Introduction

If you've ever had a severe knee injury or read about one, you're likely familiar with an account like the following: "I moved in the wrong way, I heard a pop, and then I felt the pain." The knee, like pretty much all other joints in our body, is held together (in part) by ligaments. These ligaments are strong cords that function to guide normal joint motion. But when a joint, like the knee, is forced to move in an abnormal way, one or more ligaments are stretched beyond their limit, they can tear or snap (the "pop"), resulting in joint instability (because the ligament can no longer function properly) and pain. This example illustrates the basic principle that ligament function, joint motion, and joint injury are all causally connected:

• If ligament A functions to limit excess motion in direction X, then...
• the motion that can injure ligament A is excess motion in direction X
• and injury of ligament A can cause pain or instability when moving in direction X.

Thus, if you want to fully understand knee joint injuries, you need to also understand the functions of the knee ligaments and the motions that these ligaments normally limit.

But how do you evaluate the function of a ligament? In molecular biology, the function of a protein or other cellular component is evaluated by "knocking it out." Cells are blocked from producing the protein and the resulting phenotype gives you an indication as to what the function of that protein is. For example, if the cell has membrane abnormalities, you have a clue that the protein may help to form the cell membrane. This is the classic "knockout" experiment: an abnormal result gives you a clue about normal function.

In this activity, you'll do a knockout experiment with your knee kit. For each ligament of the knee and for a combination of two ligaments, you'll:

knock out the ligament(s) to test 

 ligaments as a pair

You'll knock out each ligament of the knee independently plus two ligaments simultaneously knocking out each of the knee ligaments to test their function. With 

ligament function - joint motion - ligament injury

function-motion-injury

In molecular biology, a common way to test the function of a protein or other cellular component is by "knocking it out." Cells are somehow blocked from producing the protein and the resulting phenotype gives you an indication as to what the function of that protein is. For example, if the cell has membrane abnormalities, you have a clue that the protein may help to form the cell membrane. This is the traditional "Knockout" experiment: an abnormal result gives you a clue about the normal function.

In this activity, you'll apply this same principle to determine the function of each of the knee ligaments. 

A ligament's function tells you both how it can be injured and the loss of function that results from its injury.