments. Overstrain of the oblique sesamoidean ligament is clinically indistinguishable from injuries of the SDFT distal branches and causes a typical swelling in a small anatomical tri- angle created by the border of the sesamoid bone proximally, margin of P1 dorsally, and the diverging edge of the SDFT branch palmarly or plantarly. Ultrasound of this region is very rewarding, and injuries of the SDFT or sesamoidean ligaments are generally easily defined and identified. Injuries of the DDFT and/or manica flexoria of the


SDFT result in a swelling of these structures above the level of the sesamoids, which are, again, clini- cally indistinguishable. Ultrasound of this region is more challenging, and a definitive diagnosis can- not always be made on ultrasound examination; usually there is an index of suspicion based on ul- trasound, but further imaging is often required, par- ticularly in the case of linear tears of the DDFT.14,15 Contrast DFTS radiography is a useful addition and will often identify damage to the DDFT or SDFT manica flexoria.16 Ultimately, magnetic resonance imaging (MRI) examination or diagnostic tenoscopy is often required for a definitive diagnosis of injury to these structures.


AAEP PROCEEDINGS  Vol. 64  2018 3


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