Knee Biomechanics
Knee BiomechanicsDr. Bhuvanesh Gopal
DefinitionBiomechanics is the science of the action of forces, internal or external on the living body.Statics is the study of forces on bodies at restDynamics is the study of the motion of bodies and the forces that produce the motion
JOINT BIOMECHANICSDegree of freedomJoint reaction forceCoupled forcesJoint congruenceInstant center of rotationFriction and lubrication
KINEMATICS Kinematics is the study of motion in terms of displacement, velocity, and acceleration with reference to the cause of the motionKinesiology is the the study of human movement and motion
KINEMATICS - Knee JointHinge typeROM Ext 10-15 degreesFlex 130-150
JOINT MOTIONJ -shaped curveBoth rolling and sliding motion
J-Curve
ROTATIONAxis lies close to medial condyleAt 90 degree flexion45 degree ER30 degree IR
Adduction and abduction0 degree at full extensionAround 10 degrees at 30 degrees of knee flexion
Adduction and abduction0 degree at full extensionAround 10 degrees at 30 degrees of knee flexion
MENISCI
MENISCI
MENISCIFibrocartilagenous crescent; triangular in cross-sectionLateral meniscus is more circular; medial meniscus more c-shapedLateral meniscus has twice the excursion of the medial meniscus during knee motion.Anterior horn of LM & post horns of both menisci attach to the intercondylar eminence
MENISCI
MENISCI ContdAnterior horns attached to each other by the intermeniscal ligamentPopliteus muscle is attached to lateral meniscus (not the tendon)Semimembranosis is attached to medial meniscus
MENISCI
MENISCI ContdProvision of stabilityShock absorptionProvision of increased congruityAids lubricationPrevents synovial impingementLimits extremes of flexion & extensionTransmits loads across the joint 50- 100% of load is transmitted through menisciReduces contact stresses
MENISCI ContdThe compression of the menisci by the tibia and the femur generates outward forces that push the meniscus out from between the bones.The circumferential tension in the menisci counteracts this radial force.
HOOP STRESSHoop stress is the stress in a direction perpendicular to the axis of an itemAs the thickness of the item decreases the hoop stress increases
MENISCI ContdThese hoop forces are transmitted to the tibia through the strong anterior and posterior attachments of the menisci.This hoop tension is lost when a single radial cut or tear extends to the capsular margin and that in terms of load-bearing, a single radial cut through the meniscus is equivalent to meniscectomy.
MENISCECTOMYDecrease in TF contact area and increase in contact stress.Partial Meniscetomy 65% increase in contact stress.Total Meniscetomy 235%
SCREW HOME MECHANISMLocking Femur internally rotates( external tibial torsion) during last 10-20 degrees of extension
FEMORAL ROLL BACKPosterior roll back of femur on tibia increases during knee flexionPCL0.5cm of excursion of the medial meniscus and 1.1cm of excursion of lateral meniscus during a 0- 120 degree arc of knee motion
KINETICSExtension quadriceps mech via patellar apparatusFlexion hamstrings Knee stablizers
ACL
ACLIntraarticular extrasynovialAnteromedial fibers - tight in flexion - limits anterior translation of tibia on femurPosterolateral fibers - tight in extension - limits anterior translation plus external rotationBl.supply - middle genicular a. (post) & synovial vv (ant)Mechanoceptors with a proprioceptive roleAcl strength = 50% pcl strengthLoad to failure = 1700n
ACL
ACL
PCL
PCL2 bundles: posteromedial and anterolateralFunction:Limits hyperextensionPrevents post translation of tibia on femur especially during flexion
AXIS OF LOWER EXTREMITY
MECHANICAL AXES OF LOWER EXTREMITY
Hip joint CentreMechanical AxisAnkle Joint Centre
Knee Joint
VERTICAL AXIS
VerticalAxisFemoral ShaftAxisMechanicalAxisTransverseKnee AxisTransverseAnkle Axis
6933390
ANATOMICAL AXIS
Anatomic AxisTibiofemoral AngleMechanical Axis6
MECHANICAL AXIS OF TIBIA
from the centre of tibial plateau to the centre of tibial plafond
MECHANICAL VALGUS/ VARUS ALIGNMENT
AXfmAXfmAXtmAXtmAXIImFemurTibiaAnkleKneeFemur Head
Tibial articular surface is normally 3 degree varus with respect to mechanical axisFemoral articular surface is normally 9 degree valgus
The mechanical axis of the lower extremity is in 3 degree of valgus from the vertical axisThe anatomic axis of the femur is in 6 degrees of valgus, the mechanical axis(9 degree valgus with the vertical axis)The anatomic axis of the tibia is in 2-3 degrees of varus from the mechanical axis
ArthrodesisThe position for knee arthrodesis should be 0-7 degrees of valgus and 10-15 degreees of flexion
Neutral Femoral Rotational Axis
Whitesides LineTEAPCA
PATELLOFEMORAL JOINTPatella Pulley / changes the direction of pullEnhances the moment arm of quadricepsVaries from 6cm at full extension to about 4cm at 120 degree flexionForces at the patellofemoral jt. tend to increase with quadriceps muscle force except during the last 15-20 degrees of ext.
Q - ANGLEBrattstromQ angle formed by the line of pull of the quadriceps mechanism and that of the patellar tendon as they intersect at the centre of the patellaMales: 8-10 degreesFemales: 15 degrees 5 degrees
Q - Angle
Q-AngleLine 1ASIS to midpoint Of patellaLine 2Tibial tubercle to midpoint of patellaMidpoint of patellaTibial tubercleAnterior Superior Iliac Spine (ASIS)Line 1Line 2
FACTORS INCREASING Q ANGLEGenu valgumIncreased femoral anteversionExternal tibial torsionLaterally positioned tibial tuberosityTight lateral retinaculum
GOALS OF KNEE REPLACEMENTRestoring mechanical alignmentRestoring the joint lineBalancing ligamentsMaintaining a normal Q-angle
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