Equine stifle disorders (Proceedings)

Problems originating from the stifle joint are a common cause of rear limb lameness and reduced performance. This is particularly true in the western performance horse and horses in other disciplines that require a lot of work off of the hind end, including reiners, cutters, dressage horses, and jumpers.

Lameness originating from the stifle joint can be characterized as a reduced cranial phase of the stride, and can range from mild to severe. Often the lameness is worse when the affected leg is on the outside of the circle. Additionally, the shortness of stride may be more evident at a walk than at a jog.

Accurate palpation of the stifle joint is crucial in all cases of rear limb lameness. Once you become familiar with the amount of synovial fluid in the normal stifle joint, it is relatively easy to detect an excess amount of fluid. The stifle joint is composed of 3 synovial cavities. The medial femorotibial joint is the most commonly affected, and particular attention should be paid to this joint space during palpation. This joint can be palpated just caudal to the medial patellar ligament. An excess of joint fluid in this pouch can be the first sign of a stifle problem. The femoropatellar joint pouch can also be easily palpated medial and lateral to the middle patellar ligament. The lateral femorotibial joint space is not as easily palpated, but is the joint space least likely to have a problem.

Many horses with stifle problems will be positive to all 3 rear limb flexions, but the upper limb or stifle flexion is the most sensitive. Many horses with stifle problems will be very resistant to this flexion, and may attempt to abduct the limb or even hop up in an attempt to relieve pressure on the medial femorotibial joint.

When the clinical signs, palpation, and response to flexion suggest a stifle problem, I generally proceed with diagnostic imaging prior to performing diagnostic anesthesia. Radiography is an important step in the diagnostic workup of equine stifle problems, but usually requires sedation to get good cassette placement. I find diagnostic ultrasound of the equine stifle to be more sensitive than radiographs, and this usually does not require sedation so that I can still pursue diagnostic anesthesia if necessary. My typical diagnostic workup in the horse that I suspect has a stifle problem is to ultrasound the joint first. When the results of the ultrasound are not conclusive, I usually then proceed with diagnostic anesthesia of the stifle joint. I begin by blocking the medial femorotibial joint. There is an anatomic communication between the medial femorotibial joint and the femoropatellar joint, but blocking these pouches should be done independently as there tends to be a variable response to diagnostic anesthesia. The lateral femorotibial joint is an anatomically distinct joint and should be blocked separately. I prefer to wait a minimum of 10-15 minutes after each block to determine if there is improvement. If some improvement is noted, I will perform serial examinations at 10 minute intervals. Some of the soft tissue structures of the stifle may take longer than typical to be anesthetized, so do not be too hasty in assessing the response to stifle joint anesthesia.

As stated before, diagnostic imaging of the stifle includes radiology and ultrasonography. The standard radiographic projections include a caudocranial, lateral, flexed lateral, and oblique (caudal lateral-cranial medial oblique). In certain cases where a patellar fracture is suspected, a skyline view of the patella and trochlear ridges can be performed. As a lot of horses resent having the radiographic cassette placed between their back legs, adequate sedation is necessary for the safety of the people involved and equipment. The areas of particular concern in the stifle include the medial femoral condyle, the lateral and medial trochlear ridges, proximomedial tibia, and the articular surface of the patella.

Ultrasound examination of the stifle is a useful complementary procedure and is performed regularly with radiography in my practice. The stifle joint is very amenable to sonographic examination as it is a large joint with many soft tissue structures. The soft tissue structures that can be imaged include the patellar ligaments, the menisci and the respective cranial ligaments, the collateral ligaments, the cranial and caudal cruciate ligaments,the meniscofemoral ligaments, the long digital extensor tendon of origin, and the popliteal tendon. The articular cartilage and bony outline of the femoral trochlear ridges and the cranial and caudal one third of the femoral condyles may also be imaged. I pay particular attention to the medial femorotibial joint as this is where greater than 90% of the stifle problems in the athletic horse will be located. The medial femorotibial joint is evaluated for excess fluid, synovitis, peri-articular bone remodeling, and medial meniscal problems. Once you become familiar with the normal appearance of this joint, it is easy to detect excess fluid or changes in the medial meniscus. The joint is first evaluated in the weight bearing position. The abaxial edge of the medial meniscus and the articular surface of the medial femoral condyle can be better evaluated with the leg in a flexed position. The lateral femorotibial joint and femoropatellar joints are similarly evaluated.

Sometimes radiographs and ultrasound of the equine stifle are normal, even when there is pain originating from this area. Nuclear scintigraphy can be a sensitive test to detect an abnormal radiopharmaceutical uptake in the medial femoral condyle. Horses in this category do not respond to intra-articular anesthesia or medication, as the pain is originating from the subchondral bone. In the typical normal horse, the radiopharmaceutical uptake in the medial femoral condyle will be equal to or less than the lateral femoral condyle. I feel that an increase in radiopharmaceutical uptake in the medial femoral condyle over the lateral femoral condyle is a highly sensitive method of detecting subchondral bone pain in this area.

MRI evaluation of the equine stifle is probably the gold standard for evaluating both bone and soft tissue structures, however availability, size of the animal, and anatomic placement of the equine stifle joint are major hurdles to the routine use of this diagnostic method. Recently several large bore MRI units have been placed in large equine clinics that will be able to image the stifle in patients up to a certain size. This modality should help our knowledge of pathology in the equine stifle.

Diagnostic arthroscopy is an additional method of evaluate the equine stifle joint. Some horses with persistent effusion and synovitis of the medial femorotibial joint have no obvious pathologic findings on radiographs, ultrasound, or nuclear scintigraphy. In these horses diagnostic arthroscopy usually shows cartilage erosions or fibrillation of the medial femoral condyle or mild medial meniscal tears. I will use diagnostic arthroscopy in those horses that have lameness isolated to the stifle joint by diagnostic anesthesia, have no obvious lesions as detected by the standard diagnostic procedures, and fail to respond to conventional therapy.

The most common etiology of stifle lameness in my practice is a synovitis caused by mild to moderate medial meniscal damage. This causes effusion and pain in the joint, producing a mild to moderate lameness. Radiographically these horses are normal, but the effusion, synovitis, and meniscal damage can be seen with diagnostic ultrasound. Many horses in my practice have some degree of medial femoral condyle abnormality detected by radiology. Large subchondral bone cysts are always significant and almost always cause some degree of lameness at some stage in the horses career, usually early in the training process. Smaller subchondral lucencies or flattening of the medial femoral condyle can be common incidental findings in western performance horses. However, I do think that these findings do increase the risk of stifle related injury and lameness, particularly medial meniscal damage.

Medial meniscal tears generally cause a moderate to severe lameness. This is accompanied by effusion in the medial femorotibial joint. The tear may or may not be visible using ultrasound, however some degree of medial meniscal abnormality will usually be present. These horses will block sound to the medial femorotibial joint, but fail to respond to intra-articular anti-inflammatory therapy. In cases of obvious medial meniscal tearing or in horses that fail to respond to intra-articular anti-inflammatory therapy, I generally recommend diagnostic arthroscopy. In addition to medial meniscal tears, some other common arthroscopic findings include cartilage roughening or erosions on the medial femoral condyle, proliferative synovitis, and tears of the cranial meniscal ligament. Additionally, sometimes there is chondromalacia or softening of the cartilage over the medial femoral condyle in addition to abnormal subchondral bone in this area. With severe medial meniscal tears, there may be obvious narrowing of the medial femorotibial joint space on radiographs, severe joint effusion, and prolapse of the meniscus outside the profile of the distal medial femur and proximal medial tibia.

Lateral meniscal tears are much less common. Diagnostic anesthesia of the lateral femorotibial joint is necessary to alleviate lameness from lateral meniscal problems. In my experience, a small portion of the lateral meniscus can be visualized with ultrasound than the medial meniscus. However, most lateral meniscal problems will manifest with obvious ultrasonographic synovitis and effusion. These cases are generally managed similarly to medial meniscal tears.

Diseases of the femoropatellar joint include osteochondritis dessicans or traumatic fractures of the trochlear ridges. Patellar fractures can also occur secondary to external trauma. Fragmentation of the distal patella is a common sequela to medial patellar ligament desmotomy. These cases are usually accompanied by severe effusion in the femoropatellar joint which is most often obvious both visually and with digital palpation. Radiographs are usually diagnostic.

Other soft tissue injuries involving the stifle include middle, medial, or lateral patellar ligament desmitis. In these cases, palpable pain and swelling are usually present. Ultrasound is diagnostic. Fortunately, tears of the cranial cruciate ligament is uncommon in the horse. When it does occur, the lameness is severe. Occasionally the cranial cruciate ligament will avulse part of its tibial attachment. This will be evident radiographically on the caudocranial view. It is possible to visualize part of the cranial cruciate with ultrasound, but I find this very difficult and generally not helpful. Diagnostic arthroscopy is the diagnostic technique of choice. Cranial cruciate rupture or tears in the horse is also usually accompanied by additional soft tissue pathology, particularly the medial meniscus. These horses generally only be salvaged for breeding purposes.

The treatment of stifle problems depends largely on the underlying cause of the problem. When there is a synovitis of the medial femorotibial joint with mild medial meniscal edema or inflammation, I will typically inject the medial femorotibial joint with a combination of hyaluronic acid and triamcinolone. I like to place these horses in a 2-3 week or longer period of reduced exercise if possible, and then follow-up this treatment with every other week injections of IRAP if the horse is sound. This treatment can be very effective, but often times needs to be repeated at various stages in the training period. For horses that fail to get sound using this protocol, the diagnosis needs to be re-checked. If there still no obvious meniscal tears or other pathology, I may try injecting the medial femorotibial joint with adequan. This can be repeated every 2-3 weeks for 3-4 treatments if necessary. When there is no response to adequan, I generally recommend diagnostic arthroscopy.

Evaluating medial meniscal tears during diagnostic arthroscopy can be challenging, as sometimes the tear is not evident from the articular surface of the joint, or in a part of the joint that is not accessible to the arthroscope. When pathology is evident with the arthroscope it is often surrounded by extensive proliferation and thickening of the synovium. It is often necessary to debride or resect this synovium so that an adequate evaluation of the torn meniscus can be made. The torn meniscus is similarly debrided using mechanical resectors or radiofrequency. The entire medial femorotibial joint should be explored as a lot of cases have concurrent medial femoral condyle cartilage lesions adjacent to the meniscal tear that will also have to be addressed. Some of these medial femoral condyle lesions over time may lead to an acquired subchondral bone cyst. Often times medial meniscal tears have accompanying cranial meniscal ligament pathology. Post-operatively, I like to treat these horses with either bone marrow derived stem cells or adipose derived stem cells, as I think it aids in the healing of these soft tissue disorders.

Many horses that I see in my practice have unilateral or bilateral subchondral bone cysts of the medial femoral condyle, yet are sound and remain sound. Many of these are found at pre-purchase exam during the yearling or two year old year. Therefore I find it difficult to predict the long term prognosis based only on radiographic appearance of the cyst, as different horses with similarly sized bone cysts may behave completely differently. This makes this disease particularly challenging when determining what to do in the sound horse or horse intended for purchase.

When I have determined that a lameness is due to a subchondral bone cyst in the medial femoral condyle, the prognosis for their next year is not good. This generally precludes treatment for the young horse that is aimed at the large futurities. In these horses, conservative treatment with weekly systemic adequan injections or intra-articular injection with anti-inflammatories combined with rest may be attempted. However, I generally recommend intra-lesional injection with a steroid alone or in combination with bone marrow or stem cells. This procedure is best performed using arthroscopic guidance, but can also be performed using ultrasound guidance. This therapy is based on the finding that these subchondral bone cysts have a lining that secretes inflammatory mediators capable of inducing bone lysis, as well as causing pain and lameness. I also like to concurrently treat with systemic tiludronate. I find that this technique carries a 60-70% chance of returning the horse to soundness within approximately 60 days. Additionally, many of these cysts will be smaller radiographically; however they do not disappear entirely. This technique is particularly useful for the lame yearling that has not yet started training. In my experience, if the horse responds to treatment, they generally remain sound long term. I prefer to return these horses to work approximately 4 months post injection.

For those horses that do not improve after intra-lesional steroid therapy, I perform arthroscopic enucleation of the cyst. It is important to remove the entire lining of the bone cyst. A similar percentage of these horses will return to soundness (60-70%) post-surgery, but a minimum of 6 months and as long as 1 year may be necessary. Post-operatively the cyst fills in with fibrous tissue and will always be visible radiographically, even though clinical soundness exists. It is my opinion that over a long term these large bone cysts predispose to osteoarthritis of the medial femorotibial joint and medial meniscal problems, probably due to remodeling and shape changes of the articular surface of the medial femoral condyle.Osteochondritis dessicans in the stifle occurs most frequently on the lateral trochlear ridge of the femur, but is also seen on the medial trochlear ridge. Special radiographic views may be necessary, as these ridges are superimposed over each other and the patella in the lateral projection. The pathogenesis may be developmental in nature or traumatic. I believe that distinguishing these two etiologies is important in determining the best treatment method. Small defects in the ridges and caused by trauma are generally associated with a mild lameness and effusion in the femoropatellar joints. With conservative treatment including rest and anti-inflammatories, the lameness and effusion usually resolves within several weeks. In this situation, I generally recommend re-evaluating the radiographs and lameness every 60 days. Many of these horses will show evidence of healing radiographically and will stay sound through their athletic careers. In contrast, I think that defects in the trochlear ridges that are developmental in nature are usually manifested by severe effusion in the femoropatellar joints with no lameness. Sometimes the effusion can get so bad that it will cause a mechanical type of lameness. In these cases, arthroscopic debridement is generally the best method, as conservative treatment will often times fail to resolve the persistent effusion. The prognosis for these horses is better than those with subchondral bone cysts. This is because the trochlear ridges are not major weight bearing surfaces, whereas the medial femoral condyle is.

Disorders of the stifle are a common cause of lameness and decreased performance in the equine athlete. The stifle is a complex joint composed of 3 synovial joints and numerous soft tissue structures. An accurate diagnosis involving diagnostic anesthesia and diagnostic imaging is essential in determining the most beneficial course of therapy.