Imaging the equine stifle joint

Ultrasonography of the stifle: cranial, medial and lateral approaches

The stifle of the horse is large and anatomically complex. Consequently, the approach to ultrasonography of the region is commonly divided into discrete areas to ensure a complete evaluation. The cranial approach is used for evaluation of the femoropatellar joint, the medial approach for the medial femorotibial joint, and the lateral approach for the lateral femoropatellar joint. The main diagnostic information is obtained from these three approaches. A fully comprehensive ultrasonographic evaluation would additionally include a cranial approach with the limb in flexion and a caudal approach to the medial and lateral femorotibial joints. However, evaluation from these aspects provides limited diagnostic information, typically requires access to a micro-convex and/or convex transducer in addition to a linear transducer and may prove more challenging for the user.1-3

Cranial aspect

The middle, medial and lateral patellar ligaments, cranial surface of the patella and craniodistal articular surface of the femur are evaluated from the cranial aspect of the stifle.

Middle Patellar Ligament

Image 1: Transverse (A-D) and composite longitudinal (E) images of the middle patellar ligament of a horse. A) Level of the origin in a depression on the distal aspect of the patella (hyperechoic line). B) Level of the middle of the ligament. Note that the ligament is rounded. This horse has a hypoechoic lesion in the caudal aspect of the ligament (red arrowheads).  C) Level of the distal one third of the ligament, where it appears more triangular. Note the well-defined hypoechoic striations, which are a normal finding. D) Level of the insertion of the ligament onto the tibial tuberosity (hyperechoic line). Note the presence of normal striations as in C.

Image 2: Palpation of the middle patellar ligament on the right stifle of a horse.

The middle (intermediate) patellar ligament originates from a small depression on the distal aspect of the patella and inserts immediately distal to a concavity on the tibial tuberosity (Image 1). It is readily palpable as a taut, vertically orientated structure on the most cranial aspect of the stifle (image 2). The transducer should be placed directly over the palpable structure. Transverse and then longitudinal images should be acquired through the full length, including at the origin and insertion. The bone interface at both entheses (origin and insertion) should be smooth. In transverse section the ligament is typically oval proximally and triangular distally.4 It is homogeneously echogenic throughout, although faint, well-defined hypoechoic lines (striations representing the junctions between fascicle bundles) are often seen in the distal portion of the ligament in normal horses and should not be mistaken for a lesion.2,4,5 It is surrounded by the echogenic infrapatellar fat pad which provides poor contrast with the patellar ligaments in many horses. This may make the middle patellar ligament appear indistinct and poorly marginated. Off-incidence scanning in transverse planes can help to define the margins of the middle patellar ligament (Image 3).

Image 3: Off incidence view of the proximal portion of the middle patellar ligament. Note that the ligament is hypoechoic (arrowheads); in comparison, the surrounding infrapatellar fat pad retains a brighter echogenicity

To do this, fan (tilt) the probe proximally and/or distally by a few degrees so that the ultrasound beam becomes non-perpendicular to the fibres of the ligament. The ligament (if normal) should become hypoechoic whilst the infrapatellar fat pad should not change in echogenicity, thereby helping to resolve the middle patellar ligament from the surrounding fat pad. To evaluate the entire ligament in longitudinal section it is important to fan or slide the probe through the width of the ligament at each level.

Medial Patellar Ligament

Image 4: Transverse images of the medial patellar ligament (arrowheads) at the levels of the middle one third (A) and insertion onto the tibial tuberosity (B). Note the triangular shape in A, and the proximity to the insertion of the middle patellar ligament in B.

The medial patellar ligament originates from the medial parapatellar fibrocartilage and inserts onto the medial aspect of the tibial crest. It is located medial to the medial trochlear ridge of the femur. It is rounded or triangular in shape, and homogeneously echogenic (Image 4). Evaluate in transverse and longitudinal sections as per that described for the middle patellar ligament.

Lateral patellar ligament

The lateral patellar ligament originates from the distolateral aspect of the patella, at which level it usually appears poorly marginated and heterogeneous in echogenicity.6 It courses distally over the lateral trochlear ridge, where it becomes a flattened, crescent-shaped structure which is well-defined and homogeneous (Image 5). Distal to the lateral trochlear ridge it is oval to triangular in shape and may have a variable pattern such as striations or invaginations of the deep margin, which are frequently observed in normal horses and should be interpreted with caution because this is also the most frequent site of lateral patellar desmitis.6 It inserts immediately lateral to the middle patellar ligament, on the tibial crest. Evaluate in transverse and longitudinal sections as per that described for the middle patellar ligament.

Articular surfaces of the femur

Image 6: Composite transverse image of the cranial articular surface of the femur. LTR= lateral trochlear ridge, TG= trochlear groove, MTR= medial trochlear ridge. There is a focal area of hyperechogenicity within the articular cartilage over the lateral trochlear ridge, indicating mineralisation (arrowhead).

Evaluation of the cranial and craniodistal articular surfaces of the femur can yield important diagnostic information. Ultrasonography has been shown to be more sensitive (100%) than radiography (84-88%) at detecting osteochondrosis lesions of the trochlear ridges of the femur when arthroscopy is used as the gold standard for comparison.7 The U-shaped trochlear groove, the narrow, angular lateral trochlear ridge and the larger, rounded medial trochlear ridge should first be evaluated with the transducer in transverse orientation, and then in longitudinal orientation (Image 6). The regularity of the subchondral bone (highly reflective interface seen as a hyperechoic line with acoustic shadowing deep to it) and the regularity, echogenicity and thickness of the articular cartilage (the overlying well-defined anechoic [black] layer) should be assessed. The articular cartilage is normally thickest over the lateral trochlear ridge (3-4mm) and thinner over the medial trochlear ridge. It is not unusual for the cartilage at the bottom of the trochlear groove to be irregular; this is often a normal finding. The joint capsule of the femoropatellar joint is readily visualised, however fluid is not normally appreciated in this region, even in horses with distended femoropatellar joint capsules.2

Medial aspect

The structures of the medial aspect of the stifle include the medial femorotibial joint (periarticular margins and joint capsule), medial meniscus and medial collateral ligaments of the femorotibial joint.


Medial meniscus

Image 7: The proximomedial aspect of the tibia is easily palpable. This is a useful landmark for transducer placement in order to identify the medial meniscus.

Image 8: The medial meniscus (MM) is visible between the medial femoral condyle (MFC) proximally and the tibial plateau (TP) distally.  The medial collateral ligaments (MCL) are superficial to the MM. Note the anechoic layer between the MM and MFC, which is cartilage (*). The MM does not extend beyond an imaginary line between the most medial margins of the MFC and TP, indicating normality of size and position.

The easily palpable bony prominence of the proximomedial aspect of the tibia is a useful landmark for identification of the structures (Image 7). Place the probe over or just proximal to the bony prominence in a longitudinal orientation, with proximal to the left of the image. The triangular shaped echogenic medial meniscus should be visible between the femur proximally and the tibia distally (Image 8). From this position, the probe must be slid cranially, whilst fanning towards the caudal aspect to assess the cranial aspect of the meniscus, and then slid caudally whilst fanning towards the cranial aspect of the stifle to assess the caudal aspect of the meniscus. The echogenicity, regularity of the margins, and size/position of the meniscus should be evaluated. The medial border of the meniscus should not extend beyond an imaginary line between the medial extent of the tibial plateau and the femoral condyle (Image 8). Linear hypoechogenic areas within the meniscus can be seen in sound horses.

Collateral ligament

To locate the collateral ligament, position the probe over the palpable bony eminence in longitudinal orientation and slide the probe caudally. The collateral ligament lies superficial to the meniscus and is formed of two parts; the long or superficial portion, which is vertically orientated, and the short, deep portion which is oblique in orientation (Image 9).

Image 9: The medial collateral ligament (CL) is superficial to the medial meniscus (MM).

Because of the difference in fibre orientation, the two parts of the collateral ligaments may need to be imaged separately in longitudinal section. For both portions, the ligaments must be evaluated in their entirety, including the entheses of the origin and insertion. To assess the fibre pattern at the origin on the femur, apply pressure to the proximal aspect of the probe to push it into the muscle of the inner thigh, in order to orientate the probe parallel to the fibres of the ligament and therefore avoid off-incidence artefact.

Medial femorotibial joint

Image 10: A pouch of the medial femorotibial joint in two horses. Normal synovial fluid is anechoic (black, *). In one horse there is moderate synovial proliferation (arrowheads).

A pouch of the medial femorotibial joint is located cranial and proximal to the bony eminence (Image 10). Normal synovial fluid is anechoic (black).

TIP: The regions where distension of the medial femorotibial joint would be seen are cranial and/or caudal to collateral ligaments, and proximal to the medial meniscus (because the joint capsule inserts on to the abaxial aspect of the meniscus).

Lateral aspect

The structures of the lateral aspect of the stifle include the lateral femorotibial joint (periarticular margins and joint capsule), lateral meniscus, lateral collateral ligaments of the femorotibial joint, the tendons of the origin of the long digital extensor muscle and peroneus tertius, popliteal tendon, and the lateral pouch of the lateral femorotibial joint. The overlying muscles make evaluation of the lateral aspect more difficult; to orientate the probe parallel to the structures of the lateral aspect of the stifle, more pressure often needs to be applied to the proximal aspect of the probe. Alternatively, and if available, a micro-convex probe can be used.

Tendon of the long digital extensor and peroneus tertius

With the transducer orientated longitudinally over the lateral patellar ligament, slide the probe caudally. The first structure that will come into the image will be the joined tendons of the origins of the long digital extensor and peroneus tertius muscles, which originate from the craniolateral aspect of the femur, on the lateral epicondyle and courses distally in the extensor groove of the tibia (image 11).

Image 11: A composite image showing the tendon of the long digital extensor and peroneus tertius muscles (LDE) in longitudinal section.

This structure is deeper than the lateral patellar ligament and so a lower frequency, increased depth, and lower focal point position may need to be selected. A pouch of the lateral femorotibial joint is located deep to the tendon.

Collateral ligament

Continue to slide the transducer caudally to the lateral collateral ligament of the femorotibial joint. This originates from the lateral femoral epicondyle and inserts (mostly) onto the head of the vestigial fibula (Image 12).

Image 12: the lateral collateral ligament (CL) overlies the popliteal tendon (Pop) and the lateral meniscus (LM).

The lateral collateral ligament is also formed of two parts, the superficial vertical portion and the deep oblique portion (orientated cranioproximally to caudodistally). Reasonable pressure must be applied to the proximal aspect of the probe in order to bring the probe in line with the origin of the ligament and allowing visualisation of the fibre pattern.

Popliteal tendon

Image 13: The popliteus tendon (arrowheads) shown in on-incidence (left) and off-incidence (right) images. Note the proximity to the lateral meniscus (LM). If not careful, in on-incidence images the popliteal tendon can sometimes be mistaken for a torn portion of the lateral meniscus. The use of off-incidence scanning can help resolve the two separate structures because whilst the popliteal tendon will become hypoechoic, the LM is not subject to off-incidence artefact and so retains its echogenicity. LFC= lateral femoral condyle.

The popliteal tendon is located deep to the lateral collateral ligament and immediately superficial to the lateral meniscus. It is an elongated, triangular shaped echogenic structure which runs obliquely, in a cranioproximal to caudodistal direction (Image 13). The popliteus tendon is subject to off-incidence artefact (appears hypoechogenic when a transversely orientated ultrasound beam is not perpendicular to the fibres (Image 13), versus the lateral meniscus, which does not demonstrate off-incidence artefact.

TIP: Care must be taken to not mistake the popliteal tendon for a torn portion of the lateral meniscus.

Lateral meniscus

The lateral meniscus is a small triangular structure located between the lateral femoral condyle and the tibia (Image 14). Increased depth, decreased frequency, and a deeper position of the focal point are often required for proper visualisation. It should be assessed from its cranial to caudal extent by sliding and fanning the probe through its length, as per the medial meniscus. 


  1. Dyson S.J. (2002) Lameness associated with the stifle and pelvic regions. Proc Am Ass Equine Pract 48: 387-411.
  2. Cauvin E.R.J., Smith R.K.W. (2019) Ultrasonography of the stifle. UK-Vet Equine 3: 86-92.
  3. Nelson B.B., Kawcak C.E., Goodrich L.R., Werpy N.M., Valdés-Martínez A., McIlwraith C.W. (2016) Comparison between computed tomographic arthrography, radiography, ultrasonography, and arthroscopy for the diagnosis of femorotibial joint disease in western performance horses. Vet Radiol Ultrasound 57: 387-402.
  4. Dyson S.J. (2002) Normal ultrasonographic anatomy and injury of the patellar ligaments in the horse. Equine Vet J 34: 258-64.
  5. Hoaglund E.L., Barrett M.F. Daglish J., Contino E.K. (2018) Intermediate patellar ligament desmopathy often occurs in conjunction with other stifle abnormalities. Vet Radiol Ultrasound 60: 416-22.
  6. Gottlieb R., Whitcomb M.B., Vaughan B., Galuppo L.D., Spriet M. (2015) Ultrasonographic appearance of normal and injured lateral patellar ligaments in the equine stifle. Equine Vet J 48: 299-306.
  7. Beccati F., Chalmers H.J., Dante S., Lotto E., Pepe M. (2013) Diagnostic sensitivity and interobserver agreement of radiography and ultrasonography for detecting trochlear ridge osteochondrosis lesions in the equine stifle. Vet Radiol Ultrasound 54:176-84.

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