Knee Anatomy: Bones, Ligaments, Muscles & Nerves

The knee is the largest joint in the body and the joint most often injured and imaged after a fall, sports injury or motor vehicle collision. Knee anatomy comes down to three bones (the femur, tibia and patella), four main ligaments (the ACL, PCL, MCL and LCL), two menisci that cushion the joint, the quadriceps and hamstring tendons that move it, and the nerves and vessels that pass behind it. Knowing how these structures fit together is the first step to reading a knee MRI report, explaining a diagnosis to a patient, or illustrating a personal injury claim.

Bones of the knee

The knee joint is where the femur meets the tibia, with the patella riding in front and the fibula alongside. The view above shows a right knee from the front, bent, with the kneecap lifted away so the inside of the joint is visible.

• Femur: the lower end of the thigh bone widens into two rounded femoral condyles, medial and lateral, covered with smooth articular cartilage. Between them at the front is the trochlear groove, where the patella glides, and between them at the back is the intercondylar notch, which holds the cruciate ligaments.
• Tibia: the top of the shin bone forms the flat tibial plateau, with a medial and lateral side that carry the femoral condyles. The tibial tuberosity, a bump on the front of the tibia below the joint, is where the patellar tendon attaches.
• Patella: the kneecap, the largest sesamoid bone in the body. It sits inside the quadriceps tendon and acts as a pulley, increasing the leverage of the quadriceps when it straightens the knee.
• Fibula: runs along the outer side of the tibia. It does not touch the femur and carries little weight, but its head is the attachment of the lateral collateral ligament and the biceps femoris tendon, and the common fibular nerve wraps around its neck.

Joints of the knee

The knee is really two joints inside one joint capsule:

• Tibiofemoral joint: between the femoral condyles and the tibial plateau. It works as a modified hinge, bending to about 135 degrees and straightening fully. In the last few degrees of straightening the femur rotates slightly on the tibia, which locks the knee for standing (the screw-home mechanism); the small popliteus muscle unlocks it to start bending.
• Patellofemoral joint: between the back of the patella and the trochlear groove of the femur. The kneecap slides up and down this groove as the knee moves. Pain at the front of the knee, kneecap dislocation and patellar arthritis all come from this joint.

Ligaments of the knee

Four main ligaments hold the knee together, two inside the joint and two along its sides.

Cruciate ligaments

The two cruciate ligaments cross each other in an X inside the intercondylar notch ("cruciate" means cross-shaped).

• Anterior cruciate ligament (ACL): runs from the front of the tibial plateau, between the menisci, up and back to the inner side of the lateral femoral condyle. It stops the tibia from sliding forward under the femur and controls rotation. It is the most commonly torn knee ligament in sports, usually in a non-contact pivot or landing.
• Posterior cruciate ligament (PCL): runs from the back of the tibial plateau up and forward to the inner side of the medial femoral condyle. It is thicker and stronger than the ACL and stops the tibia from sliding backward. It is classically torn when the shin hits the dashboard in a car crash.

Behind the PCL, the small meniscofemoral ligament of Wrisberg runs from the back of the lateral meniscus to the medial femoral condyle.

Collateral ligaments

• Medial collateral ligament (MCL): a broad band on the inner side of the knee, from the femur to the tibia. It resists forces that push the knee inward (valgus stress), such as a tackle from the outside, and it is the most commonly sprained knee ligament. Its deep fibers attach to the medial meniscus.
• Lateral collateral ligament (LCL): a cord-like band on the outer side, from the femur to the head of the fibula. It resists forces that push the knee outward (varus stress). Unlike the MCL, it is not attached to its meniscus.

The front of the joint is also held by the patellar tendon and the patellar retinacula on either side of the kneecap, and the back by the oblique popliteal ligament and the joint capsule.

Menisci of the knee

Seen from above, the two menisci sit on the tibial plateau like a pair of cushioned rims.

• Medial meniscus: the larger, C-shaped meniscus on the inner side. It is firmly attached to the joint capsule and the MCL, so it moves less and is torn more often.
• Lateral meniscus: smaller and almost circular, on the outer side. It is more mobile, and the popliteus tendon passes next to it.

The transverse ligament joins the front of the two menisci, and the ACL and PCL attach to the tibia between them. The menisci spread load across the joint, absorb shock and deepen the shallow tibial plateau to stabilize the femur. Only the outer third of each meniscus (the red zone) has a good blood supply, so tears there can be repaired, while tears in the inner part are often trimmed instead.

The knee in cross-section

A sagittal cut through the middle of the knee, the plane an MRI is usually read in, shows the joint from the side. The quadriceps tendon comes down the front of the femur, wraps the patella, and continues as the patellar tendon to the tibia. Behind the patellar tendon sits the infrapatellar fat pad (Hoffa's fat pad), which cushions the front of the joint. The femoral condyle rolls on the tibial plateau, with articular cartilage capping both bones, and the cruciate ligaments run through the center of the joint.

The knee also has several bursae, small fluid-filled sacs that let tendons and skin glide: the prepatellar bursa in front of the kneecap (inflamed in "housemaid's knee"), the infrapatellar bursae around the patellar tendon, the suprapatellar bursa above the patella, which is continuous with the joint, and the pes anserine bursa on the inner side of the upper tibia.

Muscles and tendons of the knee

• Quadriceps (rectus femoris, vastus lateralis, vastus medialis and vastus intermedius): the four muscles on the front of the thigh join into the quadriceps tendon, which wraps the patella and continues as the patellar tendon to the tibial tuberosity. They straighten the knee. The lowest fibers of the vastus medialis help keep the kneecap centered in its groove.
• Hamstrings (biceps femoris, semitendinosus and semimembranosus): on the back of the thigh. They bend the knee and help control rotation. Their tendons form the upper borders of the popliteal fossa.
• Iliotibial band: a thick band of fascia on the outer side of the thigh that crosses the knee to attach to the tibia. Friction where it crosses the lateral femoral condyle causes iliotibial band syndrome in runners.
• Pes anserinus: the joined tendons of the sartorius, gracilis and semitendinosus, which attach to the inner side of the upper tibia. Its bursa can become inflamed (pes anserine bursitis).
• Gastrocnemius: the calf muscle starts from the back of the femoral condyles with two heads, so it also helps bend the knee.
• Popliteus: a small muscle deep behind the knee that unlocks the joint from full extension.

Back of the knee: the popliteal fossa

The back of the knee is a diamond-shaped hollow called the popliteal fossa. Its borders are the semimembranosus and semitendinosus above and on the inner side, the biceps femoris above and on the outer side, and the medial and lateral heads of the gastrocnemius below. The main nerve and vessels to the lower leg pass through it. From the surface inward, they are the tibial nerve, the popliteal vein and the popliteal artery, which lies deepest, against the back of the femur and the joint capsule. The common fibular nerve runs along the inner edge of the biceps femoris tendon toward the head of the fibula.

A Baker's cyst (popliteal cyst) is a fluid swelling of the bursa between the semimembranosus and the medial head of the gastrocnemius, often linked to arthritis or a meniscus tear.

Nerves and blood supply of the knee

The sciatic nerve divides just above the knee into the tibial nerve, which continues straight down through the popliteal fossa to the calf and the sole of the foot, and the common fibular (peroneal) nerve, which wraps around the neck of the fibula to reach the front and outer side of the leg. The common fibular nerve is the nerve most often injured around the knee; damage causes foot drop. The femoral nerve supplies the quadriceps, and its saphenous nerve branch supplies skin on the inner side of the knee. The infrapatellar branch of the saphenous nerve is often cut in knee surgery, leaving numbness below the kneecap.

Blood reaches the knee through the popliteal artery and a ring of genicular arteries around the joint. Because the popliteal artery is tethered close to the bone, a knee dislocation can tear it, which is why the foot pulses are checked after every serious knee injury.

Common knee injuries and how they appear on imaging

Knee injuries are imaged with X-rays first to look for fractures, then MRI for ligaments, menisci and cartilage. CT is used to plan surgery for complex fractures.

• ACL tear: usually a non-contact twist or awkward landing, often with a pop and rapid swelling. On MRI the ligament is discontinuous or wavy, often with bone bruises on the lateral femoral condyle and the back of the tibia. A small avulsion fracture on the outer tibia (Segond fracture) on X-ray strongly suggests an ACL tear.

• Meniscus tear: a twisting injury in younger people or gradual wear in older adults. The back (posterior horn) of the medial meniscus is the most common site. On MRI, a tear appears as a bright line reaching the meniscus surface. A large bucket-handle tear can flip into the notch and lock the knee.

• Patella fracture: from a direct blow, such as a fall onto the knee or the knee hitting the dashboard, or from a violent quadriceps contraction. Seen on X-ray, especially the side view. A displaced fracture that breaks the extensor mechanism is usually fixed surgically, commonly with wires or screws.

• Tibial plateau fracture: a break into the top of the tibia, usually from a fall from a height or a car bumper striking the outer knee. CT shows the fracture pattern and how far the joint surface is depressed. These fractures are graded with the Schatzker classification. Types I to III involve the lateral plateau: a split (I), a split with depression (II), and a pure depression (III). Types IV to VI are higher-energy injuries: a medial plateau fracture (IV), a fracture of both plateaus (V), and a fracture that separates the plateau from the shaft of the tibia (VI).

• MCL sprain: a blow to the outer side of the knee. Graded I to III; most heal without surgery.
• PCL tear: classically a dashboard injury; on MRI the normally dark, curved ligament is thickened or disrupted.
• Patellar dislocation: the kneecap slips out to the outer side, often tearing the medial patellofemoral ligament.
• Knee dislocation: a rare high-energy injury that tears several ligaments at once and can injure the popliteal artery and common fibular nerve.

How the knee is shown in a personal injury case

Knee injuries are among the most common injuries in personal injury claims: dashboard injuries in frontal collisions (patella fractures and PCL tears), pedestrian bumper strikes (tibial plateau fractures), and twisting falls on wet floors and stairs (ACL and meniscus tears), each a distinct mechanism of injury. A labeled illustration of the normal knee is the baseline exhibit. It shows a jury where the ACL, the meniscus or the tibial plateau sits in an uninjured knee before the injury-specific exhibit is introduced: the torn ligament, the depressed plateau, the plate and screws, or the knee replacement. Pairing the normal anatomy with the plaintiff's own MRI or CT slices helps separate an acute tear from pre-existing degeneration, which is often the central dispute in knee claims, and an illustration of the damaged joint surface explains why post-traumatic arthritis can develop years later.

Natomy creates custom medical legal illustrations of knee injuries for demand packages and trial exhibits. For more on how normal anatomy is used in court, see demonstrative evidence. Related atlas pages: leg nerves and dermatomes, hip anatomy, ankle anatomy, foot bones, elbow anatomy, shoulder anatomy, hand and wrist anatomy, back muscles anatomy and skull anatomy.