10-14b Knee Biomechanics I Flashcards
Why is the tibiofemoral joint so commonly injured?
Lots of sagittal plane motion (not as much transversal/frontal plane motion)
Lots of force (longest bones in the body = lots of force); if they go in the opposite directions = bad
Above and below: the knee is the symptom to hip, ankle, and foot motion. Caught in between more mobile joints
What are the two articulations of the tibiofemoral joint?
medial femoral condyle articulating with the medial tibial plateau
lateral femoral condyle articulating with the lateral tibial plateau
Which condyle on the femur projects more distally?
medial condyle by about 2/3 of an inch
Where do the cruciate ligaments attach?
intercondylar fossa
What does the patella engage with on the femur during early flexion?
trochlear groove
In which plane are the femoral condyles more convex? Frontal or sagittal?
Sagittal
How do the tibial condyles project in the frontal plane?
slightly concave
How do the tibial condyles project in the sagittal plane?
medial: slightly concave
lateral: slightly convex
Which condyle has a longer anterior-posterior length?
medial condyle
What attaches at the intercondylar tubercles?
cruciate ligaments
What fills the joint space of the tibiofemoral joint?
hyaline cartilage
meniscus
How does the medial meniscus compare to the lateral meniscus shape-wise?
medial meniscus: C shape
lateral meniscus: circular O shape
How does the meniscus help the articulation of the femur and tibia?
gives the femur a concave shape to rest in
What are the primary functions of the menisci?
Enhance TF congruency: distribution of forces by increasing the contact area to absorb 40-60% of the normal load: shock absorbers
help with friction/shear reduction
provide joint stability
assist in lubrication of the joint: hyaline cartilage has poor joint supply, so another structure forces more synovial fluid against the hyaline cartilage and provides more nutrients
What are the secondary directions in which the meniscus stabilizes?
restrains movement in A/P directions
and with combined valgus and rotation by providing stability
What are the implications of a menisectomy?
contact area decreases and adds stress to the femur (Stress = Force/Contact Area)
long-term consequences are arthritis (14 times more likely for OA w/ menisectomy)
Describe Meniscal mobility. Which one is more mobile? How does weight-bearing affect mobility?
Medial meniscus is more restricted due to greater ligamentous/capsular restraints (MCL)
Implications: more medial meniscus problems due to lack of mobility
In non-weightbearing: not much difference in mobility (less need for deformity)
In weightbearing: more deformity and sliding
How do the menisci move during knee extension?
deform and slide anteriorly
How do the menisci move during knee flexion?
deform and slide posteriorly
What muscle(s) help the menisci move during knee flexion?
medial meniscus by semimembranosis
lateral meniscus by popliteus
What are the most common mechanisms for meniscal tears? Which meniscus is more likely to tear? What causes a springy end-feel/locked knee?
twisting/pivoting on loaded limb
medial meniscus
local synovitis (assoc. with inflammation of synovium)
bucket handle tear: meniscus flipped up inside the joint
What would a yellowish aspiration from menisci mean vs. red aspiration?
yellowish fluid: synovitis so not good for surgery b/c cannot heal
Where do the menisci get their nutrition? Where do the different parts receive their nutrients?
lateral third gets blood supply up into the pt’s 50s (after fifties only periphery)
peripherally: supplied from capillaries
centrally: relies on diffusion from the synovium (aided by cyclic loading, while immobilization/NWB is problematic
What are the types of articular cartilage injuries?
focal lesions (acute): traumatic in origin, focal surface injury, peripheral tissue is normal
degenerative lesions: multiple causes (aging), peripheral tissues affected, OA
What does normal TF alignment consist of?
slight genu valgum
170-175 degrees
males > females due to wider pelvis in women (more genu valgum/knock-knee)
What are the two abnormal alignments of TF alignment?
genu valgum: knock-knee at <= 165 degrees
genu varum: bow-legged at >= 180 degrees
In bilateral stance, what are the mechanical axes?
Femur (FM): femoral head to knee center
Tibia (TM): knee center to ankle center
How does a varus situation affect the LBA? Implications? Torque?
axes are off and LBA comes medial to the knee
compression of medial side can cause wearing of the medial joint compartment
Varus adduction moment
How does a valgus situation affect the LBA? Torque?
LBA comes outside the knee
causes valgus abduction moment
How much does the articular cartilage deform during weight bearing? Single leg stance?
deforms by 22-30%
Single leg stance:
Compartment loads: medial bears greatest load at 2.25x BW (lateral: 0.91x BW)
natural varus (adduction) torque due to medial-leaning COM; more pronounced varus torque in bow-legged individuals = medial compression
How do we reduce adduction moments?
surgically:
osteotomy: surgical wedge to correct alignment (for one with very isolated arthritis); TKA
and conservatively: wedging in someone’s shoe/bracing/gait modification (strengthen quads) to reduce adduction moment
What are we referring to with knee osteokinematics? What are the 6 DOF?
Flexion/extension
Internal/external rotation
Abduction/adduction (valgus/varus)
tibial and femoral displacement: med/lat, sup/inf, ant/post.
How do we measure adduction (varus) and abduction (valgus)?
relationship of the distal segment relative to the midline of the proximal segment
When the tibia moves medial to the femur’s midline what kind of moment is it?
varus (adduction) moment
When the tibia moves lateral to the femur’s midline what kind of moment is it?
valgus (abduction) moment
When the femur moves lateral to the tibia
varus (adduction) moment
When the femur moves medial to the tibia
valgus (adduction) moment
If the femur internally rotates, what kind of rotation happens to the tibia?
external rotation
When femur externally rotates, the tibia moves in what direction relative to the femur?
internal rotation
How do we describe the relationship of distal and proximal segments in rotation?
rel. distal segment relative to midline of the proximal segment
During flexion/extension, where is the axis of motion?
What movements occur?
horizontal line passing through the femoral epicondyles (axis)
flexion: posterior roll and anterior glide of femur on tibia
extension: anterior roll and posterior glide
as we flex and extend does the point of most contact with the tibia and the femur remain or change?
changes
Where is the point of contact of the TF throughout motion? What is this called?
always directly beneath the cruiciate ligaments
four bar linkage system?
How do arthrokinematics of the TF work during weight-bearing?
weight bearing:
flexion: femoral condyles roll posteriorly and glide anteriorly on the tibial plateaus
extension: femoral condyles roll anteriorly and glide posteriorly on the tibial plateaus
How do arthrokinematics of the TF work during non-weight-bearing?
non-weight bearing:
flexion: tibial plateaus roll and glide posteriorly on femoral condyles
extension: tibial plateaus roll and glide anteriorly on the femoral condyles
What is the ROM for flexion/extension in passive ROM, during gait, and stairs/sitting?
Passive: 20-0-160 degrees
Gait: 0-70 degrees
Stairs/sitting: 0-90 degrees
How is knee flexion/extension ROM affected by hip position?
knee flexion in supine (rectus femoris = quad) is different than prone (hamstrings) b/c they cross both joints
Where do most functional activities fall under with Flexion/extension ranges?
0-90 degrees
get person 0-90 ASAP after injury
What degree of adduction/abduction do you have in full knee extension? in 20 degrees flexion? Why is one more than the other?
8 degrees available ROM during extension b/c closed pack position; 13 available during 20 deg. flexion b/c open packed position
When sitting at 90 degrees, what is the total amount of internal rotation available? external? Combined?
Internal: 0-30 deg.
External: 0-40 deg.
total: 60 to 70 deg.
What is the screw home mechanism for the tibia? What kind of chain?
External rotation of the tibia on the femur during the last 20° of extension (open chain)
What is the screw home mechanism for the femur? What kind of chain?
Internal rotation of the femur on the tibia during the last 20° of extension (closed chain)
What are the three main reasons for the screw home mechanism?
Bony/meniscal structure: (medial femur and tibial condyles are longer AP)
Ligament restrictions: ACL/PCL
slight lateral pull (quads)
What is unlocking the knee?
internal rotation of the tibia on the femur during early flexion (NWB) via popliteus m.
Where is close packed position? What causes it?
Full extension creates maximal
bony congruence and
ligamentous tautness
Where is loose packed position? What causes it? Relevance to injury?
25 degrees of knee flexion
minimal bony congruence
ligaments lax
minimal intra-articular pressure (ex. when landing from jump)
What provides TF joint stability?
joint capsule goes above the patella
retinacula
synovial lining
What are the cruciate ligaments’ location and attachment?
intra-articular (inside joint), extra-synovial (synovial lining b/w ligament and synovial fluid; no blood supply)
named according to their tibial attachment
ACL: from anterior aspect of tibia posteriorly and laterally
PCL: from posterior tibia and travels anteriorly and medially
ACL has how many bands?
two
Primary job of ACL?
Primary restraint to anterior translation of the tibia on the femur
primary restraint to hyperextension
Secondary jobs (2) of ACL?
Assists with resistance to internal rotation of the tibia on the femur
Assists with resisting varus and valgus forces
What is the maximum excursion at 30 degrees?
normal max ant. tibial translation at 5-8 mm
What is normal strain on the ACL? Failure?
2 to 4 %
6 to 8 % is failure
At 90 degrees of knee flexion of isometric contraction of quad as hard as you can, any ACL strain?
no
posterior translation of tibia instead
At 30 degrees of knee flexion of isometric contraction, any ACL strain? Why is the strain the level it is?
yes
anterior translation
activation of quad
less than full extension b/c hamstrings pull posteriorly and protect ACL (no quad pull anteriorly)
10 lbs knee extension 0 to 90 strain amt.? Why is the strain the level it is?
peak strain farther in extension (10 deg.)
peak is 3.8% strain
tibia is pulled anteriorly by patellar tendon (quads have larger vector and hamstrings have smaller vector)
how soon do open change post ACL reconstruction
start isolated 90 to 45 bc little strain after 3 to 4 weeks
then full arcs
How prevalent are ACL injuries?
20% of all knee injuries
70% non-contact
Why is the ACL more prone to injury in women?
women b/c structural factors: smaller in length, cross-sectional area, and volume
biomechanical factors: Less stiff and fails at lower loads
neuromuscular factors: Area occupied by collagen fiber is lower; peak in quad strength but less hamstring strength after puberty, slower m. activation pattern
What biomechanical differences can aid in preventing ACL injuries?
Higher knee valgus angles and moments
Decreased Hip flexion angles and knee flexion stiffness during cutting
Greater hip adduction
Trunk adaptations (teach proper trunk position for knee loading)
Hip transverse and frontal plane angles influence knee valgus moments
PCL location? Differences from ACL?
Runs from superior-anterior-medial (femur) to inferior-posterior-lateral (tibia)
Shorter
Less oblique
Greater CSA (120-150%)
(shorter, broader, less likely to tear than ACL)
What is the primary function of the PCL?
Restricts posterior translation of the tibia on the femur
tear caused by plantar flexion with fall on tibia, sending it posteriorly
most common mechanism for ACL injury?
valgus collapse, anterior translation
precautions after PCL surgery?
limit active hamstring contraction for 3 to 4 mo.
What are the secondary functions of the PCL?
Assists with resistance to external rotation of the tibia on the femur
Assists with resistance of varus and valgus forces
