IMAGING Magnetic Resonance Imaging Findings in Horses With Recent and Chronic Bilateral Forelimb Lameness Diagnosed as Navicular Syndrome Sarah N. Sampson, DVM; Robert K. Schneider, DVM, MS, Diplomate ACVS; and Patrick R. Gavin, DVM, PhD, Diplomate ACVR High field strength MRI is an effective method for diagnosing injury to structures within the feet of horses with navicular syndrome. The prevalence of different pathologies was determined in 151 horses with normal radiographs but clinical signs of bilateral navicular syndrome. Authors’ address: Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, WA 99164; e-mail: [email protected]. © 2008 AAEP. 1. Introduction individual variation between horses as well as the Navicular syndrome is one of the most common variety of pathologic changes that can occur in each 4–10,12–17,19,21,22 causes of forelimb lameness in many types of ath- horse. letic horses.1–4 Before magnetic resonance imag- Navicular syndrome can now be defined as multi- ing (MRI), definitive diagnosis of pathologic change ple disease processes resulting in damage to the within the horse’s foot was determined only at post- navicular bone (NB), the collateral sesamoidean lig- mortem examination. Radiography continues to be ament (CSL), the impar ligament (IL), the navicular the first diagnostic imaging step used to try to define bursa, and/or the deep digital flexor tendon 4–6,10,13–14,17,23 pathology associated with navicular syndrome, but (DDFT). Navicular syndrome is de- it has become more apparent with the availability of scribed as heel pain that blocks to a palmar digital MRI that radiography is inadequate in this region nerve (PDN) block on both forelimbs; radiographs both for diagnosis of bone and soft-tissue patholo- may or may not have visible pathology. Further- gies. This is made clear by the reports of pathology more, it has been reported that horses with navicu- diagnosed with MRI over the last 10 yr.4–17 The lar syndrome can have a variety of other structures ability to use MRI to further define navicular syn- involved that are not directly associated with the drome in live horses has enabled a greater under- navicular region itself. Examples of these are standing of the disease processes in the foot of the pathologic changes in the distal digital annular lig- horse that contribute to heel pain. The normal ament, proximal, middle, and distal phalanges, dis- anatomy of the foot seen with MRI has been de- tal interphalangeal joint cartilage and subchondral fined,18–20 and the increasing number of horses im- bone, proximal interphalangeal joint cartilage and aged continues to improve our understanding of subchondral bone, collateral ligaments of the distal NOTES AAEP PROCEEDINGS ր Vol. 54 ր 2008 419 IMAGING and proximal interphalangeal joints, lamina (includ- examination findings, results of lameness examina- ing keratoma formation), and digital flexor tendon tions, previous treatments, MRI findings, treat- sheath.4,5,7,9,10,12,17 Pathologic change can occur in ments based on MRI findings, and the suggested any of these structures as the only manifestation of rehabilitation program. disease within the foot; however, multiple struc- A thorough lameness evaluation was performed in tures can be affected within the foot of one horse.4 all horses. Injection of 1.5–2 ml local anesthetic In the past, radiography and ultrasonography in over the medial and lateral PDN of the lame leg some cases have been able to define pathologic proximal to the collateral cartilage caused each change within the bone or soft tissues, respectively, horse to switch to lameness in the opposite forelimb. within the horses foot,1,24 but negative findings with Injection of local anesthetic over the medial and these modalities are not enough to diagnosis a lack lateral PDN of the opposite forelimb then eliminated of pathology.4,15,21 Furthermore, even with find- the horse’s lameness. All horses were examined at ings of pathologic change in one structure using a trot in a straight line and in small circles in each these imaging modalities, it is now readily apparent direction before and after PDN blocks. Lameness with MRI that the diagnosis of lesions within the grades were 0 (no lameness observed), 1 (mild lame- horse’s foot has been missed when only radiography ness without an observed head nod), 2 (subtle head and/or ultrasonography are used as diagnostic nod observed), 3 (obvious head nod observed), 4 (se- aids.4,10,23 To fully define navicular syndrome in vere head nod with Ͻ50% of normal weight sup- each individual horse, it is necessary to evaluate ported on the limb), and 5 (limb not bearing weight). these horses using MRI.4–7,13,14,16,17,20,25 Radiographs, including a 60° dorsopalmar projec- MRI remains a valuable diagnostic tool for orthope- tion, a palmaroproximal to palmarodistal oblique dic problems in humans for both bone and soft-tissue (skyline) projection of the distal sesamoid (navicu- injury.26–28 MRI of the live horse has only been avail- lar) bone, and a lateromedial projection of the foot able for the last 10 yr,17 but in this time, it has become were evaluated and found to be normal for the age increasingly apparent how much MRI has to offer in and use of the horse for all horses included in the the diagnosis of orthopedic injury in the horse.4–17 study. Multiple MRI studies have been performed on cadaver MRI was performed on both front feet of every specimens to evaluate horses with chronic, end-stage horse after general anesthesia. Each horse had a navicular disease.14,15,21,29–32 Many of the horses in thorough physical exam, including an electrocardio- these post-mortem studies have had flexor cortical ero- gram (ECG), complete blood count (CBC), and chem- sion or other severe bone abnormalities visible on ra- istry profile before anesthesia. Anesthesia was diographs.21 MRI has enabled evaluation of the soft induced with ketamine and diazepam and main- tissues associated with the NB in live horses for the tained with 2–5% isoflurane in oxygen. All horses first time, and, surprisingly, it has proven how little were placed in right lateral recumbency, and their information we often obtain from radiographs regard- front feet were positioned in a 1.0-Tesla magnet.a ing the changes that are occurring in the bony struc- A human knee quadrature receiver coil was posi- tures of the foot.5–7,14,16–18 tioned around the foot, and multiple image se- The objective of this study of horses with recent quences were obtained using proton density (PD), and chronic signs of navicular syndrome was to de- T2-weighted (T2W), short tau inversion recovery termine the prevalence of injury to different struc- (STIR), and gradient echo (3DGE) sequenc- tures within the horse’s foot when lameness has es. Axial, sagittal, and dorsal sections were ob- been present for variable periods of time. Addition- tained using standard MRI protocols for the foot of ally, this study will compare these findings on two the horse developed at Washington State University groups of horses based on whether or not they had (Table 1).17 All MRIs were stored on magnetic op- recent or chronic signs of navicular syndrome. tic discs for later retrieval and evaluation. All MRI observations in these two specific groups of horses were rope recovered from anesthesia in a horses restricted to those with bilateral forelimb padded recovery stall. lameness diagnosed as navicular syndrome without A standardized format was used to record MRI radiographic changes have not been reported. observations, and images were compared with im- ages previously obtained from normal horses. A 2. Materials and Methods primary finding, based on severity and lack of other The horses in this study had MRI performed on both observed abnormalities, was made in horses where front feet at Washington State University Veteri- it was clear that one principal abnormality was nary Teaching Hospital between May 1998 and No- present. In the other horses, a primary location of vember 2007, because they had developed clinical inflammation could not be identified from the mul- signs of bilateral navicular syndrome. Horses that tiple abnormal findings identified. Based on MRI had been diagnosed with clinical signs of navicular findings, the limb with the most severe findings was syndrome were accepted if they did not have abnor- recorded before the lamest limb was known, and malities on radiographs of the front feet. Informa- these results were later compared with lameness tion recorded from medical records of horses exam findings. If MRI findings were too similar included in the study consisted of history, physical between limbs, this information was recorded. 420 2008 ր Vol. 54 ր AAEP PROCEEDINGS IMAGING Table 1. Magnetic Resonance Imaging Sequences Used for Evaluation of the Foot and Pastern of the Horse Using a 1.0-T Gyroscan Magnet Image TR TE FOV/ Matrix Slice #/ Gap Time Orientation Sequence (ms) (ms) FA FOV Size Width(mm) (mm) (min) Axial DE TSE T2 2116 100 90 15/15 512 ϫ 512 30/4 mm 0.5 2:34 TSE PD 2116 10.5 90 15/10 512 ϫ 512 30/4 mm 0.5 2:34 Axial STIR 1725 35 90 15/15 256 ϫ 256 30/3.5 mm 1.0 4:42 Axial 3D GE 47 9 25 10/10 256 ϫ 256 30/3.0 mm Ϫ1.5 3:13 Navicular Sagittal DE TSE T2 3395 110 90 14/10 256 ϫ 512 22/4 mm 0.5 2:21 TSE PD 3395 13.8 90 14/10 256 ϫ 512 22/4 mm 0.5 2:21 Sagittal STIR 1500 35 90 14/10 256 ϫ 256 22/3.5 mm 0.5 5:48 Coronal 3D GE 47 9 25 10/10 256 ϫ 256 30/3.0 mm Ϫ1.5 3:13 Navicular Coronal DE TSE PD 3395 13.8 90 13/13 512 ϫ 512 16/3.5 mm 0.3 2:21 DIP CL TSE T2 3395 110 90 13/13 512 ϫ 512 22/3.5 mm 0.3 2:21 TSE, turbo spin echo; T2, T2-weighted; PD, proton density; STIR, short tau inversion recovery; 3D GE, three dimensional gradient echo; TR, repetition time; TE, echo time; FA, flip angle; rFOV, relative field of view; DIP, distal interphalangeal joint; CL, collateral ligament; DE, dual echo (PD and T2).