Normal Biomechanics of Knee and Movements

Knee allows locomotion with minimum energy requirements from the muscles and stability for accommodating for different terrains. The knee joint has biomechanical roles in allowing gait by flexing and rotating and at the same time, provides stability during the activities of daily life. It shortens and extends lower limb as required and transmits forces across it.

It also functions to transmit, absorb and redistribute forces caused during the activities of daily life.

It is important, therefore, to understand the normal biomechanics of knee.

Movements of Knee

Knee Joint produces Functional shortening and Lengthening of extremity.

The knee is comprised tibiofemoral joint and patellofemoral joint.

The knee joint is a modified hinge joint with gliding function too. It has got six degrees of freedom

•  3 rotations
• 3 translations

In sagittal axis  it has flexion-extension movement, in frontal axis, it has a varus-valgus rotation and whereas in transverse axis there is internal-external rotation)

• Flexion-Extension:  3 degrees of hyperextension to 155 degrees of flexion
• Varus-valgus: 6-8 deg in extension
• Internal-external rotation: 25-30 deg in flexion
• Translation
  • Anterior-posterior: 5–10 mm
  • Compression: 2–5 mm [patellar compression]
  • Medio-lateral: 1-2 mm

Anatomic Axis of Knee

A line is drawn along the shaft of the Femur and shaft of tibia form angle of 170 to 175 degree. When the angle is less than 165 degree an abnormal condition called genu valgum. This subjects the medial aspect of the knee is subjected to distraction force

When the angle is more than 180 degree an abnormal condition called genu varum.

The medial aspect of the knee is subjected to Increase compression loading

Quadriceps Angle or Q Angle

It is the angle formed by a resultant vector of Quadriceps and the pull of ligamentum patella.

It is found by drawing two lines

• From anteroposterior iliac spine to the midpoint of Patella
• From Tibial tubercle to the midpoint of Patella

The normal angle is about 13 degrees. When the angle is large, lateral pull on patella is increased.

[More on Q-angle]

Patellofemoral Joint

It transmits tensile forces generated by the quadriceps to the patellar tendon and increases the lever arm of the extensor mechanism. Removal of patella decreases extension force by 30%.

The motion of the patellofemoral joint is sliding articulation. In full flexion, the patella moves 7cm caudally. Maximum contact between femur and patella is at 45 degrees of flexion.

Forces acting on patella are

• Laterally – Lateral retinaculum, vastus lateralis, iliotibial tract
• Medially – Medial retinaculum and vastus medialis
• Superior – Quardiiceps via quadriceps tendon
• Inferior – patellar ligament

Passive restraints to lateral subluxation are medial patellofemoral ligament [60% of the restraining force], medial patellomeniscal ligament [13% of the retraining force] and retinaculum [10%].

Dynamic  restraint is by quadriceps muscles

Tibiofemoral Joint

Tibiofemoral joint functions to transmit the body weight from femur to tibia.

The tibiofemoral joint reaction force is three times body weight with walking and four times body weight with climbing.

The range of motion of knee is 3 degrees of hyperextension to 155 degrees of flexion. The flexion is limited by the size of thigh-calf becomes  contact is usually the limiting factor to full flexion

Normal gait requires a range of motion from 0 to 70 degrees.

Posterior Rollback

Instant center of rotation is the point at which the joint surfaces are in direct contact.

Posterior rollback is a phenomenon whereas the knee flexes, the instant center of rotation on the femur moves posteriorly.

This occurs due to the unique shape of the femoral condyle.

Rollback allows knee flexion by avoiding impingement.

Change in center of Rotation

Change of Centre of rotation for femur motion during flexion-extension.

• Extension: contact is located centrally
• Early flexion: posterior rolling – contact continuously moves posteriorly.
• Deep flexion: femoral sliding – contact is located posteriorly

The unlocking of the ACL prevents further femoral rollback

Screw Home Mechanism

During the last 20 degrees of knee extension- anterior tibial glide persists on the tibia’s medial condyle because its articular surface is longer in that dimension than the lateral condyle’s. Prolonged anterior glide on the medial side produces external tibial rotation, the “screw-home” mechanism. This locks knee and serves to decrease the work of the quadriceps while standing.

Range of Motion at the Knee in Different Activities

The patellofemoral joint reaction is one-half of body weight during normal walking, increasing up to over three times body weight during stair climbing and descending.

Knee joint loading in Different Activities

Role of Menisci

Menisci have following  functions

• Load bearing
• Stability
• Joint lubrication
• Prevent capsule, synovial impingement
• Shock absorbers

As the femur compresses through the meniscus onto the tibia, it pushes the meniscus out of the joint cavity. The meniscus deforms to conform with the femoral condyle and allows for the contact to be distributed over a larger area.

Also, the meniscus increases its circumference and moves radially outwards and posteriorly with knee flexion, following the rolling and sliding of the femoral condyle with flexion.

During radial deformation, the meniscus is anchored by its anterior and posterior horns. During loading, tensile, compressive, and shear forces are generated.

Ascending and Descending Stairs

During ascending of stairs, the actual degree of knee flexion required to ascend stairs is determined not only by the height of the step but also by the height of the patient.  For the standard 7″ step approximately 65° of flexion will be required.

Lever arm during climbing the stairs can be reduced by leaning forward. Also, in stair climbing, the tibia is maintained relatively vertical, which diminishes the anterior subluxation potential of the femur on the tibia.

Descending stairs also required 85° of flexion.

The tibia is steeply inclined toward the horizontal, bringing the tibial plateaus into an oblique orientation. The force of body weight will now tend to sublux the femur anteriorly.

This anterior subluxation potential will be resisted by the patellofemoral joint reaction force and the tension which develops in the posterior cruciate ligament.

Stability of Knee

Stability is provided by ligaments and other structures. Main ligaments in different stresses, important for stability  as follows

Varus stress

Lateral collateral ligament

Valgus stress

Superficial portion of the medial collateral ligament

Anterior translation

The anterior cruciate ligament is a primary static restraint to anterior translation and also plays a roll in axial rotation

It has two components

• Anteromedial bundle
  • tight in flexion
• Posterolateral bundle
  • tight in extension

Posterior Translation

The posterior cruciate ligament is the primary static restraint to posterior translation and external rotation. Posterolateral corner is the primary stabilizer of external tibial rotation.

This ligament originates from anterolateral medial femoral condyle and inserts on the tibial sulcus.

It has an anterolateral component which tightens in flexion and a posteromedial component that tightens in extension.