Knee biomechanics dr.bhuvanesh

KNEE BIOMECHANICS
DR. BHUVANESH GOPAL

Definition
 Biomechanics is the science of the action of forces, internal or
external on the living body.
 Statics is the study of forces on bodies at rest
 Dynamics is the study of the motion of bodies and the forces that
produce the motion

JOINT BIOMECHANICS
 Degree of freedom
 Joint reaction force
 Coupled forces
 Joint congruence
 Instant center of rotation
 Friction and lubrication

KINEMATICS
 Kinematics is the study of motion in terms of displacement, velocity,
and acceleration with reference to the cause of the motion
 Kinesiology is the the study of human movement and motion

KINEMATICS - Knee Joint
 Hinge type
 ROM
• Ext 10-15 degrees
• Flex 130-150

JOINT MOTION
 J -shaped curve
 Both rolling and sliding motion

ROTATION
 Axis lies close to medial condyle
 At 90 degree flexion
• 45 degree ER
• 30 degree IR

Adduction and abduction
• 0 degree at full extension
• Around 10 degrees at 30 degrees of knee flexion

MENISCI
 Fibrocartilagenous crescent; triangular in cross-section
 Lateral meniscus is more circular; medial meniscus more c-shaped
 Lateral 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 Contd…
 Anterior horns attached to each other by the intermeniscal ligament
 Popliteus muscle is attached to lateral meniscus (not the tendon)
 Semimembranosis is attached to medial meniscus

MENISCI Contd…
 Provision of stability
 Shock absorption
 Provision of increased congruity
 Aids lubrication
 Prevents synovial impingement
 Limits extremes of flexion & extension
 Transmits loads across the joint – 50- 100% of load is transmitted
through menisci
 Reduces contact stresses

MENISCI Contd…
 The 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 STRESS
 Hoop stress is the stress in a direction perpendicular to the axis of an
item
 As the thickness of the item decreases the hoop stress increases

MENISCI Contd…
 These 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.

MENISCECTOMY
 Decrease in TF contact area and increase in contact stress.
 Partial Meniscetomy  65% increase in contact stress.
 Total Meniscetomy  235%

SCREW HOME MECHANISM
• Locking
• Femur internally rotates( external tibial torsion) during last 10-20
degrees of extension

Knee biomechanics dr.bhuvanesh

FEMORAL ROLL BACK
 Posterior roll back of femur on tibia increases during knee flexion
 PCL
 0.5cm of excursion of the medial meniscus and 1.1cm of excursion of
lateral meniscus during a 0- 120 degree arc of knee motion

KINETICS
 Extension – quadriceps mech via patellar apparatus
 Flexion – hamstrings
 Knee stablizers

ACL
 Intraarticular extrasynovial
 Anteromedial fibers - tight in flexion - limits anterior translation of
tibia on femur
 Posterolateral fibers - tight in extension - limits anterior translation
plus external rotation
 Bl.supply - middle genicular a. (post) & synovial vv (ant)
 Mechanoceptors with a proprioceptive role
 Acl strength = 50% pcl strength
 Load to failure = 1700n

PCL
 2 bundles: posteromedial and anterolateral
 Function:
• Limits hyperextension
• Prevents post translation of tibia on femur especially during flexion

MECHANICAL
AXES OF
LOWER
EXTREMITY
Hip joint Centre
Mechanical Axis
Ankle Joint Centre
Knee Joint

VERTICAL
AXIS
Vertical
Axis Femoral Shaft
Axis
Mechanical
Axis
Transverse
Knee Axis
Transverse
Ankle Axis
6◦
9◦3◦
3◦ 3◦
90◦

ANATOMICAL
AXIS Anatomic Axis
Tibiofemoral Angle
Mechanical Axis
6◦

MECHANICAL AXIS OF TIBIA
from the centre of tibial plateau to the centre of tibial plafond

MECHANICAL VALGUS/ VARUS
ALIGNMENT
AXfm
AXfm
AXtm
AXtm
AXIIm
Femur
Tibia
Ankle
Knee
Femur Head

 Tibial articular surface is normally 3 degree varus with respect to
mechanical axis
 Femoral articular surface is normally 9 degree valgus

 The mechanical axis of the lower extremity is in 3 degree of valgus
from the vertical axis
 The 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

Arthrodesis
• The position for knee arthrodesis should be 0-7 degrees of valgus and
10-15 degreees of flexion

Neutral
Femoral
Rotational
Axis
Whiteside’s Line
TEA
PCA

PATELLOFEMORAL
JOINT
 Patella
• Pulley / changes the
direction of pull
• Enhances the moment arm of
quadriceps
• Varies from 6cm at full
extension to about 4cm at
120 degree flexion
 Forces at the patellofemoral
jt. tend to increase with
quadriceps muscle force
except during the last 15-20
degrees of ext.

Q - ANGLE
 Brattstrom
• Q 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 patella
 Males: 8-10 degrees
 Females: 15 degrees ± 5 degrees

Q - Angle
Q-Angle Line 1
ASIS to midpoint
Of patella
Line 2
Tibial tubercle to
midpoint of patella
Midpoint
of patella
Tibial tubercle
Anterior Superior
Iliac Spine (ASIS)
Line 1
Line 2

FACTORS INCREASING Q ANGLE
 Genu valgum
 Increased femoral anteversion
 External tibial torsion
 Laterally positioned tibial tuberosity
 Tight lateral retinaculum

GOALS OF KNEE REPLACEMENT
 Restoring mechanical alignment
 Restoring the joint line
 Balancing ligaments
 Maintaining a normal Q-angle

Knee biomechanics dr.bhuvanesh

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