Biomechanics of knee

Presenter: Dr.Sudheer kumar

Introduction The knee is a mechanism of three joints

and Four bones - the femur, tibia, patella and fibula

Interact in separate joints - the tibiofemoral & patellofemoral

The function of these joints is to allow certain movements, restrict others, and to provide load transfer through the lower limb.

Biomechanics

Tibiofemoral joint

rotations

translations

screw home mechanism Axial & rotational alignment of knee Patello femoral joint Joint forces

Tibiofemoral joint Rotations

› Flexion/extension-0 to 1350

› varus valgus - 6-8o in extension

› Int/ext rotation - 25 – 300

in flexion

Translations› AP 5 - 10mm› comp/dist 2 - 5mm› medio-lateral 1-2mm

Knee motion during normal gait

Instant centre of motion

flexion axis varies in a helical fashion in a normal knee, with an average of 2 mm of posterior translation of the medial femoral condyle on the tibia during flexion compared with 21 mm of translation of the lateral femoral condyle.

Relevance :posterior rollback› as the knee flexes, the instant center of rotation on

the femur moves posteriorly 

Instant centre of motion flexion axis as varying in a helical

fashion

Femoral roll back

allows for increased knee flexion by avoiding impingement

Screw home mechanism the external rotation of the tibia on

the femur during extension and internal rotation of the tibia during knee flexion.

cause› medial tibial plateau articular

surface is longer than lateral tibial plateau(Medially based pivoting of the knee.)

  relevance

› "locks" knee decreasing the work performed by the quadriceps while standing

Axial & rotational alignment of knee

mechanical axis of the lower limb is defined as the line drawn on a standing long leg antero posterior radiograph from the center of the femoral head to the center of the talar dome

anatomical axes of the femur and the tibia form a valgus angle of 6 ± 2 degrees.

the tibial articular surface is in approximately 3 0of varus with respect to the mechanical axis, and the femoral articular surface is in 90 of valgus.

Axial & rotational alignment of knee

Patellofemoral joint

"sliding" articulation› patella moves 7cm caudally

during full flexion  maximum contact between

femur and patella is at 45 degrees of flexion

The primary function of the patella is to increase the lever arm of the extensor mechanism around the knee, improving the efficiency of quadriceps contraction.

The quadriceps and patellar tendons insert anteriorly

on the patella, with the thickness of the patella displacing their respective force vectors away from the center of rotation of the knee .

This displacement or lengthening of the extensor lever arm changes throughout the arc of knee motion.

the extensor lever arm is greatest at 20 degrees of flexion, and the quadriceps force required for knee extension increases significantly in the last 20 degrees of extension

The length of the lever arm varies as a function of the geometry of the trochlea, the varying patellofemoral contact areas, and the varying center of rotation of the knee.

Patella stabilizers passive restraints to lateral

subluxation› medial patellofemoral

ligament   primary passive restraint to

lateral translation in 20 degrees of flexion

60% of total restraining force

› medial patellomeniscal ligament 13% of total restraining

force› lateral retinaculum 

10% of total restraining force

dynamic restraint› quadriceps muscles

Q angle The angle between the extended

anatomical axis of the femur & the line between the center of the patella & the tibial tubercle

normal Q angle› in flexion

males 13 degrees

females 18 degrees

› in extension 8 degrees

Limbs with larger Q angles have a greater tendency for lateral patellar subluxation.

Because the patella does not contact the trochlea in early flexion, lateral subluxation of the patella in this range is resisted primarily by the vastus medialis obliquus fibers.

Tibio femoral joint force:

Position force acting on joint Standing on both feet - equal to body wt Swing phase - 1/2 x b.wt u/l stance phase – 2-4 x b.wt Jogging – 6x b.wt

PatelloFemoralJoint Loading

Walking › 0.3 x body weight

Ascending Stairs› 2.5 x body weight

Descending Stairs› 3.5 x body weight

Squatting› 7 x body weight

Bio mechanics of ACL Prevent anterior tibial displacement on femur Secondarily, prevents hyperextension, varus &

valgus stresses Least stress on ACL between 30-60 degrees of

flexion

Anteromedial bundle tight in flexion & extension

Posterior lateral bundle tight only in extension

PCL Biomechanics Primary stabilizer of the knee against posterior

movement of the tibia on the femur resists rotation, esp.internal rotation of tibia on

femur

Two bundles Anterolateral, taut in flexion Posteromedial, taut in extension

Thank you