Anatomy biomechanics & kinematics of the knee

Knee Anatomy

Femoral Anatomy

The largest and most complicated

joint in the body

Consists of three joints

( compartment)

medial & lateral tibio-femoral joints

patello-femoral joint

Sustains large forces between the

body’s two longest lever arms

Femoral Anatomy

Femoral Anatomy

Femoral Anatomy

The medial & lateral femoral condyles

have different Sagittal radii

The distal medial condyle is shorter, narrower

and more oblique than the lateral condyle

Medial Lateral Lateral Medial

The Patella

Oval shape wider medial to lateral

Diameter 30 – 55mm

Thickness 19 – 26mm

Bi-concave posterior surface with 4 – 5mm thick articular cartilage

Articulates with the trochlear groove

Tibial Anatomy

Tibial Anatomy

Tibial Anatomy

Lateral

The medial condyle is concave making the medial compartment more stable than the convex lateral side

The metaphysis is angled posteriorly and the plateau slops posteriorly from 3° – 15°

Lateral

Medial

Tibial Anatomy

The intercondylar eminence divides the tibial plateau

contributes to M/L stability and provides attachment for

the menisci and the ACL

The lateral side is more circular than the longer medial side

The patella ligament inserts into the tibial tuberosity

The Menisci

Maintain contact between the femur and the

tibia and bear 60% of the loads in the knee

Lateral: moves 10 – 12mm A/P

Medial: moves 4 – 5mm A/P

Lateral co-lateral ligament

Lateral meniscus

Posterior cruciate ligament

Synovial membrane

Infrapatellar fat padAnterior cruciate ligament

Patellar ligament

Medial co-lateral ligament

The Menisci

Coronal cross section

Lateral Medial

Knee Stabilisers

Static:Congruent Articular Geometry Co-

lateral ligaments Cruciate ligaments Capsule

Dynamic:MusclesMenisci

Cruciate Ligaments

So called because they cross in the coronal and sagittal planes

Provide antero-posterior and some medio-lateral stability

Interact with the MCL LCL and the menisci to control motion

In flexion the ACL is almost horizontal and the PCL vertical this reverses in extension

The Anterior Cruciate Ligament

Originates in the intercondylar notch on internal aspect of the lateral femoral condyle

The tibial insertion is anterior and medial and consists of three distinct groups of fibres

Prevents anterior displacement of the tibia

The Posterior Cruciate Ligament

Originates in the intercondylar notch on the postero-medial aspect of the femoral condyle

The tibial insertion is long extending from the intercondylar eminence on the posterior tibial plateau inferiorly for 1 – 2cms

Consists of four distinct groups of fibres

Prevents posterior displacement of the tibia

Collateral Ligaments

Taut in extension to provide medio-lateral stability and looser in flexion to allow rotation of the tibia

Lateral co-lateral ligament

Medial co-lateral ligament

The Medial Collateral Ligament

Broad & fan-shaped originates on the medialfemoral epicondyle inserts 4 – 5cm distal tothe tibial plateau

Consists of two bundles

Anterior free of capsular attachment

Posterior blends with the medial meniscusand the joint capsule

The Lateral Collateral Ligament

Narrow and cord like

originates on lateral femoral epicondyle

Inserts on the head of the fibula

free of any meniscal or capsular attachments

Limb and Joint Alignments

Limb & Joint Alignment

Anatomic AxisA line connecting the centre of a bone proximally to the centre of a bone distally

Mechanical AxisA line connecting the point of input of a load on a bone to its output to an associated structure

e,g. The centre of the femoral head to the centre of the knee

Limb & Joint Alignment

Anatomic Axis Mechanical Axis

The HKA Axis

A line connecting the centre of the femoral head the centre of the knee and the centre of ankle

This line runs inferiorly medial forming an angle of approx. 3° to the midline in normal stance

The joint line is perpendicular to the midline and therefore lies approx. 3° medially oblique to the HKA axis

Limb & Joint Alignment

Varus = Towards the Midline

Valgus = Away from the Midline

The tibia & femur do not form a

straight line but form an obtuse

angle of 170° – 175° the average

being 173° which is the physiological

Valgus of the knee 173°

Limb & Joint Alignment

Valgus deformity Varus deformity

Neutral Alignment of the femoral A/P cut will usually produce a trapezoidal Flexion Gap

3° external rotation of the femoral A/P cut will usually produce a parallel flexion gap

Femoral Alignment

Kinematics

Kinematics The Study of Joint Motion

The knee does not flex around a fixed centre it is capable of axial rotation and transverse movements

During the first 20° of flexion the femur moves posteriorly on the tibia femoral " roll-back "

Roll-back is initiated and controlled by the cruciate ligaments

As flexion increases roll-back stops and the femoral condyles slide on the the tibial plateau allowing the knee to flex

Femoral Roll-Back

Rolling only would cause the knee to dislocate as the distance around the femoral condyles is approximately twice the A/P width of the tibial plateau

Sliding only would cause impingement of the posterior femoral shaft on the posterior tibial plateau and block flexion

Rolling and sliding together allow the knee to remain stable and flex fully

Range of Motion

Active FlexionWhen the hip is in extension 120°

When the hip is in flexion 140°

Passive Flexion 160°

Rotation Is only possible in flexion

40° lateral 30° medial at 90° of flexion

Angle of flexion required for daily activities

Walking : 0° – 67° Climbing stairs : 0° – 83°Descending stairs : 0° – 90°

Sitting down : 0° – 93° Tying a shoe : 0° – 106°

Lifting an object : 0° – 117°

Biomechanics

Forces during gait

Heel strikegenerates 2 – 3 x bodyweight associated with the

contraction of the hamstrings

Stance phasegenerates 2 x bodyweight and is associated with

contraction the of the quadriceps

Toe offgenerates 2 – 4 x bodyweight

and is associated with contraction of gastrocnemius

Forces during gait

Ground reaction force (GRF) occurs

during gait from heel strike to toe off

GRF is counterbalanced the joint reaction force and the patella tendon force

For 1 bodyweight the GRF is 700N The patella ligament exerts a force of 2100N Therefore the tibio-femoraljoint reaction force is 2800N

Biomechanics

Loads transmitted across the knee

Walking 2 – 4 BW

Running 3 – 5 BW

Stairs 5 – 7 BW

Parachute jump 20 BW

The Extensor Mechanism

Made up of the 4 quadriceps muscles and the patella

The quadriceps muscles are responsible for knee extension

Help to prevent posterior displacement of the tibia

Vastus medialis

Vastus lateralis

Rectus femoris

Vastus intermedius

The Extensor Mechanism

The patella increases the

efficiency and guides the pull

of the quadriceps

The patella stays with the femur

when the tibia rotates it is

stabilised by it’s congruent fit

in the trochlear groove and the

medial and lateral retinaculae

Lateral retiaculum

Medial retinaculum

Joint Reaction Force

Patello-femoral joint reaction force

is a vector force ranging from

0.5 BW at 9° of flexion to

7 – 8 BW at 130° of flexion

Joint Reaction Force

The patellar moment arm r can be changed during patellar reconstruction

Excessive bone resection will reduce r and the quadriceps will have to pull harder

Insufficient bone resection will

increase r producing high patello-femoral contact forces

Both increase the PFJRF and may lead to patellar instability, pain, patella fracture, loosening, and excessive polyethylene wear

The ‘Q’ Angle

The angle between a line drawn from the centre of the patella to the anterior superior iliac crest and a line drawn from the centre of the tibial tuberosity through the centre of patella normally 15°

Any increase in the Q angle will predispose the patella to instability

Tibial rotation has the greatest effect on the Q angle

Summary

The knee is capable of complex motion and sustains high

dynamic loads during daily activities

Both tibio-femoral and patello-femoral articulations play a part

in the function of the knee

The knee is able to dissipate high loads through the muscles

and ligaments as well as the more compliant tissues of the

menisci and cartilage

Summary

If the knee is damaged the biomechanics change

the natural knee can compromise to an extent

Prosthetic replacements must restore function and be

capable of sustaining high dynamic loads in both the

aligned and mal-aligned condition

Prosthetic designs focus around load dissipation and

lowering wear in the tibio-femoral and patello-femoral

articulations

Anatomy Biomechanics & Kinematics of the Knee

Femoral Component 6° of Freedom

Anterior / Posterior

Anterior: Not enough posterior condyles,

Patella Kinematics

Posterior: Anterior Notch, elongation of

Posterior Condyles = Tight in Flexion

Medial / Lateral

Proximal / Distal

Femoral Component 6° of Freedom

4. Varus / Valgus5. Flexion / Extension:

Gross flexion: The prosthesis has to hyper extend in extensionGross Extension: Tends to notch the anterior cortex

6. Internal / External

Tibial 6° of Freedom

1. Anterior / Posterior

2. Medial / Lateral

3. Proximal / Distal (Resection Level)

4. Varus / Valgus Rotation

5. Flexion / Extension (Posterior Slope)

6. Internal / External Rotation

The Varus / Valgus position is the most important