The knee is the largest synovial joint in the body and functions as a modified hinge joint. It primarily allows flexion and extension, with small degrees of rotation when flexed. Although superficially simple, it is biomechanically complex, relying heavily on ligaments and surrounding muscles for stability rather than bony congruence.

🦴 Articulating Bones

Femur – medial and lateral condyles.
Tibia – tibial plateau (weight-bearing surface).
Patella – sesamoid bone within quadriceps tendon; improves mechanical advantage of knee extension.

🧵 Menisci

Medial meniscus – C-shaped, less mobile, attached to MCL (more commonly injured).
Lateral meniscus – more circular, more mobile.
Functions:
- Load distribution and shock absorption.
- Joint stability and proprioception.

🛡️ Ligaments (Primary Stabilizers)

Intracapsular (Cruciate Ligaments)

ACL – prevents anterior translation of tibia on femur; taut in extension.
PCL – prevents posterior translation of tibia; stronger than ACL.

Extracapsular

MCL – resists valgus stress; attached to medial meniscus.
LCL – resists varus stress; not attached to lateral meniscus.
Patellar ligament – continuation of quadriceps tendon to tibial tuberosity.
Oblique popliteal ligament – posterior reinforcement.
Arcuate ligament complex – posterolateral stability.

💪 Muscles Acting on the Knee

Anterior Compartment (Extension)

Quadriceps femoris (rectus femoris, vastus medialis, vastus lateralis, vastus intermedius) – primary extensors; femoral nerve.

Posterior Compartment (Flexion)

Hamstrings (biceps femoris, semitendinosus, semimembranosus) – flex knee; sciatic nerve.
Gastrocnemius – assists flexion; tibial nerve.
Popliteus – “unlocks” knee from extension; tibial nerve.

🧵 Important Tendons

Quadriceps tendon (above patella).
Patellar tendon (below patella).
Pes anserinus (sartorius, gracilis, semitendinosus insertion).
Iliotibial band (lateral stabiliser).

🩸 Blood Supply

Genicular branches of the popliteal artery form periarticular anastomosis.
Descending genicular artery (from femoral artery).

⚡ Nerve Supply

Femoral nerve – anterior capsule.
Tibial nerve – posterior capsule.
Common peroneal nerve – lateral aspect.
Obturator nerve – medial contribution.

🔄 Biomechanics

The knee exhibits a “screw-home” mechanism: during the last 20° of extension, the tibia externally rotates relative to the femur, locking the joint. Popliteus reverses this during flexion. Stability is greatest in full extension due to ligament tension.

⚠️ Clinical Correlations

ACL tear – pivoting injury; positive Lachman test.
PCL injury – dashboard injury.
Meniscal tear – locking/catching sensation.
MCL injury – valgus stress injury.
Osteoarthritis – medial c

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Knee Joint Structure and Form