- Discussion:
- bones forming the knee joint are femur, tibia, & patella;
- both the left and right femurs converge toward the knee and each tibia is
nearly vertical, femur and tibia meet
angle of some 5-12 deg;
- greater angle results in genu valgum;
- lesser angle results in genu varum;
-
patello-femoral function:
- knee from 0-20 deg of is accompanied by internal rotation of the
tibia, which decr Q angle & lateral
directed
quadriceps muscle vector;
- patella is drawn into trochlear notch of femur, & patellofemoral
contact is made from 0 to 20 deg of
knee
flexion;
- the initial contact is made at the lateral facet of the patella;
- further flexion of knee moves patella anterior relative to center
of rotation of the knee, which improves
the
mechanical advantage of quadriceps mechanism;
- patella continues to move laterally at 90 deg of knee flexion, and
lateral border of the patella provides
the primary
loading site;
-
condyles:
- femur ends in two rounded condyles joined anteriorly to from
patellar articular surface and
separated
posteriorly by a deep intercondylar fossa;
- condyles are almost in line w/ front of shaft, but they project
backward well beyond shaft, as in letter
J,
circumstance of significance in movement of the joint;
- medial condyle is larger, more curved, & projects further than
lateral condyle, accounting for angle
between
femur & tibia;
- sides of condyles are roughened and project somewhat as medial and
lateral epicondyles;
- of interest, the bigger the posterior bulge of the femoral condyle,
the greater can be ROM into flexion as
tibia
glides around convexity;
- this allows full flexion w/o contact between the posterior
joint margins of the tibia and femur;
- thick cartilaginous surfaces of knee helps to spread out joint
reactive load over a wide area and
helps
contribute to cam shape of condyles which maximizes the extensor lever arm;
- in degenerative arthritis the quality of the articular cartilage is
lost;
- as wear occurs, the patello femoral joint is reduced to a
cylindrical outline;
- the mechanical outline is lost, but wear in the bone to bone
contact area is reduced;
-
tibial plateua:
- on its expanded upper end, tibia has 2 slightly concave condyles
separated by an intercondylar eminence
and the sloping areas
in front and behind it;
- low elevation, tibial tuberosity, for insertion of quadriceps, is
situated at the junction of anterior border of
shaft with expanded
upper end of the tibia;
- Roll Back and Sliding of the Knee Joint:: (
role
of knee joint in locomotion)
- for reason of contour of femoral & tibial condyles, flexion & extension of
knee joint are not simple hinge
movements that occur at the elbow joint;
- flexion & extension do not occur about a fixed transverse axis of rotation
but rather about a constantly
changing center of rotation, that is, polycentric
rotation;
- when plotted, path of this changing center of rotation describes a
J shaped curve about the femoral
condyles;
- for example, in full flexion, posterior portions of femoral condyles are
in contact w/ posterior portions of the
tibial condyles;
- knee is extended, femoral condyles roll on tibial condyles & meniscii,
movement resembling that of a rocking
chair.
- there is also sliding of the femur backward.
- as extension progresses, shorter, more highly curved lat condyle
exhausts its articular surface & is checked
by ACL,
whereas larger & less curved medial condyle continues its forward roll & skids
backward,
assisted by
tightening of PCL;
- result is medial rotation of femur that tightens collateral
ligaments, and the joint is "screwed home;"
- flexion of extended knee is preceded by lateral rotation of femur (or
medial rotation of tibia), usually produced
by popliteus muscle;
- this rotation relaxes tension of collateral ligaments suffic to
permit flexion;
- exact ratio of rolling to gluiding differs between individuals and does
not remain constant thruout all deg of
flexion;
- it is estimated to be one to two in early flexion & to increase to
one to four by end of flexion; ;
- during normal gait, tibia undergoes internal rotation during swing phase
and external rotation during stance
phase;
- because the medial femoral condyle is larger than lateral fem condyle, the
distance from extreme
flexion contact point to extreme extension contact
point of medial femoral condyle is about 17 mm > that of
lateral femoral condyle;
- as tibia travels from flexion to extension the medial tibial plataeu must
cover a greater distance;
- ref:
Does the femur roll-back with flexion?
- Knee Rotation:
- as extension progresses, shorter, more highly curved lateral condyle
exhausts its articular surface and is checked
by ACL, whereas larger and less curved medial
condyle continues its forward roll & skids backward,
assisted by tightening of PCL;
- result is a medial rotation of femur (external tibial rotation) that
tightens collateral ligaments, & joint is
"screwed home", to use mechanical phraseology;
- flexion of extended knee is preceded by lateral rotation of femur (or
medial rotation of tibia), usually produced
by
popliteuss;
- this rotation relaxes the tension of the collateral ligaments
sufficiently to permit flexion;