Download bio-mechanics of knee joint

Lecture-2
 Located between the body’s two largest lever arms.
 Susceptible to injury.
 Largest motion in the sagittal plane.
 In extension, rotation limited by interlocking of tibial
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and femoral condyles.
Rotation increases as the knee is flexed, reaching
maximum at 90 degrees.
Ext. rot 0-45
Int. rot 0-30
Beyond 90 degrees, range of int & ext rot is decreased
due to soft tissues.
 Limited abd & add in full extension
 Increases with knee flexion up to 30 degrees.
 Motion in frontal plane again decreases beyond 30
degrees due to soft tissues.
 Range for ADLs= full ext to 117 degrees of flexion.
 The asymmetrical medial and lateral tibial condyles or
plateaus constitute the distal articular surface of the
knee joint
 The
medial tibial plateau is longer in the
anteroposterior direction than is the lateral plateau;
however, the lateral tibial articular cartilage is thicker
than the articular cartilage on the medial side.
 The proximal tibia is larger than the shaft and,
consequently, overhangs the shaft posteriorly
 Accompanying this posterior overhang, the tibial
plateau slopes posteriorly approximately 7° to 10°.
 The medial and lateral tibial condyles are separated by
a roughened area and two bony spines called the
intercondylar tubercles
 These tubercles become lodged in the intercondylar
notch of the femur during knee extension.
 The tibial plateaus are predominantly flat, with a slight
convexity at the anterior and posterior margins,
which suggests that the bony architecture of the tibial
plateaus does not match up well with the convexity of
the femoral condyle.
 Because of this lack of bony stability, accessory joint
structures (menisci) are necessary to improve joint
congruency.
 The anatomic (longitudinal) axis of the femur, as
already noted, is oblique, directed inferiorly and
medially from its proximal to distal end.
 The anatomic axis of the tibia is directed almost
vertically.
 Consequently, the femoral and tibial longitudinal axes
normally form an angle medially at the knee joint of
180° to 185°;
that is, the femur is angled up to 5° off vertical,
creating a slight physiologic (normal) valgus angle at
the knee
 If the medial tibiofemoral angle is greater than 185, an
abnormal condition called genu valgum (“knock
knees”) exists.
 If the medial tibiofemoral angle is 175° or less, the
resulting abnormality is called genu varum (“bow
legs”).
 Each condition alters the compressive and tensile
stresses on the medial and lateral compartments of the
knee joint.
 An alternative method of measuring tibiofemoral
alignment is performed by drawing a line from the
center of the femoral head to the center of the head of
the talus
 This line represents the mechanical axis, or weight
bearing line, of the lower extremity, and in a normally
aligned knee, it will pass through the center of the
joint between the intercondylar tubercles.
 The weight-bearing line can be used as a simplification
of the ground reaction force as it travels up the lower
extremity.
 In bilateral stance, the weight-bearing stresses on the
knee joint are, therefore, equally distributed between
the medial and lateral condyles (or medial and lateral
compartments).
 However, once unilateral stance is adopted or dynamic
forces are applied to the joint, compartmental loading
is altered.
 In the case of unilateral stance (e.g., during the stance
phase of gait), the weight-bearing line must shift
medially across the knee to account for the now
smaller base of support below the center of mass
 This shift increases the compressive forces on the
medial compartment
 Abnormal compartmental loading may be also be
caused by frontal plane malalignment (genu varum or
genu valgum).
 Genu valgum, for instance, shifts the weight-bearing
line onto the lateral compartment, increasing the
lateral compressive force while increasing the tensile
forces on the medial structures
 whereas the tensile stresses are increased laterally
 The presence of genu valgum or genu varum creates a
constant overload of the lateral or medial articular
cartilage, respectively, which may result in damage to
the cartilage and the development of frontal plane
laxity.
 Genu varum, for instance, may con-tribute to the
progression of medial compartment knee
 In the case of genu varum,
the weight-bearing line is shifted medially,
increasing the compressive force on the medial
condyle,
causes osteoarthritis and lead to excessive medial joint
laxity as the medial capsular ligament’s attachment
sites are gradually approximated through the erosion
of the medial compartment’s articular cartilage.
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