The hock (tarsus) is a complex of joints of the lower rear limb of the horse. It is composed of six bones (tarsal bones) that comprise four different joints. The uppermost or most proximal joint (tarsocrural) has the widest range of motion. The other three joints are less mobile (low motion joints) but do experience torsional forces that affect the overall health of the hock. Bone spavin usually affects the two most distal lower joints of the hock [distal intertarsal (DIT) and tarsometatarsal (TMT) joints].
What is bone spavin? Bone spavin is a common, often painful condition of the two most distal joints of the hock (DIT and TMT); it is also called true spavin or jack spavin and is a manifestation of osteoarthritis or degenerative joint disease. The lameness associated with bone spavin usually has a slow onset with progressively worsening clinical signs. What causes bone spavin? Bone spavin usually occurs in mature or older horses that are asked to work excessively off the hind limbs.
Repeated compression and rotation of the bones (as with high speed stops and turns) and excessive tension on the ligaments and tendons is a prominent component of the whole disease process. Multiple repetitive traumatic insults (“use trauma”) to the hock as associated with day to day training, activities, and competition of these horses is a common factor in the pathogenesis of osteoarthritis. Although some horses do not have observable changes that can be detected or measured, they still suffer from pain in the hock. When this is the case, the term blind or occult spavin is instituted.
These horses display the clinical signs of bone spavin (such as pain and/or a reluctance to perform or a reluctance to perform at the previous level), but not all of the documentable changes are necessarily seen. Conformation of the horse is also a factor in the development of bone spavin. Horses with sickle hocks (increased angle of flexion) or cow hocks (hocks close together and almost touching with the toe pointing outward) have increased stress on the bony structures of the joint. Bone spavin can also occur in conjunction with fractures, infectious joint disease, metabolic bone disease, and developmental bone diseases of young horses.
How is bone spavin diagnosed? Bone spavin can be diagnosed by a veterinarian by assessing the history of the horse (age, breed and use of horse), performing a physical exam (joint effusion or swelling, heat, pain on palpation of the joint), performing a lameness exam (hock flexion, trotting and circling the horse on a hard surface, intraarticular anesthesia (joint blocks), and radiographs. Radiography of the hock may reveal periarticular osteophytes (bony proliferation in proximity with joint spaces), bone lysis (decreased density of bone), and/or bone remodeling (increased density of tarsal bones).
Nuclear scintigraphy (bone scan) can also be performed which may reveal “hot” spots indicative of joint/bone injury. Not all of these procedures are necessary to diagnose bone spavin. Consideration is given to the owner’s wishes to either invest in the diagnosis or to invest in treatment. Although some horses do not have observable changes that can be detected or measured, they still suffer from pain in the hock; when this is the case, the term blind or occult spavin is instituted.
These horses display the clinical signs of bone spavin (such as pain and/or a reluctance to perform or a reluctance to perform at the previous level), but not all of the documentable changes are necessarily seen. Treatment Treatment of bone spavin can be divided into three main types: conservative, medical, and surgical. Conservative and medical treatments focus on reduction of inflammation with or without protection of the joint cartilage, while surgical treatment involves arthrodesis (joint fusion), or tendon cutting.
The goal of any treatment for bone spavin is to make the horse pain free. The choice of treatment is dependent on a variety of factors including the degree of lameness, other causes of lameness coincidentally present such as back problems, the progression to ankylosis (natural joint fusion), the type of work the horse is used for, time and financial constraints, and response to other treatments. Conservative treatment makes sense when there are financial constraints, only mild lameness, radiographic evidence of self-ankylosis, or simply if the wait-and-see approach is desired.
Corrective trimming and shoeing, controlled exercise, and oral phenylbutazone comprise conservative therapy. The hoof must be trimmed so that it is balanced, and it should be trimmed regularly. A flat steel or aluminum shoe is used. To ease breakover, the toe is squared off and the heel can be elevated with pads or wedged shoes. In general the horse should be encouraged to perform as much moderate exercise as possible with the emphasis placed on straight line walking and trotting four to five times per week.
Exercise facilitates cartilage degeneration once it has begun, and this will lead to desirable ankylosis of the distal hock joints. However, in juvenile cases of bone spavin, cartilage preservation rather than ankylosis is the priority, and exercise is discouraged. Because the distal hocks joint have a low range of motion, fusion will not significantly affect gait or athletic ability but will relieve pain. Oral phenylbutazone can be given for an extended period of time (four to six weeks) if used at a low dose or given only when the horse will be ridden.
Medical treatment may be either systemic (intravenous or intramuscular) or intraarticular (injected into the joint). Medical treatment is often recommended in the early stages of bone spavin. Polysulfated glycosaminoglycans such as Adequan ® can be given intramuscularly. These drugs are chondroprotective (protect cartilage) and appear to have anti-inflammatory activity. Intravenous sodium hyaluronate is also chondroprotective and may be more beneficial than polysulfated glycosaminoglycans in an acute flare up of bone spavin.
If the results of systemic treatment are unsatisfactory, intraarticular long-acting corticosteroids are indicated. It is important to continue systemic therapy, because it maximizes the effect and reduces the frequency of intraarticular therapy. The addition of intraarticular sodium hyaluronate may increase the corticosteroid’s duration of effect. Reduced but continued exercise should follow intraarticular therapy. Corticosteroids may actually slowly encourage cartilage erosion, but they also impair healing; the net effect is slower ankylosis.
Therefore, if rapid ankylosis is the top priority and soundness can be sacrificed until the joint fusion occurs, both systemic chondroprotectives and intraarticular therapy may be avoided. However, chondroprotective drugs as well as nutraceuticals such as chondroitin sulfate and glucosamine may be beneficial in prevention of bone spavin in a mature, athletic horse. Several surgical therapies for bone spavin are available. There are advantages to performing surgery early in the course of bone spavin although it is usually not recommended until the horse’s pain is unresponsive to other therapies.
For example, surgery can reduce the time to achieve ankylosis in the lower hock joints. However, if advanced radiographic evidence of ankylosis exists, surgery is not recommended. One surgical technique that does not require general anesthesia is the cunean tenectomy. The cunean tendon (a branch of the tibialis cranialis tendon) is cut, and pressure applied to the inside of the lower hock joints is relieved. This procedure does not fuse the hock joints. Joint drilling and plating can be used in conjunction with cunean tenectomy or separately. Three types of joint arthrodesis are available.
However, these require general anesthesia. Bone drilling destroys the cartilage in the joint and initiates new growth between the upper and lower bones of the joint which arthrodeses the joint. Alternatively, chemical arthrodesis can be performed with sodium monoiodoacetate (MIA), which blocks an enzyme pathway, resulting in cartilage death and joint collapse. Laser arthrodesis can also be performed to destroy the cartilage; this is the least invasive procedure for fusing the joint. A new therapy for bone spavin has recently been investigated: Extracorporeal shock wave therapy.
High energy shock waves similar to the ones used for breaking up kidney stones are applied to the joint and surrounding structures. The technique is non-invasive but does require general anesthesia for the large focused systems to ensure exact positioning. The exact mechanism by which shock waves work on bone is unknown. A potential explanation is that the therapy increases the activity of osteoblasts (bone making cells) which strengthens the bone under the cartilage. This activity may help maintain joint shape and shock absorption thus protecting the cartilage, or it may enhance ankylosis.
The therapy is safe and appears effective in the short-term, but long-term effects are unknown. Prognosis The prognosis for bone spavin is always guarded. A prognosis cannot be given until the gamut of therapies has been attempted, including surgery, especially when ankylosis is in progress. Of the arthrodesis options, chemical arthrodesis has been reported as more successful than surgical arthrodesis (drilling technique) but with many more potential side effects. Laser arthrodesis and shockwave therapy hold promise, but long-term results are not yet known.
Regardless of the means by which it is achieved, fusion of the lower hock joints usually results in soundness. There is clearly no single universally accepted technique to achieve permanent relief of pain in horses that have developed bone spavin. The variety of treatments available reveals that each technique has its limitations, and the unique circumstances of each case must determine the choice of treatment regime. Every horse is different, but with an individualized treatment schedule, a horse with bone spavin can often be maintained as a useful athlete for an extended period of time.