Simulating ligament injuries through a knock-out experiment

Now that you're familiar with the normal motions of the knee, you're ready to knock out some ligaments. On page 1 of the activity worksheet, you'll find a table. There's a row for each of the motions that you practiced in the previous step and a column for each of the major ligaments of the knee plus a combination of two ligaments, the MCL and ACL. For this experiment, you'll remove each ligament(s) listed at the top of the column and then go down the rows, checking whether the motion corresponding to each row is normal or abnormal.

If you simulate abnormal motion (motion outside the normal range of motion) with the removal of a ligament, that means:

The ligament functions to limit excess motion in that direction,
The abnormal motion you are simulating is the same motion that can injure that ligament,
and if the ligament is injured, the abnormal motion you are simulating could cause pain or instability in the knee.

For example, if you remove a ligament and the tibia is now able to rotate about its long axis laterally beyond its normal range, you could conclude that:

The ligament you removed functions to limit lateral longitudinal rotation,
Extreme lateral longitudinal rotation can injure that ligament,
and once that ligament is injured, there could be pain and/or instability with lateral longitudinal rotation.

For this activity, aan abnormal motion test for flexion-extension (e.g., beyond the normal range of ~~motion~~extension (or hyperextension of the knee) is not included just to keep the activity simpler.

Knocking out your knee's collateral and cruciate ligaments

To knock out your knee kit's collateral and cruciate ligaments, watch the video or follow the steps listed below.

Video of removing the femur access door, unclipping a ligament, replacing the femur access door, holding the door in place with one hand while simulating motion with another, reattaching the door screw.

Locate the access door on the posterior aspect of the femur.

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Use the Allen wrench to remove the screw from the access door. Place the screw in a place where it won't roll off the table or get lost.

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Inside, from medial to lateral, you'll see the anchoring clips for the: MCL, PCL, ACL, and LCL.

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To knock out a particular ligament, remove its clip from the socket.

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Replace the femur access door.

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To hold the door in place for your experiment, simply hold the access door in place with one hand while you simulate motions with your other hand. This activity goes much faster if you don't have to screw on and off the door each time.

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Medial collateral ligament (MCL)

Start by knocking out the medial collateral ligament (MCL). It's easiest to remove the clip for the MCL when the knee flexed to at least 90º; extending the knee tightens the MCL, making it more difficult to unclip.

Anterior-posterior translation

With your MCL knocked out, simulate anterior-posterior translation of the tibia at near full extension and write one of the following results in the corresponding cell of the table on page 1 of your activity worksheet.

ANT: The tibia translates anteriorly by more than 5 mm
POS: The tibia translates posteriorly by more than 5 mm
✓: The tibia translates the normal amount along the AP translation axis

Now repeat with the knee flexed to 90º and once you're done, check your work below.

ASSESS

Your simulated AP translations without the MCL should look like the video below.

Video of simulated AP translation at near full extension and 90º flexion with IT band knocked out

At both angles and with the tibia in a neutral long-axis position, the range of AP translation is normal. However, if you flex the knee to 90º and laterally rotate the tibia at the same time, you can observe excess anterior translation.

Video of simulated AP translation at 90º and with the tibia laterally rotated

For this activity, you don't need to test all possible combinations of motions for each test. So it's fine to put "✓" in both boxes. But if you want, you can write "ANT when the tibia is laterally rotated" for AP translation at 90º.

Screenshot of worksheet filled with the first 2 rows for the MCL.

Remember that for each result, you can draw conclusions about the ligament's function and, if you observe abnormal motion, the motion that would injure that ligament and cause pain or instability. For example, you can draw the following conclusions about the MCL from this experiment:

The MCL does not function on its own to limit anterior translation of the tibia when the knee is near full extension
the MCL functions to limit anterior translation of the tibia when the tibia is laterally rotated and the knee is flexed,
anterior translation and lateral rotation of the tibia when the knee is flexed can damage the MCL,
and if your MCL were damaged, you could experience pain or instability if your tibia moves anteriorly while laterally rotated during flexion.

Knock-out experiments are powerful!

Longitudinal rotation

Next, rotate the tibia about its long axis at near full extension and with the knee flexed to 90º, writing one of the following results in the corresponding cells on page 1 of your activity worksheet.

LAT: Lateral rotation of the tibia causes condylar disarticulation (no cartilage at one or both points of contact)
MED: Medial rotation of the tibia causes condylar disarticulation
✓: There is always cartilage on both bones at the point of contact

Once you've finished, check your work below.

ASSESS

Your simulated motions should look like the video below.

Video of simulated lateral and medial rotation of the tibia at near full extension and 90º flexion with MCL knocked out

As you can see from the video above, knocking out the MCL causes excess (abnormal) lateral rotation of the tibia both near full extension and at 90º of flexion. So, you should have "LAT" in both rows for longitudinal rotation.

Screenshot of worksheet filled with first 4 rows for the MCL

Varus-valgus rotation

Lastly, rotate the tibia along the varus-valgus axis at near full extension and with the knee flexed to 90º, writing one of the following results in the corresponding cells on page 1 of your activity worksheet.

VAL: Valgus rotation causes lift off between the medial condyles of more than 1 mm
VAR: Varus rotation causes lift off between the lateral condyles of more than 2 mm at near full extension or more than 10 mm at 90º of flexion
✓: Normal amount of lift off

Once you've finished, check your work below.

ASSESS

Your simulated motions should look like the video below.

Video of simulated varus-valgus rotations of the tibia at near full extension and 90º flexion with MCL knocked out

As you can see from the video above, knocking out the MCL causes excess (abnormal) valgus rotation of the tibia both near full extension and at 90º of flexion. So, you should have "VAL" in both rows for varus-valgus rotation.

Screenshot of worksheet filled with all 6 rows for MCL

As you can see the MCL pulls a lot of weight in stabilizing the knee! By working through this ligament first with opportunities to check your work along the way, you hopefully have a good idea of how to simulate the same motions and recognize abnormal motions for the remaining ligaments. Note, that just because the MCL has many different functions doesn't mean that the other ligaments will too. For some ligaments, you'll observe just a single abnormal motion in completing your table, for others you'll find more than one.

Posterior cruciate ligament (PCL)

Reattach the MCL, knock out the posterior cruciate ligament (PCL), and simulate all the motions to complete the next column. You'll use the same abbreviations for each motion test as for the MCL. Here are those abbreviations again for easier reference:

Anterior-posterior translation

ANT: The tibia translates anteriorly by more than 5 mm
POS: The tibia translates posteriorly by more than 5 mm
✓: The tibia translates the normal amount along the AP translation axis

Longitudinal rotation

LAT: Lateral rotation of the tibia causes condylar disarticulation (no cartilage at one or both points of contact)
MED: Medial rotation of the tibia causes condylar disarticulation
✓: There is always cartilage on both bones at the point of contact

Varus-valgus rotation

VAL: Valgus rotation causes lift off between the medial condyles of more than 1 mm
VAR: Varus rotation causes lift off between the lateral condyles of more than 2 mm at near full extension or more than 10 mm at 90º of flexion
✓: Normal amount of lift off

Anterior cruciate ligament (ACL)

Reattach the PCL, knock out the anterior cruciate ligament (ACL), and simulate all the motions to complete the next column. Finding the excess motions that result from an ACL knock out is a bit trickier than for the other ligaments because of an interaction with the menisci. Do the anterior-posterior translation test closer to near full extension (about 180º rather than 160º) and with a bit more force than for the other ligaments.

Lateral collateral ligament (LCL)

Reattach the ACL, knock out the lateral collateral ligament (LCL), and simulate all the motions to complete the next column.

Iliotibial (IT) tract/band

Reattach the LCL, knock out the iliotibial (IT) tract/band, and simulate all the motions to complete the next column. To detach the IT tract, simply unhook it from its attachment on the cross-section plate.

Video of unhooking the IT tract from its attachment on the CS plate

MCL + ACL

For your last knock-out test, you'll knock out two ligaments at the same time. Reattach the IT tract, knock out both the MCL and the ACL, and simulate all the motions to complete the next column. Make a note if any of the excess motions that you observe are greater than when the MCL or ACL is knocked out on its own—indicate this in your table by writing "greater" along with the direction of excess motion.

Reattaching all of the ligaments and the femur access door

Once you've finished running all of your simulations, reattach all of the ligaments and the femur access door using the screw and Allen wrench to secure it into place.

Same image as above with the Allen wrench in the screw socket