Knee Anatomy: Structure, Ligaments, and Biomechanics

The knee is the largest and most complex joint in the human body, subjected daily to loads that can reach 3-4 times body weight during walking and up to 7-8 times during running or jumping. Its anatomy must simultaneously guarantee stability under load and mobility to allow locomotion, which makes it one of the joints most vulnerable to injuries and degenerative pathologies.

Understanding knee anatomy — from bones to ligaments, from menisci to cartilage, from muscles to biomechanics — is essential for anyone wishing to prevent injuries, understand a diagnosis, or consciously approach a rehabilitation pathway.

Table of Contents

• The Bones of the Knee
• The Menisci
• The Ligaments of the Knee
• Articular Cartilage
• The Muscles of the Knee
• Knee Biomechanics
• Biomechanics of the Patellofemoral Joint
• Frequently Asked Questions (FAQ)
• Conclusion
• Frequently Asked Questions
• Resources
• Sources and Scientific References

The Bones of the Knee

The knee joint comprises three bones—femur, tibia, and patella—that articulate to form the largest weight-bearing joint, located between the hip and ankle. The knee is formed by the articulation of three bones: the femur, tibia, and patella.

Femur

The femur is the longest and strongest bone in the body. Its distal (lower) end features two voluminous rounded prominences, the femoral condyles (medial and lateral), covered with cartilage. The femoral condyles articulate with the tibial plateaus and are separated posteriorly by the intercondylar fossa, through which the cruciate ligaments pass.

• The medial condyle is larger and extends more distally, contributing to the physiological valgus of the knee.
• The lateral condyle is wider in the sagittal plane and articulates with the lateral meniscus.

Anteriorly, the condyles converge into the femoral trochlea, the groove in which the patella slides.

Tibia

The tibia is the medial bone of the lower leg. Its proximal (upper) end widens to form the tibial plateaus (medial and lateral), two relatively flat surfaces that accommodate the femoral condyles. Between the two tibial plateaus are the intercondylar eminences (tibial spines), which contribute to joint stability and are the insertion point for the cruciate ligaments.

• The medial tibial plateau is concave and wider, offering greater joint congruity.
• The lateral tibial plateau is convex, less stable, and more dependent on the lateral meniscus for support.

The tibial tuberosity, an anterior bony prominence, is the insertion point of the patellar tendon and represents a fundamental clinical landmark.

Patella

The patella is the largest sesamoid bone in the body, located within the quadriceps femoris muscle tendon. Its posterior surface is covered by the thickest cartilage in the entire body (up to 7 mm) and articulates with the femoral trochlea, forming the patellofemoral joint.

The functions of the patella are:

• Increase the lever arm of the quadriceps by 30-50%, improving its mechanical efficiency in knee extension.
• Protect the anterior surface of the joint.
• Distribute compression forces over a wider area of the femoral trochlea.

The Menisci

The menisci are two crescent-shaped fibrocartilaginous structures interposed between the femoral condyles and the tibial plateaus. They are fundamental for knee function.

Medial Meniscus

• Open “C” shape, wider posteriorly.
• Adherent to the joint capsule and the medial collateral ligament, which makes it less mobile and more vulnerable to injury.
• Covers approximately 50% of the medial tibial plateau.

Lateral Meniscus

• Nearly circular “O” shape, more uniform.
• Less adherent to the capsule, more mobile (moves about 11 mm during flexion, compared to 5 mm for the medial meniscus).
• Covers approximately 70% of the lateral tibial plateau.

Functions of the Menisci

The menisci play multiple and fundamental roles:

• Load distribution: they increase the femorotibial contact area by 60-70%, reducing stress on the cartilage. Without the menisci, contact stress increases by 200-300%.
• Shock absorption: they dissipate impact forces during walking and running.
• Stability: they contribute to anteroposterior and rotational stability.
• Lubrication: they facilitate the distribution of synovial fluid.
• Proprioception: they contain mechanoreceptors that inform the nervous system about the joint’s position.

The vascularization of the menisci is limited to the peripheral third (red zone), while the central two-thirds (white zone) are avascular and nourished by diffusion from the synovial fluid. This distribution explains why peripheral lesions can heal, while central ones have poor reparative capacity.

The Ligaments of the Knee

Ligaments are the primary passive stabilizers of the knee. They are distinguished into cruciate ligaments (intra-articular) and collateral ligaments (extra-articular).

Anterior Cruciate Ligament (ACL)

The ACL is the main stabilizer against anterior tibial translation and controls rotation. It originates from the medial surface of the lateral femoral condyle and inserts into the anterior intercondylar area of the tibia.

• It is composed of two bundles: anteromedial (taut in flexion) and posterolateral (taut in extension).
• It ruptures most frequently with a valgus + rotation mechanism of the knee (typical in soccer, basketball, skiing).
• Its injury leads to rotational instability and increases the risk of meniscal lesions and cartilage degeneration over time.

Posterior Cruciate Ligament (PCL)

The PCL is the strongest ligament in the knee, about 30% more resistant than the ACL. It stabilizes the knee against posterior tibial translation. It originates from the lateral surface of the medial femoral condyle and inserts into the posterior intercondylar fossa of the tibia.

• It is typically injured by a direct blow to the tibia with the knee flexed (e.g., dashboard injury in a car accident).
• Isolated PCL injuries are often treated conservatively.

Medial Collateral Ligament (MCL)

The MCL is a fibrous band located on the inner side of the knee that resists valgus forces (medial opening). It consists of:

• Superficial layer: from the medial aspect of the medial femoral condyle to the proximal tibia, about 4-5 cm below the joint line.
• Deep layer: adherent to the capsule and the medial meniscus.

It is the most frequently injured ligament of the knee. Knee sprains often involve the MCL.

Lateral Collateral Ligament (LCL)

The LCL is a cord-like structure that resists varus forces (lateral opening). It originates from the lateral femoral epicondyle and inserts onto the head of the fibula.

• Unlike the MCL, it does not adhere to the capsule or the lateral meniscus.
• Its injuries are less frequent and often associated with high-energy trauma.

Articular Cartilage

Hyaline cartilage covers the articular surfaces of the femur, tibia, and patella with a variable thickness from 2 to 7 mm (maximum on the posterior surface of the patella). Its functions are:

• Reduce friction: the coefficient of friction of articular cartilage is lower than that of ice on ice.
• Absorb and distribute loads: its viscoelastic structure allows it to deform under load and recover its shape.
• Allow movement: the smooth surface permits the gliding of articular surfaces.

Cartilage is avascular, aneural, and alymphatic: it is nourished by diffusion from the synovial fluid and has an extremely limited capacity for regeneration. This explains why cartilage lesions and knee osteoarthritis are so difficult to treat.

The Muscles of the Knee

The muscles acting on the knee are the main dynamic stabilizers and motors of movement.

Quadriceps Femoris

The quadriceps is the largest muscle in the body, composed of four heads:

• Rectus femoris: the only biarticular head, originates from the anterior inferior iliac spine and flexes the hip in addition to extending the knee.
• Vastus lateralis: the largest head, on the lateral aspect of the femur.
• Vastus medialis: on the medial aspect of the femur. Its oblique bundle (vastus medialis obliquus or VMO) is crucial for patellar stabilization, counteracting the tendency for lateralization.
• Vastus intermedius: the deepest head, on the anterior aspect of the femur.

The four heads converge into the quadriceps tendon which inserts onto the superior pole of the patella, continues as the patellar tendon, and inserts onto the tibial tuberosity.

The quadriceps is the prime mover of knee extension and the main dynamic stabilizer during standing and walking. Its weakness or imbalance (especially of the VMO) is associated with patellar chondropathy and patellofemoral pain.

Hamstrings

The hamstrings occupy the posterior region of the thigh and are the main knee flexors:

• Biceps femoris: long head (biarticular) and short head. It inserts onto the head of the fibula. It is also an external rotator of the tibia.
• Semitendinosus: biarticular, inserts onto the medial aspect of the proximal tibia (pes anserinus). Internal rotator of the tibia.
• Semimembranosus: biarticular, inserts onto the medial tibial condyle. Internal rotator.

The hamstrings also play a synergistic role with the ACL, limiting anterior tibial translation. Their relative weakness compared to the quadriceps (hamstring/quadriceps ratio less than 0.6) is a risk factor for ACL injury.

Calf Muscles

• Gastrocnemius: biarticular, originates from the femoral condyles and inserts onto the calcaneus via the Achilles tendon. It flexes the knee and plantarflexes the ankle.
• Popliteus: a deep muscle in the popliteal fossa, it is an internal rotator of the tibia and a posterolateral stabilizer of the knee. It “unlocks” the knee from full extension.

Pes Anserinus

The pes anserinus is the common insertion point on the proximal medial tibia for three muscles: sartorius, gracilis, and semitendinosus. It contributes to medial stability and internal rotation. Its bursa can become inflamed, causing pes anserinus bursitis.

Knee Biomechanics

Flexion and Extension

The knee is primarily a modified hinge joint that primarily allows flexion (0°-140°) and extension. The active range of flexion is about 140°, while the passive range can reach 160°.

The movement is not a simple rotation on a fixed axis, but a complex mechanism of rolling-gliding (roll-back):

• At the beginning of flexion (0°-20°), the femoral condyle rolls posteriorly on the tibial plateau.
• In advanced flexion (beyond 20°), anterior gliding of the condyle on the tibia predominates to prevent the femur from “falling off” the tibia posteriorly.
• This combined mechanism allows a wide range of motion while maintaining articular contact.

Axial Rotation

The knee allows axial rotation of the tibia on the femur, possible only when the knee is flexed (when the collateral ligaments are relaxed):

• External rotation: approximately 40° at 90° of flexion.
• Internal rotation: approximately 30° at 90° of flexion.
• In full extension: rotation is almost zero, as the collateral and cruciate ligaments are taut (locked position or “close-packed position”).

“Screw-Home” Mechanism

During the last 20° of extension, the tibia rotates externally by about 10-15° relative to the femur. This mechanism, called screw-home, “locks” the joint in full extension, providing maximum stability for standing, with minimal muscular energy expenditure. The popliteus muscle is responsible for “unlocking” the knee by internally rotating the tibia at the beginning of flexion.

The Q-Angle

The Q-angle (quadriceps angle) is the angle formed by:

• A line from the anterior superior iliac spine to the center of the patella.
• A line from the center of the patella to the tibial tuberosity.

An increased Q-angle (frequent in women due to a wider pelvis) generates a lateralizing force vector on the patella, predisposing to:

• Patellar chondropathy and patellofemoral pain
• Subluxation or patellar dislocation
• Alterations in patellar tracking

Compression Forces

The forces crossing the knee vary enormously depending on the activity:

These forces explain why the knee is so susceptible to overuse pathologies and degenerative osteoarthritis.

Biomechanics of the Patellofemoral Joint

The patella slides in the femoral trochlea during flexion-extension, with a vertical movement of about 7-8 cm. The patellofemoral compression force progressively increases with flexion:

• 0° of flexion: almost no compression force.
• 45° of flexion: the force equals body weight.
• 90° of flexion: the force reaches 3-4 times body weight.
• Deep flexion (full squat): the force can reach 7-8 times body weight.

This explains why activities with a flexed knee (stairs, squats, crouching position) are the most painful in the presence of patellar chondropathy or knee osteoarthritis.

Frequently Asked Questions (FAQ)

How many bones make up the knee?
The knee is formed by three bones: the femur, tibia, and patella. The fibula, although nearby, does not directly participate in the knee joint.

What is the difference between the medial and lateral meniscus?
The medial meniscus is “C”-shaped, more adherent to the capsule, and less mobile, making it more vulnerable to injury. The lateral meniscus is “O”-shaped, more mobile, and covers a larger tibial surface.

What is the ACL and why does it rupture so often?
The anterior cruciate ligament (ACL) stabilizes the knee against anterior tibial translation and rotation. It frequently ruptures during movements involving changes of direction, deceleration, or landing from a jump, typical of sports like soccer, basketball, and skiing.

What is the Q-angle and why is it important?
The Q-angle is the quadriceps angle, formed by the axis of the anterior superior iliac spine, the center of the patella, and the tibial tuberosity. An increased Q-angle (over 20°), more frequent in women, increases the lateral force on the patella and predisposes to patellofemoral pain.

Why does the knee “creak”?
Joint crepitus can result from the formation of bubbles in the synovial fluid, the sliding of tendons over bony prominences, or cartilage roughness. If not accompanied by pain or swelling, they are generally harmless.

How much load does the knee bear?
Forces on the knee vary from 2-3 times body weight during walking up to 7-8 times during activities like running or deep squats. These enormous forces explain the knee’s vulnerability to wear and tear and osteoarthritis.

Conclusion

The knee’s anatomy reveals a sophisticated engineering structure, where stability and mobility are balanced through an integrated system of bones, menisci, ligaments, cartilage, and muscles. Knowledge of biomechanics — from the roll-back mechanism to the screw-home, from the Q-angle to compression forces — is essential for understanding knee pathologies and for setting evidence-based prevention and rehabilitation strategies.

In case of knee pain, swelling, instability, or functional limitation, it is advisable to consult your doctor or physical therapist.

Frequently Asked Questions

Why is understanding knee anatomy crucial for injury prevention and rehabilitation?

A comprehensive understanding of the knee’s complex anatomy, encompassing its bones, ligaments, menisci, and cartilage, is fundamental for recognizing potential vulnerabilities. This knowledge is vital for implementing effective injury prevention strategies and for developing a targeted and successful rehabilitation plan.

What are the primary structures responsible for knee joint stability?

Knee joint stability is maintained by a sophisticated interaction of its ligaments, menisci, and surrounding muscles. The cruciate and collateral ligaments provide static stability by limiting excessive movement, while the menisci distribute loads and the quadriceps and hamstrings offer dynamic support.

What is the role of the menisci in knee function?

The menisci, comprising the medial and lateral structures, are crucial C-shaped cartilages within the knee joint. They function as shock absorbers, distributing compressive forces across the joint, and also contribute to joint lubrication and overall stability.

How does the patella contribute to knee movement and protection?

The patella, or kneecap, significantly enhances the mechanical advantage of the quadriceps muscles, thereby increasing their efficiency during leg extension. Furthermore, it acts as a protective shield for the anterior knee joint, helping to dissipate forces and reduce friction during movement.

For a broader overview of related conditions, see our complete guide to knee pain.

Sources and Scientific References

1. Meyer JJ et al. (2017). Interprofessional approach for teaching functional knee joint anatomy. Ann Anat. 210:155-159. DOI | PubMed
2. Sharma L (2021). Osteoarthritis of the Knee. N Engl J Med. 384:51-59. DOI | PubMed
3. Neuhaus C et al. (2021). A systematic review on conservative treatment options for OSGOOD-Schlatter disease. Phys Ther Sport. 49:178-187. DOI | PubMed
4. Dragoo JL et al. (2012). Evaluation and treatment of disorders of the infrapatellar fat pad. Sports Med. 42:51-67. DOI | PubMed
5. Cheuy VA et al. (2017). Arthrofibrosis Associated With Total Knee Arthroplasty. J Arthroplasty. 32:2604-2611. DOI | PubMed