Knee Joint Anatomy: Ligaments, Menisci, Bursae, and Clinical Tests Explained

The knee is the largest synovial joint in the human body, and it is arguably the one that causes the most clinical problems. ACL tears, meniscal injuries, osteoarthritis, patellar dislocations, bursitis — the list of common knee pathologies is long, and every single one requires a thorough understanding of the underlying anatomy to diagnose and manage effectively. The knee is also one of the most frequently tested joints in anatomy, orthopedic surgery, emergency medicine, and sports medicine examinations. Despite being classified as a "hinge joint," the knee is far more complex than a simple door hinge — it allows flexion, extension, and a small but clinically important amount of rotation when the knee is flexed. This complexity comes from its unique combination of ligaments, menisci, and bursae working together to provide both stability and mobility. Let us work through each component systematically.

The knee joint is formed by three bones: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). The fibula does not participate in the knee joint itself, though the lateral collateral ligament attaches to the fibular head. The joint actually consists of two articulations within one joint capsule: the tibiofemoral joint (between the femoral condyles and the tibial plateau) and the patellofemoral joint (between the patella and the femoral trochlear groove). The femoral condyles are the two large, rounded prominences at the distal end of the femur. The medial condyle is larger and extends more distally than the lateral condyle — this explains the normal slight valgus angle of the knee (the "physiological valgus" of about 5-7 degrees). The tibial plateau has medial and lateral condyles that are relatively flat, creating an inherent mismatch with the convex femoral condyles. This incongruity is partly compensated by the menisci (discussed below), which deepen the tibial surface and improve contact area. The patella is the largest sesamoid bone in the body, embedded within the quadriceps tendon. It improves the mechanical advantage of the quadriceps by increasing the moment arm for knee extension — biomechanical studies show the patella increases quadriceps efficiency by approximately 30-50%. Its deep surface is covered by the thickest articular cartilage in the body (up to 7mm), reflecting the enormous compressive forces it endures. During deep knee bends, the patellofemoral joint experiences forces up to 7 times body weight.

The cruciate ligaments are the two intracapsular (but extrasynovial) ligaments that cross inside the knee, forming an X shape — "cruciate" comes from the Latin crux, meaning cross. They are the primary stabilizers against anterior-posterior translation of the tibia on the femur. The anterior cruciate ligament (ACL) runs from the anterior intercondylar area of the tibia upward and posteriorly to the medial surface of the lateral femoral condyle. Its primary function is to prevent anterior translation of the tibia relative to the femur — in other words, it stops the shin bone from sliding forward under the thigh bone. The ACL also resists internal rotation and valgus stress to a lesser degree, and it is the primary restraint against hyperextension. The ACL is the most commonly injured ligament of the knee, especially in sports that involve pivoting, cutting, and sudden deceleration — soccer, basketball, skiing, and football account for the majority of ACL tears. The classic mechanism is a non-contact injury: the athlete plants the foot, the knee is slightly flexed and in valgus, and the femur internally rotates over the fixed tibia. The patient often reports hearing or feeling a "pop," followed by immediate swelling (hemarthrosis within 2-4 hours, because the ACL has a blood supply that bleeds into the joint when torn). The posterior cruciate ligament (PCL) is the stronger and thicker of the two cruciates. It runs from the posterior intercondylar area of the tibia upward and anteriorly to the lateral surface of the medial femoral condyle. Its primary function is to prevent posterior translation of the tibia — it stops the shin from being pushed backward. The classic mechanism of PCL injury is the "dashboard injury": in a car accident, the knee strikes the dashboard, driving the tibia posteriorly. PCL tears are less common than ACL tears and often can be managed non-operatively because of the PCL's superior healing capacity.

The collateral ligaments are the primary stabilizers against varus and valgus stress (side-to-side angulation) at the knee. The medial collateral ligament (MCL, also called the tibial collateral ligament) runs from the medial epicondyle of the femur to the medial surface of the tibia, about 6-7 cm below the joint line. It is a broad, flat band that is tightly adherent to the medial meniscus — and this connection is clinically important because it means a severe MCL injury can also damage the medial meniscus. The MCL resists valgus stress (force pushing the knee inward). It is most commonly injured by a blow to the lateral side of the knee (such as a tackle in football), which forces the knee into valgus. MCL tears are the most common knee ligament injury overall and are graded I through III based on severity. Most MCL injuries heal without surgery because the ligament has a good blood supply and is extra-articular. The lateral collateral ligament (LCL, also called the fibular collateral ligament) is a cord-like structure that runs from the lateral epicondyle of the femur to the head of the fibula. Unlike the MCL, the LCL is not attached to the lateral meniscus — the popliteus tendon passes between them. The LCL resists varus stress (force pushing the knee outward). Isolated LCL injuries are uncommon because varus forces at the knee are relatively rare in daily activities and sports. When the LCL is torn, it is often part of a more complex posterolateral corner injury that also involves the popliteus, arcuate ligament, and popliteofibular ligament — these complex injuries are much more difficult to treat and often require surgical reconstruction.

The menisci are two C-shaped (or more accurately, crescent-shaped) fibrocartilaginous structures that sit on top of the tibial plateau, deepening the articular surface and dramatically improving the congruence between the round femoral condyles and the flat tibial surface. The medial meniscus is C-shaped and larger, covering about 50% of the medial tibial plateau. It is firmly attached to the deep surface of the MCL and to the joint capsule along its entire periphery. This relative immobility is why the medial meniscus is torn about 3-5 times more frequently than the lateral meniscus — it cannot escape when abnormal forces are applied. The lateral meniscus is more circular (almost O-shaped) and covers a larger proportion (about 70%) of the lateral tibial plateau. It is more mobile because it is not attached to the LCL, and the popliteus tendon separates it from the capsule posterolaterally. This mobility allows it to move out of the way during rotation, making it less susceptible to injury. The menisci serve multiple functions beyond shock absorption: they distribute load over a larger area of the tibial plateau (increasing contact area by about 60%), they contribute to joint stability, they assist with lubrication by spreading synovial fluid across the articular cartilage, and they provide proprioceptive feedback. Removing a meniscus (meniscectomy) increases contact pressure on the articular cartilage by 200-300%, which is why total meniscectomy dramatically accelerates osteoarthritis development and why surgeons now favor meniscal repair over removal whenever possible. The blood supply to the menisci is critical for understanding their healing potential. The outer third (the "red zone") has a good blood supply from the perimeniscal capillary plexus and can heal after repair. The inner two-thirds (the "white zone") is avascular and receives nutrition only through diffusion from the synovial fluid — tears in this zone generally cannot heal and may require partial meniscectomy.

The unhappy triad is one of the most famous injury patterns in all of orthopedics, and it is a perennial exam favorite. The classic description, attributed to Don O'Donoghue in the 1950s, involves simultaneous injury to three structures: the ACL, the MCL, and the medial meniscus. The mechanism is typically a valgus force applied to a flexed, externally rotated knee — picture a football player whose planted leg is struck from the lateral side. The valgus stress tears the MCL first (it is the primary valgus restraint), then as the force continues, the ACL ruptures (it is the secondary valgus restraint and also resists the anterior translation that occurs as the tibia subluxes forward). The medial meniscus, tethered to the MCL, is torn along with it. However, more recent arthroscopic studies have shown that the lateral meniscus is actually injured more frequently than the medial meniscus in this combination injury pattern. Some authors now refer to the updated triad as ACL + MCL + lateral meniscus tear. For exam purposes, know both: the classic triad (ACL, MCL, medial meniscus) and the updated finding (lateral meniscus may be more commonly involved). The clinical presentation is dramatic: immediate severe pain, rapid swelling (hemarthrosis from the ACL tear), instability, and inability to bear weight. This is a surgical injury — the ACL is reconstructed, the meniscus is repaired or partially removed, and the MCL is usually allowed to heal non-operatively in a brace.

A bursa is a fluid-filled sac lined by synovial membrane that reduces friction between moving structures. The knee has more bursae than any other joint — at least 11 named bursae, reflecting the complexity of the forces acting around this joint. The most clinically important bursae include the following. The suprapatellar bursa is the largest, extending about 5-6 cm above the patella between the quadriceps tendon and the femur. It communicates with the knee joint cavity (it is essentially an extension of it), which means fluid in the knee joint also fills the suprapatellar bursa — this is where you aim the needle during knee aspiration (arthrocentesis). The prepatellar bursa lies between the patella and the overlying skin. Inflammation of this bursa (prepatellar bursitis) is known as "housemaid's knee" because it classically resulted from prolonged kneeling on hard surfaces — today it is common in carpet layers, roofers, plumbers, and anyone who spends extended time on their knees. It presents as a fluctuant, warm swelling directly over the patella. The infrapatellar bursae include a superficial bursa (between the patellar tendon and the skin, whose inflammation is called "clergyman's knee" because it results from kneeling in a more upright position than housemaid's knee) and a deep bursa (between the patellar tendon and the tibial tuberosity). The pes anserinus bursa lies between the pes anserinus (the combined tendons of sartorius, gracilis, and semitendinosus — remembered by the mnemonic "Say Grace before Tea") and the MCL. Pes anserine bursitis is a common cause of medial knee pain, especially in overweight patients and runners, and is frequently misdiagnosed as a medial meniscus tear.

The muscles acting on the knee can be organized by their primary action. Extensors: the quadriceps femoris is the sole knee extensor and is the most powerful muscle group in the body. It consists of four heads — rectus femoris (the only one that crosses the hip, making it a hip flexor as well), vastus lateralis, vastus medialis, and vastus intermedius. They converge into the quadriceps tendon, which envelops the patella and continues as the patellar tendon (technically a ligament, since it connects bone to bone) to insert on the tibial tuberosity. The vastus medialis obliquus (VMO, the distal fibers of vastus medialis) is critically important for patellar tracking — weakness or atrophy of the VMO allows the patella to track laterally, leading to patellofemoral pain syndrome, one of the most common knee complaints in young, active patients. Flexors: the hamstrings are the primary knee flexors — biceps femoris (long and short heads), semimembranosus, and semitendinosus. Biceps femoris inserts on the fibular head (laterally), while semimembranosus and semitendinosus insert medially (semitendinosus is part of the pes anserinus). Other knee flexors include gracilis, sartorius, popliteus, and the gastrocnemius (which crosses the knee posteriorly). Rotators: when the knee is flexed (which unlocks the rotation), popliteus is the key medial rotator of the tibia (or lateral rotator of the femur on a fixed tibia). It also "unlocks" the knee from full extension by laterally rotating the femur at the initiation of flexion. Biceps femoris laterally rotates the tibia, while semimembranosus, semitendinosus, sartorius, and gracilis medially rotate the tibia. These rotation functions become clinically important during rehabilitation and when understanding mechanisms of rotational knee injuries.

The physical examination of the knee uses specific tests to assess each ligament and structure. For the ACL, the three main tests are the anterior drawer test, the Lachman test, and the pivot shift test. The anterior drawer test is performed with the patient supine, hip flexed 45 degrees, knee flexed 90 degrees, and the foot stabilized on the table. The examiner sits on the patient's foot and pulls the tibia anteriorly — excessive forward translation compared to the other knee suggests ACL insufficiency. The Lachman test is the most sensitive clinical test for ACL tears (sensitivity approximately 85-95%). The knee is flexed only 20-30 degrees, one hand stabilizes the femur, and the other pulls the tibia anteriorly. A soft or absent endpoint indicates ACL rupture. The reason Lachman is more sensitive than the anterior drawer is that at 20 degrees of flexion, the hamstrings are more relaxed and the secondary restraints are less engaged, so ACL deficiency is more apparent. The pivot shift test is the most specific test for ACL tear — it recreates the actual instability event. With the tibia internally rotated and a valgus force applied, the knee is slowly extended from flexion. In an ACL-deficient knee, the lateral tibial plateau subluxes anteriorly as the knee approaches extension, then dramatically reduces ("shifts" back) at about 30 degrees of flexion. A positive pivot shift essentially confirms functional ACL insufficiency. For the PCL, the posterior drawer test is performed similarly to the anterior drawer but pushing the tibia posteriorly. In a PCL-deficient knee, the tibia sags posteriorly even at rest (posterior sag sign) — this is important to recognize because a posterior sag can make the anterior drawer test falsely appear positive (you think you are pulling the tibia forward, but you are actually just reducing a posterior subluxation back to neutral). For the collateral ligaments, valgus stress testing (at 0 and 30 degrees of flexion) assesses the MCL, while varus stress testing assesses the LCL. Testing at 30 degrees of flexion isolates the collateral ligament, while testing at full extension also tests the cruciates and posterior capsule — laxity at full extension suggests a more severe, multi-ligament injury.

Meniscal tears present with joint line tenderness, mechanical symptoms (catching, locking, or giving way), and effusion. The McMurray test is the most well-known clinical test for meniscal tears, though its sensitivity is moderate (approximately 60-70%). To perform it: with the patient supine and the knee fully flexed, the examiner holds the heel with one hand and places the other hand on the knee joint line. For a medial meniscus tear, the tibia is externally rotated and the knee is slowly extended while applying a valgus force — a palpable click or clunk at the medial joint line with pain suggests a medial meniscal tear. For a lateral meniscus tear, the tibia is internally rotated and a varus force is applied during extension. The Apley grind test (also called Apley compression test) is another useful test: the patient lies prone with the knee flexed 90 degrees. The examiner pushes down on the heel (compressing the meniscus) while rotating the tibia. Pain during compression and rotation suggests meniscal pathology, while pain during distraction (pulling the tibia upward) suggests ligamentous injury. Thessaly test is a newer, weight-bearing test where the patient stands on one leg with the knee flexed 20 degrees and rotates the body and knee internally and externally. Pain, locking, or giving way is a positive result. A locked knee — one that cannot fully extend — suggests a displaced bucket-handle meniscal tear, where a longitudinal tear flips into the intercondylar notch and physically blocks extension. This is an indication for urgent arthroscopic surgery to reduce the displaced fragment.

The beauty of knee anatomy is that it directly translates into clinical practice in a way that is immediate and tangible. When a patient walks into a clinic or emergency department with a knee injury, the history and physical examination — guided by anatomy — can often establish the diagnosis before any imaging is ordered. A young soccer player who heard a "pop" during a cutting maneuver, with rapid swelling and instability: ACL tear until proven otherwise. A middle-aged runner with medial joint line tenderness and a positive McMurray: medial meniscal tear. A football player tackled from the lateral side with valgus instability, hemarthrosis, and inability to bear weight: unhappy triad. A roofer with a painless, fluctuant swelling over the front of the kneecap: prepatellar bursitis. Each diagnosis maps directly back to the anatomy we have covered. When you study the knee, resist the temptation to memorize structures in isolation — instead, connect each structure to its function and its injury pattern. Think: "The ACL prevents anterior tibial translation → testing for anterior translation (Lachman) diagnoses ACL tears → the mechanism that loads the ACL (pivot, deceleration) is the mechanism that tears it." This kind of functional thinking, where anatomy, biomechanics, and clinical presentation form a continuous chain, is what separates a student who can pass an exam from a clinician who can figure out what is wrong with a patient. Use AnatomyIQ to test yourself on knee structures — photograph a cadaveric or model knee, label the ligaments and menisci, and get instant feedback on accuracy. Building visual-spatial familiarity with the knee through repeated labeling practice makes the clinical reasoning flow naturally.