Biomechanics Biomechanics of of

the Kneethe Knee

Bony StructuresBony Structures

• Complex joint consists of– Femur– Tibia– Fibula– Patella

• Hinge joint w/ a rotational component

Joints of the Knee

• Tibiofermoral joint

– Dual condyloid

articulations

between the medial

and lateral condyles

of the tibia and the

femur

• Patellofemoral joint

– Articulation between the patella and the femur

– The patella improves the mechanical advantage of the knee extensors by as much as 50%

Joints of the knee

• Collateral ligaments - cross the

medial and lateral aspects of the

knee

– Medial Collateral ligament

– Lateral Collateral ligament

Ligamentous Support

lateral (fibular)

medial (tibial)

Collateral Ligaments

Prevents abduction and adduction movement of the knee

Ligamentous Support

• Cruciate ligaments - cross each

other in connecting the anterior

and posterior aspects of the knee

– Anterior cruciate ligament

– Posterior cruciate ligament

Anterior Cruciate Ligament

• Prevent anterior luxation

• Primary static restraint to anterior

displacement

• Limit tibia rotation upon femur

– Esp, internal rotation

• Limit valgus and varus stress upon

knee

Anterior Cruciate (ACL)

Anterior Cruciate Ligament

• Knee stabilizer

• Primary restraint to posterior

displacement 90%

• Limit internal rotation of tibia upon

femur during weight bearing

• Resist hyperextension

Posterior Cruciate Ligament

Posterior Crucuate (PCL)

shorter and stronger thanACL

Posterior Cruciate Ligament

FEMUR

TIBIA

PATELLA

The ACL prevents the femur from sliding posteriorly on the tibia or the tibia from sliding anteriorly on the femur

The PCL prevents the femur from sliding anteriorly on the tibia or the tibia from sliding posteriorly on the femur

Additional Ligamentous

Support

•iliotibial bandthick, strong band of tissue connecting tensor fascia latae to femur and tibia

Menisci

• Cartilaginous discs located between

the tibial and femoral condyles

• The menisci distribute the load at

the knee over a large surface area

and also help absorb shock

• The lateral meniscus is smaller and

more mobile than the medial

meniscus

• The inner portion of the menisci are

avascular

Menisci

Menisci Function• Increases stability by deepening tibial

plateaus

• Decreases friction by 20%

• Increases contact area by 70%

• Absorbs shock

• Distribute pressure between femur and tibia in weight bearing

• Balance intra-articular pressure of muscle action

Menisci Function• Increases stability by deepening

tibial plateaus• Decreases friction by 20%• Increases contact area by 70%• Absorbs shock• Distribute pressure between femur

and tibia in weight bearing• Balance intra-articular pressure of

muscle action

Bursae & Fat Pad of the Knee

• Stability is due primarily to

ligaments, joint capsule and

muscles surrounding the joint

• Designed for stability w/ weight

bearing and mobility in locomotion

Knee Articulations

Knee Articulations

• Tibiofemoral Joint– Modified hinge

joint

– 2-3 degrees of freedom

– Articulating surfaced are not congruent

Knee Articulations

• Proximal

Tibiofibular Joint

– Syndesmosis joint

Knee Articulations

• Patellofemoral Joint– Medial and lateral facets of the femoral

condyles articulate with patella

Knee Movements

Axis of Rotation

Axis of Rotation

Knee Goniometry

• Flexion– 0-130-140 degrees

• Extension– 0 degree

• Screw Home Mechanism

Screw Home Mechanism• Locking mechanism as the knee nears its

final extension– Automatic rotation of the tibia externally

(approx. 10 degrees) during the last 20 degrees of knee extension

• Forms a close-packed position for the knee joint

• Femoral condyles are a different size– Causes internal rotation when the knee

is flexed and external rotation when the knee is extended

Knee Rotation

FlexionExternalRotation

InternalRotation

Extension

Patella migrates posteriorly from extension to flexion

30 60 90

Patellar Translation

• Normal length of patellar tendon = patellar height: 1:1 ratio

Patellar Contact Areas

1200

Patellar Contact Areas

Mechanical Function of Patella

(1) Increases angle of pull of quads on tibia, improves the ratio of motive: resistive torque by 50%

(2) Centralizes divergent tension of quads into a single line of action

(3) Some protection of anterior aspect of knee

without patella

with patella

Mechanical Function of Patella

• Patella contributes to quadriceps moment arm– 13% at 90o

– 31% at 0o

• No angle without patella & therefore no compressive resultant force

• Stabilizes patella in trochlea groove

• Patella assures “some” compression in full extension

• Patella not in femoral sulcus however

Sagittal Plane PF Joint Sagittal Plane PF Joint Mechanics: Mechanics: PF Compressive PF Compressive Force FunctionForce Function

Patellofemoral Compressive Force

Mechanics

• PFC force with flexion– 0.5 x BW gait– 3.4 x BW stairs– 8.8 x BW squatting

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

– A line from the middle of

the patella to the tibial

tuberosity

Q-angle

• Knee in extension– Normal – males 13 degrees– Normal - females – 18 degrees

• Knee in 90 degrees flexion– Both genders – 8 degrees

Atypical Q-anglesbowleggedness

knock-knees

Genu Recurvatum

knee hyperextension

Posture & WB Forces

• The mechanical axis of TF joint is the weight bearing line from the center of femoral head to superior talus center

• Allows WB in stance of the medial = lateral Tibiofemoral compartments

• Increase in valgus results: – Compression

overload to the lateral Tibiofemoral compartment

– Distraction overload to medial Tibiofemoral compartment

Posture & WB Forces

• Decrease in valgus results – Compression

overload to the medial Tibiofemoral compartment

– Distraction overload to lateral Tibiofemoral compartment

Posture & WB Forces

Joint Mechanics

• Resultant force has a tendency to laterally translate the patella

• Laterally displace tibial tubercle– external tibial rotation– external tibial torsion

• Medially displace patella– internal femoral rotation– femoral anteversion

• Laterally displace ASIS (ASIS)– females

Joint Mechanics

Tibial Torsion

• Tibial torsion– An angle that

measures less than 15 degrees is an indication of tibial torsion

– I nward twisting of the tibia (and is the most

common cause of intoeing

Movements of the knee

• Flexion– hamstrings– assisted by:

• gracilis• sartorius• popliteus• gastrocnemius

Muscle Pull

Gracilis

Sartorious

Popliteus

Gastrocnemius

Movements of the knee

• Extension– quadriceps:

• rectus femoris• vastus lateralis• vastus medialis• vastus intermedius

Rectus femoris

Vastus lateralis

Vastus intermediate

Vastus medialis

Muscle Pull

Loads on Knee

• Forces at tibiofemoral Joint

– Shear stress is greater during open

kinetic chain exercises such as knee

extensions and knee flexions

– Compressive stress is greater during

closed kinetic chain exercises such as

squats and weight bearing exercises

Loads on Knee

• Forces at Patellofemoral Joint

– With a squat, reaction force is 7.6

times BW on this joint

•Beneficial to rehab of cruciate

ligament or patellofemoral surgery