Anterior cruciate ligament tear and forward sliding of the tibia
The anterior cruciate ligament (ACL; cruciate means cross-like) stops the tibia (shinbone) sliding forwards under the femur (thigh bone), so a tear lets the tibia slide forwards.
Remember
Because the ACL restrains forward sliding of the tibia, a tear lets the tibia slide forwards, and the story is usually a twist or a sudden stop.
The two cruciate ligaments cross each other inside the knee and join the femur to the tibia. From the front of the tibia, the ACL runs up and backwards to the femur, so it is the main brake that stops the tibia sliding forwards under the femur. Once it tears, that brake is lost and the tibia moves too far forwards.
The ACL usually tears during a sudden stop or twist rather than a tackle. Deceleration (suddenly slowing), hyperextension (the knee forced past straight) and rotation, such as a change of direction on a planted foot, are the classic triggers. A large force to the back of a partly bent knee can also tear it.
Cruciate tears usually present with a pop, severe pain, rapid swelling, reduced movement and a knee that feels as if it gives way when taking weight.
Forward-pull tests look for the lost brake. In the anterior drawer test the examiner pulls the shin forwards, and Lachman's test is also a forward pull, made from a different position. More forward movement of the tibia than in the other knee points to a torn ACL, although these tests are good but not perfect. The other knee is the comparison, since the aim is to find more movement than is normal for that person. The drawer test can also be used for the posterior cruciate ligament (PCL), whose tear gives excess backward movement, but Lachman's test is for the ACL only.
The PCL does the opposite job and stops the tibia sliding backwards.
Key facts
- Role of the ACL
- it stops the tibia sliding forwards under the femur, so a tear gives abnormal forward sliding of the tibia.
- Usual cause
- a sudden stop, a twist or the knee forced past straight, often with little or no contact.
- Usual presentation
- a pop, severe pain, rapid swelling, reduced movement and a knee that feels as if it gives way when taking weight.
- Sign
- more forward movement of the tibia than in the opposite knee on the anterior drawer test or Lachman's test.
- Opposite movement
- the PCL stops the tibia sliding backwards, so a tear gives excess backward movement, classically after a force to the front of the bent knee (a dashboard injury).
- Collateral ligaments
- they run down each side of the knee and resist sideways opening, outwards (varus) or inwards (valgus).
Exam traps
- A tibia that slides backwards suggests a torn posterior cruciate ligament, for example after a dashboard injury, and not an ACL tear, because the ACL is the ligament that stops forward sliding.
- Excessive valgus opening of the knee (the inner side of the joint opening as the knee is pushed inwards) suggests a medial collateral ligament tear, since that ligament restrains valgus opening. That ligament can be torn with the ACL, but valgus opening alone does not show that the ACL has gone.
- A direct blow to the side of the knee harms the collateral ligaments, which resist sideways movement, whereas the ACL is typically torn without contact during a twist or sudden stop.
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