A New Navicular Vantage
By Nancy S. Loving, DVM • Jan 01, 2012 • Article #28520
The MRI image reveals a deep digital flexor tendon injury (the white spot on the black tendon area) contributing to navicular pain.
Photo: R. Stuart Shoemaker, DVM, Dipl. ACVS
The great thing about technological advances is that they provide new perpectives on old problems. Magnetic resonance imaging (MRI), for instance, has given equine veterinarians an improved vantage point for examining the underlying disease process in navicular syndrome cases. Historically, the term navicular syndrome referred to lameness resulting from pain in the podotrochlear apparatus in the back of the foot that includes the navicular bone, bursa (sac cushioning the navicular bone from the deep digital flexor tendon), supportive ligaments, and deep digital flexor tendon.
"Everybody used to think that navicular disease meant the problem related to degeneration in the bone and surrounding structures. That's because on radiographs all we could see was the bone," explains Robert Schneider, DVM, Dipl. ACVS, professor and equine orthopedic surgeon at Washington State University's College of Veterinary Medicine.
Veterinarians began recognizing, however, that this process involves more than just the navicular bone. The lameness that started out being called "navicular disease" was renamed "navicular syndrome" and later called "caudal heel syndrome."
"Even still, there were a variety of treatment failures and opinions about what and why things happen and how and why to treat this 'syndrome,' including various shoeing strategies," says Stuart Shoemaker, DVM, Dipl. ACVS, owner of Idaho Equine Hospital, in Nampa. "This indicated that there was little understanding about what was going on within the equine foot." The modality that revolutionized Shoemaker's understanding of this foot lameness was MRI.
"These days a lameness diagnosis within the foot points to a very specific entity or combination of structural injuries," says Schneider, who has been examining MRI studies of horses with foot pain for a dozen years. "We need to get rid of the term 'navicular disease' to describe every horse that blocks to palmar digital nerve (PDN) anesthesia in the front feet. Through MRI findings, pain is now localized as coming from specific soft tissue structures rather than grouping the lameness under a generalized heading as navicular disease."
Shoemaker best describes the syndrome as "palmar digital-responsive lameness," meaning that any of a variety of problems responds to regional anesthesia of the palmar digital nerves. "MRI has added new diagnoses to what used to be considered a single disease," he explains.
Causes of Foot Pain
Shoemaker has observed that injury to certain foot structures is often tied to the type of work the horse performs. "MRI studies are giving us a database to substantiate that different (riding) disciplines elicit specific injuries because of what horses are asked to do in their particular sport," he explains.
While all horses have the potential to be affected with problems in the back of the foot, Schneider says "Arabians stand out as having the lowest incidence of palmar heel pain, while Quarter Horses show increased incidence. There may be a genetic predisposition based on conformation, but it is likely strongly related to athletic use--the sports in which Quarter Horses participate feature hard stops, quick turns, and sprints, often in difficult footing."
Shoemaker notes that the lamest of the Western horses he sees are Western pleasure Quarter Horses, likely due to 1) a large body mass to small foot size ratio, which has some genetic implications; 2) long, slow repetitive trauma from persistent loping; and 3) nutrition and diet that spur rapid growth.
"We used to attribute injury to repetitive pressure of the deep digital flexor tendon against the navicular bone," says Schneider. "MRI has changed my view to now believe that pathology is not just one process but arises from multiple etiologies. Injury is related to different biomechanical forces that we don't yet fully understand."
The Diagnostic Process
Owners of horses with navicular region pain often first notice that the horse has a shortened stride or increased discomfort when moving over hard ground or downhill. A veterinarian might base his diagnosis of this problem on a careful clinical exam and diagnostic nerve blocks that localize the lameness to the feet. Often, lameness affects both front feet--a successful nerve block in one limb might make lameness more obvious in the opposite limb.
The veterinarian might perform PDN anesthesia by infusing a small amount of local anesthetic over the palmar digital nerves in the lowest part of the pastern. "It used to be thought that regional PDN anesthesia only blocks the navicular bone," says Schneider. "In fact, anesthetic diffuses both proximally (upward) and deeply to numb many structures within the foot and pastern, as far up as the fetlock. The idea that what is anesthetized below the point of where you stick the needle is inaccurate."
Shoemaker agrees, explaining, "There are a number and combination of structures involved in pathology of the lower digit. Diagnostic nerve blocks we learned in our veterinary education were previously overinterpreted. We now know that PDN blocks have the potential to numb the entire foot and podotrochlear apparatus including straight and oblique sesamoidean ligaments, fascial sheaths, pastern joint, collateral ligaments of coffin and pastern joints, deep and superficial digital flexor tendons, navicular bursa, suspensory ligament of navicular bone, impar ligament, coffin bone, short and long pastern bones--all these structures are involved in varying combinations."
This previous misunderstanding led many practitioners to assume a diagnosis that turned out to be very unspecific, with horses responding to a PDN block lumped under the generalized heading of navicular syndrome. In fact, each horse might have a very different structure involved in his lameness, each requiring a different therapeutic approach.
After diagnostic nerve blocks have helped localize a horse's lameness to the lower limb, the next step in the veterinarian's diagnostic workup is to take radiographs (X rays) of the foot and/or pastern. Radiographs pose another dilemma since information gleaned from foot images doesn't correlate well to the presence of pathology. Many problems detected by successful anesthesia with PDN blocks reside in the soft tissues (tendon, ligament, bursa, cartilage), which radiographs aren't able to evaluate. And, many horses have remodeling changes in their navicular bones related to age, use, genetics, and living conditions that aren't associated with any pathologic or lameness problem.
"As an industry, we have documented unequivocally that the correlation between clinical disease and radiographs does not exist as it relates to the podotrochlear apparatus in the foot," Shoemaker emphasizes. However, bony changes seen on radiographs might provide evidence of osteoarthritis, a fracture, or a cystic lesion in the bone and are worth pursuing.
"If radiographic imaging doesn't yield a diagnosis, then I'll discuss with the client to pursue MRI," says Schneider. "In many cases the extent of the problem is often more serious than suspected based on clinical exam and/or radiographic findings."
Schneider adds that it's important to remember the navicular bone and its supporting structures are all part of the coffin joint. He also stresses that horses with articular cartilage abnormalities, subchondral (underlying) bone damage, or collateral ligament injury respond to anesthetic nerve blocks similar to a horse with a podotrochlear apparatus injury.
"This is significant because collateral ligament injury creates coffin joint instability with joint cartilage wearing unevenly," observes Shoemaker. An MRI, he notes, can help sort these out.
The lameness' bilateral nature (generally showing up in both feet) allows veterinarians to compare one foot to the other on MRI. "Interestingly, we find different lesions in both front feet as often as we do the same lesions," Shoemaker says. "In some horses a conformational or discipline-related cause creates similar lameness in both front feet. In others, a primary lameness may cause overloading of the opposite support limb, with subsequent injuries due to different mechanisms."
Not every horse owner, however, can afford pricey MRI imaging for his or her horse. So when treating horses without an MRI study, Shoemaker follows PDN blocks and radiographs with ultrasound imaging as far into the pastern joint as ¬possible to detect pathology.
Also, MRI is not available in every equine hospital; however, Shoemaker is part owner of MREquine, a company that has three traveling, custom-built coaches providing MRI access to practitioners throughout the country. Sharing this technology makes it more affordable for clinics and clients alike, he notes.
Including MRI in a diagnosis can lead to a very different treatment plan than if considering other imaging results. "A truly foot-sore horse usually has MRI findings, which makes us rest horses more often than we previously would have done," says Schneider. "What we might have considered a chronic degenerative condition might actually be an acute injury of a collateral ligament or deep digital flexor tendon that could resolve with rest and rehabilitation."
Before MRI was introduced as a diagnostic tool, veterinarians used intra-¬articular therapy (joint injections) as both a diagnostic and therapeutic strategy to relieve clinical signs of pain and lameness. "Coffin joint injection medication diffuses into surrounding tissue, including collateral ligaments," Shoemaker explains. "This enables horses to perform on mildly unstable joints, which possibly accelerates joint deterioration with adverse implications for a horse's career longevity."
Schneider emphasizes, "The No. 1 consideration is that an MRI study gives 90% or greater chance of achieving an accurate diagnosis and answer for the problem. With this information, better decisions can be made for treatment, duration of rest time, where to put injections, and what kind of healing stimulants to use relative to the specific structures involved."
Ultimately, MRI can provide a definitive diagnosis leading to targeted therapy and, thus, a better prognosis for an athletic future.
Schneider stresses that good shoeing and training practices are the main methods of preventing development of foot problems in the navicular region. Equine athletic pursuits involve repetitive movements to which the horse must adapt. Thus, he urges trainers to take time to develop horses' ligament, tendon, and bone strength.
"MRI has not changed shoeing strategies for caudal heel pain," says Schneider. "The principles are the same as they've been for decades: Eliminate the broken-back hoof-pastern axis, shorten and roll the toe, assist breakover, balance the hoof, and pad to protect from concussion when necessary." Success relies on hiring a capable farrier, especially one who has an ¬excellent working relationship with a veterinarian to collaborate on the best shoeing strategies for each horse.
Due to the advent of MRI, veterinarians are viewing the previously named navicular disease or caudal heel syndrome quite differently. This condition is now considered a lameness diagnosis based on injury to specific anatomical structures rather than being viewed only as a generalized degenerative condition. MRI continues to reveal obscure reasons for lameness in equine athletes. With an accurate diagnosis, horses can receive therapy targeted to their specific problem to improve their chance of recovery.