Knee Joint — Ligaments, Menisci & Bursae

The knee is the largest and most complex synovial joint in the body — a compound modified hinge (bicondylar) joint that permits flexion–extension plus a small but critical rotation. For NEET PG it is the single most fertile Anatomy–Orthopaedics crossover, so master the ligament attachments, the meniscal differences, the popliteus unlocking mechanism, the unhappy triad and the contents of the popliteal fossa.

Classification & basic architecture

The knee is a synovial joint of the modified hinge / condylar variety. It is functionally made of three articulations sharing one joint cavity:

1. Two femorotibial compartments (medial and lateral condyles of femur on the tibial condyles, cushioned by menisci).
2. One femoropatellar (patellofemoral) joint — a saddle articulation between the patella and the trochlear (patellar) surface of the femur.

The fibula does not participate in the knee joint. The articulating ends are covered by hyaline cartilage; stability depends almost entirely on muscles and ligaments rather than bony congruity (unlike the hip).

High-yield: The knee is a modified hinge / bicondylar / condylar joint. The patella is the largest sesamoid bone in the body and develops in the tendon of quadriceps femoris; the part of the femoral articular surface for the patella is the trochlear groove.

Movements and their muscles

Movement	Prime movers	Range/Notes
Flexion	Hamstrings (biceps femoris, semitendinosus, semimembranosus), gracilis, sartorius, popliteus, gastrocnemius	~120–140°
Extension	Quadriceps femoris (vastus medialis "screws home")	0° (slight hyperextension possible)
Medial rotation of leg (flexed)	Popliteus, semimembranosus, semitendinosus, sartorius, gracilis	Only when knee flexed
Lateral rotation of leg (flexed)	Biceps femoris	Only when knee flexed

Ligaments of the knee

Ligaments are grouped as extracapsular (patellar, collaterals, oblique & arcuate popliteal) and intracapsular (cruciates, menisci, transverse). The cruciates are intracapsular but extrasynovial (outside the synovial membrane, which is reflected anteriorly over them).

Cruciate ligaments — the examiner's favourite

The cruciates are named according to their tibial attachment. They cross each other like the limbs of an X.

Feature	Anterior Cruciate (ACL)	Posterior Cruciate (PCL)
Tibial attachment	Anterior intercondylar area	Posterior intercondylar area
Femoral attachment	Posteromedial aspect of lateral femoral condyle	Anterolateral aspect of medial femoral condyle
Prevents	Anterior displacement of tibia on femur (or posterior displacement of femur on tibia)	Posterior displacement of tibia on femur (or anterior displacement of femur on tibia)
Taut in	Extension	Flexion
Strength	Weaker, less vascular → poor healing	Stronger, thicker (the principal stabiliser)
Blood supply	Middle genicular artery	Middle genicular artery
Clinical test	Anterior drawer / Lachman test (most sensitive)	Posterior drawer / posterior sag sign

High-yield mnemonic — "LAMP": Lateral femoral condyle → Anterior cruciate; Medial femoral condyle → Posterior cruciate. Also remember ACL = Anteriorly on tibia, prevents Anterior tibial slide.

High-yield: The PCL is the strongest ligament of the knee and the chief stabiliser when the knee is flexed (e.g., weight-bearing downhill). It tightens in flexion; ACL tightens in extension.

Mechanism of injury: ACL tears with a sudden deceleration/pivot or a blow to the back of an extended knee (common in football, skiing). PCL tears with a "dashboard injury" — force driving the proximal tibia posteriorly with the knee flexed.

Collateral (extracapsular) ligaments

Feature	Tibial / Medial Collateral (MCL)	Fibular / Lateral Collateral (LCL)
Shape	Broad, flat band	Rounded cord
Capsule relation	Fused with capsule and medial meniscus	Separate from capsule and lateral meniscus (separated by popliteus tendon)
Attachment	Medial epicondyle of femur → medial condyle/shaft of tibia	Lateral epicondyle of femur → head of fibula
Structure passing deep	—	Tendon of popliteus passes between LCL and lateral meniscus
Resists	Valgus (abduction) stress	Varus (adduction) stress
Injury	Common; involved in unhappy triad	Less common

High-yield: Because the MCL is firmly attached to the medial meniscus, a valgus blow tears both together — the anatomical basis of the unhappy triad. The LCL is not attached to the lateral meniscus, so the lateral meniscus is more mobile and less often torn.

Other ligaments

• Patellar ligament — continuation of the quadriceps tendon from apex of patella to tibial tuberosity; site of the knee-jerk reflex (L3–L4).
• Oblique popliteal ligament — expansion of the semimembranosus tendon reinforcing the capsule posteriorly; pierced by the middle genicular vessels and posterior division of obturator nerve.
• Arcuate popliteal ligament — arches over the popliteus tendon posterolaterally.
• Transverse ligament — joins the anterior horns of the two menisci.

Menisci (semilunar cartilages)

The menisci are C-shaped fibrocartilaginous wedges that deepen the tibial articular surface, distribute load, absorb shock and aid lubrication. Each is thick peripherally, thin centrally.

Feature	Medial meniscus	Lateral meniscus
Shape	Larger, broader C (semicircular)	Smaller, nearly circular (O-shaped)
Mobility	Less mobile (fixed to MCL & capsule)	More mobile
Attachments	Fused with MCL and capsule	Separated from LCL by popliteus tendon; gives slip to popliteus
Injury frequency	Far more commonly torn	Less commonly torn

High-yield: Medial meniscus = larger, less mobile, more frequently injured. Lateral meniscus = smaller, more mobile, protected by popliteus. The classic exam line: "tightly adherent to MCL" → medial meniscus.

Blood supply: Only the peripheral outer ⅓ (red–red zone) is vascular (from genicular arteries); the inner ⅔ (white zone) is avascular and heals poorly — hence peripheral tears can be repaired but central tears are usually excised. Vascular supply to the knee overall is the genicular anastomosis around the joint, formed by branches of the femoral, popliteal (5 genicular branches), and anterior/posterior tibial arteries.

The locking and unlocking mechanism

In full extension the femur medially rotates on the fixed tibia (or the tibia laterally rotates on the femur in the open chain) during the last few degrees — the "screw-home" mechanism. This locks the knee into its close-packed, most stable position with minimal muscular effort, important for prolonged standing. Locking is helped by the vastus medialis.

To begin flexion the knee must first be unlocked, and this is the job of the popliteus:

Popliteus contracts → laterally rotates the femur on the fixed tibia (unlocks) → ACL relaxes → flexion can begin.

High-yield: Popliteus is the "key that unlocks the knee." It originates from the lateral femoral condyle (intracapsular, below LCL) and inserts on the posterior tibia above the soleal line. Acting from the tibia it medially rotates the leg; the popliteus also retracts the lateral meniscus during flexion, protecting it.

Numbered flow of standing → walking:

1. Standing: knee fully extended, locked (screw-home), quadriceps relaxed, energy-efficient.
2. To walk: popliteus fires → lateral rotation of femur (unlocking).
3. ACL slackens, knee flexes via hamstrings.
4. Quadriceps re-extend and re-lock at heel strike.

Bursae around the knee

There are numerous bursae; those that communicate with the joint cavity and those linked to clinical eponyms are tested.

Bursa	Location	Communicates with joint?	Clinical eponym
Suprapatellar	Between femur & quadriceps tendon	Yes (an extension of the cavity)	Effusion fills here
Popliteus bursa	Beneath popliteus tendon	Yes	—
Gastrocnemius (semimembranosus) bursa	Posteromedial	Often yes	Baker's (popliteal) cyst
Prepatellar	Subcutaneous, over patella	No	"Housemaid's knee" (kneeling upright)
Superficial infrapatellar	Subcutaneous, over patellar ligament	No	"Clergyman's knee" (kneeling erect)
Deep infrapatellar	Between patellar ligament & tibia	No	—

High-yield: Prepatellar bursitis = housemaid's knee; (superficial) infrapatellar bursitis = clergyman's knee. The suprapatellar bursa communicates with the joint, so a knee effusion produces a fluctuant swelling above the patella and a positive patellar tap.

High-yield: A Baker's (popliteal) cyst is a distension of the gastrocnemius–semimembranosus bursa, often secondary to osteoarthritis or rheumatoid arthritis; rupture can mimic deep vein thrombosis (pseudothrombophlebitis).

The unhappy triad of O'Donoghue

A lateral (valgus) blow to the abducted, flexed, laterally-rotated knee — classically a clipping tackle in football — produces the famous triad:

1. Tibial (medial) collateral ligament tear
2. Medial meniscus tear (because it is attached to the MCL)
3. Anterior cruciate ligament tear

High-yield: Classic unhappy triad = MCL + medial meniscus + ACL. (Some texts substitute the lateral meniscus because the impact loads the lateral compartment, but the traditional NEET PG answer is the medial meniscus, owing to its MCL attachment.)

Clinical features & examination tests (anatomy basis)

• McMurray test — knee fully flexed, then extended while the leg is rotated. A palpable/audible click suggests a meniscal tear. External rotation + extension tests the medial meniscus; internal rotation tests the lateral meniscus. (Reason: rotation traps the torn fragment of the opposite-side meniscus against the condyle.)
• Apley grind test — patient prone, knee 90°; compression + rotation = meniscal pain; distraction + rotation = ligament pain.
• Anterior drawer / Lachman — ACL integrity (Lachman at 20–30° flexion is the most sensitive).
• Posterior drawer / posterior sag — PCL integrity.
• Valgus stress test — MCL; Varus stress test — LCL.
• Patellar tap / bulge sign — effusion in the suprapatellar pouch.

Popliteal fossa — the crossover star

The diamond-shaped popliteal fossa is consistently asked alongside knee anatomy.

Boundaries:

• Superomedial — semimembranosus & semitendinosus.
• Superolateral — biceps femoris.
• Inferomedial & inferolateral — medial and lateral heads of gastrocnemius (plantaris laterally).
• Roof — skin & fascia (pierced by small saphenous vein).
• Floor — popliteal surface of femur, oblique popliteal ligament, popliteus fascia.

Contents — superficial to deep (the key arrangement): the artery is deepest, the nerve most superficial.

Tibial nerve (most superficial) → popliteal vein → popliteal artery (deepest, on the floor).

Mnemonic: "NerVe–Vein–Artery from superficial to deep" (or remember the artery hugs the bone, so it is deepest and most protected).

High-yield: Order from superficial (skin) to deep (bone): tibial nerve → popliteal vein → popliteal artery. The common peroneal (fibular) nerve runs along the medial border of biceps femoris/tendon, then winds around the neck of the fibula — the most commonly injured nerve in the lower limb (foot drop, loss of eversion & dorsiflexion).

The popliteal artery is the continuation of the femoral artery (from the adductor hiatus) and gives 5 genicular branches forming the periarticular anastomosis; it ends at the lower border of popliteus by dividing into anterior and posterior tibial arteries. It is the deepest structure, hence a popliteal aneurysm presents as a pulsatile swelling, and popliteal artery injury is feared in posterior knee dislocation.

Nerve & blood supply (Hilton's law in action)

• Innervation: femoral, obturator, tibial and common peroneal nerves — illustrating Hilton's law (nerves crossing a joint supply the muscles moving it and the skin over them).
• Arterial supply: genicular anastomosis from descending genicular (femoral), 5 genicular branches of popliteal, descending branch of lateral circumflex femoral, and recurrent branches of the tibial arteries.

Complications & applied points

• ACL deficiency → rotational instability, "giving way," secondary meniscal and chondral damage, early osteoarthritis.
• Meniscal tears → locking, effusion, joint-line tenderness; bucket-handle tear may block extension.
• Posterior knee dislocation → popliteal artery and common peroneal nerve at risk (always check distal pulses).
• Genu varum / valgum → altered load on compartments → unicompartmental osteoarthritis.
• Common peroneal nerve palsy (fibular neck) → foot drop, sensory loss over dorsum of foot and lateral leg.

Key differentials of a swollen/painful knee

Presentation	Likely diagnosis	Anatomical/diagnostic clue
Acute haemarthrosis after pivot injury	ACL tear	Positive Lachman; lipohaemarthrosis on X-ray suggests fracture
Locking, joint-line tenderness	Meniscal tear	Positive McMurray/Apley; MRI is investigation of choice
Anterior knee swelling on kneeling	Prepatellar bursitis	Swelling over patella, joint line spared
Posteromedial cystic swelling	Baker's cyst	Trans-illumination negative; USG confirms
Diffuse effusion with valgus laxity	MCL injury ± triad	Valgus stress test positive

Investigation of choice for soft tissue (ligament/meniscus) = MRI knee. Plain X-ray (AP, lateral, skyline) is first for bony injury, effusion (lipohaemarthrosis), and avulsions (Segond fracture of lateral tibia → highly associated with ACL tear).

Recently asked / exam angle

• Which femoral condyle gives attachment to the ACL? → Lateral femoral condyle (medial surface). PCL → medial femoral condyle. (Repeatedly asked; mnemonic LAMP.)
• Strongest ligament of the knee? → Posterior cruciate ligament.
• Which meniscus is more commonly injured and why? → Medial — larger and fixed to the MCL, hence less mobile.
• Muscle that unlocks the knee? → Popliteus.
• Unhappy triad components? → MCL, medial meniscus, ACL.
• Contents of popliteal fossa superficial to deep? → Tibial nerve, popliteal vein, popliteal artery.
• Bursa communicating with the joint cavity? → Suprapatellar bursa.
• Housemaid's knee? → Prepatellar bursitis. Clergyman's knee? → Superficial infrapatellar bursitis.
• Nerve winding around neck of fibula / most commonly injured in lower limb? → Common peroneal (fibular) nerve.
• McMurray external rotation tests which meniscus? → Medial meniscus.
• Sesamoid bone in quadriceps tendon? → Patella (largest sesamoid).

Rapid revision

1. Knee = largest synovial joint; modified hinge / condylar type; fibula not part of it.
2. Cruciates named by tibial attachment; ACL → lateral femoral condyle, PCL → medial femoral condyle (LAMP).
3. ACL prevents anterior tibial slide (taut in extension); PCL prevents posterior tibial slide (taut in flexion, strongest ligament).
4. MCL is flat, fused to capsule and medial meniscus; LCL is cord-like and free (separated by popliteus tendon).
5. Medial meniscus = larger, less mobile, more often torn; lateral = smaller, mobile, protected by popliteus.
6. Only the peripheral red zone (outer ⅓) of meniscus is vascular and can heal.
7. Popliteus unlocks the knee by laterally rotating the femur on the fixed tibia; vastus medialis aids locking (screw-home).
8. Unhappy triad = MCL + medial meniscus + ACL (valgus, flexion, lateral rotation injury).
9. Suprapatellar bursa communicates with the joint → site of effusion & patellar tap.
10. Prepatellar = housemaid's knee; superficial infrapatellar = clergyman's knee; Baker's cyst = gastrocnemius–semimembranosus bursa.
11. Popliteal fossa contents superficial→deep: tibial nerve → popliteal vein → popliteal artery (artery deepest).
12. Common peroneal nerve winds around fibular neck → foot drop; MRI is the investigation of choice for ligament/meniscal injury.