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.
Soft-tissue damage that is not severe enough to cause instability in the joint has a much better prognosis, but only if it is diagnosed before progressing to the point of instability.
Don’t ignore hind-end problems until they reach the point the horse’s performance is severely compromised. Diagnosis will likely require a thorough and painstaking ultrasound evaluation. This may be something you need a lameness specialist or university clinic to do, but it’s well worth the investment. Once the problem is defined, the veterinarian can give you a much more accurate idea of prognosis, and you can follow the healing process with serial ultrasounds, just like you would for an injury in the suspensory ligament or flexor tendons.
It’s also important to realize that healing in ligaments and tendons takes a long time. A few days of stall rest and a short course of bute probably won’t get the job done. Some general tips are:
• First, get a diagnosis.
• A short period of stall rest or paddock turnout, with local cold hosing or anti-inflammatory drugs, if the treating vet thinks this is advisable, should be used to get control of any active inflammation in the joint.
• Liniment topically is also helpful, or the vet may recommend local injections.
• Unless the vet feels there is a mechanical problem that requires special shoeing, most horses are most comfortable barefoot, with a carefully balanced foot, short toe and rounded hoof wall at ground surface to aide with easy breakover. Avoid studs, borium, trailers or any traction devices that tend to hold the foot to the ground.
• As with tendon and ligament injuries in the lower leg, controlled exercise during the healing period is important to the strength of the structures supporting the stifle and holding everything in place. Work with your veterinarian on this. Serial examinations of the joint and ultrasounds will allow you to make wise decisions about what level of exercise is safe and beneficial for the horse.