In some cases we observe moderate periarticular osseous proliferation associated with the proximal (top) aspects of the third metatarsal (hind cannon) bones (i.e. "spurs") within the tarsometarsal joints of the hock. Although these lesions are somewhat intimidating to visualize from a radiographic standpoint, they may not necessarily be associated with joint instability, arthritis, pain, and/or lameness. In fact, we generally observed a fairly poor correlation between clinical signs (i.e. evidence of lameness) and radiographic changes (i.e. what we see on the x-ray film) in regard to low-motion joints. In other words, the radiographic changes (the spur) may or may not be associated with future pain and/or lameness.
Radiographic studies (x-rays) are an integral part of the equine lameness evaluation. Certain radiographic abnormalities, such as bone spurs in the hocks or enlarged channels in the navicular bones can be intimidating to look at from a visual standpoint.
However, we must realize that radiographs (x-rays) reveal only one small part of the whole picture. Radiographs provide structural information; in other words, they tell us what things look like. More specifically, they tell us what bone looks like. Since soft tissues all have a similar density on x-ray film, we often don’t get a lot of insight into problems that effect tissues such as tendons, ligaments, muscles, synovial membranes, etc. Therefore, a normal radiographic study doesn’t confirm normal/ healthy soft tissues.
Secondly (and more importantly), the true source of pain and lameness (i.e. inflammation) is radiographically invisible. In other words, we can’t “see” inflammatory cells on x-ray film. What we can see are potential effects that inflammation may or may not have on associated bony structures. Therefore, the normal radiographic appearance of bone does not confirm the absence of active inflammation. Likewise, the abnormal appearance of bone does not always confirm the presence of active inflammation. This is especially true in low-motion areas, such as the distal tarsal (hock) joints and the navicular region.
It is not to say that radiographic examination does not provide the veterinarian with valuable information and that taking x-rays is a waste of time. It is very important to know what bone looks like, especially in suspect areas. However, since there is poor correlation between what is observed radiographically (i.e. what the tissue looks like) and what is observed clinically (i.e. what tissue acts like), the veterinary examiner must be careful during interpretation of the x-ray study (especially when assessing low-motion areas).
Therefore, we typcially use our clinical impression to confirm the source of discomfort/ lameness in lower-motion areas such as the distal hock joints. Horses with distal tarsitis (hock pain) will exhibit a characteristic hind limb gait.