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