Normal vs abnormal motions of the knee

In testing for ligament function, you'll need to be able to identify abnormal motions of the knee. To do this, you'll need to be able to identify normal motions of the knee. So, before removing any of the ligaments, you'll first practice simulating the normal motions of the knee. The sections below will walk you through simulating anterior-posterior translation, longitudinal rotation, and varus-valgus rotation, each at near full extension of the knee and at 90 degrees of flexion. Practice each motion multiple times with all of the ligaments in place until you get a good feel for what is the normal range of motion. For this entire activity, you'll hold the femur still and move the tibia relative to a static femur.

Anterior-posterior (AP) translation

Normal motion

Simulate AP translation of the tibia at near full extension of the knee and at 90º of flexion, using the video below as a guide.

Video of AP translation of the tibia at near full extension of the knee and at 90º of flexion

With all of the ligaments intact, you should only be able to translate the tibia approximately 1 cm (10 mm) in total. You can use your ruler to verify this. You can also think of this as the tibia moving 5 mm anterior and 5 mm posterior relative to its "neutral position" (i.e., when the tibia is in the middle of its motion path).

We're using the term "near full extension" rather that "at full extension" because when the knee is fully extended, it is technically "locked" and has reduced mobility along the other motion axes. Exactly what "locking your knees" means is covered fully in a different activity - don't worry about that now. For this activity, just know that the simulations you're doing with the knee extended are just shy of full extension

Abnormal motion

Abnormal AP translation will look something like the following, with the tibia moving greater than 5 mm either in the anterior or posterior direction.

Renderings of AP translation with the tibia too far anterior and too far posterior

Longitudinal rotation

Normal motion

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Video of AP translation of the tibia at near full extension of the knee and at 90º of flexion

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Video or rendering showing that point of contact between bones occurs at a point where there is articular cartilage on both bones (continuous cartilage contact)

Abnormal motion

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Video or rendering showing example of no articular cartilage at point of contact on one or both bones

Varus-valgus rotation

Normal motion

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Abnormal motion

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