THE MERCK VETERINARY MANUAL
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Bone Spavin in Horses

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Bone spavin refers to osteoarthritis or osteitis of the distal intertarsal and tarsometatarsal articulations, and occasionally the proximal intertarsal joint. Lesions involve degenerative joint disease, particularly on the dorsomedial aspect of the hock with periarticular new bone proliferation, which eventually leads to ankylosis. Lytic lesions, which are part of the degenerative joint disease complex, can occasionally be seen; animals with such lesions are difficult to treat. Although bone spavin usually causes lameness, this may be obscured if the lesions are bilateral. Among the different theories proposed to explain this condition, poor hock conformation, excessive concussion, specific athletic events (cutting, dressage, pulling), and mineral imbalance are the most frequently mentioned. All breeds can be affected, but it is most prevalent in dressage horses, Standardbreds, and Quarter horses.

The lame horse tends to drag the toe. The forward flight of the hoof is shortened, and hock action is decreased. Frequently, horses “warm out” of the lameness after a few minutes' work, but in some cases the lameness persists because the bone lesions involve the articular surfaces. The heel may become elongated. Standardbreds develop soreness in the gluteal musculature (so-called trochanteric bursitis) secondary to spavin. In advanced cases, the bony proliferation may be visible on the distal dorsomedial aspect of the hock (seat of spavin). When standing, the horse may rest the toe on the ground with the heel slightly raised. The lameness often disappears with exercise and returns after rest. The spavin test (ie, trotting after limb flexion for ∼60 sec) may be a useful aid to diagnosis but, because of the reciprocal apparatus in the rear limb, this test is not specific for this condition or even this joint. In so-called occult spavin, there are no visible or radiographic lytic lesions or exostoses. Local anesthesia of the individual tarsal joints is necessary to localize the exact site of pain responsible for the lameness.

The disease is self-limiting, ending with spontaneous ankylosis of the affected joint(s) and a return to soundness. In the early stages, intra-articular injection of corticosteroids or sodium hyaluronate (or both) may be beneficial. NSAID (eg, phenylbutazone) eliminate or reduce the clinical signs. Working the horse after this treatment is aimed at accelerating ankylosis and resolution of lameness. Chemical arthrodesis using monoiodoacetate or alcohol has been advocated as a less traumatic management option. These agents induce a severe arthritis, and NSAID must be administered for a few days. Additionally, it must be confirmed prior to the injection of the agent that there is no communication with the talocrural joint. Otherwise, the talocrural joint will also lose its function. Surgical arthrodesis is another means of accelerating ankylosis of the affected joint. Insertion of oblique screws crossing the tarsometatarsal and distal intertarsal joints facilitate faster resolution of the lameness. These screws can be inserted through surgical plate(s) or next to them. If only the plate and not the screws bridge the joint(s) involved, the repair is mechanically less stable. Preparation of fan-shaped holes along the involved joint surface(s) has been advocated alone or in conjunction with a plate. The drill bit should not be swiped along the joint surface to remove as much cartilage as possible; this technique induces a severe lameness as a result of the instability created. Cunean tenotomy is commonly used but of questionable value by itself. Deep-point firing used to be advocated for hastening ankylosis, but it is very doubtful that it has any beneficial effect beyond encouraging rest. Corrective shoeing by raising the heels and rolling the toe may help but is unlikely to eliminate lameness on its own.

Last full review/revision March 2012 by Stephen B. Adams, DVM, MS, DACVS; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS; James K. Belknap, DVM, PhD, DACVS; Jane C. Boswell, MA, VetMB, CertVA, CertES (Orth), DECVS, MRCVS; Peter Clegg, MA, Vet MB, PhD, CertEO, DECVS, MRCVS; Andrew L. Crawford, BVetMed, CertES (Orth), MRCVS; Jean-Marie Denoix, DVM, PhD, Agregé; Marcus J. Head, BVetMed, MRCVS; C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS, DACVSMR; James Schumacher, DVM, MS, DACVS, MRCVS; John Schumacher, DVM, MS, DACVIM; Roger K. W. Smith, MA, VetMB, PhD, DEO, DECVS, MRCVS; Chris Whitton, BVSc, FACVSc, PhD

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