Bone spavin is a term that denotes excessive osseous (bone) growth/ proliferation associated with the lower three hock joints of the horse (the proximal intertarsal, distal intertarsal and tarsometatarsal joints). In most cases the lower two joints (the distal intertarsal and tarsometatarsal joints) are primarily involved.
Excessive bone proliferation is one of the clinical and radiographic markers of osteoarthritis (OA), an advanced phase of degenerative joint disease (DJD). Extra bone production, however, does not confirm the presence of active inflammation and pain, especially with regard to the lower two hock joints.
Several TYPES of bone spavin have been described:
JACK SPAVIN is a term used to describe the presence of unusually large osseous lesion(s).
HIGH SPAVIN denotes osseous pathology higher (more proximad) in the joint than is typical.
OCCULT SPAVIN does not produce any significant bony projections (exostoses) associated with the distal tarsal joints. This term, therefore, is used to define clinical evidence of arthritis/ pain rather than the presence of visible bony abnormalities.
JUVENILE SPAVIN is used to describe the occurrence of bone spavin in young horses (less than 3 years of age). Obviously, this form of bone spavin occurs before the animal has done much work. While osteochondrosis (developmental orthopaedic disease) is the most common cause of juvenile spavin, cuboidal bone distortion in premature or dysmature foals has also been implicated in several cases. Like other forms of spavin, juvenile spavin may occur in the presence or absence of clinical pain and/or lameness.
CAUSES OF BONE SPAVIN
Cartilage compression. Chronic excessive compression of the tarsal bones can cause erosion of cartilage lining adjacent joints. Erosion of overlying cartilage eventually results in the exposure of (underlying) subchondral bone, which then proliferates into the joint space(s).
Uneven loading. Like most joints, the distal tarsal joints are designed to bear weight uniformly from one side to the other. Medial (inside) - to - lateral (outside) imbalance causes uneven loading across the articular surface(s). Excessive compression (of cartilage) is experienced along one side of the joint and excessive tension (of the joint capsule and supporting ligaments) is experienced along the other side. This inherently causes instability, which in turn triggers excessive bone production and proliferation along the margins of the joint. Simply put, unstable joints tend to grow extra bone along their edges in an attempt to stabilize themselves.
Other contributing factors. Several conformational abnormalities have been implicated as predisposing factors towards developing bone spavin. Those that cause uneven loading of the hocks, such as "sickle-hocked" and "cow-hocked" angular deformities, are especially noteworthy.
As you might guess, poor trimming or shoeing can also contribute to bone spavin, especially if distal limb balance is affected.
Certain activities may also contribute to abnormal, uneven, repeated and/or excessive loading of the lower hock joints, and therefore bone spavin. Such disciplines can impose excessive forces on the distal hock joints in the form of flexion (dressage), shearing stress (jumping), sudden stopping (reining), rotation (western events) or concussion (Standardbred racing).
“Bone spavin” has been used interchangeably with the term “distal tarsitis”, although in the strict sense bone spavin denotes (osseous) bony changes that are visibly apparent with no reference to the clinical status of the joint(s). Such visible changes, therefore, may or may not occur in conjunction with joint inflammation, pain or lameness.
Distal tarsitis, on the other hand, is a clinical diagnosis. Horses with distal tarsitis exhibit clinical evidence of lower hock joint pain (usually manifesting as lameness with a characteristic gait signature). In many cases of distal tarsitis, lameness exists in the absence of any visible abnormalities associated with the hock joints, both grossly and radiographically.
To learn more about DISTAL TARSITIS, click HERE.