The Knee Complex

💡 Core Concept: The Knee is a "Double Condyloid" joint designed to sustain massive weight-bearing loads while allowing large ranges of mobility. It is functionally divided into two articulations: the Tibiofemoral Joint and the Patellofemoral Joint.

1. Structure of the Tibiofemoral Joint

A. Alignment

• Normal Alignment: The femur angles inward (170-175°), creating a physiological Genu Valgum.
• Mechanical Axis: Weight-bearing line passes from the femoral head to the center of the talus. It passes slightly medial to the knee joint center, causing greater compression on the medial compartment.

B. The Menisci

Fibrocartilage discs that deepen the joint, absorb shock, and distribute weight.

[Image of knee meniscus anatomy superior view]

• Medial Meniscus: "C" shaped. firmly attached to the MCL and capsule. (Less mobile, more prone to injury).
• Lateral Meniscus: "O" shaped. Loosely attached. More mobile.
• Vascularity:
  • Red-Red Zone (Outer 1/3): Vascularized, can heal.
  • White-White Zone (Inner 1/3): Avascular, cannot heal (requires meniscectomy).

C. Ligaments (Cruciate & Collateral)

Ligament	Function (Checks)	Mechanism of Injury
ACL	Anterior translation of Tibia on Femur.	Hyperextension, Valgus + Rotation (Plant and twist).
PCL	Posterior translation of Tibia on Femur.	"Dashboard Injury" (Direct blow to proximal tibia when knee is flexed).
MCL	Valgus Stress + External Rotation.	Blow to lateral knee (Valgus force).
LCL	Varus Stress.	Blow to medial knee (Varus force).

2. Tibiofemoral Kinematics

A. The "Screw Home" Mechanism

Locking of the knee in full extension requires rotation due to the larger surface area of the medial femoral condyle.

[Image of Screw Home Mechanism knee biomechanics]

• Open Chain (Tibia moving): Tibia rotates Externally on Femur during the last 30° of extension.
• Closed Chain (Femur moving): Femur rotates Internally on Tibia.
• Unlocking: The Popliteus muscle initiates flexion by rotating the femur externally (or tibia internally) to unlock the knee.

3. The Patellofemoral Joint (PFJ)

A. Function

• The Patella acts as an Anatomic Pulley.
• It increases the Moment Arm of the Quadriceps muscle, increasing extension torque.

B. Joint Reaction Forces (JRF)

Compression forces on the patella vary greatly depending on the exercise:

Activity	Angle of Max Stress	Clinical Implication
Open Chain (Leg Ext)	0° to 30° Flexion	Avoid terminal extension exercises in PFJ pain patients.
Closed Chain (Squat)	60° to 90° Flexion	Avoid deep squats in PFJ pain patients. Safe zone is 0-45°.

C. Patellar Tracking & The Q-Angle

The patella has a natural tendency to track Laterally due to the pull of the Quads and the Q-Angle.

• Q-Angle: Line from ASIS → Midpoint of Patella → Tibial Tuberosity. Normal is 10-15°. Increased Q-angle increases lateral bowstringing force.
• Stabilizers:
  • Dynamic: VMO (Vastus Medialis Obliquus).
  • Static: Lateral Femoral Condyle (bony buttress) and Medial Retinaculum.

[Image of Q angle measurement knee]

⚠️ The "Unhappy Triad" (O'Donoghue's): A severe injury resulting from a valgus force applied to a planted foot with external rotation. Traditionally involves injury to the: 1. ACL 2. MCL 3. Medial Meniscus

🏆 Key Points: Clinical Biomechanics

• Menisectomy: Removal of the meniscus increases joint contact stress by 200-300%, leading to rapid Osteoarthritis.
• Active Insufficiency: The Hamstrings cannot produce full force when the hip is extended and the knee is flexed (shortened at both ends).
• Genu Recurvatum: Hyperextension of the knee. Often due to weak quadriceps or tight plantarflexors.

📝 20 High-Yield MCQs

Test your knowledge for Exams.

Q1. The "Screw Home Mechanism" in Open Kinetic Chain involves:

A. External Rotation of the Tibia on the Femur B. Internal Rotation of the Tibia on the Femur C. External Rotation of the Femur on the Tibia D. Internal Rotation of the Femur on the Tibia

Rationale: In open chain extension (kicking a ball), the concave tibia rolls/slides anteriorly and spins externally to lock.

Q2. Which part of the Meniscus has the potential to heal if torn?

A. Inner 1/3 (White-White) B. Middle 1/3 C. Outer 1/3 (Red-Red) D. None

Rationale: Only the outer periphery has a direct blood supply from the genicular arteries.

Q3. The primary function of the Patella is to:

A. Increase the moment arm of the Quadriceps B. Protect the ACL C. Prevent knee hyperextension D. Connect tibia to femur

Rationale: By displacing the tendon away from the joint axis, it increases the leverage (moment arm) of the quads.

Q4. Which ligament attaches to the Medial Meniscus?

A. Medial Collateral Ligament (MCL) B. Lateral Collateral Ligament (LCL) C. PCL D. Iliotibial Band

Rationale: The deep fibers of the MCL attach to the medial meniscus, which is why they are often injured together.

Q5. A "Dashboard Injury" (Direct blow to proximal tibia) typically ruptures the:

A. ACL B. PCL C. MCL D. Patellar Tendon

Rationale: A posterior force on the tibia pushes it backward relative to the femur, stressing/tearing the PCL.

Q6. The Closed Packed Position of the knee is:

A. Full Extension B. 90 degrees Flexion C. 25 degrees Flexion D. Full Flexion

Rationale: In full extension, the ligaments are taut, and the screw home mechanism locks the bony anatomy.

Q7. Which muscle is responsible for "unlocking" the knee from full extension?

A. Biceps Femoris B. Popliteus C. Gastrocnemius D. Semimembranosus

Rationale: The Popliteus internally rotates the tibia (or externally rotates the femur) to reverse the screw home mechanism.

Q8. During Closed Kinetic Chain exercises (e.g., Squat), Patellofemoral compression forces are highest at:

A. 90 degrees flexion B. 0-30 degrees flexion C. Full extension D. Compressive forces are constant

Rationale: As the knee flexes deeply, the quads pull harder and the angle of pull presses the patella firmly into the trochlear groove.

Q9. An increased Q-Angle results in a tendency for the patella to track:

A. Medially B. Laterally C. Superiorly D. Inferiorly

Rationale: A large Q-angle increases the lateral "bowstring" force vector on the patella, predisposing to dislocation or PFPS.

Q10. The Anterior Cruciate Ligament (ACL) primarily checks:

A. Anterior translation of Tibia on Femur B. Posterior translation of Tibia on Femur C. Varus stress D. Pure Flexion

Rationale: The ACL prevents the tibia from sliding forward out from under the femur.

Q11. Genu Valgum is commonly known as:

A. Bow Legs B. Knock Knees C. Hyperextended Knees D. Patella Alta

Rationale: Valgum = Distal segment goes lateral (knees touch). Varum = Distal segment goes medial (Bow legs).

Q12. The lateral meniscus is shaped like an:

A. O (Circle) B. C (Semicircle) C. L D. Triangle

Rationale: The Lateral meniscus is nearly circular (O-shaped), while the Medial is C-shaped.

Q13. Which muscle group acts as a dynamic restraint to anterior tibial translation (assisting the ACL)?

A. Quadriceps B. Hamstrings C. Adductors D. Iliopsoas

Rationale: The Hamstrings pull the tibia posteriorly, counteracting the anterior shear force. Quads pull the tibia anteriorly (stressing the ACL).

Q14. The Pes Anserinus is the insertion point for which three muscles?

A. Sartorius, Gracilis, Semitendinosus B. Semimembranosus, Biceps Femoris, Gracilis C. Rectus Femoris, Vastus Medialis, Sartorius D. Glute Max, TFL, Piriformis

Rationale: "Say Grace Before Tea" (Sartorius, Gracilis, Semitendinosus) attach at the medial proximal tibia.

Q15. Which structure provides static resistance to Lateral Patellar tracking?

A. Vastus Lateralis B. Lateral Femoral Condyle (Bony Buttress) C. Medial Meniscus D. PCL

Rationale: The lateral femoral condyle projects more anteriorly than the medial, creating a bony wall that stops the patella from sliding off laterally.

Q16. During meniscal tear surgery, removal of the meniscus (meniscectomy) results in:

A. Decreased contact area and increased joint stress B. Increased contact area and decreased stress C. Increased stability D. Better shock absorption

Rationale: Without the meniscus, load is concentrated on a smaller area of cartilage, dramatically increasing pressure (Stress = Force/Area).

Q17. Active Insufficiency of the Hamstrings occurs when:

A. Hip is Flexed and Knee is Extended B. Hip is Extended and Knee is Flexed C. Hip is Flexed and Knee is Flexed D. Standing

Rationale: The muscle is shortened across both joints simultaneously, losing the ability to generate tension.

Q18. The Lateral Collateral Ligament (LCL) is unique because:

A. It is cord-like and not attached to the capsule or meniscus B. It attaches to the lateral meniscus C. It is broad and flat D. It checks valgus stress

Rationale: The LCL is extracapsular and separated from the lateral meniscus by the Popliteus tendon.

Q19. For Open Kinetic Chain exercises (e.g., Knee Extension machine), PFJ stress is LOWEST at:

A. 0 degrees (Full extension) B. 90 degrees flexion C. 30 degrees flexion D. It is always high

Rationale: In open chain, the patella barely contacts the femur at 90 degrees. As the leg extends (0-30), contact area decreases but quad force increases, causing max stress/pain.

Q20. Functional knee flexion required for normal walking (Swing phase) is approximately:

A. 90 degrees B. 60 degrees C. 20 degrees D. 120 degrees

Rationale: You need about 60-65 degrees of flexion to clear the foot during the swing phase of gait. Stairs require 80-90 degrees.