knee biomechanics
DESCRIPTION
knee biomechanicsTRANSCRIPT
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