This article discussed knee biomechanics, for a discussion on the anatomy of the joint see Knee Joint.
The knee motion is more complex than a simple hinge. The contours of the medial and lateral femoral condyles are oblique rather than round and this causes the centre of knee rotation to change with flexion and extension. The axis of rotation moves in a J-like pattern. It follows the contour of the femoral condyles from anterior in extension, to posterior in full flexion.
In addition to this rotational movement there is also sliding movement. The femur slides anteriorly on the tibia during extension, and slides posteriorly on the tibia during flexion. In full flexion the posterior aspects of the femoral condyles are adjacent to the posterior aspects of the tibial plateaus, and vice versa.
In the arc of active function, condylar motion may be summarised as follows. The medial femoral condyle can be viewed as a sphere which rotates to produce a variable combination of flexion, longitudinal rotation (and minimal varus if lift-off occurs laterally). It hardly translates and thus is analogous to a somewhat constrained ball-in-socket joint such as the hip. The lateral condyle behaves like the wheel of a sidecar on ice: it rolls but also “slides” antero-posteriorly 
Tibiofemoral Joint Movement
- Sagittal plane motion dominates along with quadriceps muscle group action.
- The typical range of motion is from 3° of hyperextension to 155° of flexion. In cultures where squatting is common flexion can reach beyond 155°.
- Rotation is restricted by the interlocking of the femoral and tibial condyles. This is because the medial femoral condyle is longer than the lateral condyle; and also because there is tightening of the collateral ligaments, ACL, and posterior capsule.
- Rotation is maximum at 30-40° of flexion where external tibial rotation is approximately 18° and internal tibial rotation is approximately 25°. Rotation is constant up to approximately 120° of flexion and then reduces up to full flexion due to soft tissue tightening.
- Frontal plane (abduction/varus and adduction/valgus).
- At full extension there is almost no frontal plane motion. Passive frontal plane motion increases with knee flexion up to 30°, but only up to a few degrees. With flexion past 30° there is reduced frontal plane motion due to soft tissue limitation.
- For normal activities of daily living a range of motion of 117° of flexion is required. However squatting and kneeling require higher ranges of motion. During gait, the range of flexion needed increases from 0-6° with slow walking up to 18-30° with running.
- There is sliding throughout the range of motion. Medially there is sliding with a close to constant contact point on the tibia. Laterally there is rolling and sliding as the contact point on the tibia moves posteriorly.
Patellofemoral Joint Movement
- Surface motion occurs primarily in the sagittal plane with respect to axes fixed in the femur.
- Anterior Cruciate Ligament
- The ACL is the primary restraint to anterior tibial displacement.
- Resists 75% of anterior force at full extension and addition 10% up to 90° of knee flexion.
- The bulk of the fibres are tight in maximal extension.
- In extension: the posterolateral bundle is tight and the anteromedial bundle is moderately lax
- In flexion: the femoral attachment of ACL is more horizontal, causing the anteromedial bundle to tighten and the posterolateral bundle to relax
- The anteromedial bundle is the predominant restraint to anterior tibial displacement.
- The posterolateral bundle stabilises the knee near full extension especially against rotatory loads.
- Posterior Cruciate Ligament
- The two bundles function to resist posterior tibial translation and rotation at different angles of knee flexion.
- Resists 85-100% of posterior force at both 30° and 90° of knee flexion. Secondary stabiliser for rotation at high flexion angles.
- The bulk of the fibres are taut in the intermediate positions and in maximal flexion
- In extension: the anterolateral bundle is lax and the posteromedial bundle is tight.
- In flexion: the anterolateral bundle is tight and the posteromedial bundle is lax.
- Lateral Collateral Ligament
- Primary restraint to varus angulation. Resists 55% of applied load at full extension
- Medial Collateral Ligament
- Superficial portion is primary restraint to valgus angulation. Resists 50% of applied valgus load. Capsule, ACL, and PCL share remaining valgus load.
- MCL along with ACL also resists rotation at 20-40°of knee flexion.
- Limits AP displacement of medial femoral condyle and provides medial pivot action in function.
- Knee opens with varus moments with opening of the lateral knee more so than on the medial side. The MCL is paradoxically weaker than the LCL, but the ITB helps to dynamically stabilises the lateral side.
- Muscle contraction and co-contraction of quadriceps and hamstrings contribute to knee stability by increasing the stiffness.
- Scree-home mechanism of the tibiofemoral joint in extension adds stability in full extension. The tibia rotates externally and the contact point shifts anteriorly which acts as a brake to further extension and provides a stable knee position.
Knee Locking and Unlocking
Screw Home Mechanism in Extension
The screw home mechanism occurs with rotation in the transverse plane is essentially a passive increased motion of the medial compartment during extension.
When the knee actively extends, the smaller and more curved lateral femoral condyle reaches a point of terminal extension before the medial condyle. Further lateral extension is inhibited by the lateral condyle and the ACL. During the last 20° of knee extension, the medial condyle of the tibia continues to glide anteriorly because of the longer articular surface compared to the lateral side. Prolonged anterior gliding of the medial side leads to external tibial rotation, and this is termed the screw-home mechanism.
In the extended position the ACL and PCL are tangled and tightened if the tibia is externally rotated. This locks the knee joint thereby increasing stability in the extended position.
When the knee begins to flex (0° of extension to 20° of flexion), posterior tibial glide begins first on the longer medial condyle, and produces relative tibial internal rotation, a reversal of the screw-home mechanism.
Unlocking in Flexion
In the initial stages of flexion of between 0 to 20°, the longer medial condyle of the tibia glides posteriorly. This causes relative tibial internal rotation and is a reversal of the screw-home mechanism.
Flexion of the knee first requires lateral rotation of the femur on the tibia which unlocks the joint. Some authors believe that the unlocking motion is done by action of the popliteus muscle. The popliteus releases the tension in the collateral ligaments to allow smooth knee flexion under action of the hamstrings.
This motion occurs in the pre-swing and late-swing phase of gate, but a paradoxical external tibial rotation appears to occur in the remaining stance phase.
- Lengthens the lever arm of the quadriceps muscle force about the centre of rotation of the knee, increasing the mechanics and efficiency of the quadriceps.
- Quadriceps muscle force increases with knee flexion to counterbalance flexion moment, starting from minimal quadriceps action with standing. The torque around the patellofemoral joint is increased with flexion.
- Quadriceps and ligament forces aren't parallel, which produces a laterally directed force on the patella, putting it at risk of laterally subluxing.
- This is prevented by the slope and height of the patella groove on the lateral side, while the medial groove is shallow.
- At flexion beyond approximately 90° the patella sinks into the intercondylar notch which has high slopes laterally and medially.
- With knee extension, the lower aspect of the patella sits against the femur. With flexion to 90° the contact between the patella and femur moves cranially and increases in area With a tight ITB, the patellofemoral joint force can shift laterally.
- Freeman & Pinskerova. The movement of the normal tibio-femoral joint. Journal of biomechanics 2005. 38:197-208. PMID: 15598446. DOI.
- Kim, Ha Yong et al. “Screw-Home Movement of the Tibiofemoral Joint during Normal Gait: Three-Dimensional Analysis.” Clinics in orthopedic surgery vol. 7,3 (2015): 303-9. doi:10.4055/cios.2015.7.3.303