MRI FINDINGS IN 232 HORSES WITH LAMENESS LOCALIZED TO THE METACARPO(TARSO)PHALANGEAL REGION AND WITHOUT A RADIOGRAPHIC DIAGNOSIS JENNIFER N. KING,CHAD J. ZUBROD,ROBERT K. SCHNEIDER,SARAH N. SAMPSON,GREG ROBERTS Two hundred and thirty-two horses with lameness localized to the metacarpo(tarso)phalangeal (MCP/MTP) region without a radiographic diagnosis were evaluated. All horses had high-field magnetic resonance (MR) imaging of the MCP/MTP region performed for the lame limb and the contralateral limb for comparison. There were 46 horses that had bilateral abnormalities in the forelimbs; 27 of these horses were not lame in the contralateral limb at the time of examination. Bilateral hind limb abnormalities were observed in 37 horses; 22 horses were not lame in the contralateral limb. Soft tissue abnormalities alone were observed in 218 limbs (162 horses). Subchondral bone and articular cartilage abnormalities alone were observed in 43 limbs (34 horses). A combination of soft tissue, subchondral bone, and cartilage abnormalities were observed in 64 limbs (36 horses). The distribution of primary abnormalities was as follows; oblique distal sesamoidean ligament desmitis (73 limbs in 56 horses), straight distal sesamoidean ligament desmitis (44 limbs in 38 horses), chronic subchondral bone injuries (15 limbs in 12 horses), suspensory ligament branch desmitis (14 limbs in 12 horses), collateral ligament desmitis (12 limbs in 12 horses), tendonitis of the superficial and deep digital flexor tendons (10 limbs in 10 horses), osteochondral defects greater than 1 cm (nine limbs in six horses), osteochondral defects less than 1 cm (eight limbs in seven horses), bone marrow lesions (six limbs in five horses), intersesamoidean ligament desmitis (five limbs in four horses). MR imaging is useful in diagnosing bone and soft tissue injuries when radiographs and ultrasound fail to yield a diagnosis. C  2012 Veterinary Radiology & Ultrasound. Key words: equine, fetlock, lameness, metacarpophalangeal, metatarsophalangeal, MRI. Introduction T HE BONES and supporting soft tissues in the metacarpo(tarso)phalangeal (MCP/MTP) region are commonly injured in equine athletes. This joint has the greatest range of motion in the horse, and the support- ing soft tissues absorb most of the forces of weight bear- ing during high-speed exercise. 1–3 Single traumatic events and microdamage caused by repetitive cyclic loading are responsible for bone and articular cartilage damage in this joint. Bone injuries like third metacarpal/metatarsal condylar fractures, sesamoid fractures, and osteochondral fragmentation of the proximal phalanx are commonly di- agnosed using radiography. 4–6 The soft tissue components of this joint are injured as a consequence of hyperextension, medial–lateral instability, or subluxation. Ultrasonography has been the most common modality used to image the ten- dons and ligaments in this region. 7, 8 Soft tissue lesions that From the Department of Veterinary Clinical Sciences, College of Veteri- nary Medicine, Washington State University, Pullman, WA 99164 (King, Schneider, Sampson, Roberts) and Oakridge Equine Hospital, 6675 East Waterloo Road,, Edmond OK 73034 (Zubrod) Address correspondence and reprint requests to Jennifer N. King, at the above address. Email: jking_dvm@hotmail.com Received January 18, 2012; accepted for publication August 7, 2012. doi: 10.1111/j.1740-8261.2012.01983.x are difficult to identify using ultrasonography can be identi- fied with magnetic resonance (MR) imaging. 9–11 Magnetic resonance imaging has been used to detect subchondral bone damage and osteochondral defects when there are no detectable radiographic abnormalities. 12 Magnetic resonance imaging is useful for diagnosing in- jury to equine athletes when conventional imaging fails to yield a definitive cause of lameness. 9–12 Magnetic reso- nance imaging abnormalities in the MCP/MTP region have been summarized and reported in a group of 40 horses, in- cluding horses with radiographic abnormalities. 13 Because the MCP/MTP joint is a common location for lameness problems in all types of equine athletes and radiographs frequently do not yield a diagnosis, it is important to char- acterize abnormalities in a large group of horses. Our pur- pose was to describe the MR imaging findings in horses with lameness localized to the MCP/MTP region when radiographs were normal. The objectives were to identify the most frequent problems in a large population of per- formance horses and describe the clinical features associ- ated with MR imaging abnormalities. Our hypothesis was that injury to the supporting soft tissue structures would be more frequent than abnormalities in subchondral bone Vet Radiol Ultrasound, Vol. 54, No. 1, 2013, pp 36–47. 36