Biomechanics of the Knee
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Basics
Meniscus
Patella Motion
Collateral ligaments
Basics
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The hips are wider apart than the ankles
thereby making the mechanical axis of the
lower limb running obliquely inferiorly and
medially and forms an angle of 3 degrees with
the vertical.
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The wider the pelvis, the greater the angle, as in the
case of females. This also explains why the
physiological valgus of the knee is more marked in
women than in men.
Basics
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The structure of the knee makes axial rotation
impossible when the knee is fully extended,
the axis of the leg coincides with the
mechanical axis of the lower limb and axial
rotation occurs not at the knee but at the hip,
which is complimentary to the knee.
Basics
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There is strictly no absolute extension. It is
possible to achieve passive extension (5-10
degrees) from the position of reference, this is
wrongly called ‘hyperextension’.
Active flexion attains a range of 140 degrees if
the hip is already fixed and only 120 degrees
if the hip is extended.
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This difference is due to the fact that the hamstrings
lose some of their efficiency with extension of the
hip.
Basics
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Passive flexion of the knee attains a
range of 160 degrees and allows the
heel to touch the buttock.
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Pathologically, passive flexion is limited by
retraction of the extensor apparatus essentially the quadriceps - or by
shortening of the capsular ligaments.
Meniscus
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Extension, the menisci are pulled
forward by the meniscopatellar fibers,
which are stretched by the anterior
movement of the patella and this draws
the transverse ligament forward.
The posterior horn of the lateral
meniscus also is pulled anteriorly by the
tension developed in the
meniscofemoral ligament, as the
posterior cruciate ligament becomes
taut.
Medial Lateral
Medial Lateral
Meniscus
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In flexion, the medial meniscus is drawn
posteriorly by the semimembranosus expansion
which is attached to its posterior edge, while the
anterior horn is pulled anteriorly by the fibers of
the anterior cruciate ligament attached to it
The lateral meniscus is drawn posteriorly by the
popliteus.
Lateral rotation
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Medial rotation
During axial rotation, the menisci follow exactly
the displacements of femoral condyles. Starting
from the neutral position they can be seen to
move on the tibial condyles in the opposite
direction
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Lateral rotation - the lateral meniscus is pulled
towards the anterior part of the tibial condyle while
the medial meniscus is drawn posteriorly.
Medial rotation - the medial meniscus moves
forward while the lateral meniscus recedes.
Lateral rotation
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Medial rotation
The menisci become distorted about their
attachments of the horns. The total range of
movement of the lateral meniscus is twice as
great as that of the medial meniscus. These
displacement of the menisci during axial rotation
are mostly passive
Patella Motion
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The extensor apparatus of the knee slides on
the lower end of the femur like a cable on a
pulley.
During flexion the patella is vertically displaced
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Normally the patella moves only in the vertical plane
and not transversely.
At the end of extension this appositional force is
diminished.
In hyperextension it tends to be reversed separating the patella from the femur because
the quadriceps tendon and the ligamentum
patellae form an obtuse angle laterally.
Patella Motion to the Tibia
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During flexion and extension
the patella recedes while
moving along the arc of a
circle.
During movements of axial
rotation the patella moves
relative to the tibia in a frontal
plane
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Neutral position the ligamentum patellae runs slightly
obliquely inferiorly and laterally.
In medial rotation, the femur is laterally rotates relative to
the tibia and this drags the patella laterally, the
ligamentum patella now runs inferiorly and medially.
Lateral rotation the femur draws the patella medially so
that the ligamentum patellae runs obliquely inferiorly and
laterally
Collateral Ligaments
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Collateral ligaments strengthen the
articular capsule medially and laterally.
Medial
Lateral
Collateral Ligaments
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Collaterals become taut during
extension.
Medial
Lateral
Collateral Ligaments
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Collaterals slacken during flexion.
Medial
Lateral