Knee Mechanics

what is the true knee articulation?

the tibiofemoral joint

what are the characteristics of the tibiofemoral joint?

the femoral condyles roll and slide across the tibial plateau

has poor bony stability

what are the soft tissue stabilizers of the tibiofemoral joint?

medial collateral ligament

lateral collateral ligament

anterior cruciate ligament

posterior cruciate ligament

menisci

what is the Screw Home Mechanism?

during the last few degrees of extension, the tibia rotates laterally on the femur, because the tibia continues to contact the larger medial condyle

this mechanism helps “lock” the knee in place, bringing the knee into it’s most stable position of full extension

unlocked by popliteus

what is the function of the MCL?

provides support on the medial side of the knee against valgus forces placed upon the knee

what are the two layers of the MCL?

superficial: tight with knee extended

deep: provides support and is tight when the knee is slightly flexed

what is the function of the LCL?

provides support on the lateral side of the knee against varus forces placed upon the knee

what are the characteristics of the LCL?

it is a pencil-like band of tissue

tight in knee extension and becomes loose as the knee moves into flexion

where is the ACL located?

it runs from the lateral femur to the medial tibia inside the joint

what is the function of the ACL?

provides support to prevent the tibia from sliding forward on the femur

works with the MCL to stabilize against valgus stress as well, with assistance from the hamstrings

when this is deficient, the hamstrings become more active in stabilizing the knee

what are the characteristics of the PCL?

one of the strongest ligaments in the body

prevents posterior translation of the tibia on the femur

very rarely injured in athletics

what are the two menisci of the knee?

medial meniscus - C shaped, larger and longer

lateral meniscus - oval shaped, more mobile

what are the functions of the menisci?

triple the joint contact area

decreases pressure (force is distributed over a larger area

weight bearing, shock absorption

crucial to joint stability and health

what would occur without menisci?

the tibial and femoral condyles would come together in a small area

increased pressure, increased compression on the articular cartilage

what is the outcome of a menisectomy?

it will increase compressive loads on the articular surface

i.e. a complete lateral menisectomy increases contact pressure by 200+%

what is the shape of the menisci?

they are wedge-shaped

what is the Q angle and its normal values?

the angle between the tibia and femur (quadriceps angle)

from the ASIS to the middle of the patella

middle of patella to the tibial tuberosity

normal in males: 10-14 degrees

normal in females: 15-17 degrees (difference is due to a wider pelvis in females)

what does the Q angle represent?

the line of pull of the quadriceps muscle on the patella

what is tibial torsion and its function?

it is used as a sign of femoral anteversion or retroversion

the person kneels on a table, imaginary line is drawn along the midline of thigh-tibia-foot

second line is drawn from the 2nd toe through the middle of the heel

angle should be about 12-15 degrees (angles greater or less than this indicate a tibial torsion)

what is genu valgum and its causes?

“knock knees”

the distal segment is angle away from the midline

typically associated with excessive pronation of the feet

what are the complications with genu valgum?

places chronic tension on medial structures of the knee

places chronic compression on lateral structures of the knee

places excessive tension on the IT band

may be an indication of weak hip external rotators

will distribute body weight through the lateral joint

what is genu varum?

bowlegged appearance

distal segment is angled towards the midline

will distribute bodyweight through the medial joint

what is the structural cause of genu varum?

deviation of the femoral and tibial alignment

associated with other hip/knee malalignments

what is the functional cause of genu varum?

associated with hyperextension of the knees

tibia tends to internally rotate with hyperextension

when hyperextension is corrected genu varum typically disappears

what is genu recurvatum?

hyperextension of the knees

common compensation for lordosis, or “sway back”

weakness and stretching of the hamstrings are classic components of genu recurvatum

chronic recurvatum causes excessive compressive loads on the anterior structures of the knee and excessive tensile loads on the posterior ligaments and tendons

what does the muscular stability of the knee depend on?

joint position and which muscles contract

anterior stability from the quads via patellar ligament

posterior stability from the hamstrings

lateral stability from TFL

medial stability from gracilis and sartorius

what is the role of the quads at the knee?

they always stabilize

this is important when the joint angle is greater than 90 degrees

at these joint angles the hamstrings provide a dislocating component

what is the function of the hamstrings regarding anterior shear?

the contraction of the hamstrings decreases shear at higher joint angles, which is one reason closed chain exercises are preferred in rehab of the knee

what are the active movers for knee flexion?

biceps femoris, semitendinosus, semimembranosus

substitute with sartorius and gracilis

what are the prime movers of knee extension?

rectus femoris, vastus intermedius, vastus lateralis, vastus medialis

what is the anatomy of the patellofemoral joint?

the patella tracks in the intercondylar groove

will track superior and slightly lateral with quad contraction

posterior patella has the thickest articular cartilage found in the body

what are the seven facets of the patella?

the vertical ridge of the articular cartilage separates the posterior patella into medial and lateral facets

each of these can be separated into superior, middle, and inferior facets

a seventh facet lies on the far medial side of the patella and is called the odd facet

which facet is in contact with the femur at 20 degrees?

the inferior facet

which facet is in contact with the femur at 45 degrees?

the middle facets

which facet is in contact with the femur at 90 degrees?

the superior facet

which facet is in contact with the femur at 135 degrees?

the odd facet → the most commonly injured because of the lateral pull of the quads and Q angle

what results from the patella tracking superiorly and laterally as the quads contract?

increased Q angle which increases the lateral pull of the quads and increases contact forces on the posterior patella and femoral condyles

why does patellofemoral compression occur and where is it the greatest?

as the angle of the knee increases, compressive forces increase (even though the forces of the quads and patellar tendon stay the same)

the greatest compressive force will be at approximately 90 degrees

there is a resultant vector sideways due to the pull of the quads laterally (due to the Q-angle)

as the Q-angle increases, the resultant lateral force vector will increase

what does patellofemoral pain syndrome result from?

patellofemoral mechanics (natural pull of the patella superiorly and laterally)

patellofemoral compression

vastus lateralis is much stronger than the medialis

V. medialis only functions in patella tracking in the last 15 degrees of extension (VMO)

increased Q-angle will throw off pull of quads even more

at what position does the knee become more unstable?

as the knee becomes more flexed, ligamentous stability decreases

if an external force is applied to the knee as it approaches the limits of its ROM, injury is likely

in terms of the three joints of the lower extremity, the knee becomes the weak link in the chain

what is IT band friction syndrome and its cause?

during knee flexion in weight bearin, the TFL contracts to stabilize the pelvis

the increased tension on the IT band can increase the friction as the IT band slides over the lateral femoral epicondyle, causing inflammation of the IT band

why are deep knee squats bad for the knees?

with squat motion, the COG is behind the axis of rotation of the knee

as the squat gets deeper and past 90 degrees, the posterior tissue begins to touch, causing the axis of rotation to move back towards the line of action of the COG

this causes a separating force at the knee joint, which places excessive stress in the non-contractile stabilizers of the knee

which side of the body should a cane be used?

opposite side of the injury → increases the moment arm and the torque, and decreases the force needed to take weight off the injured side