The tarsus is the source of hind limb lameness in up to 80% of affected horses.1 Because of its complex structural arrangement, lesions can be difficult to effectively image or definitively diagnose with standard imaging techniques. Pathological changes can involve bone, cartilage, tendon, ligament, or synovium. The distal tarsal bones have a minimal range of motion2 and primarily undergo compression.3 Changes in the distal tarsal bones are frequently implicated as the source of lameness within the tarsus; however, there are few studies that have examined pathological changes of these bones in vivo or compared the usefulness of various imaging modalities available to practitioners.
Radiography can be useful when examining the tarsus, especially for osteoarthritis,4,5 but the relationship between radiographic changes and signs of lameness is considered inconsistent. There is a substantial amount of superimposition of the distal bony structures, and up to a 30% to 50% decrease in bone density must occur before it is evident on radiographic images.6 Despite this, radiography is often one of the first imaging modalities chosen because of its availability, low cost, and effectiveness in detecting osteochondrosis and fractures.6
Nuclear scintigraphy is also often used, particularly to help localize hind limb lameness. With respect to the distal tarsal bones, much work has been done to describe patterns of RU in sound7 and lame8 horses. Abnormal RU demonstrates an increase in osteoclast and osteoblast activity, and although this is a sensitive imaging modality (for acute and chronic lameness), the findings are not specific in terms of lesion type.
Magnetic resonance imaging in horses has received much attention in the past 10 years.9 Gross normal anatomy of the tarsus has been shown to correspond well with MRI findings obtained with high-field10 and low-field11 scanners. Little work has been done to describe pathological changes of the tarsus in vivo, although cadaver specimens have been used to describe SCB thickness patterns in horses with12 and without13 previous lameness caused by signs of tarsal pain.
Equine practitioners must consider which of these diagnostic imaging modalities is the most appropriate to recommend to their clients when treating horses with lameness attributable to the tarsus. The purposes of the study reported here were to describe pathological findings identified with MRI in the distal tarsal bones of horses with unilateral hind limb lameness attributable to tarsal pain and to compare the usefulness of MRI with that of radiography and nuclear scintigraphy in the evaluation of this region. We hypothesized that radiography would not be a sensitive imaging modality, compared with nuclear scintigraphy and MRI, for the detection of specific conditions of the tarsal region.
Region of interest
Short-tau inversion recovery
Is2 digital camera, Is2 Medical Systems Inc, Fishers, Ind.
Mirage Image Processing Software, Segami Corp, Columbia, Md.
Eklin Medical Systems Inc, Santa Clara, Calif.
MAGNETOM Espree 1.5-T MRI system, Siemens Medical Solutions USA Inc, Malvern, Pa.
Tomoscan M/EG, Philips Medical Systems North America Inc, Irvine Calif.
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