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.

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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

A frequent viva failure is reciting ligament functions in isolation. Examiners reward candidates who translate the biomechanics into the clinical instability pattern, its provocative test, and the abnormal motion produced. This is effectively the biomechanical "differential diagnosis" of the unstable knee.

Knee biomechanics is mature but several practice-relevant debates remain genuinely unsettled and are favourite higher-order viva probes:

• Alignment philosophy in TKA. Mechanical alignment is the validated reference standard, but kinematic and functional alignment aim to better reproduce native rollback and soft-tissue balance. Registry-level evidence of superior survivorship for kinematic alignment is not yet established, so it remains under evaluation rather than standard of care.
• CR versus PS TKA. Both replicate rollback differently (intact PCL versus cam-post), and registries show comparable survivorship. The "correct" choice is patient-specific (PCL competence, deformity) rather than universally superior.
• The medial-pivot concept. Although described as the "normal" kinematic pattern, in-vivo fluoroscopy shows a true medial pivot in only about half of normal knees during deep flexion, questioning how faithfully any single implant design should enforce it.
• Posterior tibial slope. Increased slope facilitates rollback and flexion but raises ACL/graft strain and anterior tibial translation; the optimal slope, especially in ACL-deficient and revision knees, is debated.
• The "isometric" ACL. The ACL is not truly isometric; anteromedial and posterolateral bundles tension reciprocally, fuelling the single- versus double-bundle reconstruction debate, where double-bundle has not shown consistent clinical superiority.

The biomechanical principles in this topic rest on a small number of classic cadaveric and in-vivo studies that remain the most-cited primary sources in knee biomechanics. Each card below is linked to its PubMed record for independent verification.

Knee biomechanics is examined across every major fellowship (FRCS Tr&Orth, FRACS, EBOT/FEBOT, ABOS, DNB/MS, MRCS, SICOT). The numbers and mechanisms are universal; the clinical translation differs mainly in implant selection and alignment philosophy, where the national arthroplasty registries provide the strongest comparative evidence.

Global Epidemiology and Examination Relevance

• The knee is the largest synovial joint and the most commonly replaced joint worldwide; symptomatic knee osteoarthritis affects a large and rising share of adults over 60, driven by ageing and obesity.
• Core examined concepts are consistent globally: ACL load sharing (approximately 85% primary restraint), femoral rollback (20-25mm at 90°), the screw-home mechanism (around 10° tibial external rotation), and patellofemoral force escalation with flexion.
• The four-bar cruciate linkage, instant centre of rotation, and coupled rotation are standard basic-science viva material in all curricula.

Alignment Philosophy: Side-by-Side

Registry Evidence (Arthroplasty Kinematics)

• NJR (UK), AJRR (US), AOANJRR (Australia), SHAR (Sweden), Norwegian and NZJR all track TKA implant survival and revision by design.
• Registries consistently show that cruciate-retaining (CR) and posterior-stabilised (PS) TKA achieve comparable long-term survivorship; the choice is driven by PCL competence, deformity and surgeon preference rather than a clear kinematic superiority.
• Registry surveillance was central to identifying poorly performing designs early, reinforcing the principle that kinematic theory must be validated by real-world revision data.

High- vs Limited-Resource Practice Variation

• High-resource settings: fluoroscopic and optical gait analysis, robotic/navigated alignment, and routine MPFL reconstruction for patellar instability are available; meniscus repair is prioritised over resection.
• Limited-resource settings: clinical kinematic assessment (Lachman, pivot-shift, screw-home palpation) and plain radiographs carry the diagnostic load; meniscectomy may still be performed where repair facilities or follow-up are unavailable, accepting the higher long-term osteoarthritis risk demonstrated by contact-mechanics studies.
• The biomechanical reasoning is identical everywhere; resource constraints alter how aggressively native kinematics can be restored.