SEVEN: Ultrasonography of the Stifle

Ultrasonography of the Stifle

Eddy R.J. Cauvin

AZURVET Referral Veterinary Centre, Cagnes sur Mer, France

Stifle injuries are increasingly recognized as a major cause of hind limb lameness. Ultrasonography has tremendously improved our ability to confirm or rule out stifle lesions, which primarily involve soft tissue structures. Radiography is often disappointing or inaccurate, and it will only allow us to detect late degenerative changes. The equine stifle is a large, complex region and its ultrasonographic examination requires a thorough knowledge of the anatomy.

Preparation and Scanning Technique

The stifle region should be finely clipped from distal to the tibial tuberosity up to the stifle skin fold, and all around the limb, then prepared as usual. Most of the examination is performed with the limb weight bearing, although, in very lame horses, examination can be adequately performed with the limb partially flexed. To scan the cranial aspect of the femorotibial joints, however, the limb must be flexed. It may be useful to use a wooden block to stabilize the foot. It is not necessary to achieve full flexion, which is often not tolerated by the injured horse. Intra-articular analgesia will dramatically interfere with evaluation of the joint, as it causes hemorrhage, inflammation, and effusion, and because air may be introduced into the joint space or periarticular soft tissues. Gas can persist in the joint for up to 2 weeks, so it is therefore preferable to perform an ultrasonographic examination prior to diagnostic intra-articular analgesia whenever possible.

A high-frequency (7–15 MHz) linear probe is best to evaluate the femoropatellar joint and the medial aspect of the stifle. A standoff pad may be used but can be fiddly and is not necessary. To examine the cranial, lateral, and caudal aspects of the stifle, a 6–12 MHz micro-convex, curved array transducer is preferred.

Ultrasonographic Anatomy

The anatomy of the stifle region has been described elsewhere and the reader is encouraged to study anatomy texts in detail to better understand the three-dimensional arrangement of this complex joint.

Femoropatellar Joint

The femoropatellar joint is best imaged with the joint extended, as most of the trochlea is hidden by the patella when the limb is flexed. In the extended position the patella points craniolaterally, and the large medial trochlear ridge is palpable under the skin cranially. Starting in transverse planes, the probe is placed cranioproximally over the quadriceps muscles and examination is continued from proximal to distal, down to the tibial tuberosity. The quadriceps muscle is relatively homogeneous with a typical hypoechogenic muscular pattern. There is no insertion tendon as such, so that there is a sharp interface between the muscle belly and the proximal surface of the patella (Figure 7.1). The cranial surface of the patella forms a sharp, smooth hyperechogenic interface. Distally, it forms a shallow groove at the origin of the middle patellar ligament (PL) (see below). Medially, it is prolonged by the hypoechogenic, crescent-shaped parapatellar fibrocartilage, which curves around the medial trochlear ridge and terminates via the thin medial collateral patellar ligament (Figure 7.2). It also gives rise to the medial PL (see below).

Figure 7.1    Normal proximal patella and distal crus. (A) Position of the transducer cranio-proximal to the patella (sagittal plane) and (B) corresponding ultrasonographic image. The arrow points to the virtual position of the suprapatellar pouch of the femoropatellar joint. F: femur; Q: quadriceps.
Figure 7.2    Cranial view of the normal femoropatellar joint. (A) Position of the transducer and corresponding ultrasonographic images in 7.2B–E. (B) Transverse plane image showing the medial aspect of the patella (pat) and parapatellar fibrocartilage (pfc). (C) Transverse plane image over the cranial aspect of the trochlea. The cartilage over the center of the trochlear groove (TG) is often irregular (yellow arrow), it is smooth over the ridges (red arrows). The fat pad (FP) is echogenic and heterogeneous. The middle patellar ligament (MiPL) is often difficult to discern from the fat pad in transverse images. The lateral patellar ligament (LPL) is flattened and curves over the sharp lateral trochlear ridge (LTR) (double arrows). (D) Longitudinal image of the medial trochlear ridge (MTR), showing the smooth subchondral bone outline and thin overlying cartilage (yellow arrows). The fibrous part of the capsule (red arrows) is tightly applied against the ridge surface. SC: subcutis. (E) Longitudinal plane image of the LTR. The cartilage (yellow arrows) is thicker than on the MTR and is thickest at the apex of the ridge. The capsule (red arrow) adheres to the LPL (white arrows).

Distal to the apex of the patella, the trochlear groove appears as a wide, U-shaped bone interface covered by anechogenic cartilage (Figure 7.2). In many horses, the latter is irregular in the center of the groove, which is considered to be a normal feature. The medial trochlear ridge is broad, smooth, and rounded, and is covered by relatively thin cartilage (0.8–1 mm thickness). This becomes irregular medially. The lateral trochlear ridge is much narrower and triangular in cross-section. Its cartilage is thicker (2–2.5 mm). In longitudinal sections (parasagittal), the ridges form a smooth, convex bone interface, topped by anechogenic cartilage, which should be perfectly regular in thickness. The lateral PL is seen as a striated structure lying directly over the lateral ridge. The capsule is tightly applied against the cartilage surface, and no fluid is normally visible over the trochlea.

The patellar ligaments actually anatomically correspond to the quadriceps tendon and are therefore similar in appearance to digital flexor tendons. Their borders are, however, ill defined due to poor contrast with the surrounding infrapatellar fat pad. The middle PL is round to oval in cross-section (Figure 7.3) and is the largest of the three PLs. It runs from the apex of the patella to the cranial-most part of the tibial tuberosity, within the fat pad and in the center of the trochlear groove. Distally, it becomes more triangular and often contains thin, hypoechogenic lines, giving it a webbed appearance. This should not be mistaken for a tear. Tilting of the probe to create an off-incidence artifact will cause the ligament to become hypoechogenic, enhancing its outline within the fat pad. The medial PL is triangular in cross-section and, in the extended limb, lies over the medial aspect of the medial trochlear ridge, 4 or 5 cm caudal to the apex of the ridge (Figure 7.4). Proximally it becomes more heterogeneous as its fibers spread out into the parapatellar fibrocartilage. Distally, it inserts approximately 2 cm medial to the middle PL and receives a tendinous branch from the sartorius muscle. The lateral PL is thinner and crescent shaped in cross-section (Figure 7.2). It caps the apex of the lateral trochlear ridge, outlining the cartilage. It inserts on the tibial tuberosity, immediately lateral to the middle PL’s insertion. There is often a small to moderate amount of anechogenic synovial fluid in the lateral and medial joint recesses caudal to the respective PLs, over the surface of the lateral and medial trochlear ridges.

Figure 7.3    Normal middle patellar ligament (MiPL). Longitudinal (A) and transverse (B,C,D) ultrasonographic images. At the patellar origin (B) the cranial surface of the distal patella (pat) forms a trough. The MiPL is homogeneous and finely striated (arrows). (C) Central portion of the MiPL (yellow arrows): the edges contrast poorly with the surrounding FP. The two vertical anechogenic lines are edge refraction artifacts. (D) Distal insertion of the MiPL on the tibial tuberosity (TB): this portion is triangular and contains thin, hypoechogenic reticulations due to thicker endotendon tissue trabecula (red arrows).
Figure 7.4    Normal medial patellar ligament (MPL). Longitudinal (A) and transverse (B,C) ultrasonographic images. A and C are obtained at mid-distance, caudomedial to the medial trochlear ridge. The ligament lies within the joint capsule, making its outline indistinct (yellow arrows). B is obtained just distal to the parapatellar fibrocartilage (pfc). There it broadens and becomes more heterogeneous as the fibers spread into the thickness of the fibrocartilage. (C) Further distally, the MPL (calipers) is poorly defined from the fibrous capsule.

Medial Femorotibial Joint: Medial Aspect

A linear transducer is best, as the joint is very close to the skin. The topography of the medial aspect of the joint is reviewed in Figure 7.5. The proximal edge of the tibial condyle and the round, smooth surface of the medial femoral condyle form a triangular space filled by the echogenic medial meniscus (Figure 7.6). Linear, anechogenic artifacts are caused by refraction at the insertion of the capsule on the outer surface of the meniscus. These are easily mistaken for tears but they should remain perpendicular to the probe when the latter is tilted. The capsule inserts over the whole abaxial aspect of the meniscus, so that fluid can only accumulate proximal to it, over the abaxial surface of the medial femoral condyle. Discrete distension of the joint is common but the synovial membrane should remain very thin. Small villi may be seen in the pouch in normal horses. Transverse plane images of the menisci are obtained by rotating the probe 90°. These images are useful to better evaluate the configuration of certain tears.

Figure 7.5    Computed tomography (CT) scan three-dimensional (3-D) reconstruct of the stifle showing the topography of the medial femorotibial joint. The medial femoral condyle (MFC) and medial tibial condyle (MTC) are separated by the wedge-shaped meniscus. The latter is topographically divided into body (1), at the level of the medial collateral ligament (black arrows), and cranial (2) and caudal (3) horns. The cranial horn inserts onto the craniomedial aspect of the tibial eminence (TE) via a cranial meniscotibial ligament (yellow arrows). A very short caudal meniscotibial ligament links the caudal horn to the caudomedial edge of the tibial plateau. The medial collateral ligament (MCL) is flat but strong and extends from the medial femoral epicondyle (E) to the medial aspect of the MTC and is divided into two distinct branches. MTR: medial trochlear ridge; TT: tibial tuberosity.
Figure 7.6    (A) Position of the transducer over the medial aspect of the medial femorotibial joint. (B) Longitudinal (frontal) plane ultrasonographic image obtained cranial to the collateral ligament. The meniscus is echogenic, wedge shaped and amorphous. It sits axial to an imaginary line drawn between the edges of the condyles (dotted line). The underlying cartilage is smooth and even (white arrows) and the fibrous capsule (red double arrows) inserts over its abaxial border. Note the hypoechogenic artifacts running perpendicular to the transducer (yellow arrows. (C) Image obtained proximal to that in B. Note the mild distension of the joint pouch by anechogenic fluid. No fluid is seen over the meniscus (B). The membrane is thin and even (yellow arrows) in the absence of inflammation. MFC: medial femoral condyle; Tib: tibia.

The medial collateral ligament (MCL) is a strong, flattened structure, approximately 5–6 mm in thickness. As in most joints it is made up of two poorly differentiated branches (Figure 7.7). It is necessary to rotate the probe to image each branch individually, as they run obliquely to each other, crossing over the meniscus. The superficial branch runs vertically; it originates on the femoral epicondyle several centimeters proximal to the joint, and inserts on the abaxial surface of the tibia. It receives part of the insertion of the adductor muscle proximally. The deep branch originates caudal to it, and runs in a craniodistal direction to insert on the tibia, cranial to the superficial branch.

Figure 7.7    Longitudinal (frontal) plane ultrasonographic images over the medial aspect of the stifle showing the superficial branch of the collateral ligament (yellow arrows), recognized by its striated pattern. It runs from the medial femoral epicondyle (MFE) over the medial meniscus (m) and abaxial borders of the medial femoral condyle (MFC) and medial tibial condyle (MTC). The deep branch (red arrows) runs obliquely to it, hence a grainier pattern as the fibers are not aligned with the transducer. It separates the superficial branch from the meniscus to which it adheres. Note that the ligaments are poorly distinguished from the rest of the fibrous capsule.

Lateral Femorotibial Joint: Lateral Aspect

The general topography is presented in Figure 7.8. The joint is covered by a thicker layer of muscle and the bone surfaces are very oblique to the skin. It is consequently difficult to obtain images using a linear transducer. A micro-convex probe provides better images. Craniolaterally, the tendon of origin of the long digital extensor and peroneus tertius muscles originates on the lateral femoral epicondyle and runs within the deep extensor groove of the proximal tibia (Figure 7.9). The groove is covered by cartilage and a synovial recess extends from the lateral femorotibial joint (LFT), between the bone surface and the tendon. Similarly to the MCL, the lateral collateral ligament (LCL) is divided into two branches, although they are more difficult to tell apart. They both insert on the tibia and fibular head. The popliteal tendon originates immediately cranial to the LCL. It runs obliquely in a caudodistal direction, over the meniscus and underneath the LCL. It is triangular in cross-section and should not be mistaken for a torn portion of the meniscus. It fans out over the caudal proximal tibia as a hypoechogenic muscle. Fluid may be seen proximal to the meniscus and cranial or caudal to the collateral ligament, but this is less common than in the medial compartment.

Figure 7.8    CT scan 3-D reconstruct showing the topography of the lateral femorotibial joint. The lateral femoral (LFC) and tibial (LTC) condyles are separated by the lateral meniscus (m). The lateral collateral ligament (yellow arrows), is made up of two closely related branched originating on the lateral femoral epicondyle (LFE) and inserting both on the tibia and fibula (yellow arrows). The common tendon of origin (black arrowheads) of the peroneus tertius and long digital extensor muscles (ext) originates on the abaxial aspect of the lateral trochlear ridge and runs over the cranial horn of the mm, within the extensor fossa of the tibia. The tendon of origin of the popliteus muscle originates on the femoral epicondyle, cranial to the collateral ligament and runs between the latter and the lateral meniscus in a caudodistal direction (white arrows). U: ulna.
Figure 7.9    (A) 3-D reconstruct showing the position of the transducer in the two positions (B and C) corresponding to the ultrasound images over the extensor groove of the tibia (B) and lateral aspect of the lateral femorotibial joint (C). (B) Longitudinal (frontal) plane ultrasonographic image showing the common tendon of origin of the peroneus tertius and long digital extensor muscles (ext) (arrows) and extensor groove of the tibia (T). (C) Longitudinal (frontal) plane ultrasonographic image showing the lateral collateral ligament (LCL, yellow arrows). The striation is lost proximally because of the use of a curved array transducer inducing an off-incidence artifact. The hypoechogenic artifact helps, however, to better see the ligament contours. At this level the popliteus tendon (POP: calipers) is flat and located between the meniscus (m) and LCL. LFC: lateral femoral condyle; LFE: left femoral epicondyle; LTR: lateral trochlear ridge.

Cranial Aspect of the Femorotibial Joints

Imaging the cranial aspect of the femorotibial articulation requires that the stifle be flexed, in order to expose the intercondylar space and the cruciate ligaments (Figure 7.10). A micro-convex transducer must be used. The surface of the femoral condyles, overlying cartilage, and cranial horns of the menisci are easily visualized deep to the fat pad. The round condylar surfaces can be assessed for cartilage or subchondral defects.

Figure 7.10    3-D CT-scan reconstructs showing the topographic anatomy of the femorotibial articulation. The stifle is fully flexed to expose the intercondylar space. The ligaments have been drawn to show their general topography. The cranial cruciate ligament (1) runs in a caudoproximolateral to distocraniomedial direction, between the axial surface of the lateral femoral condyle (LFC) and the tibial eminence; the caudal cruciate ligament (2) runs in the sagittal plane, in a cranioproximal to caudodistal direction, crossing its cranial counterpart medially between the two condyles. It originates distal to the trochlea and inserts on the caudal aspect of the tibial plateau; the two arrows indicate the cranial meniscotibial ligaments. lm: lateral meniscus; MFC: medial femoral condyle; mm: medial meniscus; TT: tibial tuberosity.

The cranial cruciate ligament (CrX) can be identified with the ultrasound beam perpendicular to the ligament fibers (Figure 7.11): the probe is placed immediately distal to the patella, over or medial to the middle PL. The probe is angled downward in a sagittal plane to image the surface of the tibial eminence. The beam is then rotated 15–20° clockwise in the left limb and anticlockwise in the right limb until the linear pattern of the ligament is recognized. The CrX originates caudally on the axial (medial) aspect of the lateral femoral condyle, and inserts in a dip between the medial and lateral prominences of the tibial eminence. Cross-sectional images can be obtained by rotating the probe 90° in the same position. The CrX is hyperechogenic with a regular striated pattern.

Figure 7.11

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Jun 8, 2017 | Posted by in EQUINE MEDICINE | Comments Off on SEVEN: Ultrasonography of the Stifle

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