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Compendium January 2011 (Vol 33, No 1)

Surgical Views: Treating Navicular Syndrome in Equine Patients

by R. Wayne Waguespack, DVM, MS, DACVS, R. Reid Hanson, DVM, DACVS, DACVECC

    CETEST This course is approved for 3.0 CE credits

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    Abstract

    Navicular syndrome is a chronic, progressive condition affecting the navicular bone and bursa, deep digital flexor tendon (DDFT), and associated soft tissue structures composing the navicular apparatus. The treatment options for navicular syndrome are as varied as the proposed causes of the condition. The severity of clinical signs, intended use and workload of the horse, and owner compliance with therapy are important considerations in developing a treatment plan. Nonsurgical treatment of navicular syndrome consists of rest, hoof balance and corrective trimming/shoeing, and medical therapy, including administration of systemic antiinflammatories, hemorheologic medications, and intraarticular medications. While surgical therapy can include desmotomy of the collateral (suspensory) ligaments of the navicular bone, palmar digital neurectomy is more commonly performed when medical therapy is ineffective. Recently, adjunct therapies, such as acupuncture and extracorporeal shock wave therapy, have also been used to treat horses with navicular syndrome. Because of the wide range of pathologic changes and the lack of a definitive cure associated with navicular syndrome, treatment is directed toward the individual horse, focusing on the management of clinical signs to alleviate stress and retard degenerative changes of the navicular bursal and navicular regions.

    Navicular syndrome is a chronic progressive condition affecting the navicular bone and bursa as well as the associated soft tissue structures (FIGURE 1) .1 This article discusses treatment options and future research involving navicular syndrome.

    The treatment options for navicular syndrome are as varied as the proposed causes of the condition. As new ideas regarding the etiology have been proposed, new treatment regimens have followed. Recently, with the use of computed tomography (CT) and magnetic resonance imaging (MRI), it appears that the deep digital flexor tendon (DDFT) at the level of the navicular bone may play a more significant role than previously understood (FIGURE 2) .1,2 Despite new information regarding navicular syndrome, many treatments remain based on modification of pain rather than elimination or reduction of pathologic processes. In patients with navicular syndrome, changes in the navicular bone and the surrounding soft tissues have been reported, primarily involving the palmar fibrocartilage of the navicular bone, its underlying subchondral and trabecular bone, and the apposing DDFT. In addition, erosions and fibrillation of the fibrocartilage, remodeling of the underlying subchondral and trabecular bone, palmar cortical bone erosion, DDFT fibrillation, core lesions, and adhesions have been recognized.3–5 These changes are remarkably similar to those seen in osteoarthritis; however, in navicular syndrome, the palmar fibrocartilage of the navicular bone and the apposing DDFT are involved instead of two apposing hyaline cartilage surfaces.

    Knowledge of these changes may eventually help clarify why some treatment regimens produce clinical improvement and allow development of more specific therapeutic alternatives. In lieu of this, treatment options are focused on management of clinical signs and should be designed to alleviate stress on the navicular region and prevent or retard degenerative changes within the navicular bone and bursal region.

    Nonsurgical Therapy

    The severity of clinical signs, the horse’s intended use and workload, and the owner’s compliance with therapy are important considerations in developing a treatment plan. Young horses and horses with less severe clinical signs are often candidates for conservative therapy. Nonsurgical treatment of navicular syndrome consists of rest, hoof balance and corrective trimming/shoeing, and medical therapy, including systemic antiinflammatories, hemorheologic medications, and intraarticular medications.6–8

    Rest

    One of the most overlooked components of treatment is rest. Continued stress on the navicular region leads to structural failure and eventual breakdown. Time is necessary to allow soft tissue inflammation to subside and bone remodeling to take place. The time necessary to reduce inflammation varies on an individual basis. Absolute confinement is contraindicated; pasture or paddock turnout is sufficient.

    Corrective Shoeing and Hoof Balance

    Careful evaluation of foot conformation and balance is the focal point in nonsurgical treatment of navicular syndrome. Many horses respond to correction of hoof abnormalities.9 The goal is to reduce forces on the navicular region by (1) correcting hoof balance and the hoof–pastern axis, (2) allowing the use of all weightbearing structures of the hoof by maintaining the heel mass and protecting the palmar aspect of the foot from concussion, and (3) decreasing the work of the moving foot by either shortening the toe length of the foot to permit an easier breakover or rolling the toe of the shoe.

    Horses with navicular syndrome often have unbalanced hooves consisting of long toes with low, underrun, and contracted heels (FIGURE 3) . One of the first requirements of hoof balance is hoof–pastern alignment. Proper hoof–pastern alignment is assumed if a straight line can be drawn through the dorsal pastern and dorsal hoof wall (FIGURE 3). The foot should be trimmed to maintain heel mass and shorten the toe to facilitate breakover if the hooves are characterized by long toes. Use of the so-called four-point or natural-balance trim has recently been favored by some clinicians, but the same principles of breakover can probably be achieved with more traditional trimming if the toe is shortened sufficiently. Radical changes in foot trimming may temporarily increase lameness; therefore, correct foot balance may need to be achieved in stages.

    In principle, a hoof with proper hoof–pastern alignment has the appropriate amount of tension or force exerted on the navicular bone by the DDFT. In horses with low heels, correct alignment can be established with selective trimming or, more commonly, by elevating the heels with a wedge-heel shoe.9 In a study of normal Dutch Warmblood horses, heel elevation with a 6° wedge reduced the maximal force on the navicular bone by 24% compared with use of flat shoes.10 In another study, force-plate analysis indicated that lameness significantly decreased in horses with navicular syndrome that were shod to produce a 3° heel elevation alone or in combination with phenylbutazone administration.11 Lateromedial radiographs may be necessary to demonstrate whether a horse needs more or less heel elevation. If the horse’s lameness worsens after elevation of the toe using the wedge test, some degree of heel elevation may be beneficial.

    The hoof should also be maintained in medial-to-lateral balance. This is determined either by obtaining a dorsopalmar radiograph of the foot or watching the horse walk to ensure that the medial and lateral aspects of the hoof hit the ground simultaneously. If the hoof cannot be trimmed to establish normal hoof–pastern alignment or lateromedial balance, shoes can be applied to encourage the hoof to grow correctly. Many types of shoes have been recommended for horses with navicular syndrome. Full-bar, egg-bar, natural balance, and wide-web aluminum or steel shoes with or without elevated heels have been used to support the heels and move the weightbearing axis in a palmar direction in horses with low, collapsed, and underrun heels (FIGURE 4) . A recent study showed that using egg-bar shoes with no heel elevation in clinically normal Dutch Warmblood horses with well-conformed feet did not reduce the force on the navicular bone compared with using regular flat shoes.10 However, these findings cannot necessarily be translated to lame horses with less-than-ideal foot conformation. In the same study, heel wedges did reduce the force on the navicular bone compared with flat shoes.10 In a study evaluating 55 horses with clinically diagnosed navicular syndrome, 53% had permanent relief of lameness 12 to 40 months after application of egg-bar shoes.12 In another study, horses shod with egg-bar shoes showed histomorphometric evidence of altered navicular bone modeling compared with untreated controls.13 In any case, shoes should be set (1) beyond the heels to provide heel support and (2) full or wide to allow proper hoof expansion.7,10

    Medical Therapy

    Antiinflammatories

    Intraarticular Medications

    Corticosteroids (e.g., methylprednisolone acetate, triamcinolone acetonide, betamethasone sodium phosphate) have been injected into the navicular bursa and the distal interphalangeal joint (DIPJ) to treat navicular syndrome, and the results have varied.7,14 These powerful antiinflammatories are routinely used in combination with sodium hyaluronate to control clinical signs associated with navicular syndrome. Most clinicians medicate the DIPJ when treating navicular syndrome.15 Medications injected into the DIPJ, which is near the navicular bursa, have been shown to diffuse to the navicular region. A study by Pauwels et al15 showed that clinically effective concentrations of methylprednisolone and triamcinolone diffused between the DIPJ and the navicular bursa. This is also supported by the response to treatment of clinical cases of navicular syndrome.

    Over time, some horses stop responding to intraarticular therapy of the DIPJ. Intrabursal therapy may provide another option for reducing inflammation in the navicular region of the foot. One study showed that 80% of horses that no longer responded to traditional therapy for navicular syndrome (i.e., corrective shoeing, administration of systemic antiinflammatories, intraarticular therapy of the DIPJ) were sound 2 weeks after intrabursal injections of corticosteroids, sodium hyaluronate, and amikacin.16 The mean duration of soundness in this study was 4.5 months.

    In recent years, interleukin-1 receptor antagonist protein (IRAP) therapy (Vet Arthrex Biosystems, Bonita Springs, FL) has been used increasingly in veterinary practices to treat osteoarthritis in horses. Several studies have investigated the effects of IRAP therapy for this purpose.17–19 While the efficacy of IRAP therapy for treating navicular syndrome has not been investigated, some clinicians have noted positive effects from IRAP injections either in the DIPJ or directly in the navicular bursa. IRAP is a naturally occurring protein that works by occupying receptor sites on the membrane of cartilage cells, preventing the normal cytokine-initiated inflammatory cascade.19,20 Blockage of matrix metalloproteinases and cytokines (e.g., interleukin-1) reduces their degenerative effects on cartilage, preventing degenerative joint disease and osteoarthritis.18,19,21–23

    Systemic Antiinflammatories

    NSAIDs have been the mainstay of navicular syndrome therapy for many years.7,8,11 The analgesic and antiinflammatory properties of NSAIDs make them attractive therapeutic agents. Phenylbutazone is the most commonly used NSAID. Flunixin meglumine, diclofenac, carprofen, naproxen, ketoprofen, meclofenamic acid, and aspirin are also used in clinical situations. NSAID toxicosis may develop with long-term use of certain NSAIDs because of the nonselective inhibition of cyclooxygenases 1 and 2. In a study comparing oral firocoxib (a highly selective cyclooxygenase-2 inhibitor) and oral phenylbutazone in controlling pain associated with naturally occurring osteoarthritis, there was no statistically significant difference.24

    Other drugs with potential antiinflammatory properties that may be useful for treating navicular syndrome are glucosamine–chondroitin sulfate compounds, polysulfated glycosaminoglycans (PSGAGs), and sodium hyaluronate.16,25 Glucosamine–chondroitin sulfate compounds are given orally, sodium hyaluronate can be given intravenously or intrasynovially, and PSGAGs can be given intrasynovially or intramuscularly. Administration of glucosamine–chondroitin sulfate compounds and intramuscular PSGAGs has been shown to reduce clinical signs associated with navicular syndrome.25–27 These medications are effective only while they are administered. Lameness reappears after the therapy is discontinued. The bioavailability of glucosamine–chondroitin sulfate compounds in horses had been questioned, but it has now been documented.28

    Hemorheologic Drugs

    Isoxsuprine hydrochloride (a β agonist) is a peripheral vasodilator that has been successfully used in relieving the clinical signs of navicular syndrome.29,30 The drug can also decrease blood viscosity and platelet aggregation.31 Isoxsuprine is given orally, and numerous dosages have been recommended. The specific mode of action of isoxsuprine in treating navicular syndrome is unknown. In a study by Ingle-Fehr and Baxter,30 oral isoxsuprine did not increase blood flow to the equine foot. Isoxsuprine also binds strongly to α adrenoreceptors and, therefore, may be active despite insignificant but measurable levels in plasma.32,33 The drug may also have antiinflammatory and hemorheologic properties, which may be the source of its beneficial effects in horses with navicular syndrome. A clinical trial that evaluated the clinical efficacy of isoxsuprine for treating navicular syndrome in horses demonstrated a decrease in lameness.29 The length of the treatment period varies, with some horses showing clinical improvement for months after discontinuing therapy. One author noted that some horses that did not respond to an isoxsuprine dosage of 0.6 mg/kg PO q12h did respond satisfactorily to 0.9 mg/kg PO q12h.34

    Pentoxifylline and propentofylline are other hemorheologic agents that have been used for treating navicular syndrome. In one study on the efficacy of propentofylline in treating navicular syndrome, the lameness scores of horses receiving propentofylline significantly improved.35 In a study evaluating the oral administration of pentoxifylline, therapeutic levels were achieved in horses.36 This was not supported in an earlier study in which oral administration of pentoxifylline had no significant effects on digital blood flow to the equine foot.30 The findings in these two studies may be due to the use of different dosages of pentoxifylline.

    Bone Metabolism Medications

    Bisphosphonates, such as tiludronate, are used to normalize bone metabolism through inhibition of bone resorption; however, bisphosphonates approved for horses are not available in the United States. Areas of increased bone resorption and formation are common on radiographs of horses with navicular syndrome. Delguste et al37 showed that tiludronate reduces bone resorption and prevents long-term osteopenia in healthy horses. Several studies have evaluated the dosages (0.1 mg/kg q24h IV for 10 days versus 1 mg/kg IV in a single dose), bioavailability, and pharmacologic effects of tiludronate in horses.38,39 In a clinical trial by Denoix et al,38 horses treated with tiludronate (1 mg/kg IV in a single dose) showed significant improvement in lameness and returned to their normal level of activity within 2 to 6 months after treatment.

    Surgical Therapy

    Palmar Digital Neurectomy

    Palmar digital neurectomy (PDN) involves disrupting the nerve supply to the palmar one-third of the hoof to alleviate heel pain (FIGURE 5; FIGURE 6). PDN is considered a palliative procedure because it only alleviates the pain associated with navicular syndrome and does not prevent or retard progression of the damaging processes. The various PDN methods share the common goal of atraumatically sectioning the nerve to minimize the likelihood of the most common complication: neuroma formation.40–45 Research suggests that the simple guillotine technique results in the longest period of desensitization and least chance of neuroma formation compared with epineural capping and carbon dioxide laser division.40,43,44 The guillotine technique can be performed in several different ways. Some clinicians make a 1- to 2-cm incision over the nerves for each affected foot, just proximal to the collateral cartilages, and remove approximately 1 cm of palmar digital nerve.41 Other clinicians make either a 3- to 4-cm incision per nerve or two 1-cm incisions (one just proximal to the collateral cartilage, and the other just distal to the proximal sesamoid bones)34,42,45 (FIGURE 6). In this method, approximately 3 to 4 cm of each palmar digital nerve is removed.42,45

    Postoperative care is extremely important in the success of this procedure. Horses are confined to a stall and handwalked for 1 month. Support bandages are maintained while the horse is on stall rest. Some clinicians support postoperative perineural injection of the proximal transected nerve root with a corticosteroid to reduce postoperative inflammation. Limited exercise, bandaging, and an atraumatic surgical procedure all have a significant effect on preventing neuroma formation.

    Clinical interpretation of the long-term resolution of lameness after PDN varies, as do reported success rates. Published reports indicate that up to 92% of horses were in work 1 year after PDN.43 In the same study, 77% of horses in work 1 year after surgery were sound.43 In another study, 74% of horses were sound 1 year after PDN; however, this percentage decreased to 63% after 2 years.41 Complications associated with PDN include failure to alleviate lameness due to remaining accessory branches of the palmar digital nerve, rupture of the DDFT, subluxation or luxation of the DIPJ, and deep hoof infection; therefore, PDN should not be recommended casually. Recurrence of lameness is most commonly associated with reinnervation or neuroma formation. Development of a painful neuroma may be managed with surgical excision or perineural injections of Sarapin (High Chemical Company, Levittown, PA) and triamcinolone acetate.

    Alternative Surgical Options

    Navicular bursoscopy is usually a diagnostic tool but may be used for treatment, depending on the pathology noted on endoscopic examination.46,47

    Desmotomy of the collateral (suspensory) ligaments of the navicular bone has been recommended as a surgical treatment for navicular syndrome.48 In a study evaluating 118 horses, 76% of treated horses were sound 6 months after surgery, but only 43% remained sound after 3 years.49 In a study from New Zealand, 12 of 17 horses were sound at least 6 months after surgery.50

    Surgical drilling of cyst-like lesions involving the navicular bone via an arthroscopic approach has been described, but no follow-up data are available at this time.51

    Some clinicians advocate core decompression of the navicular bone in horses with navicular syndrome.52 No clinical trials have been performed to date, but anecdotal evidence suggests that this may be a treatment option in horses with severe bone edema identified on MRI.

    Desmotomy of the accessory ligament of the DDFT has been described as a treatment for navicular syndrome in horses with a markedly upright foot conformation.53

    These alternative surgical therapies need further research and clinical evaluation to determine their merit.

    Adjunct Therapy

    Temporary resolution of palmar foot pain can be achieved by chemical ablation of sensory fibers in the palmar digital nerves. Some products (Sarapin [a plant alkaloid that is thought to alter transmission in type-C fibers] and cobra venom35) can cause temporary loss of sensation and are usually injected in combination with a corticosteroid. Liquid nitrogen can be injected percutaneously to freeze the digital nerves.35,54

    The combination of nontraditional therapies (e.g., acupuncture) with traditional ones (e.g., rest, corrective shoeing) may be beneficial.55

    Several studies have evaluated the efficacy of extracorporeal shock wave therapy for treating navicular syndrome, and the results have varied.56–58

    Conclusion

    Currently, there is no standardized treatment for navicular syndrome. The response to various therapies indicates that affected horses most likely experience pain from several different sources, and many of the therapies improve the associated lameness to some degree. Treatment is often directed at pain relief rather than preventing further damage. Focusing on maintaining a balanced hoof through proper trimming and shoeing in combination with improved diagnostic techniques during the period of soft tissue inflammation may help prevent this devastating condition.

    Key Facts

    • Navicular syndrome is a chronic, progressive condition affecting the navicular bone and bursa, deep digital flexor tendon, and associated soft tissue structures composing the navicular apparatus.
    • Because of the wide range of pathologic changes associated with navicular syndrome and lack of a cure for it, treatment is directed toward palliative care.
    • Treatment of navicular syndrome should focus on managing clinical signs, alleviating stress on the navicular region, and preventing or retarding degenerative changes in the navicular bursal region.

    Downloadable PDF

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