Osteoarthritis of the tarsometatarsal, distal intertarsal, and, less commonly, proximal intertarsal joint—colloquially known as “bone spavin”—is a common cause of lameness or poor performance in horses from all disciplines. Lameness can be unilateral or bilateral, and pathology can develop in one joint only, or in two or even three concurrently.
The pathogenesis of distal tarsal osteoarthritis is not completely known. It is thought that compression and rotation, resulting in uneven or excessive loading of these joints during exercise, can contribute to its development.
Osteoarthritis of the distal tarsus frequently develops in joints that have normal conformation. However, conformation abnormalities (sickle hocked, cow hocked, or excessively straight hock conformation) and incomplete ossification of the distal tarsal bones during development can lead to abnormal loading of the joints and to the development of osteoarthritis.
Other causes of bone spavin include tarsal bone fractures, septic arthritis, osteochondrosis, and excessive repetitive exercise.
In most horses, the most common sign of distal tarsal osteoarthritis is lameness, which varies from subtle changes in performance to severe lameness. Horses with pain localized to the distal tarsal joints can also display pain on palpation of the lumbar region.
Few clinical signs of distal tarsal osteoarthritis are evident on physical examination. In horses with more chronic distal tarsal pathology, however, the soft tissues over the medial aspect of the distal tarsal joints can be appreciably thickened, and in chronic cases, new bone formation of the dorsomedial aspect of the tarsus can be appreciated.
A proximal limb flexion test exacerbates lameness in horses with distal tarsal joint pain.
Diagnostic analgesia should be used to localize the source of pain. Most horses with osteoarthritis of the distal tarsal joints improve after intra-articular analgesia of those joints. However, accurate needle placement into the distal tarsal joints can be difficult, especially when osteophytes are present at the injection site, and improper placement can result in injection of fluid into the surrounding soft tissues and an incomplete response to diagnostic analgesia; false-negative results are possible as a result.
Perineural analgesia of the superficial and deep peroneal (fibular) and tibial nerves can be useful to confirm tarsal pain when intra-articular injection is too difficult or results are ambiguous.
Radiography confirms the diagnosis of distal tarsal osteoarthritis. Radiographic changes include narrowing or loss of joint space, sclerosis of the subchondral bone, lysis of the subchondral bone, periarticular osteophyte formation, and periosteal new bone formation (see distal intertarsal osteoarthritis image).
Courtesy of Dr. Hilary Rice.
The severity of lameness and extent of radiological change are poorly correlated. For horses in which distal tarsal joint pain is suspected but there is little radiographic change, nuclear scintigraphy of the tarsus can reveal an increased focal uptake of radionuclide in the distal tarsal bones.
The aim of treatment for distal tarsal osteoarthritis is to provide pain relief so that the horse can remain in work. Conservative treatment includes systemic and intra-articular anti-inflammatory medications, trimming and farriery, extracorporeal shock wave therapy, bisphosphonate injections, and adaptation of the horse's work program.
Systemic NSAIDs can be used in conjunction with other forms of medical management for distal tarsal pain. When used as the sole treatment for distal tarsal osteoarthritis, NSAIDs can help to alleviate associated pain; however, lameness typically returns after these medications are discontinued.
Corticosteroids are the most commonly used intra-articular anti-inflammatories for the treatment of distal tarsal osteoarthritis. Other intra-articular therapies include hyaluronan, polysulfated glycosaminoglycans (PSGAGs), and orthobiological therapies such as platelet-rich plasma (PRP), autologous conditioned serum (ACS), and autologous protein solution (APS). Intra-articular injection of ethyl alcohol can be used for chemical ankylosis of the distal tarsal joints; however, chemical ankylosis can take a long time and can produce undesirable adverse effects.
The most common surgical technique used to treat distal tarsal osteoarthritis is facilitated ankylosis. Drilling across the distal intertarsal and tarsometatarsal joints promotes distal tarsal ankylosis. Initial techniques were more aggressive, with 60% of the articular cartilage removed; however, substantial postoperative pain was associated with this procedure. A more conservative approach, using three drill tracts, is currently recommended. Return to full athletic performance usually takes 10–12 months. The prognosis is considered fair to good.
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Also see pet owner content regarding disorders of the tarsus and joint disorders in horses.
