The knee is a complex joint composed of the tibiofemoral and patellofemoral joints. It functions to provide mobility and support body weight during both static and dynamic activities. The knee joint contains menisci that increase joint congruence and distribute weight forces. It also contains cruciate and collateral ligaments that restrict motion and provide stability. During flexion and extension, the tibia glides and rotates on the femur through rolling and sliding motions controlled by the ligaments and menisci.

1. BIOMECHANICS
THE KNEE COMPLEX

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KNEE JOINT

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INTRODUCTION
FUNCTIONS OF KNEE ARE:
1. Provide mobility
2. Support body during Dynamic & Static
activities.
3. In closed K chain it works with hip & ankle
joints to support body wt. in static erect
posture.
4. Dynamically, support during sitting & squatting
activities and transferring body wt. during
locomotor activities.
5. In open K chain knee provides mobility for foot
in space.

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ARTICULATIONS
Knee complex composed of 2
articulations within a single
capsule:
1. The tibiofemoral joint
2. The patellofemoral joint

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TIBIOFEMORAL JOINT
1. Is a double condyloid joint with
medial & lateral articular surfaces.
2. Flexion and Extension occur in
sagittal plane around a coronal
axis.
3. Medial & Lateral rotation occur in
transverse plane & vertical axis.

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FEMORAL ARTICULAR SURFACE
Medial & Lateral femoral condyles
forms proximal articular surface.
Patellar groove.

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TIBIAL ARTICULAR SURFACE
This corresponds to femoral articular
surface.
They are 2 concave medial and
lateral asymmetrical plateaus.
Medial condyle is 50% larger than
lateral condyle.

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MENISCI
Two asymmetrical fibro cartilaginous joint
disks called MENISCI are located on tibial
condyles that enhances the congruence of
knee joint.
Med.Men. is semi circle.
Lat.Men. is 4/5 of a ring.
Both men. open towards intercondylar
area.
They are thick peripherally & thin centrally
forming concavities.

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MENISCI
USES.
1. Increases joint congruence.
2. Distribute weight bearing forces.
3. Reduces friction between joints.
4. Serve as shock absorbers.

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MENISCI
ATTATCHMENTS:
1.Both Men. are attached to,
Intercondylar tubercles of tibia.
Tibial condyle via coronary lig.
Patella via patellomeniscal & patellofemoral ligs.
Transverse lig.
ACL.
2.Med. Men. attached to,
MCL.
Semitendinous muscle.
3.Lat. Men. attached to,
Ant. & Post. menisco femoral ligs.
PCL.
Popliteus muscle.

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MENISCI
OTHER FACTS
Meniscal complex is well established in 8 weeks old
embryo.
Well vascularised in 1st yr. of life.
Vascularity gradually reduces from 18 months to 18 yrs.
Over age 50 only periphery is vascularised.
The horns remain vascularised throughout life.
Whole meniscal complex is well innervated.
Innervation is denser in post. horns. May be due to greater
load on post. horns.
The meniscal innervation is a source of information about:
1. Joint position.
2. Movt. Direction.
3. Movt. Velocity.
4. Tissue formation.

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TF ALLIGNMENT & WEIGHT
BEARING FORCES
Anatomical or Longitudinal axis of
femur is oblique falling medially &
inferiorly.
Anatomical axis of tibia is almost
vertical.
These 2 lines forms an angle of
185-190° medially at the knee
i.e. physiologic valgus angle.
Mechanical axis of lower
extremity falls from head of
femur to sup. Surface of head of
talus.
This line passes through the knee
joint in bilateral static stance
giving equal forces to both the
condyles.
If the med.TF angle is more than
195˚(>165˚lat.) it is Genu
Valgum or Knock Knee.
If the med.TF angle is less than
180˚(<180˚ lat.) it is Genu
Varum or Bow Legs.

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TIBIOFEMORAL JOINT
REACTION FORCE
JRF reach 2-3 times body weight in
normal gait.
JRF reach 5-6 times B W in running,
stair climbing etc.
This time menisci assume 40-60% of
imposed load.
In the absence of menisci JRF
doubles on femur & increase 6-7
times on tibial condyle.

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KNEE JOINT CAPSULE
Restrict various movts.
To maintain joint integrity and normal joint function.
ATTATCHMENTS
Postly. :proximally to post. margins of femoral condyles
and inter condylar notch. Distally to post.tibial condyle.
Med. & Lat. :Prox. above femoral condyle & dist. margins of
tibial condyle.
Collateral ligs. reinforce sides of capsule.
Antly. : Patella,Q’ceps muscle superiorly and patellar lig.
inferiorly.
Anterolaterally & anteromedially expansions from vastus
med. & vastus lat. From patella to collateral ligs. And tibial
condyles

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EXTENSOR RETINACULUM
Anteromedial & anterolateral portions of capsule
are known as Extensor retinaculum or medial &
lateral patellar retinacula.
Have 2 layers. Deeper and Superficial.
Deeper longitudinal fibers connects capsule
anteriorly to menisci and tibia via coronary
ligs.This is known as patellomeniscal bands.
Superficial transverse fibers blend with fibers of
vastus medialis & lateralis and to post. tibial
condyles.
The transverse fibers connecting patella and
femoral condyle are known as patellofemoral ligs.

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SYNOVIAL LINING
Most extensive and involved in the body.
Anteriorly synovium adheres to inner wall of the
joint.
Posteriorly synovium invaginate anteriorly
following the contour of femoral intercondylar
notch & adheres to the ant. aspect and sides of
ACL & PCL.
Embryonically synovial lining is divided by septa
into 3 compartments.
• Superior PF compartment.
• Medial TF compartment.
• Lateral TF compartment.

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SYNOVIUM (cont )
By 12 weeks of gestation synovial septa resorbed
resulting in a single joint cavity.
The superior compartment remain as a superior
recess of capsule known as Suprapatellar bursa.
Posteriorly the synovial lining may invaginate :
• Laterally between popliteus muscle and lat. Femoral
condyle.
• Medially invaginate between semimembranosus tendon,
med. head of gastrocnemius tendon and med. femoral
condyle.

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SYNOVIUM (cont )
PLICAE
Synovial septa which are not resorbed into adulthood exist as
folds or pleats of synovial tissue known as Plicae.
They are composed of loose, pliant and elastic fibrous
connective tissue.
They easily passes back and forth over femoral condyles as the
knee flexes and extends.
Observed in 20 to 60% of population.
Commonly known plicae are:
1. Inferior/Infrapatellar plica extends from ant. portion of
intercondylar notch to infrapatellar fat pad.
2. Superior/Suprapatellar plica is located between suprapatellar
bursa and knee joint.
3. Medial/ Mediopatellar plica arises from medial wall of
retinaculum to infrapatellar fat pad.
Plica can become irritated and inflamed, leads to pain, effusion
and changes in joint structure and function.

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KNEE JOINT LIGAMENTS
The knee ligaments are credited with
restricting and controlling:
1. Excessive knee motion
2. Varus and valgus stresses at knee
3. Anterior and posterior displacement of
tibia beneath femur
4. Medial and lateral rotation of tibia
beneath femur
5. Stabilization in anteroposterior
displacements and rotations of tibia
known as rotary stabilization.

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COLLATERAL LIGAMENTS

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CRUCIATE LIGAMENTS

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Anatomy of the ACL
3 strands
Anterior medial tibia to
posterior lateral femur
Prevent anterior tibial
displacement on femur
Secondarily, prevents
hyperextension, varus &
valgus stresses

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Biomechanics of the ACL
Most injuries occur in Closed
Kinetic Chain
Least stress on ACL between
30-60 degrees of flexion
Anteromedial bundle tight
in flexion & extension
Posterior lateral bundle
tight only in extension

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Posterior Cruciate Ligament
Two bundles
• Anteromedial, taut in
flexion
• Posterolateral, taut in
extension
Orientation prevents
posterior motion of tibia
PCL larger & stronger
than ACL

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PCL Biomechanics
Functions:
• Primary stabilizer of the
knee against posterior
movement of the tibia
on the femur
• Prevents flexion,
extension, and
hyperextension
Taut at 30 degrees of
flexion
• posterior lateral fibers
loose in early flexion

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PCL Biomechanics
PCL
• Primary restraint to
post. Displacement -
90%
• relaxed in extension,
tense in flexion
• reinforced by
Humphreys/ant. MF lig.
or Wrisberg/post.MF lig.
• restraint to varus
/valgus force
• resists rotation,
especially int. rotn. of
tibia on femur

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KNEE JOINT MOTION
Flexion/Extension
The axis for these movements lies medially oblique
through the joint
This causes the tibia to move from a position slightly
lateral to the femur in full extension to a position
medial to the femur in full flexion
Passive ROM 0-130˚
Normal gait on level ground needs 60˚ of flexion
80° for stair climbing
90° for sitting down on a chair
Excessive knee hyper extension is termed as Genu
recurvatum

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ARTHROKINEMATICS
flexion/extension
Femur rolling &
gliding at 25°
Anterior glide is due
to
1. Tension of ACL
2. The menisci
Extension is reverse
roll, spin and glide
1. Tension of PCL
2. menisci

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LOCKING AND UNLOCKING
DURING KNEE
EXTENSION
The tibia glides
anteriorly on the femur.
During the last 20
degrees of knee
extension, anterior
tibial glide persists on
the tibia's medial
condyle because its
articular surface is
longer in that dimension
than the lateral
condyle's.
Prolonged anterior glide
on the medial side
produces external tibial
rotation, the "screw-
home" mechanism.

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LOCKING AND UNLOCKING
DURING KNEE FLEXION
When the knee begins to flex from
a position of full extension,
posterior tibial glide begins first
on the longer medial condyle.
Between 0 deg. extension and 20
deg. of flexion, posterior glide on
the medial side produces relative
tibial internal rotation, a reversal
of the screw-home mechanism.
popliteus is the muscle that
‘unlocks’ the knee at the beginning
of flexion of the fully extended
knee. As the extended and locked
knee prepares to flex (when
beginning to descend into a squat
position), the popliteus provides
an external rotation torque to the
femur that mechanically unlocks
the knee. Since the knee is
mechanically locked by a combo of
extension and slight IR of the
femur on a fixed tibia, unlocking
the knee requires that the femur
ER on the fixed tibia.

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KNEE JOINT MOTION
Rotation: in 2 different ways
Axial rotation : occurs around a
longitudinal axis. Med. and lat. rotn. are
possible. This occur in flexed position.
Approximately 60-70° of active or
passive ROM is possible
Terminal or automatic rotation :
associated with locking mechanism

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ARTHROKINEMATICS
Axial rotation
Longitudinal axis for axis
rotation lies at medial
intercondylar tubercle
i.e. med. Condyle act as
pivot point while the lat.
condyle move through a
greater arc of motion
When lat. and med.
rotation of tibia occurs at
knee joint
When lat. and med.
rotation of femur occurs at
knee joint

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Knee Muscles
Knee Extension
• Rectus Femoris
• Vastus Medialis
• Vastus Lateralis
• Vastus Intermedius

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Rectus Femoris
Two-jointed, bipennate
ACTIONS
May contribute to lateral
rotation and abduction of hip
Extends knee and flexes hip
EFFECTS OF WEAKNESS
Decreases knee extension
strength
EFFECTS OF TIGHTNESS
Limits knee flexion range of
motion (ROM) with hip
extended
Hip abduction reduces stretch
of rectus femoris.

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Vastus Medialis
Consists of longitudinal (vastus
medialis longus [VML]) and
oblique (vastus medialis oblique
[VMO]) portions
ACTIONS
Knee extension
Patellar stabilization
Active with other heads
throughout knee extension
EFFECTS OF WEAKNESS
Decreased knee extension
strength
Few data support the belief that
specific vastus medialis weakness
contributes to patellar tracking
abnormalities. And, the VMO has
been refuted to contribute to the
last 15 deg of knee extension.
EFFECTS OF TIGHTNESS
With rest of quadriceps, limits
knee flexion ROM

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Vastus Lateralis
Largest head of quadriceps
in many individuals
ACTIONS
Knee extension
EFFECTS OF WEAKNESS
Significant weakness in
knee extension
EFFECTS OF TIGHTNESS
Decreased knee flexion
ROM
May contribute to
patellofemoral dysfunction

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Vastus Intermedius
Unipennate and deep
ACTIONS
Extends knee and pulls
capsule proximally
EFFECTS OF WEAKNESS
Significant loss of knee
extension strength 15-
40% of PCSA
EFFECTS OF TIGHTNESS
Decreased knee flexion
ROM regardless of hip
position

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Knee Muscles
Knee Flexion—
•Biceps Femoris
•Semitendinosous
•Semimembranosus
•Gastrocnemius
•Plantaris

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Hamstrings
ACTIONS
Knee flexion
Active knee flexion without
rotation requires simultaneous
contraction of medial and lateral
hamstrings.
Contribute to knee joint stability
Hip extension throughout ROM of
hip
May also adduct and rotate hip
EFFECTS OF WEAKNESS
Knee flexion weakness
Larger impairment may be
weakness of hip extension.
EFFECTS OF TIGHTNESS
Decreased knee extension ROM
with the hip flexed
May contribute to knee flexion
contractures and posterior
rotation of pelvis
Considerable evidence from cadaver
studies indicating that the hamstings
muscles decrease the stress/strain on
the ACL

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Hamstring and Gait

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Rotation of the Knee and
hamstrings

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Tightness of
hamstrings can pull
the pelvis into a
posterior pelvic tilt,
flattening the
lumbar spine

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Popliteus
ACTIONS
Small muscle
Increased activity with
combined knee flexion and
medial rotation of tibia
Medially rotates knee
Adds stability to tibiofemoral
joint
EFFECTS OF WEAKNESS
Difficult to determine
Injured with extensive
posterolateral ligamentous
structures
EFFECTS OF TIGHTNESS
Difficult to discern but would
contribute to knee flexion
contractures

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MEDIAL ROTATORS OF THE
KNEE
Sartorius and
Gracilis,
in addition to
medial hamstrings
and popliteus

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Sartorius
Strap muscle with very long
muscle fibers
ACTIONS
Hip flexion with large moment
arms to also abduct and
laterally rotate hip
Knee flexion
Inactive with isolated medial
rotation of knee
EFFECTS OF WEAKNESS
Isolated weakness may have
little effect
EFFECTS OF TIGHTNESS
Small effect but may
contribute to flexion
contracture

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Gracilis
ACTIONS
Medial rotation and
flexion of knee
Adduction of hip
EFFECTS OF
WEAKNESS
No reports but may
affect hip adduction
and knee flexion and
medial rotation
strengths

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Pes Anserinus
Semitendinosus,
sartorius, and
gracilis attach to
tibia by a common
tendon on the
anteromedial
aspect of the tibia.
They effectively
stabilize the medial
aspect of the
knee.

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LATERAL ROTATORS OF THE
KNEE
Tensor fasciae
latae with lateral
hamstrings

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Tensor Fasciae Latae
ACTIONS
Flexes, abducts, and medially
rotates hip
Extends and laterally rotates
knee
Participates in gait with other
abductors, perhaps to
stabilize pelvis or progress
pelvis over stance limb
EFFECTS OF WEAKNESS
Effects small, but reduced hip
abduction, flexion, and medial
rotation strength and
decreased knee extension
strength
EFFECTS OF TIGHTNESS
Decreased hip adduction and
lateral rotation ROM
Associated with knee and
patellofemoral pain and
dysfunction

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Patellofemoral Joint
Patella is a true sesamoid
bone
Posterior surface of the
patella is covered with
thick hyaline cartilage
The patella slides within
the trochlear groove
Function of the patella
• 1) Aids knee extension by
producing anterior
displacement of quadriceps
tendon and lengthening
the lever arm of the quad
muscle force
• 2) Allows wider distribution
of compressive stress on
the femur by increasing
area of contact between
patellar tendon and femur

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PATELLAR MOVEMENTS
In full extension the patella
sits on ant. Surface of femur
In flexion , the patella slides
distally on the femoral
condyles
In full flexion the patella sinks
into the intercondylar notch
The patella in extended knee
has little contact with femoral
sulcus beneath it
First contact made at 10-20°
of flexion on inferior margin
of patella
By 90° flexion all patella got
some contact except odd
facet
Above 90° odd facet gain
contact
At 125° flexion contact is on
lateral facet

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PMJ STABILITY
PFJ is under control of two
restricting mechanisms that
cross each other at right
angles
1. Transverse group of
stabilizers
2. Longitudinal group of
stabilizers
In a so called patellar
tracking both the transverse
and Longitudinal structures
will influence the medial and
lateral positioning of the
patella within the femoral
sulcus
Medial – lateral forces
1. Pull of VL is 12-15° lateral
on long axis of femur
2. Pull of VM is 15-18° medial
with 50-55° pulling of VMO

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Q-Angle
The Q-angle is the angle
formed by a line from the
anterior superior spine of
the ilium to the middle of
the patella and a line from
the middle of the patella to
the tibial tuberosity.
Males typically have Q-
angles between 10 to 14°
Females between 15-17°
A Q- angle of more than
20° or more is considered
to be abnormal creating
excessive lateral forces on
the patella.

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PF joint reaction forces
The patella is pulled
simultaneously by the
quadriceps tendon
superiorly and by the
patella tendon inferiorly
In normal full extension
the patella is suspended
between them
Even a strong contraction
of quadriceps produce no
PF compression
– Minimal quad forces are
required during upright
and relaxed standing
(center of gravity almost
directly above knee)

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PF joint reaction forces
As knee flexion increases,
the center of gravity shifts
posterior, increasing flexion
moments required
Knee flexion affects angle
between patellar tendon
force and quadriceps tendon
force
The total joint reaction force
depends on
1. Magnitude of active or
passive pull of quadriceps
2. Angle of knee flexion
10-15deg flexion: 50% of
BW
Increase flexion, increased
muscle activity up to 3.3xBW
Mechanical advantage is the
biggest between 30-70deg
flexion