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The knee joint primarily involves three bones:
– Femur (thighbone)
– Tibia (shinbone)
– Patella (kneecap)
These bones form two main articulations:
a) Tibiofemoral joint: Between the femur and tibia b) Patellofemoral joint: Between the patella and femur
The fibula, while not directly involved in the knee joint, plays a role in stabilizing the lateral aspect of the knee.
The knee contains two C-shaped fibrocartilaginous structures called menisci:
– Medial meniscus: Larger and less mobile
– Lateral meniscus: Smaller and more mobile
Functions of the menisci include:
– Shock absorption
– Load distribution
– Joint stabilization
– Improved congruency between femoral and tibial surfaces
The knee is stabilized by four main ligaments:
a) Cruciate Ligaments (intracapsular but extrasynovial):
– Anterior Cruciate Ligament (ACL): Prevents anterior translation of the tibia on the femur
– Posterior Cruciate Ligament (PCL): Prevents posterior translation of the tibia on the femur
b) Collateral Ligaments:
– Medial (Tibial) Collateral Ligament (MCL): Provides medial stability
– Lateral (Fibular) Collateral Ligament (LCL): Provides lateral stability
Additional ligamentous structures include:
– Popliteal ligaments
– Arcuate ligament complex
– Oblique popliteal ligament
Numerous bursae surround the knee joint, reducing friction between moving structures. Key bursae include:
– Suprapatellar bursa
– Prepatellar bursa
– Infrapatellar bursa
– Pes anserine bursa
Several muscle groups act on the knee joint:
a) Quadriceps femoris:
– Rectus femoris
– Vastus lateralis
– Vastus medialis
– Vastus intermedius
These muscles converge to form the quadriceps tendon, which inserts onto the patella and continues as the patellar tendon to attach to the tibial tuberosity.
b) Hamstrings:
– Biceps femoris
– Semitendinosus
– Semimembranosus
c) Gastrocnemius: The medial and lateral heads originate from the posterior femoral condyles.
d) Popliteus: Aids in “unlocking” the knee during flexion.
a) Arterial Supply:
– Genicular arteries (branches of the popliteal artery)
– Descending genicular artery (branch of the femoral artery)
– Anterior and posterior tibial recurrent arteries
– Circumflex fibular artery
b) Venous Drainage:
– Genicular veins
– Great saphenous vein
– Small saphenous vein
– Popliteal vein
c) Innervation:
– Femoral nerve (L2-L4)
– Obturator nerve (L2-L4)
– Sciatic nerve (L4-S3) – Common peroneal nerve (L4-S2)
– Tibial nerve (L4-S3)
a) Patella: The patella is a sesamoid bone embedded in the quadriceps tendon. It serves to:
– Increase the leverage of the quadriceps muscle
– Protect the anterior aspect of the knee joint
– Guide the quadriceps tendon during knee flexion and extension
The undersurface of the patella has a vertical ridge that articulates with the trochlear groove of the femur, forming the patellofemoral joint.
b) Femoral Condyles: The distal femur consists of two rounded condyles:
– Medial condyle: Larger and more prominent posteriorly
– Lateral condyle: Smaller and more prominent anteriorly
These condyles articulate with the tibial plateau and are separated posteriorly by the intercondylar notch.
c) Tibial Plateau: The proximal tibia forms the tibial plateau, which consists of:
– Medial tibial plateau: Larger and concave
– Lateral tibial plateau: Smaller and convex
The plateaus are separated by the intercondylar eminence, which serves as an attachment point for the cruciate ligaments.
d) Anterior Cruciate Ligament (ACL): The ACL originates from the posteromedial aspect of the lateral femoral condyle and inserts on the anterior intercondylar area of the tibia. It consists of two bundles:
– Anteromedial bundle: Taut in flexion
– Posterolateral bundle: Taut in extension
The ACL prevents anterior translation of the tibia on the femur and provides rotational stability.
e) Posterior Cruciate Ligament (PCL): The PCL originates from the anterolateral aspect of the medial femoral condyle and inserts on the posterior intercondylar area of the tibia. It also has two bundles:
– Anterolateral bundle: Taut in flexion
– Posteromedial bundle: Taut in extension
The PCL prevents posterior translation of the tibia on the femur and is the primary stabilizer of the knee.
f) Medial Collateral Ligament (MCL): The MCL is a broad, flat ligament on the medial aspect of the knee. It has two layers:
– Superficial MCL: From the medial femoral epicondyle to the medial tibial surface
– Deep MCL: From the medial femoral epicondyle to the medial meniscus and tibia
The MCL provides stability against valgus stress and external rotation of the tibia.
g) Lateral Collateral Ligament (LCL): The LCL is a cord-like structure on the lateral aspect of the knee. It runs from the lateral femoral epicondyle to the fibular head, providing stability against varus stress.
h) Menisci: The menisci are C-shaped fibrocartilaginous structures that sit on the tibial plateau. They are thicker at the periphery and thinner centrally, creating a concave surface for femoral condyle articulation.
Medial Meniscus:
– Less mobile due to attachments to the MCL
– More commonly injured due to its decreased mobility
Lateral Meniscus:
– More mobile
– Forms part of the posterolateral corner complex
The menisci serve several functions:
– Shock absorption
– Load distribution
– Joint lubrication
– Proprioception
– Limiting extreme flexion and extension
i) Posterolateral Corner (PLC): The PLC is a complex of structures that provide stability to the lateral and posterior aspects of the knee. Key components include:
– LCL
– Popliteus tendon
– Popliteofibular ligament
– Arcuate ligament
– Fabellofibular ligament (when present)
The PLC resists posterior translation, varus angulation, and external rotation of the tibia.
j) Quadriceps Mechanism: The quadriceps femoris muscle group is the primary extensor of the knee. It consists of four muscles:
– Rectus femoris: Originates from the anterior inferior iliac spine
– Vastus lateralis: Originates from the lateral intermuscular septum and linea aspera
– Vastus medialis: Originates from the medial intermuscular septum and linea aspera
– Vastus intermedius: Originates from the anterior femoral shaft
These muscles converge to form the quadriceps tendon, which envelops the patella and continues as the patellar tendon to insert on the tibial tuberosity.
k) Hamstring Complex: The hamstrings are the primary flexors of the knee and consist of three muscles:
– Biceps femoris (long and short heads): Inserts on the fibular head
– Semitendinosus: Forms part of the pes anserinus
– Semimembranosus: Inserts on the posteromedial tibia
These muscles also aid in rotation of the tibia when the knee is flexed.
The knee joint allows for several movements:
– Flexion and extension (primary movements)
– Internal and external rotation (when flexed)
– Slight abduction and adduction
During the gait cycle, the knee goes through several phases:
– Heel strike: Slight flexion for shock absorption
– Stance phase: Progressive extension
– Toe-off: Maximal extension
– Swing phase: Flexion to clear the ground
The “screw-home” mechanism occurs during terminal extension, where the tibia externally rotates on the femur, providing increased stability in full extension.
Understanding the complex anatomy of the knee is crucial for:
– Accurate diagnosis of knee injuries and conditions
– Proper interpretation of physical examination findings
– Appropriate selection and interpretation of imaging studies
– Effective planning of surgical interventions
– Development of targeted rehabilitation programs
Common knee pathologies include:
– Ligament injuries (ACL, PCL, MCL, LCL tears)
– Meniscal tears – Patellofemoral disorders
– Osteoarthritis
– Tendinopathies (patellar tendinopathy, quadriceps tendinopathy)
– Bursitis
The knee joint’s complex anatomy allows for its remarkable function in weight-bearing and mobility. The interplay between bones, ligaments, menisci, muscles, and other soft tissue structures provides stability while allowing for a range of motion necessary for daily activities and athletic performance. A thorough understanding of knee anatomy is fundamental for all aspects of knee care, from prevention and diagnosis to treatment and rehabilitation of knee disorders.
©2025 Dr Frank McCormick All Rights Reserved.
©2025 Dr Frank McCormick All Rights Reserved.
