Normal knee MRI

Patellofemoral joint

When we look at the knee in the coronal plane, the first thing that we see is the patella as the most anterior structure. The patella is a triangular shaped sesamoid bone that is embedded in the quadriceps tendon. With its posterior surface, it articulates with the anterior surface of the femur, forming the patellofemoral joint.

When examining the patella, it is important to evaluate its contours, bone marrow and articular cartilage. On T1, the patella appears as a homogenous intense white triangular signal. We want to make sure that there are no signs of fractures or cyst formation in the bone, which would be seen as hypointense lines or circles.

Next, we should evaluate the thickness and homogeneity of the patellar cartilage by looking at images taken in the sagittal and axial planes. On T2, the patellar cartilage has a uniformly homogeneous white signal. It should be noted that the thickness of the patellar cartilage is greater in the mid and lower portions, while being thinner in the upper portion. Any abnormal signal and irregularities in the articular cartilage, such as fissures, ulcerations, or osteochondral fragments, may point towards chondromalacia of the patellofemoral joint.

We also need to inspect the cavity of the patellofemoral joint for any signs of fluid collection, which on T2 would appear as a hypointense signal between the patellar and femoral articular cartilages.

Tibiofemoral joint

As we go deeper into the knee joint in the coronal plane, we can observe the medial and lateral femoral condyles superiorly articulating with the tibial plateau inferiorly, forming the tibiofemoral joint. If at this point you’re unsure about which side is medial and which is lateral, you can simply scroll deeper posteriorly until you find the fibula, which indicates the lateral side. While examining the tibiofemoral joint, we need to look at the contours of the bone, bone marrow and articular cartilage.

While evaluating the bone marrow, we should look for any sign of edema, contusion, tumor or fracture. On T1, a normal bone marrow is higher in intensity, appearing bright, while on T2 it is low in intensity and appears dark. The normal bone marrow shows a mainly homogenous intensity, with only some contrasting linear signals that correspond to blood vessels. Bone marrow edema on T1 typically appears as an increased, white signal, while fractures are suspected with the appearance of larger dark lines through the bone.

Next, we will look at the articular cartilage of the tibiofemoral joint. We should assess the thickness and intensity of their articular cartilages which are best seen in sagittal and coronal planes. On T2, normal articular cartilage appears as a thick homogenous white signal that covers the articular surfaces of the lateral and medial femoral condyle and the tibial plateaus.

Lastly, we want to look at the cavity of the tibiofemoral joint for any signs of fluid collection, which would be seen as a hypointense signal between the articular cartilages.

Test your knowledge on the anatomy of the knee joint in this quiz.

Cruciate ligaments

We will now dive into the center of the knee joint to look at the cruciate ligaments. The anterior and posterior cruciate ligaments are very important ligaments both in terms of their function, and their susceptibility to injuries. The cruciate ligaments stabilize the knee during dynamic motion and prevent rolling and displacement of the femoral condyle, as well as hyperextension and hyperflexion of the knee joint.

On a slice through the posterior compartment of the knee in the coronal plane, we can see the origin of the anterior cruciate ligament (ACL), arising from the intercondylar notch of the tibia. From here, and we can follow it by scrolling anteriorly towards its insertion on the medial surface of the lateral femoral condyle. The ACL is best seen on T1, appearing as two sets of black, low signal bands, often showing some linear striations with a higher signal near their tibial attachment.

It is important to check the ACL in the axial plane as well, where it is best visualized on a slice roughly through the intercondylar notch. On this level, the ACL is seen on the lateral femoral condyle as a black band about 1,5 cm in the anteroposterior diameter. Upon scrolling deeper towards the tibia, we can see that the ACL fans out and crosses over to the medial femoral condyle.

The posterior cruciate ligament (PCL) also contains two bands that arise from the posterior intercondylar area of the tibia and extend anteromedially to attach on the medial femoral condyle. Unlike the ACL, the PCL has a characteristic homogeneous low signal on a sagittal T1 and its two bands are not distinctly seen on the MRI. In the axial plane, the PCL is clearly seen on a slice through the tibial articular cartilage, appearing on the posterior aspect of the intercondylar area as a black ovoid signal. It can be traced by scrolling upwards as it ascends to attach onto the medial femoral condyle.

T2-weighted sequences are the gold standard for diagnosing tears of the ACL, while tears in the PCL are often not visible on this sequence. A tear in the ACL would be seen as a high signal intensity within the substance of the ligament. The assessment of pathological changes in the cruciate ligaments is best performed in the sagittal plane, with the axial and coronal planes being complementary.

Menisci

As we scroll a bit deeper in the coronal view from the central part of the knee into the posterior compartment, we can identify the menisci. These are fibrocartilaginous C-shaped plates found between the articular surfaces of the femur and tibia that provide congruence and shock absorption to the knee joint.The menisci are best examined on T1 coronal and sagittal images, where it is important to assess their contours, position and intensity.

On a T1 sagittal image, we can see that the menisci are uniformly low in signal, normally appearing as a black bow tie-shaped structure in the joint cavity. On an axial image however, we see the medial and lateral meniscus as black semicircular structures just underneath the medial and lateral femoral condyles, respectively. We should look out for any increased intensity of the signal in the menisci. A hyperintense signal that reaches the signal of the articular cartilage can point us towards a meniscal tear or degeneration.

The medial collateral ligament (MCL) is best examined on a T2 coronal image, where it is seen as a long, thin uniformly low-intensity band on the medial side of the joint capsule. It originates from the medial femoral epicondyle and inserts into the medial metaphysis of the tibia.

On a slice through the intercondylar notch in the axial plane, we can find the origin point of the MCL, appearing as a short low signal band near the medial side of the femoral condyle. By scrolling downwards, we can follow the MCL to its attachment onto the tibia.

Like the medial collateral ligament, the lateral collateral ligament (LCL) is best seen on a T2 coronal image, appearing as a homogenous low-intensity structure on the lateral side of the joint capsule. The LCL originates from the lateral femoral epicondyle and inserts onto the fibular head together with the biceps femoris tendon.

If we look at the LCL in the axial plane on a cut through the intercondylar notch, we can see its origin as an ovoid low signal near the posterior aspect of the femoral condyle. By scrolling downwards, we can follow the LCL as it inserts onto the anterolateral aspect of the fibular head.

If you want to see more MRI images of the menisci, check out the following study units.