Approaches to Diagnosing Diseases of the Equine Stifle
The equine stifle is a complex joint made of articulation of the femur, tibia and patella. The articulation creating three joint spaces: the medial femorotibial joint, the lateral femorotibial joint and the femoropetellar joint. Congruency in the femorotibial joint is created by the meniscus. The joint is stabilized by the collateral ligaments, and multiple extensor and flexor muscles and their attachments. It is important to have a robust understanding of this anatomy to accurately diagnose problems associated with the stifle.
Many of the injuries or lesions in the stifle occur in the medial femorotibial joint; the lateral femorotibial joint is affected less frequently. The lesions of the equine stifle, such as osteochondrosis, subchondral cyst like lesions, osteoarthrosis, fractures, synovial membrane changes/synovial effusion, meniscal injury, meniscotibial ligament injury, cartilage damage and collateral and patellar, desmopathies may be a cause of hind limb lameness.
Imaging the equine stifle often starts with the radiographic examination, whether it be in search for pathologic changing in the lame horse for survey radiography in sale or prepurchase examination. By far and away the equine stifle is most commonly radiographed using portable equipment for radiography. The standard and complete study of the equine stifle include the lateromedial (LM), caudocranial (CC) and caudo 45° lateral-craniomedial oblique (CdLCrMO) projections. An additional oblique image to consider is the latero 5° cranio 10° distal-mediocaudoproximal oblique (flexed lateral oblique).
The caudal-cranial (CC) radiographic projections is obtained with the horse standing square. A downward angle of 5–10° with x-ray generator is needed to be tangential with the joint space. A well-positioned radiograph should superimpose the cranial and caudal aspects of the tibial condyle, which facilitating visualization of the joint space. An improperly positioned CC view can artifactually create the appearance of a narrowed joint. If the joint space appears narrow on the radiograph, particularly in the absence of other indicators of advanced joint disease, it is recommended that a repeat radiograph be obtained to determine whether this is a real finding or a positional artifact. The CC view is particularly useful for evaluating for peri-articular osteophytes (commonly medial femoral condyle and tibial condyles), examining the articular surfaces of the medial and lateral femoral and tibial condyles and femoral intercondylar fossa. The attachment sites of the collateral ligaments, cranial meniscal ligaments and cruciate ligaments should also be examined.
The lateral view is obtained with the plate positioned on the medial aspect of the stifle joint. In a well-positioned lateral view, the medial and lateral femoral condyles are superimposed. The entire patella and supra-patellar area should be included. In larger horses or using a smaller x-ray detector, two lateral views, one made more distally and caudally to include the complete proximal tibia and one made more cranially and proximally to include the entire patella may be required. The lateral view is particularly useful for evaluating the femoral trochlear ridges, patella, proximal tibia at insertion sites of the key ligaments, and caudal aspect of the joint.
The CdLCrMO view is commonly utilized for evaluating lesions of the medial femoral condyle and lateral trochlear ridge of the femur. The view is obtained by positioning the plate on the craniomedial aspect of the joint and aligning the x-ray beam at a 45° angle on the caudolateral aspect of the joint. Including a slight downward projection (approximately 10 degrees) may be needed in some cases to minimize superimposition of the lateral tibial condyle. In addition to its usefulness for evaluation of the MFC, the CdLCrMO is also excellent for evaluating the lateral trochlear ridge. Radiography may give you some clue that there are soft tissue injuries. Ultrasound lets you visualize and diagnose suspected injuries.
Ultrasound is often employed to diagnose both soft tissue and osseous injuries of the stifle in horses that respond to intra-articular anesthesia.5,8,9 A systematic approach to ultrasonographic examination facilitates visualization of common sites of the pathologic change. It is important to evaluate and document the key structures in the equine stifle that may become injured.
A linear ultrasound probe with a variable megahertz (8–13) is used for the majority of the imaging. The caudal aspect of the stifle requires a convex probe with a variable megahertz of 4–8 depending on the size of the horse. The ultrasound frequency should be set to the highest megahertz available and still be able to visualize the intended structure in its entirety. An appropriate scanning depth should be used. This is often 3–6 cm in depth for the medial, cranial and lateral and 6–8 caudally. The intended anatomic structure should fill the screen and the focal zones be set at the depth of the intended piece of anatomy. The stifle region should be properly prepared by clipping the hair, washing the skin with a mild detergent and water, and applying acoustic gel.
A starting point at the medial aspect of the joint, moving cranial and then lateral is often employed. This is followed by evaluation in a flexed position to visualize the cranial attachments of the menisci and femoral condyles. Finally, the caudally aspect of the joint is evaluated. Each structure should be imaged in cross section and longitudinally. Creating a mental checklist of structures is paramount to reduce the chance of injury oversite in the examination.
The medial femorotibial joint should contain a small amount of compressible anechoic fluid. In conditions of chronic effusion, there may be synovial hypertrophy and joint capsule thickening. The medial meniscus is triangular shaped with variably numbered hypoechoic parallel horizontal lines. The abaxial margin of the meniscus should remain with the boundaries of the tibial and femoral bone margins. The medial meniscus is much more frequently injured when compared to the lateral. Intra-substance tears may be oblique or horizontal in orientation and often are seen on the tibial margin of the meniscus but can occur in a variety of locations. These may be more apparent when the limb is in a non-weight bearing stance. Meniscal protrusion may accompany tears or present alone. The cranial attachment of the medial meniscus visualized in a flexed position is common site for tears and enthesopathy and can contribute to meniscal laxity. Placing the ultrasound transducer in the longitudinal plane in the “V” created by the medial and intermediate patellar ligament will serve as a guide to find the meniscotibal ligament in the transverse plane. This is the most sensitive plane to find injury. A common pitfall is diagnosing the normal hypoechoic striations and loose bundles as intrasubstance tears.
The medial bone margins of the tibia and femur are common sites of osteophytes. Osteophytes may occur along with meniscal injury or alone. Moderate and large femoral osteophytes will deform the femoral border of the medial meniscus.
The lateral collateral ligament is larger than the medial collateral ligament. The fiber pattern of the collateral ligaments twist from the proximal to the distal insertion. This allows the ligament to be taut in both flexed and extended positions but can result in sometimes confusing hypoechoic regions within the ligament on transverse images. The collateral ligaments of the stifle are rarely injured; injury occurs most often at the origin of the medial collateral ligament.
Three main patellar ligaments are present in the horse. These include the medial, intermediate (middle) and lateral patellar ligaments. These ligaments have distinct shapes in cross section, with the medial being more triangular, the lateral being flat and broad in the intermediate ovoid. The intermediate patellar ligament contains linear hypoechogenic striations at its insertion. The intermediate patellar ligament is more commonly injured (Figure 4).
The trochlear ridges are easily identified on ultrasound. The overlying cartilage is hypoechogenic with lateral being twice as thick as the medial. Ultrasound is sensitive at finding osteochondral fragments and intertrochlear groove osteochondrosis lesions. Along the distal aspect of the lateral femoral trochlea is the extensor fossa. This is the common origin for the fibularis (peroneus) tertius and long digital extensor tendon. From here fanning the probe slightly caudally, the lateral meniscus is visualized. The lateral meniscus is triangular in nature and lies deep to the collateral ligament and tendon of the popliteus.
In a flexed position, the articular surfaces of the femoral condyles can be examined. The cranial aspect of the medial femoral condyle is a common anatomic area for articular and subchondral bone pathologic change. Shallow articular defects as well as the cloaca of the subchondral bone cysts are seen on ultrasound. Using standard, well positioned radiographs, including the LM, CC and CdLCrM o views and knowing soft tissue attachments can aid in finding significant pathologic change in the equine stifle.
In conclusion, diagnosis of stifle injury may represent a significant challenge to the equine clinician. However, a stepwise approach to the equine stifle aids in the visualization of areas where pathologic change commonly occurs and may be seen with ultrasound and radiographs. Appropriate knowledge of anatomy and common areas of pathologic change are needed to get the most out of your diagnostic imaging.