Knee Biomechanics

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This article discusses knee biomechanics, for a discussion on the anatomy of the joint see Knee Joint. To fully understand the concepts presented here, it is helpful to first review the general principles outlined on the Joint Biomechanics page

Kinematics of the Knee Joint

The knee joint allows movement primarily in the sagittal plane (flexion and extension) but also includes crucial rotational movement in the axial plane and minor movements in other planes.

Unlike a simple hinge, knee motion involves complex coupled movements guided by bone geometry and ligamentous constraints, especially with flexion and extension. Specifically, the coupling of rotation and translation in the sagittal plane. The precise nature of this coupling (how much roll vs. glide) changes throughout the range of motion, guided by bone geometry and ligamentous control. Understanding the relative frame of reference (femur moving on fixed tibia vs. tibia moving on fixed femur) is vital, as descriptive terms like "anterior" or "posterior" translation will reverse depending on which bone is considered fixed.

Planes of Motion and Functional Requirements

Analyzing the knee using the 12-movement checklist reveals its functional capabilities and limitations:

Sagittal Plane Motion (Flexion/Extension): This is the primary movement with the largest range. However, the common terms "flexion" and "extension" describe the limb segments (thigh and leg) changing their angle relative to each other. At the joint level, this involves a combination of movements.

• Range: Typically from 3° of hyperextension to 155° of flexion, potentially more in cultures where deep squatting is common.
• Mechanism: Knee flexion/extension is not simple rotation. To maintain joint contact, it involves coupled rolling and gliding.
  • Flexion: Requires '''posterior sagittal rotation''' of the femur coupled with '''anterior sagittal translation''' of the femur relative to the tibia (or posterior translation of the tibia relative to the femur).
  • Extension: Requires '''anterior sagittal rotation''' of the femur coupled with '''posterior sagittal translation''' of the femur relative to the tibia (or anterior translation of the tibia relative to the femur).
• Axis of Rotation: Due to the oblique contours of the femoral condyles, the centre of knee rotation changes, following a J-shaped path from anterior in extension to posterior in full flexion.
• Sliding: Sliding occurs throughout the range. The femur slides anteriorly on the tibia during extension and posteriorly during flexion. In full flexion, the posterior aspects of the femoral condyles approach the posterior tibial plateaus, and vice versa.
• Functional Ranges: Normal activities of daily living typically require up to 117° of flexion. Gait requires 0-6° (slow walking) up to 18-30° (running). Squatting and kneeling demand higher ranges.

Axial Rotation (Internal/External Rotation): Essential rotation of the tibia relative to the femur (or vice versa), particularly evident when the knee is flexed. This becomes crucial for the "locking" mechanism in full extension.

• Range: Rotation is maximal at 30-40° of flexion, reaching approximately 18° of external tibial rotation and 25° of internal tibial rotation. This range remains relatively constant up to ~120° flexion, then reduces due to soft tissue tightening.
• Constraints: Rotation is limited by the interlocking shape of the condyles (the medial femoral condyle is longer), tightening collateral ligaments, the ACL, and the posterior capsule.

Frontal Plane Motion (Varus/Valgus Angulation):

• Range: Very limited, especially in full extension. Passive motion increases slightly up to 30° of flexion (only a few degrees) and then reduces due to soft tissue constraints. Generally considered undesirable motion that should be minimized in a stable knee.

Compression/Distraction:

• Compression: Occurs constantly during weight-bearing and muscle contraction (force couple principle). Though the amplitude is small (millimeters), its high frequency and magnitude contribute significantly to joint loading and potential wear (e.g., osteoarthritis).
• Distraction: A small amount of joint gapping is possible but strictly limited by ligaments.

Undesired Translations: Coronal plane translation (medial/lateral shifting) is minimal in a stable knee and resisted by ligaments

Differential Condylar Motion

The movement isn't symmetrical between the compartments:

• Medial Femoral Condyle: Behaves somewhat like a constrained ball-in-socket joint. It primarily rotates with minimal translation, allowing flexion, longitudinal rotation, and minimal varus lift-off laterally. The contact point on the medial tibia remains relatively constant during sliding.
• Lateral Femoral Condyle: Behaves more like a wheel, exhibiting both rolling and significant antero-posterior sliding. The contact point on the lateral tibia moves posteriorly during flexion.

Patellofemoral Joint Movement

Surface motion at the patellofemoral joint occurs primarily in the sagittal plane relative to axes fixed in the femur.

Stability: Keeping the Knee Aligned

The knee's structure provides stability during its wide range of motion through a combination of bony anatomy, passive ligamentous restraints, menisci, and dynamic muscular control.

Bony Anatomy Contributions

• Femoral Condyles: The rounded shape of the medial and lateral femoral condyles facilitates the large range of sagittal rotation (rolling). The curvature is not uniform; they are flatter in the central portion, providing a more stable platform for weight-bearing in extension.
• Tibial Plateau: The relatively flat articular surfaces of the tibia provide the base upon which the femoral condyles move.
• Intercondylar Notch (Fossa): This groove between the femoral condyles is not arbitrary; it is essential for housing the cruciate ligaments centrally within the joint.

Ligamentous Constraints

Ligaments act as passive restraints, limiting excessive motion, particularly translations and rotations in undesired planes.

Collateral Ligaments (MCL & LCL):

• Location: Medial (MCL) and Lateral (LCL) sides.
• Primary Function: Resist coronal plane rotation (MCL resists valgus/outward bending; LCL resists varus/inward bending). The LCL resists 55% of varus load at full extension. The superficial MCL resists 50% of valgus load (capsule, ACL, PCL share the rest). The knee tends to open more laterally under varus stress; although the MCL is weaker than the LCL, the ITB dynamically assists lateral stability.
• Secondary Function: Limit distraction, resist extreme coronal translation and axial rotation. The MCL (with ACL) also resists rotation at 20-40° flexion and limits AP displacement of the medial femoral condyle, aiding medial pivot function.

Cruciate Ligaments (ACL & PCL):

• Location: Centrally within the intercondylar notch, crossing each other.
• Need: Collaterals are poorly positioned to prevent excessive anterior/posterior sliding (sagittal translation). Bony flanges (like the ankle) would prevent necessary rotation. Thus, dedicated central ligaments are required.
• Central Placement Advantage: Placing them near the axis of axial rotation minimizes their length change during twisting, allowing axial rotation while still preventing excessive sagittal translation. Peripheral placement would block rotation. This necessitates the intercondylar notch.
• Attachments & Function:
  • ACL (Anterior Cruciate Ligament): Attaches anteriorly on the tibia, runs superiorly, posteriorly, and laterally to the medial surface of the lateral femoral condyle.
    • Primary Role: Main restraint (75% at full extension, +10% up to 90° flexion) to anterior tibial translation. The anteromedial bundle is the predominant restraint.
    • Secondary Role: Posterolateral bundle stabilizes near full extension, especially against rotation.
    • Tension Pattern: Bulk tight in max extension (posterolateral bundle tight, anteromedial moderately lax). In flexion, femoral attachment becomes more horizontal (anteromedial bundle tightens, posterolateral relaxes).
  • PCL (Posterior Cruciate Ligament): Attaches posteriorly on the tibia, runs superiorly, anteriorly, and medially to the lateral surface of the medial femoral condyle.
    • Primary Role: Main restraint (85-100% at 30° and 90° flexion) to posterior tibial translation.
    • Secondary Role: Stabilizes against rotation at high flexion angles.
    • Tension Pattern: Bulk taut in mid-range and max flexion. In extension (anterolateral bundle lax, posteromedial tight). In flexion (anterolateral bundle tight, posteromedial lax).

The Menisci

These C-shaped fibrocartilages on the tibial plateau improve stability by:

• Deepening the tibial sockets (increasing congruency).
• Distributing load over a larger area.
• Aiding lubrication.

Muscular Contributions

Muscles crossing the knee (quadriceps, hamstrings, gastrocnemius, popliteus) provide dynamic stability. Their coordinated contraction and co-contraction controls the roll-glide mechanism and significantly increases joint stiffness and stability, particularly during activity.

Screw-Home Mechanism: Locking and Unlocking the Knee

This crucial mechanism enhances knee stability in full extension, reducing the muscular effort needed for standing.

Locking the Knee (Terminal Extension)

• The Need: The curved condyles, ideal for flexion/extension, make the fully extended knee potentially unstable against collapsing into flexion.
• The Action: During the final ~20° of extension, an obligatory '''medial axial rotation''' of the femur occurs relative to the tibia (equivalent to lateral rotation of the tibia relative to the femur).
• The Mechanism:
  • As the knee extends, the smaller, more curved lateral femoral condyle finishes its rolling/gliding before the larger, longer medial condyle.
  • Continued anterior gliding of the medial tibial plateau (relative to the femur) or medial femoral condyle (relative to the tibia) causes this external tibial/internal femoral rotation.
  • This rotation misaligns the primary curvature axes of the condyles relative to the pure sagittal plane, making simultaneous rolling backward (flexion initiation) difficult.
  • The ACL tension contributes to guiding this rotation.
  • In this "locked" state, the ACL and PCL become tauter, further enhancing stability.
• The Result: A stable, locked knee resistant to unwanted flexion, allowing efficient standing with minimal muscle activity. The contact point shifts anteriorly, acting as a brake.

Unlocking the Knee (Initiating Flexion)

• The Need: To flex from the locked position, the screw-home rotation must be reversed.
• The Action: An initial '''lateral rotation of the femur''' relative to the tibia (or internal rotation of the tibia) is required. This occurs from 0° to ~20° of flexion.
• The Mechanism:
  • Posterior glide begins first on the longer medial condyle, producing the relative internal tibial rotation.
  • The Popliteus muscle is primarily responsible for actively initiating this unlocking rotation. It also helps release tension in collateral ligaments for smooth flexion onset by the hamstrings.
• Gait Context: This unlocking occurs in pre-swing and late-swing. Paradoxically, external tibial rotation may occur during the stance phase.

Patella Function

The patella plays a key role in knee mechanics:

• Efficiency: Acts as a fulcrum, lengthening the lever arm of the quadriceps tendon around the knee's center of rotation, thereby increasing the mechanical advantage and efficiency of quadriceps muscle force during extension.
• Force Transmission: Quadriceps force increases significantly with knee flexion to counteract the increasing flexion moment. This results in increased torque and compressive forces at the patellofemoral joint as flexion angle increases.
• Stability & Tracking:
  • The line of action of the quadriceps force and the patellar ligament force are not parallel, creating a net laterally directed force on the patella, predisposing it to lateral subluxation.
  • This lateral pull is counteracted primarily by the higher slope and prominence of the lateral trochlear groove on the femur compared to the shallower medial groove.
  • Beyond ~90° flexion, the patella sinks deeper into the intercondylar notch, which has high slopes both medially and laterally, providing further stability.
  • A tight Iliotibial (IT) band can increase the lateral force component on the patella.
• Contact Area: The area of contact between the patella and femur increases and moves superiorly (cranially) on the patella as the knee flexes from extension towards 90°. In full extension, the inferior aspect of the patella articulates with the femur.

Conclusion

The knee joint is a prime example of the intricate relationship between anatomical structure and biomechanical function. Its design enables a large range of motion, primarily in the sagittal plane, through a complex coupling of rolling and gliding facilitated by its condylar geometry. Stability is achieved through a multi-faceted system: the inherent shape of the bones, strong passive ligamentous restraints (collaterals for coronal stability, centrally-placed cruciates for sagittal stability allowing axial rotation), the congruency-enhancing menisci, and crucial dynamic muscular control. The screw-home mechanism provides an additional layer of passive stability in full extension via obligatory axial rotation. Understanding these principles - the fundamental movements, coupled motions, stabilizing structures, and forces involved - is vital for comprehending knee function, diagnosing pathology, and appreciating the sophisticated engineering of the human musculoskeletal system.