Whorlbone is a layman's term that refers to inflammation within the greater trochanteric bursa of the horse. The greater trochanteric bursa is a small synovial sac that lubricates the middle gluteal muscle tendon as it courses over the greater trochanter of the femur just outside of the hip joint.

Greater Trochanteric Bursa

Inflammation within this bursa often occurs as a result of chronic excessive pelvic limb adduction (i.e. pulling the limb underneath the center of the body) during movement.  This motion results in increased strain of the middle gluteal muscle and its associated tendon.

Excessive limb adduction is in turn a gait characteristic classically associated with distal tarsitis (hock pain). Therefore, greater trochanteric bursitis (or whorl bone) is a common consequence of chronic hock pain.

Some professionals also correlate the development of whorlbone with stifle problems. We have not made this association at The Atlanta Equine Clinic, presumably because stifle-affected horses will tend to swing their pelvic limbs outward (i.e. away from the body) to avoid hind limb engagement rather than pull their limbs underneath the body. This motion is known as pelvic limb abduction.

Abducting the pelvic limb typically decreases middle gluteal muscle/ tendon tension whereas adducting the pelvic limb increases it.

Whorlbone is diagnosed via clinical examination. Most horses will react positively to digital (finger) pressure applied over the affected greater trochanter(s). Pain associated with the greater trochanteric bursa often causes horses to exhibit a "hock-in/ toe-out" way of moving behind. 

In most cases, whorl bone resolves pursuant to successful treatment (injection) of the hocks.  Occasionally, however, persistence of whorl bone following hock treatment warrants local therapy in the form of injection.  Typically, a combination of hyaluronan and steroid is used in similar fashion to that of joint injection.

In less severe cases, extracorporeal shock wave therapy (ESWT) may be used to alleviate greater trochanteric bursitis until hock injections have "kicked in".

The Atlanta Equine Clinic - 2011
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