Lecture 17 (December 4, 1998)

Biomechanics of the Knee

Functions of the knee:1. Transmit loads2. Participates in motion3. Aids in conservation of momentum (GAIT)

Composed of 2 joints:1. Tibiofemoral 2. Patellofemoral

ROM: Sagittal: 0-140 degrees

Transverse (rotation): Full extension: virtually nothing 90 deg flexion (maximum): ER 45 degrees / IR

30 degrees

Frontal (ab/adduction): Full extension: virtually nothing 30 deg flexion (maximum): few degrees only

Therefore, 2 degrees of freedom double condyloid joint (medial and lateral articulating surfaces

Functional ROM: 0 – 117 degrees (Table 6-1)

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Tibiofemoral Joint

Anatomy of femur

Two condyles separated by the intercondylar notch/fossa

Notch becomes shallow patella groove

Medial condyle is longer anterior-posterior (2/3”)

Medial condyle extends further distally creating a horizontal distal femur in conjunction with oblique angle of femur

Anatomy of tibia

Medial and lateral condyles

Medial condyle is 50% larger than lateral condyle articular cartilage is 3x thicker

two intercondylar tubercles (lodge in intercondylar notch of femur)

Menisci – dynamic as opposed to static structures

Asymmetric, fibrocartilagenous disk-like structures Wedge-shaped

Medial meniscus Semicircular or C-shaped

Lateral meniscus 4/5ths of a ring

open ends of menisci are called horns susceptible to tears

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connected to tibia via coronary ligaments as well as other structures patellomeniscal/patellotibial ligaments – thickening of

anterior joint capsule transverse ligament between anterior horns lateral meniscus to PCL via coronary ligs. and post. capsule

lateral meniscus is more loosely attached to tibia than medial making it less susceptible to tears

IMPORTANT – medial meniscus attaches to the medial collateral ligament

Adult meniscus is poorly vascularized – only on periphery

What are the functions of the menisci?1. Distribute and absorb forces

Joint reaction forces 2-3x BW in walking 5-6x BW in running and stair-climbing menisci assume 40-60% of imposed load removal of meniscus

increases magnitude of forces on articular cartilage

decreases surface area over which forces are distributed

can lead to arthritic changes in cartilage

2. Enhance congruency of the tibiofemoral joint What is congruency?

Refers to the extent to which surfaces complement each other in shape and size (perfect – no gaps between 2 sides of joint)

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3. Assist in proper arthrokinematics

Alignment of Tibiofemoral Joint

Anatomic axes of tibia and femur tibiofemoral 185-190 deg. slight valgus

Given this alignment, what would you say about the compressive/tensile forces on the knee joint?

Not the case since:

Mechanical axis (femoral head talus) is only in 3 deg or less of valgus therefore w\b forces distributed evenly medially and laterally

Tibiofemoral angle > 195 degrees genu valgum

What happens to the forces on the knee joint? Compressive forces increase laterally Tensile forces increase medially

Tibiofemoral joint 180 degrees genu varum

What happens to the forces about the knee? Compressive forces increase medially Tensile forces increase laterally

Tibiofemoral Angle Effect on compressive forces on medial meniscus180 25% increase175 50% increase

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Instantaneous Axis of Rotation: moves with the degree of flexion – semicircular

See figures from Nordin’s book

Arthrokinematics of Tibiofemoral Joint

0 to 25 degrees flexion is primarily ROLLING (Ball analogy) of femoral condyles on tibia

anterior gliding occurs with continued rolling (convex on concave)

anterior gliding offsets the posterior displacement that would result from the rolling PURE SPIN beyond 25 deg. of flexion

wedge shape of meniscus forces femoral condyle to roll uphill as knee flexes anterior shear

Screw-home or locking mechanism

Due to the asymmetry of the condyles the tibia externally rotates during knee extension.

Begins at approximately 30 degrees of flexion and most evident during final 5 degrees of extension

Full extension: tibial tubercles lodged in intercondylar notch menisci tightly interposed between femur and tibia ligaments are taut Closed-packed position

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Passive Knee stabilizers: Joint capsule

Extensor retinaculum – anteromedial and anterolateral portions of the capsule (medial and lateral patellar retinacula)

MCL (runs anteriorly from post. Femur ant. Tibia) Blends with capsule Attaches to medial meniscus Resists valgus stresses esp. when knee is flexed Resists external rotation Secondary to anterior tibial displacement

LCL (runs posteriorly to fibula head) No attachments to meniscus, more distinct ligament Resists varus stresses Resists external rotation and posterior tibial

displacement

ACL Resists anterior tibial translation and internal rotation AMB – lax in extension (max tension 70 deg flexion) PLB – taut in extension Maximum excursion of tibia at 30 degrees of knee

flexion Minor contribution to resist varus and valgus stresses Can create rotation of tibia (IR) with excessive

translation. Injury – caused by flexion and rotation in either

direction With ER tightens as winds around PCL With IR tightens as winds around lateral femoral

condyle

PCL

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Primary restraint to posterior tibial translation Maximal tibial translation occurs with knee flexed at

75-90 degrees AMB – lax in extension (maximal tension in 80-90 deg

of flexion) PLB – taut in extension Minor role in resisting valgus/varus Plays role in creating and restraining rotation Posterior tibial translation is associated with tibial ER Assists in ER for screw-home mechanism Injury mechanism is a hyperextension event

ITB Fascia from TFL, glute max and medius Attaches to linea aspera on femur and lateral tubercle of

tibia Gives rise to the iliopatellar band may cause patella

tracking problems Reinforces anterolateral aspect of knee Resists posterior translation of femur via connection to

BF and VL

Posterior Capsular Ligaments Posteromedially – oblique popliteal ligament –

tendinous expansion of popliteus (lateral fermoral condyle posterior tibia) Arise from semimembranosus central aspect of

capsule Posterolaterally – arcuate ligament

Posterior fibula head tibial intercondylar area and lateral epicondyle of femur

These ligaments are taut in extension check hyperextension

Also arcuate checks varus and popliteal checks valgus

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Patella

Function:1. aids in extension by increasing the moment arm (anatomic

pulley) - > effect at 20 – 40 degrees of flexion reduces forces generation by quads

2. allows for wider distribution of contact forces and reduces friction between quad tendon and femur

3. protection

Anatomy:

Triangular shaped – largest sesamoid bone – least congruent joint

3 facets covered with articular cartilage:1. lateral2. medial (thickest articular cartilage in the body 7mm)3. odd (most medial) – reported in as much as 80% of

population

with flexion the patella translates caudally in fermoral sulcus/trochlea

full flexion sinks in intercondylar notch

from 25 to 130 degrees of flexion the patella:

tilts medially (11 degrees) about a vertical axis to accommodate the asymmetric femoral condyles with malalignment – excessive and irreducible lateral tilt

rotates laterally (7 degrees) about an anterior-posterior axis

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failure of patella to slide, tilt, or rotate properly can lead to: restricted patellofemoral ROM restricted knee ROM patellofemoral tracking problems pain tissue

damage patellofemoral instability

Knee flexion angle Patella contact0 Little or no contact

10-20 Inferior margin across medial & lateral facets flexion Moves proximal and lateral

Beyond 90 deg Medial facet intercondylar notch, odd facet makes contact

At 135 deg Contact via lateral and odd facets only

Medial facet receives most consistent contact (thickest articular cartilage).

Odd facet receives least contact

These two areas are susceptible to degenerative changes.

In general, contact moves from inferior to superior with increased flexion, and from :

Medial and lateral (before 90)Lateral (past 90)Lateral and odd (beyond 90-135)

Note: actual movement of the patella begins lateral and moves medially and then upward.

PFJRF during Gait

10-15 deg 50% of BW

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stair climbing/running hills 3.3 x BW at 60 deg.

deep squats to 130 degs 7.8 x BW

As you flex from 30 to 90 contact area increases and is mainly on medial facet

70-90 patella tendon contacts femur helping to distribute the load

Clinically, it is more important to understand contact stress rather than joint reaction forces. Why?

Stress = force/area

Closed Chain

Stress increases from 0 to 90 deg flexion 90-120 deg., stress decreases or levels off

Open chain

Forces and stress lowest at 90 deg. of flexion and full extension

Clinically Open-chain safest between 25 – 90 degrees (60-90 if distal

lesions) Closed chain safest between 0- 45 degrees especially if there are

proximal lesions

Medial-lateral stability of Patella

Transverse and longitudinal passive stabilizers:

Transverse:

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medial patellar retinacula vastus medialis lateral patellar retinacula vastus lateralis

longitudinal patellar tendon quadriceps tendon

these stabilizers influence the tracking of the patella

Forces on patella quadriceps contracting results in lateral pull of patella anything that the obliquity of the pull could cause:

1. excessive lateral compression2. subluxation and/or dislocation laterally

causes of obliquity:1. weakness of the VMO (dynamic stabilizer)2. excessive genu valgum Q angle (10-15 degrees is

normal, > 20 is abnormal)3. excessive femoral anteversion Q angle4. tight lateral retinaculum or loose medial retinaculum5. tight ITB (iliopatella band)6. diminished height of lateral femoral lip

Other factors affecting patella alignment/tracking: status of gluteal muscles anatomy of quads position of tibial tuberosity mechanics of the foot

Specific problems with VMO:1. barely reaches the top of the patella2. fibers run more vertical rather than oblique

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