A Firm Stifle-Lameness Diagnosis May Require A Specialist

High-performance horses are at risk for all types of injuries, by the nature of the demand on their bodies.

The horse's stifle is the anatomical equivalent of our knee, designed to be capable of a very large range of movement. This calls for special modifications to protect the joint surfaces from friction, and to stabilize the joint against excessive movement or movement in the wrong direction.

There are three bone structures that make up the stifle-the end of the femur from above, the top of the tibia below (with the head of the much smaller fibula bone as well), and the patella, or "knee cap," which glides along the front of the joint. The job of flexing the stifle and moving the leg forward falls on the large quadriceps femoris muscle that sits above it.

To give the stifle the ability to flex through a great range of positions, the tendons of the quadriceps don't insert directly onto the leg below, but rather onto the top surface of the patella. The patellar ligaments then travel down from the bottom of the patella to attach on the tibia.

When talking about joints and what goes wrong with them, most attention is usually focused on the bone and cartilage surfaces. However, with the stifle, there is an intricate array of soft-tissue structures that may be involved when there is stifle pain. These include:

• Menisci: The menisci are thick, crescent-shaped pads of cartilage that sit between the cartilage surfaces of the femur and tibia, providing a dense, shock absorbing cushion.
• Cruciate Ligaments: These are two thick ligaments, one anterior (front) and one posterior (back) that anchor the femur to the tibia, helping to prevent excessive bending or movement from side to side and to keep the ends of the two bones in proper alignment.

They are called cruciate because they cross from one side to the other inside the joint, creating an "X." The posterior cruciate works with the medial collateral ligament to stablize the inside portion of the joint, and the anterior cruciate works in conjunction with the lateral collateral ligament to stabilize the outside portion.

• Collateral ligaments: Like all joints, the stifle is supported, and its sideways motion restricted, by strong ligaments that run along its inside (medial) and outside (lateral surfaces).
• Patellar ligaments: There are three ligaments that attach to the patella above and the tibia below. These serve to limit the movement/excursion of the patella, "anchoring" it, and also to help it move smoothly in the groove of the femur where it travels. The pull of the quadriceps muscle, which flexes the stifle and advances the leg, is transmitted first to the patella, and then to the tibia via the patellar ligaments.
  
  As you can see from our chart of possible causes of stifle pain (pages 17-19), the soft-tissue structures of the stifle play important roles. In fact, an ultrasound examination is usually an indispensable part of the diagnosing a stifle problem.

Rehabing Stifle Injuries
It's no secret that significant stifle lamenesses have a poor prognosis. X-ray evidence of arthritic changes almost certainly means there is significant injury to one or more soft-tissue stabilizing or cushioning structures. For this reason, stifle problems usually do not respond as well to joint injections or joint nutraceuticals. Until more treatment options are available, horses with stifle problems involving joint-space narrowing and bone-spur formation will have a guarded prognosis at best, but some may return to at least pasture soundness or lighter use.