It's been my observation over the last 20 years or so that a lot of horse owners request to have their horses' hocks injected whenever a hind leg problem is suspected to be impacting performance. With some horses this procedure is very productive, but I think a more open mind relating to the source of the problem is in order.
The stifle joint, which is analogous to our knee joint, is a complex and challenging joint to evaluate. Its location and size has made it a deep, dark structure to analyze. Subtle distentions or thickening of the joint capsule are not nearly so evident as the joints of the more distal limb such as the hock and fetlock. The stifle is difficult to evaluate radiographically because of its mass, surrounding tissue and critical soft-tissue structures. Flexion of the hind leg has been historically referred to as a "spavin test" denoting a problem of the hock, but horses with stifle problems certainly can be positive to this test.
So, how does one go about evaluating the stifle? I believe it takes a series of diagnostic approaches. Careful palpation of the anterior, more accessible part of the joint can reveal suspicious thickening or distention of the joint. Diagnostic blocking of the joint by injecting local anesthetics can identify it as the source of pain. Studying the soft tissue structures such as the cruciate ligaments and menisci with ultrasound imaging can be helpful.
Advanced radiographic techniques can show things such as bone remodeling, osteochondral chips or bone cysts. The ultimate diagnostic procedure sometimes resorted to is arthroscopic visualization of the joint interior. Many of these procedures are specialized and require expensive technological aids to achieve, but are essential if you are to make an accurate diagnosis. Without an accurate diagnosis, any attempt at treatment is arbitrary and less likely to succeed.
The stifle is a common source of lameness in Western performance horse endeavors such as roping, cutting, reining and barrel racing. I think the stresses of making sharp turns while "getting in the ground" generate a lot of torque that can be damaging. These horses are the football players of the equine world, and we're all aware of how many football players have stifle problems.