ACL-Anatomy & Biomechanics

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ACL-Anatomy & Biomechanics Powered By Docstoc

      Presented by: Inbar Evron
         University of Miami
Department of Biomedical Engineering
Length of 38mm (25-41mm)
Width of 10mm (7-12mm)
 The ACL is made up of multiple collagen fiber
The major cell type is the fibroblast
 Type 1 collagen is the major constituent ( 70-
 80% dry weight)
 Type 3 (8% dry weight) and type 4 (12% dry
 weight) are other major components
 Between 65 to 75% of a ligament’s total
 weight is composed of water
Cross linked character of collagen fibrils(150
to 250 nanometer in diameter )
Fibrils are grouped into fibers (1 to 20 um in
A bundle of fibers is surrounded by an
endotenon (endotendineum)
Synovial membrane envelope encloses the
Poor blood supply
Blood supply from the middle genicular
Vessels predominantly in the soft tissues (fat
pad, synovial membrane)
Bony attachments do not provide a significant
source of blood to distal or proximal
ACL arises from the posteromedial corner of
medial aspect of lateral femoral condyle in
the intercondylar notch
An attachment is actually an
interdigitation of collagen fibers & rigid
bone thru transitional zone of fibrocartilage
and mineralized fibrocartilage
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A wide area in front of and lateral to anterior
tibial spine
 Some fibers attach to lower anterior surface
of spine and anterior horn of lateral meniscus
The ACL consists of two bundles,an
anteromedial (AM), and a posterolateral (PM)
The anteromedial (AM) bundle is tight in
flexion and the posterolateral (PM) bundle is
tight in extension
The AM is thought to be important as a
restraint to anterior-posterior translation of
the knee, while the PL is thought to be an
important restraint to rotational moments
about the knee.
 Ultimate tensile load: 2160 ± 157 N
 Stiffness: 242 ± 28 N/mm
passive knee extension produces forces along
ACL only during last 10 degrees of knee
The posterolateral (PM) bundle of the ACL is
 tight in extension
At 5 degrees of hyperextension, ACL forces
 range between 50 and 240 newtons
Hyperextension of the knee develops much
 higher forces in ACL than in the PCL
The anteromedial (AP) bundle of the ACL is
tight in flexion
 Movement from extension to 90 deg of
flexion changes the position of the tibia
relative to the femur differently for the medial
and lateral compartment
The muscles spanning the knee apply forces
and moments to the joint
 An increase in ACL strain produced by
 contraction of the quadriceps muscle group
 when the knee is between full extension and
 approximately 50 deg flexion
Contarction of the quadriceps with the knee
 near extension produces substantial anterior
 tibial translations
Contraction of the hamstring muscles
produces a posterior directed force that
acts on the tibia throughout the range of
knee flexion
Movement of the knee from a flexed to an
extended position , passively or through
contraction of the leg muscles, produces an
increase in ACL strain values.
Active extension of knee between the limits
of 50 and 110 degrees does not strain the
Unsectioned ACL's in neutral rotation, application of
 100 newtons of anterior force produces:
         - 2-5 mm of anterior translation at full
         - 5-8 mm of translation at 30 deg of
         - as flexion angle increases further,
 anterior translation decreases
Sectioning of ACL results in increased laxity at all
 flexion angles
         - 20-30 deg of flexion:
         - maximum anterior translation occurs w/
 100-newton anterior force ( 7-9 mm of increased
 translation is seen )
ACL is the predominant restraint to anterior
tibial displacement
ACL limits internal-external rotational
ACL is a restraint to adduction and abduction
at full extension
ACL is taut in full knee extension, and tends
to externally rotate tibia
Tension in ACL is least at 40 to 50 deg of
knee flexion
ACL guides the screw home mechanism near
full extension
Towards full extension the tibia& joint rotate
ACL does not remain an isometric (constant
length) structure as the knee is flexed and
Ligament increases in strain magnitude as the
lower leg is passively extended, with the
femur in a horizontal plane

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