Hind limb lameness localizing to the distal tarsus and proximal metatarsus is common among all horses and disciplines. The complexity of diagnosis of distal tarsal pain and proximal metatarsal pain lies in the in reliability in localization of lameness via local and intra-articular analgesia in this region.1,2 Additional challenges are met imaging the complex anatomy of the region.
Osteoarthritis is the most common cause of lameness associated with the tarsus in horses; most commonly seen in the distal intertarsal and tarsometatarsal joints. Radiography is generally the first diagnostic imaging modality used to diagnosis this process tarsus. Radiographic findings for osteoarthritis include periarticular osteophytes, periarticular and subchondral lysis, joint narrowing, enthesophytes at the attachment of the ligaments surrounding the tarsometatarsal and distal intertarsal joints. These changes are typically at the dorsal, dorsomedial and dorsolateral aspects of the distal intertarsal and tarsometatarsal joints. A 4-view series is needed to fully evaluate the tarsus.3-5 However, some lesions of the distal tarsus may be undetected or be underestimated by radiographic evaluation alone. There tends to be poor correlation of the degree of lameness and alteration in performance with radiographic changes of the tarsus and metatarsus.6 It is likely that this poor correlation stems in part from the under-diagnosis of pathologic changes. There is evidence to suggest that location of the pathologic change may have some bearing on clinical significance. Those horses with osteoarthritis along the medial aspect of the tarsometatarsal joint tended to lame in that limb. Horses with talocalcaneal joint osteoarthritis are typically lame because of the disease process.7 Nuclear medicine has widespread availability may help diagnose radiographically occult lesions. The use of MR and correlation back with nuclear medicine has helped the understanding of disease process and location.8 Additionally, locations thought to be uncommonly affected by osteoarthritis, such as the plantar aspect of the distal tarsal joints may be overlooked on radiographs alone. Lesions in the bone marrow are typically occult radiographically despite having a consistent moderate to severe lameness. Advanced imaging progresses this knowledge and helps target treatment.
Fractures involving the distal tarsus are not common. However, when horses present with an acute lameness suspicious of a fracture; diagnosis may be a challenge as the fractures may be radiographically occult with conventional views. The central tarsal bone tends to be more affected. Often the fracture configuration is biarticular extending in a dorsomedial to plantar lateral fashion. These may be most apparent on a dorso 15–25 lateral-plantar lateral oblique image. Fragmentation is may be seen with traumatic injury collateral ligament injuries or be benign. Fragmentation in the proximal tubercle of the talus are typically benign,9 but may be met with uncertainty due to the low frequency encountered. It is important in cases of fragmentation to have a comprehensive understanding of what soft tissue may be affected or causing the fragmentation to discern potentially significant from insignificant findings.
The suspensory ligament is a well-documented source of hind limb lameness in the horse.10-12 The bulk of the suspensory ligament originates on the plantar-proximal aspect of the 3rd metatarsal bone. At is proximal extent, the origin is bilobed and valentine or trapezoid in shape. However, there is a small bundle of fibers that continues proximally to the fourth tarsal bone and plantar aspect calcaneus.12 Hindlimb suspensory ligament injury is a common problem in the sport horse. Despite the frequency of this injury, diagnosis can be challenging as horses can vary in their presenting signs and the often bilateral nature of the disease confounds the diagnosis. Response to hind limb flexion is generally moderate to marked, which may overlap with clinical signs associated with lameness originating primarily from the tarsus. Approached to ultrasound examination is critical for making accurate diagnoses. In the authors opinion, the more sensitive approach to diagnosing injury is a non-weight bearing approach (often in a farrier sling) with the bone margin parallel to the bottom of the ultrasound screen. Imaging/pathologic findings associated with the hind suspensory ligament vary. Pathologic changes may include one or more of finding including diffuse enlargement, dorsal or plantar margin tearing, diffuse fiber damage, fibrosis/scarring and osseous changes. Persistent hypoechoic areas regardless the angle of beam incidence are typically indicative of areas of fiber damage. Variations of anatomy do occur and are important to realize. For example, there is a hypoechoic region on both on and off angle approximately just distal to the origin of the suspensory ligament. This site may be confused for a core lesion, which are less common in the hind suspensory ligament.
Focal or diffuse areas of increased echogenicity independent of angle of incidence are consistent with fibrosis/scar tissue. Dystrophic mineralization can also occur and if subtle can be difficult to distinguish from scarring. Measuring the cross-sectional area (CSA) can be helpful for comparing the size to the opposite limb. Assessing mild enlargement can be difficult, particularly as there is no guarantee that the contralateral limb is normal. Measurements on cross-sectional area of the hind suspensory ligament are not standardized for each bread. Normal CSA for a Warmblood may not be the same as a reining Quarter Horse. For the warmblood this may greater 2 cm2 and for the quarter horse it may be 1.8 cm2. It is more helpful to use imaging characteristics to diagnose injury rather than using cross sectional area as primary source of normal versus abnormal. These include loss of the normal space between the dorsal margin of the suspensory ligament and plantar bone margin, displacement of the medial plantar vessels, and extension of the ligament beyond the plantar confines of the splint bone. Pathologic changes in the suspensory ligament often do not result is marked enlargement, but focal marginal change (dorsal most frequent). It is important to realize the suspensory ligament is comprised of multiple tissue types and has different echogenicity as it is image with ultrasound.
Pathologic changes to the bone can occur in additional to ligamentous pathologic change or can be the primary abnormality. Abnormalities can include bone proliferation, resorption and avulsion fragmentation. MR imaging is very good for assessing changes to surface, cortical and medullary portions, ultrasound can actually be superior for identifying small avulsions and enthesopathies. This is part due to acoustic shadowing from fragmentation, good contrast between bone and ligament structures and thin tissue sampling. The axial margins of MT2 and MT4 should also be evaluated for proliferative changes that could impinge on the suspensory ligament.
In conclusion, a combination of history, lameness evaluation, response to diagnostic analgesia and multiple imaging modalities is needed to make a diagnosis. Imaging modalities should be viewed complementary modalities and be used as such. To get the most out of imaging findings, it is important to take into consideration clinical history, physical and moving examinations and diagnostic anesthesia findings.
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