Knee Biomechanics

The knee experiences forces of 2-3× body weight during walking (up to 6× running) and 3-4× body weight patellofemoral force during stair climbing, making biomechanical understanding crucial for clinical practice. The screw-home mechanism automatically externally rotates the tibia 10° during terminal extension due to asymmetric femoral condyles (lateral shorter), locking the knee for standing stability—the popliteus unlocks this to initiate flexion. Femoral rollback (20-25mm at 90° flexion) prevents posterior impingement and maintains extensor mechanism leverage, controlled by the PCL and posterolateral corner. The ACL resists 85% of anterior tibial translation, with secondary restraints handling the remaining 15%; the MCL is the primary valgus restraint at 30° flexion while the ACL assumes this role at full extension. Patellofemoral reaction force increases exponentially with flexion, reaching 7-8× body weight during deep squatting, explaining anterior knee pain in flexion-loading activities.

Knee biomechanics encompasses the study of mechanical forces, motion patterns, and structural relationships within the knee joint complex. Understanding these principles is fundamental to orthopaedic practice, informing diagnosis, surgical technique, rehabilitation, and implant design.

The knee is the largest joint in the body, bearing forces up to 6x body weight during activities. It must balance mobility (0-140° flexion) with stability for weight-bearing. This is achieved through the complex interplay of bone geometry, ligaments, menisci, and surrounding musculature.

Key Biomechanical Principles:

1. Four-Bar Linkage Model: The ACL and PCL form a crossed linkage system that constrains motion while allowing rollback
2. Screw-Home Mechanism: Terminal tibial external rotation locks the knee in extension
3. Coupled Motion: Flexion couples with internal rotation; extension couples with external rotation
4. Instant Center of Rotation: Shifts posteriorly with flexion, important for implant design

Joint Geometry and Congruency

The tibiofemoral joint is incongruent - the convex femoral condyles articulate with relatively flat tibial plateaus. This geometry allows large range of motion but requires menisci and ligaments for stability.

Tibiofemoral Motion Patterns

Knee motion combines rolling and gliding (sliding) of femoral condyles on tibial plateaus. The ratio changes with flexion angle.

Femoral rollback is the posterior translation of the femoral condyles on the tibial plateau during flexion. This is critical for:

• Maintaining extensor mechanism moment arm (prevents quadriceps insufficiency)
• Preventing posterior soft tissue impingement
• Allowing deep flexion without bony contact

The PCL and posterolateral corner resist excessive rollback. Loss of PCL leads to excessive posterior translation and abnormal kinematics.

The screw-home mechanism is the automatic external rotation of the tibia (10°) during the last 20-30° of extension. This locks the knee in full extension for energy-efficient standing.

Anatomical Basis

Anterior Cruciate Ligament (ACL)

The ACL is the primary restraint to anterior tibial translation, resisting 85% of anterior drawer force at 30° flexion.

Secondary restraints to anterior translation include:

• Medial and lateral menisci (posterior horns)
• MCL (medial capsule)
• Posterolateral corner
• Iliotibial band

These resist approximately 15% of anterior drawer force combined. ACL reconstruction restores primary restraint and normal kinematics.

Posterior Cruciate Ligament (PCL)

The PCL resists posterior tibial translation and maintains femoral rollback during flexion.

PCL-deficient knees develop excessive posterior translation and loss of femoral rollback, leading to:

• Patellofemoral pain (loss of extensor mechanism leverage)
• Increased medial compartment load
• Difficulty descending stairs or slopes

Collateral Ligaments

At full extension, the ACL and posterior capsule become primary restraints to valgus/varus stress. At 30° flexion, collaterals are isolated and maximally stressed.

Patellofemoral Joint Forces

The patella functions as a mechanical lever to increase the moment arm of the quadriceps, improving extension efficiency. However, this creates large compressive forces at the patellofemoral joint.

Patellar Tracking

Normal patellar tracking requires balance between medial and lateral stabilizers.

Q-angle (quadriceps angle) measures the vector of quadriceps pull relative to patellar tendon. Normal values:

• Males: 14° average
• Females: 17° average (wider pelvis)

Increased Q-angle raises lateral patellar stress and predisposes to lateral tracking disorders.

Click to expand

Click to expand

Tibiofemoral Contact Stress

The menisci are critical for load distribution. They transmit 40-60% of knee joint load and reduce peak contact stress on articular cartilage.

Meniscectomy consequences:

• Total meniscectomy: 200-300% increase in peak contact stress
• Partial meniscectomy: 65-100% increase (depends on amount removed)
• Increased risk of osteoarthritis

Alignment effects:

• Varus alignment: Shifts load medially (medial compartment overload)
• Valgus alignment: Shifts load laterally (lateral compartment overload)
• Neutral alignment: 60% medial, 40% lateral load distribution

Total Knee Arthroplasty Design Considerations

Understanding knee biomechanics guides TKA design:

Cruciate-retaining vs posterior-stabilized TKA:

• CR-TKA: Preserves PCL, maintains femoral rollback naturally
• PS-TKA: Resects PCL, uses cam-post mechanism to replicate rollback
• Both aim to restore 20-25mm posterior translation at 90° flexion

Australian Epidemiology and Practice

Knee Biomechanics in Australian Practice:

• Knee biomechanics forms a core component of FRACS Basic Science examination
• Understanding ACL function, screw-home mechanism, and patellofemoral mechanics is essential for both Part I and Part II examinations
• AOANJRR data demonstrates over 65,000 primary TKAs performed annually in Australia, making understanding of knee kinematics critical for surgical planning

RACS Orthopaedic Training Relevance:

• FRACS curriculum emphasizes four-bar linkage model, instant center of rotation, and coupled motion concepts
• ACL biomechanics examination questions focus on load sharing percentages (85% primary, 15% secondary restraints)
• Understanding femoral rollback (20-25mm) and its implications for TKA design is frequently examined
• Screw-home mechanism and popliteus function are common viva topics

Australian Knee Registry Data:

• AOANJRR tracks TKA outcomes with particular attention to kinematic alignment vs mechanical alignment techniques
• Registry data informs choice between CR-TKA and PS-TKA based on long-term revision rates
• Australian surgeons contribute to global understanding of knee kinematics through research at major centres

Clinical Application in Australian Practice:

• Major trauma centres employ fluoroscopic gait analysis for complex knee pathology assessment
• Sports medicine centres around Australia utilise biomechanical analysis for ACL injury prevention programs
• Victorian Institute of Sport and Queensland Academy of Sport integrate knee biomechanics into athlete screening protocols

PBS Considerations:

• Physiotherapy for knee rehabilitation is subsidised under Medicare chronic disease management plans
• Hyaluronic acid injections for knee OA management available with appropriate clinical indication