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The stifle is the largest and most complex joint in the horse and, as such, it is an important cause of hindlimb lameness. Equivalent to the human knee, the stifle is controlled by some of the most powerful muscles in the horse’s hindquarters and is subject to tremendous stress forces.

The cruciate ligaments, a pair of ligaments that cross each other and connect the femur to the tibia, is situated between
the two joint pouches of the femorotibial joints. Direct joint communication between the medial femorotibal and femoropatella joints is present in most horses. However, communication between the two femorotibial joints is only present in approximately 25% of horses (1).
The Menisci, which are cartilaginous C-shaped discs positioned between the femur and tibia, assist in providing shock absorption and prevent bone-on-bone contact.

There are also a number of important supporting ligamentous structures, including the medial and lateral collateral ligaments, as well as the lateral, middle and medial patella ligaments.

Stifle anatomy
The stifle is actually comprised of two joints: the joint of the femur and tibia, which is called femorotibial, and the joint of the femur and patella, which is called femoropatella. There are three synovial joint sacs or ‘joint pouches’; the femoropatella, the medial (inside) and the lateral (outside) femorotibial pouches. Each of the medial and lateral femorotibial joint pouches is further divided into cranial (front) and caudal (rear) pouches by the femoral condyles (the rounded protuberance at the end of the bone that forms the articulation with the other bone.

Stifle injuries
Stifle injuries can affect soft tissue, bone or cartilage, and are usually either traumatic, developmental or degenerative in nature. Regardless of the cause, insult to the stifle will usually also cause instability elsewhere in the joint, which makes diagnosis and treatment challenging in horses (2).

Assessment and diagnosis
Initial assessment of a horse should include a thorough lameness exam, including history, palpation and flexion tests of their joints. An adequate history should include the horse’s age, use, duration of lameness and whether a specific incident or injury caused the lameness.

Palpation should focus on establishing whether there is effusion (‘joint swelling’) centred over the femoropatella and medial femorotibial joints. There may be swelling associated with the supporting ligaments and chronic lameness results in atrophy (‘wasting’) of the hindquarter gluteal muscles, which should also be inspected. Clinical signs can vary greatly depending on the severity and chronicity of the condition.

Flexion tests
Flexion tests are a routine part of a lameness examination and assist in localising a lameness to the lower limb (that part of the limb below the knee or hock) or the upper limb (that part of the limb including and above the knee or hock). A lame horse, in which the stifle is the cause, is likely to appear worse after an upper limb flexion, i.e. the horse will appear more lame when trotted in a straight line. With the upper limb flexion test, however, the hock and hip joints are also flexed. Therefore, these will need to be ruled out as a possible cause of lameness. This is most commonly done through the use of ‘joint blocks’.

Joint blocks
Joint blocks, or more accurately ‘intraarticular anesthesia’, can be useful in confirming a suspicion that the stifle joint is the cause of your horse’s lameness. This is where local anesthetic is directly injected into a joint to numb the area causing pain. Ideally, local anesthetic should be injected into all three joint compartments to ensure all are de- sensitised effectively.

Due to the complexity and size of the stifle joint, if the lameness improves by 50% or more, this is thought to be confirmatory. It is typical for your veterinarian to wait up to an hour before making the final call whether the joint block has lessened the severity of your horse’s lameness. Once again, this is related to the time it takes for the local anaesthetic to work inside such a large and complex joint.

Other diagnostic tools
Findings from a lameness examination or through the use of joint blocks can, at times, be confusing and more sophisticated techniques may need to be used to screen for stifle injuries, which are typically only in large equine hospitals.

Nuclear scintigraphy involves the injection of a radioactive isotope (Technetium 99m) into the bloodstream of the horse. This isotope circulates around the body and is absorbed by bone which is undergoing active remodelling or inflammation. A gamma camera is used to collect and record images from different parts of the body, which allows veterinarians to pinpoint exactly where the source of lameness may be. (You can read a more detailed article about scintigraphy at: www.horsesandpeople. com.au/article/equinescintigraphy).

Once the site of lameness has been confirmed to be the stifle joint, then ultrasound are performed to determine a direct cause. Radiographs are useful to evaluate bone abnormalities, whilst ultrasound is useful for evaluating soft tissue structures. These two diagnostic modalities will, therefore, provide very useful information when utilised together to determine treatment.

Once the site of lameness has been confirmed to the stifle joint and the exact cause of lameness has been determined with diagnostics, such as radiographs and ultrasound, the veterinarian should be able to provide the owner with a prognosis and a treatment plan specific to their individual horse.

Stifle injuries, treatment options
As with all joint disease or injury, there are many treatment options available and which of these your veterinarian chooses will depend on a range of factors, including the exact nature and severity of the injury, intended future use of the horse and financial constraints.
Common treatments:
Common treatments include, but are not limited to:
• Rest, followed by a controlled exercise program.
• Direct intraarticular joint treatment: IRAP, corticosteroids, hyaluronic acid.
• Systemic treatment with drugs, such as phenylbutazone, oral joint
supplements, pentosan.
• Surgery: arthroscopy, blistering of the patella ligaments.

Complementary therapies: massage, acupuncture.

Systematic treatments

Nonsteroidal anti-inflammatory medications (NSAIDs) include:

Phenylbutazone (‘Bute’), which has been the mainstay of treatment for joint disease for many decades, and works well to decrease lameness, due to its rapid onset of action and strong anti- inflammatory action. Bute, however, is not a long-term treatment for horses involved in the competition, due to its long swabbing time, and both kidney and intestinal side effects.

Meloxican is a newer anti-inflammatory medication, which is reported to have fewer side effects than Phenylbutazone and a shorter withdrawal time.

Pentosan and Pentosan Halo: Pentosan polysulphate is derived from a plant (beechwood extract) and is registered for intramuscular administration in the horse. Pentosan Halo contains an additional ingredient, HA, which is administered in a separate syringe and given intravenously to the horse. These products aid in healing mild cartilage disease. Experimental studies performed in horses have revealed improvement in lameness and joint flexion, as well as reduced inflammation inside an arthritic joint.

Oral joint supplements: There is a vast number of oral joint supplements available for horses. The specific formulation, concentration and source of products differs considerably. However, most contain one or more of the following: chondroitin sulfate, glucosamine, hyaluronan and MSM.

Many of these products work by providing molecules that are naturally found and have important properties in cartilage or joint fluid, and which undergo depletion when there is inflammation within a joint.

Scientific studies have indicated these products are effective in the treatment of joint disease in humans. In horses, however, treatment efficacy is based more on individual opinion, rather than substantiated by quality scientific studies. An interesting report in human literature compared the label ingredient of oral joint supplements with the independent testing of the products, and found little correlation to the label claim and content, or price and content. This potentially emphasises the use of trusted brands that have at least undergone some testing.

Direct intraarticular joint treatment
Corticosteroids are the most potent anti-inflammatories available and are injected directly into a joint (termed ‘intraarticular’) to provide rapid pain relief. Furthermore, controlled scientific studies have shown that using ‘low doses’ of corticosteroids can improve the integrity of the cartilage.

Controversy exists surrounding the use of intraarticular corticosteroids, due to the reported potential side effects. Much of this controversy is based on unsubstantiated statements in the lay press. Furthermore, studies that revealed potentially harmful effects of IA steroids were conducted using normal joints and cartilage.

Current research suggests that inflamed and arthritic joints do not exhibit the same harmful effects seen in normal joints when IA ministered
unnecessarily. Some consideration should be given to the reported incidence of corticosteroid- induced laminitis. However, no direct association has ever been proved. Caution should be undertaken in horses that have had laminitis and which are prone to developing laminitis.

Hyaluronic Acid (HA) is an essential and normal component of joint fluid and cartilage. It provides joint fluid with the properties of lubrication and elasticity, which is necessary for a smooth and even gliding joint surface. HA is commonly injected in conjunction with
a corticosteroid inside a joint, and also acts as a natural anti-inflammatory and enhances the action of the steroid. HA can also be injected intravenously in conjunction with Pentosan.

IRAP (Interleukin 1 receptor antagonist protein)
Blood is taken from the horse and incubated with special glass beads that promote the production of regenerative and anti-inflammatory proteins by the white cells in the blood. The sample is centrifuged and the serum containing these proteins is harvested. This is
injected into the affected joint. Usually, a series of three to four injections are performed every 1-2 weeks. This treatment can work well for horses with mild arthritis and no significant radiographic changes. IRAP is not swabbable as it is made from the body’s own proteins.

Complementary therapies
In horses with initiating mild arthritis, all that may be required to keep the animal sound is increasing the warm-up period prior to intense work. In other cases, it is necessary to reduce the work intensity and duration, whilst treatments are initiated to allow medication time to take effect. Complementary therapies, such as massage, acupuncture and chiropractic, can also be incorporated into the
training regime. The response to these adjunctive treatments is quite variable from horse to horse and, therefore, some experimentation to find out what works may be necessary.

Surgery
Surgical treatment to remove either the offending or major contributing cause of arthritis is required in the following cases:
Bone chips: these can occur as a consequence of arthritis or intensive training over a long period of time.

OCD (osteochondrosis): is a congenital problem where, due to a variety of reasons, such as nutritional and genetic disposition, bone chips form in specific locations in the horse.

Bone cysts: these are areas where there is a lack of bone below an area of (usually abnormal) cartilage

Medicating a joint and making it pain- free in the cases listed above will only aggravate the arthritis and further its progression. For this reason, x-rays are commonly performed to assess the affected joint, in order to screen for potential surgical problems.

Arthroscopy
Arthroscopy is the most common method by which joints are assessed d and surgically treated. It involves making keyhole incisions directly into the joint and placing a special instrument called an arthroscope into the joint, so the interior can be viewed on a monitor. This technique has been adapted from humans and reduces post-operative complications, such as infection and scar tissue restriction, as used to be common after open joint surgeries in horses.

Joint re-surfacing techniques
Due to the inherent poor healing of cartilage defects, a considerable amount of research has been performed in the last decade evaluating different surgical treatments for focal cartilage defects. Despite this, problems still exist with each technique currently developed. For example, there can be great difficulty in gaining surgical access to certain areas of the joint where joint disease occurs (for example, the weight-bearing surfaces of the stifle joint).
Often, specialised equipment is required, as well as specialised surgical expertise, and this comes with a substantial cost. This is why many of the techniques scientifically evaluated have not been found to be easily applicable in the clinical setting. Furthermore, true, evidence-based follow-up of horses with naturally-occurring clinical joint disease and the outcome following treatment is lacking, making the justification of using any of these techniques very difficult.

Stem cells
An exciting, emerging application for stem cell therapy in horses is in the management of lameness associated with osteoarthritis. Stem cells are harvested from the bone marrow or fat, and are injected directly into the affected joint. Stem cell treatment has been used in conjunction with arthroscopic surgery when, typically, the results from surgery alone would be poor.

The potential benefits for the use of stem cells in osteoarthritic joints relate to the anti-inflammatory properties of stem cells and also their ability to embed within the joint, as well as their capacity for self-renewal resulting in a prolonged duration of effect. Candidates for stem cell treatment for osteoarthritis are those horses that fail to respond or become refractory to conventional treatments, or those horses that suffer side effects from non-steroidal anti-inflammatory or corticosteroid medication.

Conclusion
The most important aspect of lameness diagnosis is to apply a complete step-by-step approach to confirm the exact location of the pain.

Whether or not your horse has a stifle problem directly causing lameness can only be established through the use of a logical, systematic approach, and this is vital in order to direct targeted therapy and increase the prognosis of a horse returning to an athletic career.

References:
1. deLuhunta A, Habel RH: Applied Veterinary Anatomy,
Philadelphia,1986, WB Saunders.
2. Lati mer FG, Tarsus and Sti fl e in
Equine Sports Medicine and Surgery,
2004, WB Saunders.