This document is from a physiotherapy workshop and seminar about the anterior cruciate ligament (ACL). It discusses the knee joint's complexity with no bony stability and reliance on muscles and ligaments. It describes the knee's six degrees of freedom and muscles that bend the knee, including the hamstrings and popliteus. It also details the screw-home mechanism that provides stability through external rotation in extension and internal rotation in flexion. Finally, it examines the role, injury mechanisms, and kinematics of the ACL in providing stability against anterior tibial translation and tibial internal rotation.

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ANTERIOR
CRUCIATE
LIGAMENT
Suryo Saputra Perdana | MSc. Physiotherapy

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Knee Joint
At first glance, simple joint
More complex than other joint
No bony stability structures
Supported and maintained
only by muscles and ligaments
Lippert, 2011

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6 Degree
of Freedom

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What muscles
bend the knee?

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If you said HAMSTRINGS,
you’re mostly CORRECT

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The hamstrings are incapable of bending knee if the
knee is totally LOCKED OUT straight
They depend on a tiny muscle called the “popliteus”
to UNLOCK the knee

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Screw-home
Mechanism
External Rotate in
last 20-degree extension knee
Moesley, 2010
Internal Rotate in
early 20-degree flexion knee
Open Chain

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Locking Mechanism
External Rotate in last 20-degree extension knee
The LIGAMENTS PULLED TAUT which makes knee
a little more stable and when it comes to standing
upright that’s a huge advantage

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Unlocking Mechanism
Internal Rotate in early 20-degree knee flexion
The hamstrings pull straight up. They don’t insert
at an angle that will let them untwist a tibia that’s
been rotated by screw home mechanism.

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Screw-home
Mechanism
Internal Rotate in
last 20-degree extension knee
Moesley, 2010
External Rotate in
early 20-degree flexion knee
Close Chain

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Unlocking Mechanism
External Rotate in early 20-degree knee flexion
Locking Mechanism
Internal Rotate in last 20-degree extension knee

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The ACL
Resembling a cross
Post-Lat Condylus Femur
Ant-Med Tibia Plateu
Provide stability against
Anterior Tibial Translation (ATT)
Tibial Internal Rotation (TIT)
Tightens during extension
Extension knee end feel (firm)
Nordenval, 2012 & Griffin, 2005

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The Role
AM-PL Bundle

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ACL Injury
The frequent injury mechanism
Non-contact combined
valgus and internal rotation
Less knee flexed
More knee valgus
Tibia Internal Rotation
Excessive
anterior tibial translation

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Kinematics ATT
2 mm in extension knee
3 mm in flexion knee
5.5 mm during walking
Domnick, 2016
10-15 mm at injury at flexion 300

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Kinematics ATT

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Kinematics TIT

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