Knee Biomechanics: Complete Overview

1. Bony Architecture & Articular Geometry

• Medial tibiofemoral compartment
• Lateral tibiofemoral compartment
• Patellofemoral joint

2. Normal Range of Motion

• A few degrees of passive axial rotation are possible at 30° of flexion
• Knee motion is complex due to a changing (polycentric) instant center of rotation

3. Kinematics: The Four-Bar Linkage Model

• The four bars are: ACL, PCL, femur (between femoral insertions), and tibia (between tibial insertions)
• The intersection of the cruciate ligaments defines the instantaneous center of joint rotation
• As the knee flexes, this center moves posteriorly, producing coupled rolling and gliding at the articular surfaces

Rolling vs. Sliding

4. Posterior Femoral Rollback

• As the knee flexes, the tibiofemoral contact point moves posteriorly — this is posterior femoral rollback
• Rollback increases maximum knee flexion by preventing posterior impingement of the femoral condyle on the posterior tibial plateau
• Normal rollback requires an intact PCL
• In cruciate-sacrificing TKA, PCL sacrifice compromises rollback; this must be compensated by prosthetic cam-post mechanisms (see Section 8)
• Lateral femoral condyle rolls back more than the medial condyle

5. The Screw-Home Mechanism: Locking & Unlocking

Locking (Terminal Extension — "Screwing Home")

• During the last 15° of extension, the femur rotates internally (or equivalently, the tibia rotates externally relative to the femur)
• This occurs because:
  • The medial femoral condyle has a larger radius of curvature (longer articular arc) than the lateral condyle
  • The medial condyle must travel a greater distance; the lateral condyle finishes its arc first, so the femur pivots internally about the medial condyle
  • The axis of rotation of the intact knee lies in the medial femoral condyle
• This rotation "locks" the joint in extension, creating a close-packed position — maximally congruent, stabilised without muscle effort
• The anterior cruciate ligament tightens progressively as extension is completed

Clinical implication: The knee is most stable in full extension. Significant ligamentous and bony injury is required to disrupt a fully extended knee.

Unlocking (Initiation of Flexion — Popliteus)

• To initiate flexion from full extension, the screw-home must be reversed
• The popliteus muscle is the "unlocking" muscle
• It internally rotates the tibia (or externally rotates the femur on a fixed tibia) by ~5°
• This "unlocks" the joint from its close-packed position, allowing flexion to begin
• The popliteofibular ligament assists by providing additional posterolateral restraint during this phase

6. Meniscal Biomechanics

• Circumferentially: to disperse hoop stresses (captured by vertical mattress sutures in repair)
• Radially: to resist longitudinal tearing
• Randomly at the surface: to disperse shear stresses from flexion

Load Transmission

• Menisci reduce peak contact stresses by 100–200%
• Resection of 75% of radial width produces contact stress increases equivalent to total meniscectomy

Meniscal Movement

• Menisci move anteriorly in extension and posteriorly in flexion
• Lateral meniscus excursion (~11 mm) is twice that of the medial meniscus (~5 mm) during a 120° arc
• The lateral meniscus has fewer soft-tissue attachments and is more mobile
• The posterior horn of the medial meniscus is a major secondary stabiliser against anterior tibial translation in ACL-deficient knees

7. Ligament Biomechanics

• Ligaments anterior to the flexion axis stretch with flexion; those posterior shorten
• ACL experiences peak loads of 170 N during walking, up to 500 N with running
• Reconstructed grafts must be placed within the flexion axis to maintain isometry through the full arc of motion

Directional Stabilisers

8. Patellofemoral Biomechanics

• The patellofemoral joint has a sliding articulation
• The patella slides 7 cm caudally with full flexion
• The instant center of the patellofemoral joint is near the posterior cortex above the condyles
• The patella increases the mechanical advantage of the quadriceps by increasing the moment arm of the extensor mechanism
• At 90° of flexion, patellofemoral compressive forces reach 3–5× body weight (e.g., stair climbing)
• Maltracking of the patella is related to the Q-angle (normal: 10–15° in males, 15–20° in females)

9. Lower Extremity Axes — Relevance to Arthroplasty

10. Application in Total Knee Arthroplasty (TKA)

Biomechanical Goals of TKA

1. Restore the mechanical axis — neutral alignment (0° of the mechanical axis through the joint centre)
2. Restore joint line — mismatch alters PCL tension, gap balance, and patellar tracking
3. Balanced flexion-extension gaps — symmetric gaps avoid laxity and prevent polyethylene wear
4. Restore posterior condylar offset — maintains flexion and rollback

Component Design and PCL Management

• Normal rollback is compromised by PCL sacrifice in cruciate-sacrificing TKA — the PS prosthetic cam-post mechanism replicates this function
• Functional range needed post-TKA: ≥90° for daily activities; ≥110° to rise from a chair

Rotational Alignment in TKA

• Femoral component should be externally rotated 3° relative to the posterior condylar axis (parallel to the epicondylar axis and Whiteside's line)
• Tibial component should be rotationally aligned to the tibial tuberosity (medial third)
• Malrotation leads to patellar maltracking, flexion instability, and accelerated polyethylene wear

• Lateral placement of femoral and tibial components
• Medial placement of the patellar component
• Avoidance of tibial internal rotation

11. Application in Knee Arthroscopy

Biomechanical Basis of Arthroscopic Decisions

• Total meniscectomy increases contact stresses by 100–200%
• Partial meniscectomy: even 75% radial width resection approximates total meniscectomy in stress effects
• The peripheral 20–30% of the medial meniscus and 10–25% of the lateral meniscus are vascularised — tears in the red-red zone have the highest healing potential and should be repaired

• Femoral tunnel must be placed at the anatomic footprint (posterior wall of the lateral femoral condyle) — isometric placement within the flexion-extension axis is critical
• Tibial tunnel should avoid roof impingement in extension
• Graft tensile strengths guide choice: patellar tendon graft ~2900 N; quadrupled hamstring ~4000 N

• Arthroscopy is typically performed in figure-of-four position or with knee at 70–90° flexion — after the popliteus has unlocked the knee
• Varus stress opens the medial compartment; valgus opens the lateral — exploiting natural tibiofemoral gap dynamics

Summary Table: Key Biomechanical Concepts