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 translation
Normal motion
Simulate anterior-posterior translation of the tibia 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 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/average 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
You can recognize abnormal AP translation as movement of the tibia greater than 5 mm either in the anterior or posterior direction from its neutral position.
Renderings of AP translation with the tibia too far anterior and too far posterior
Longitudinal rotation
Normal motion
Next, simulate longitudinal rotation (i.e., long-axis rotation) of the tibia 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 near full extension of the knee and at 90º of flexion
You don't need to worry about measuring the number of degrees the tibia can rotate in each direction. Rather, pay attention to the articular cartilage surfaces where the femur and tibia contact one another. Notice how at the point of contact during normal motion (and in a healthy knee), there are is always articular cartilage on each side/bone.
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)
You do not want bone-on-cartilage or bone-on-bone contact at a joint (that's what happens with osteoarthritis—not good!). You want continuous cartilage contact for smooth (and pain-free) motion.
Abnormal motion
Abnormal longitudinal rotation will look something like the following, with the tibia rotating so far relative to the femur that there is no longer any articular cartilage at the point of contact on one or both bones.
Video or rendering showing example of no articular cartilage at point of contact on one or both bones
Varus-valgus rotation
Normal motion
Last, simulate varus-valgus rotation of the tibia near full extension of the knee and at 90º of flexion, using the video below as a guide.
Video of varus-valgus rotation of the tibia near full extension of the knee and at 90º of flexion
As you simulate this rotation, take a close look at where the femur and tibia are contacting one another inside the joint. During varus rotation, a gap forms between the lateral condyles of the femur and tibia. During valgus rotation, a very small gap opens up between the medial condyles (hardly visible). This gap formation is called lift off.
Rendering showing lift off between the lateral condyles
During varus rotation, a normal lift off between the lateral condyles is about 2 mm near full extension and about 10 mm at 90º of flexion. During valgus rotation, the amount of lift off is less than 1 mm and not easily visible.
Abnormal motion
If there's abnormal motion during varus or valgus rotation, you'll see greater than normal lift off between the lateral or medial condyles, respectively.
Video or rendering showing example of greater than normal lift off between the medial and lateral condyles