Texas Equine Veterinary Association

www.texasequineva.com • 13 Introduction Navicular syndrome, which is now more commonly referred to as "heel pain or foot pain," is one of the most common causes of lameness in adult horses. The heel region is composed of numerous synovial, soft tissue, and osseous structures functioning to form the podotrochlear apparatus. The deep digital flexor tendon is cushioned from the navicular bone by the navicular bursa. The navicular bone is suspended by the collateral sesamoidean ligament above it and the distal sesamoidean impar ligament below it. These three structures, in addition to the T-ligament, separate the distal interphalangeal joint, the navicular bursa, and the digital flexor tendon sheath from each other. Diagnosis Lameness most commonly affects the forelimbs and is frequently bilateral and insidious in onset, with one limb displaying more severe clinical signs (lameness). Hoof tester sensitivity, when present, is localized to the heel bulbs and/or across the sulcus of the frog, but many horses with heel pain have no hoof tester sensitivity. Lameness may be unilateral in some horses, and in such instances lesions within the DDFT or other structures of the hoof should be investigated (Adams and Stashak 6th Ed., Lutter et al). Traditionally, a palmar digital nerve block is used to localize lameness to the heel/navicular region. However, proximal diffusion of local anesthetic can by Lauren Richardson, DVM ABOUT THE AUTHOR Lauren Richardson, DVM completed a large animal surgery residency at Texas A&M University in 2019. She attended veterinary school at the University of California, Davis, then transplanted to British Columbia for a year internship before returning to Texas. Following completion of her residency at A&M, she accepted a position at Tryon Equine Hospital in North Carolina. Her professional interests include equine sports medicine and surgery. syndrome navicular be significant, and can complicate interpretation of this block (Nagy 2009). In addition, a recent report demonstrated that significant improvement in lameness can be seen with the use of a lidocaine palmar digital nerve block within five minutes; however, peak analgesia may not be reached for up to 30 minutes (Biavaschi Silva 2015). Imaging Modalities Radiographs are typically the first-line modality when evaluating heel pain. Standard views include lateromedial, dorsopalmar, 60-degree, dorsoproximal-palmarodistal oblique, and palmaroproximal-palmarodistal oblique (skyline) projections. The radiographic abnormalities most consistently present are cyst-like lesions of the medullary cavity, flexor cortex lesions, and medullary sclerosis which causes loss of demarcation of the corticomedullary junction (Adams and Stashak 6th Ed.). Ultrasonography may be used to evaluate a small central portion of the flexor surface of the navicular bone, distal part of the DDFT, proximal aspect of the collateral ligaments of the distal interphalangeal joint, entheses of the distal phalanx, and effusion of the navicular bursa or distal interphalangeal joint. In addition, ultrasound can provide useful information in horses without radiographic abnormalities. However, the keratinized hoof wall, sole, and frog limit the ability to further assess structures within the hoof capsule, especially those of the podotrochlear region (Rabba 2011). Magnetic resonance imaging is increasingly utilized to further characterize heel pain by identifying lesions that cannot be identified with other modalities. In fact, horses without radiographic abnormalities commonly have pathologic changes to the collateral sesamoidean ligament and deep digital flexor tendon, as well as increased synovial fluid in the distal interphalangeal joint and/or navicular bursa (Sampson 2009). Suspected adhesion formation of the DDFT to surrounding structures of the navicular apparatus may also be identified (Lutter 2015). In addition to lesions of the soft tissues of the podotrochlear region, MRI can recognize changes of the navicular bone that are often not seen on radiographs. These include more subtle medullary cavity changes, flexor cortex erosions, and distal border fragments (Sampson 2009).

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