Knee Biomechanics

The knee is the largest joint in the body and bears forces of up to 6 times body weight during activity: 2-3 times body weight in walking, up to 6 times in running. It has to balance mobility, 0-140° of flexion, against the stability needed for weight-bearing, and it does so through the interplay of bone geometry, ligaments, menisci and the surrounding musculature. The principles below inform diagnosis, surgical technique, rehabilitation and implant design.

Four ideas organise the topic:

1. Four-bar linkage. The ACL and PCL form a crossed linkage 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 tibial internal rotation; extension with external rotation
4. Instant centre of rotation. Shifts posteriorly with flexion, which matters for implant design

Joint geometry and congruency

An incongruent joint. The convex femoral condyles articulate with relatively flat tibial plateaus. The mismatch allows a large range of motion but leaves the joint with minimal intrinsic stability, so the knee depends on its menisci and ligaments, and meniscectomy or ligament injury significantly increases contact stress.

The femoral condyles. The medial condyle is longer, has the larger radius and extends further distally; the lateral condyle is shorter, has the smaller radius and flattens posteriorly. Both articular surfaces are spherical anteriorly and flatten posteriorly, and the asymmetry between the two condyles is what drives the screw-home mechanism in terminal extension.

The tibial plateaus. The medial plateau is concave, larger and more conforming; the lateral plateau is convex or flat, smaller and less conforming. In the sagittal plane the plateau slopes 5-10° posteroinferiorly.

Motion patterns

Rolling and gliding. Flexion combines rolling and gliding (sliding) of the femoral condyles on the tibial plateaus, and the ratio changes with the flexion angle: rolling dominates early flexion, gliding dominates deep flexion.

Femoral rollback. As the knee flexes the femoral condyles translate posteriorly on the tibial plateau, reaching 20-25mm at 90°. Rollback maintains the moment arm of the extensor mechanism, which prevents quadriceps insufficiency, keeps the posterior soft tissues from impinging and allows deep flexion without bony contact. The PCL and the posterolateral corner control it, maintaining rollback while resisting excess.

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What it is. During the last 20-30° of extension the tibia rotates externally on the femur by about 10°, automatically, and the knee locks in full extension. The locked position is what allows energy-efficient standing without muscle effort.

Why it occurs. The lateral femoral condyle is shorter than the medial. Tibial external rotation aligns the femoral condyles with the tibial plateaus, giving maximum bony congruency at full extension.

How it unlocks. The popliteus contracts and internally rotates the tibia. This has to happen before the knee can flex from full extension, so a popliteus injury causes difficulty unlocking.

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Anterior cruciate ligament

Primary restraint. The ACL resists 85% of the anterior drawer force at 30° of flexion, the Lachman position, which is also where its tension peaks. Its secondary functions are to resist internal rotation and valgus stress.

Two bundles. The anteromedial bundle is tight in flexion and the posterolateral bundle tight in extension, so the two tension reciprocally across the arc: the posterolateral bundle contributes most near extension, the anteromedial bundle remains an important restraint into flexion.

How it fails. The pivot-shift injury combines valgus, internal rotation and anterior translation; the non-contact mechanism is deceleration with the foot planted and the knee near extension. The result is abnormal anterior translation and rotatory instability, because the secondary restraints are insufficient on their own.

Secondary restraints. Together these resist approximately the remaining 15% of anterior drawer force:

• Posterior horns of the medial and lateral menisci
• MCL and medial capsule
• Posterolateral corner
• Iliotibial band

The ACL-deficient knee loses its anterior restraint and develops abnormal kinematics. Reconstruction restores the primary restraint and, with it, normal motion patterns.

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Posterior cruciate ligament

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

The deficient knee. Without the PCL the tibia translates posteriorly to excess and rollback is lost, which leads to:

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

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Collateral ligaments

Through the arc. At full extension the ACL and posterior capsule become the primary restraints to valgus and varus stress and the MCL drops to a secondary role. At 30° of flexion the collaterals are isolated and maximally stressed.

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Joint forces

The lever. The patella increases the moment arm of the quadriceps, which makes extension more efficient, at the cost of large compressive forces across the patellofemoral joint.

How large. The reaction force increases exponentially with flexion:

• Walking: 0.5 times body weight
• Stair climbing: 3-4 times
• Squatting at 90°: 7-8 times
• Deep squat at 135°: up to 8-10 times

Why it climbs. Greater flexion demands greater quadriceps force, and the lever arm of the patellar tendon shortens in deep flexion. Contact stress peaks at 60-90° of flexion, which is why anterior knee pain is felt on stairs and squats.

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Patellar tracking

A balance of stabilisers. Normal tracking depends on the balance between the medial and lateral stabilisers. The shares below are fractions of the total restraint measured against lateral translation, with every structure tested as the denominator, not fractions of the medial side alone.

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Q-angle. The quadriceps angle measures the line of quadriceps pull relative to the patellar tendon and averages 14° in males and 17° in females, the difference reflecting the wider female pelvis. A larger angle raises lateral patellar stress and predisposes to lateral tracking disorders.

The menisci. They transmit 40-60% of the load across the knee, more in flexion, enlarging the contact area and reducing peak stress on the articular cartilage. Remove them and the stress concentrates:

• Total meniscectomy: peak contact stress rises 200-300%
• Partial meniscectomy: 65-100%, depending on how much is removed
• Either raises the risk of osteoarthritis

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Alignment. In neutral alignment the load splits 60% medial, 40% lateral. Varus alignment shifts it further medially and overloads the medial compartment; valgus shifts it laterally and overloads the lateral compartment.

Through the gait cycle. Finite element models of gait place the highest contact stresses at loading response and push-off.

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The menisci transmit load not just by filling the joint space but through a specific mechanism, circumferential (hoop) stress, and understanding it explains why some meniscal tears are biomechanically catastrophic while others are tolerated.

The mechanism. The meniscus is built from circumferentially oriented collagen fibres, with a few radial tie fibres, anchored to the tibia at the anterior and posterior roots. Under axial compression the wedge-shaped meniscus tends to extrude peripherally; the circumferential fibres, held by the roots, resist that by developing circumferential tensile stress. Vertical compressive load is thereby converted into circumferential tension, spread over a larger area, and the articular cartilage is protected.

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Why the tear pattern matters. The pattern decides whether the hoop survives:

Total knee arthroplasty design

The femoral component must replicate normal posterior rollback. Its J-curve matches the native condylar geometry and its asymmetric condyles replicate the medial-lateral differences, with the goal of restoring normal kinematics and range of motion.

The tibial component carries a posterior slope of 0-7° to facilitate rollback. More conformity lowers contact stress but reduces mobility, and the choice between a flat and a dished insert is a trade-off between stability and constraint. The polyethylene must withstand 3-6 times body weight cyclically.

Cruciate-retaining versus posterior-stabilised. A CR design preserves the PCL and so keeps femoral rollback naturally; a PS design resects the PCL and substitutes a cam-post mechanism. Both aim to restore the 20-25mm of posterior translation at 90° flexion, though as the figures below show, no design simply reproduces it.

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The concept. During stance the ground reaction force passes medial to the knee joint centre, creating an external knee adduction moment that loads the medial compartment: the dynamic counterpart of the static 60:40 medial bias, and the single most important gait-derived determinant of medial-compartment load.

Why it matters. The magnitude of the KAM, and the KAM impulse over stance, correlates with medial-compartment load and predicts progression of medial knee osteoarthritis; a varus thrust during gait increases it. Every medial-offloading strategy therefore works by reducing the KAM:

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Reciting ligament functions in isolation is a frequent viva failure. The credit goes to translating each restraint into its clinical instability pattern, its provocative test and the abnormal motion produced, which amounts to the biomechanical differential diagnosis of the unstable knee.

Knee biomechanics is examined across every major fellowship. 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.

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.