Of the four hock joints, only the top two (i.e. the tibiotarsal and proximal intertarsal joints) are necessary for normal locomotion in the horse. The bottom two joints (i.e. the distal intertarsal and tarsometatarsal joints) are flat in shape and are bordered by cuboidal bones which move very little relative to one another. These joints, therefore, are very low-motion and therefore inessential with regard to normal pelvic limb movement.
In advanced cases of distal tarsitis, gross osseous (bony) swelling is often visible along the medial aspect (inside) of the distal joints.
Even though they do not move very much, the lower hock joints can be a source of instability, pain and lameness in the horse. This disease is often referred to as DISTAL TARSITIS or "bone spavin".
Distal = Lower
Tarsus = Hock
"-itis" = Inflammation
The low-motion nature of the distal hock joints allows veterinarians to be relatively aggressive with regard to therapeutic strategy, as is explained HERE. In most cases, your veterinarian will elect to treat distal tarsitis via intraarticular injection of the distal hock joints with a combination of corticosteroid and synthetic hyaluronan. This approach works extremely well in the vast majority of cases.
The relatively few horses which are refractory to standard intraarticular injection of antiinflammatories may require more aggressive treatment in the form of joint fusion. Since the joints have minimal clinical relevance (almost no range-of-motion), their fusion does not result in any appreciable alteration in the horse's pelvic limb gait. Moreover, the elimination of the joints removes any associated instability, pain and lameness. In short, hock fusion can be considered a CURE for distal tarsitis.
Hock fusion can occur through one of three avenues:
Spontaneous Fusion. Although many professionals believe that distal tarsal fusion is a phenomenon that will inevitably occur in every horse, its natural occurrence is actually very rare. We often hear the phrase, "My horse's hocks are 90% fused". This would be analogous to saying that "she is 90% pregnant". Either the distal hock joints are fused or they are not. If they're not fused (and no matter how close to being fused they might be), they still have the potential to cause pain and lameness.
This procedure entails direct physical removal of the cartilage lining the joint via the use of an electric drill. Obliteration of cartilage results in the exposure of subchondral bone on both sides of the joint. Subchondral bone surfaces, in the absence of an overlying layer of cartilage, eventually grow together, fusing the joint. This form of articular effacing, often called "foraging", requires the use of general anesthesia within an equine surgical facility. The procedure is considerably painful and expensive, and it may take up to 1 year for fusion to occur.
Infusion of caustic chemicals into affected joint(s) result in deterioration and dissolution of cartilage, thereby exposing the underlying subchondral osseous (bony) layers of the joint (which eventually grow together). Although this procedure is certainly more elegant than surgical arthrodesis, serious complications have been reported. Most postoperative problems arise pursuant to the inadvertent introduction of the chemicals into one or both of the upper hock joints.
Hock Fusion at The Atlanta Equine Clinic
Although associated complications have been reported, the technique of chemical fusion has worked extremely well in our hands and is currently the procedure-of-choice at The Atlanta Equine Clinic.
There are three pre-requisites for this procedure:
- The horse must be refractory (non-responsive) to other forms of treatment (such as intraarticular injection with corticosteroids)
- Marked periarticular osseous proliferation (extra bone growth) must be evident during radiographic examination. This gives us the impression that the affected joint(s) are in the process of "trying to fuse".
- The horse owner is aware of the procedure and its potential complications.
The chemical used to produce hock fusion at The Atlanta Equine Clinic is monoiodoacetic acid (MIA). This product precipitates accelerated erosion of any cartilage to which it is exposed.
Typical Course of Events:
Surgical Infusion of MIA. This procedure is performed in the standing, sedated horse. Epidural anesthesia in combination with systemic antiinflammatories and analgesics (pain killers) is often used concurrently to address initial postoperative pain. The horse can be extremely painful during the initial 16-18 hours post-treatment and must be monitored closely during this time.
Dramatic Improvement During Initial Post-Treatment Period. After about 18 hours, the horse's comfort tends to improve rapidly. This usually allows the owner to resume exercise the day following treatment. Many horses are sound after 7-10 post-treatment days. The quick improvement in comfort is attributed to presumed chemical neurectomy of the nerves (in the joint capsule) which provide sensory output from the joint. The nerves are essentially "killed off" and can no longer transmit pain signals to the brain. Consequently, the horse's lameness resolves and overall performance is dramatically improved. The fusion process has only begun to occur at this point and is not yet providing any antiinflammatory relief. The effects from the "neurectomy" can last up to 6 months following the treatment.
Regression of Clinical Signs. Some degree of stiffness may develop as the nerves within the joint capsule(s) grow back. On average, some degree of lameness recurs by 4-6 months post-treatment. Sensory output from the joints has been reestablished but the fusion process has not yet been completed. Lameness will persist until the distal tarsal joints finally stabilize (fuse).
Soundness is Achieved. Fusion typically occurs between 8-10 months following MIA introduction into the affected joint(s). Once fused, the distal tarsal joints are no longer a source of pain or lameness.