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, a test for extension beyond the normal range of motion (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) and complete the first column of the table on page 1 of your worksheet, following the same procedure as for the IT tract. Once you've finished completing the column, check out the hint below to see if you're on the right track.
Anterior-posterior translation
With your IT tract 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 motions should look like the video below.
Video of simulated AP translation at near full extension and 90º flexion with IT band knocked out
Since the range of motion is normal, you should have two checkmarks on your worksheet.
Screenshot of worksheet filled with the first 2 rows for IT band.
From this result, you can conclude that:
- the IT band does not function to limit anterior-posterior translation of the tibia at near full extension or at 90º of flexion
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 IT band knocked out
And you should have a checkmark for longitudinal rotation at near full extension and "MED" for longitudinal rotation at 90º of flexion.
Screenshot of worksheet filled with first 4 rows for IT band.
Knocking out the IT band causes excess (abnormal) medial rotation of the tibia, visible as the contact point between the femur and tibia moving beyond the edge of the articular cartilage. From this result, you can conclude that:
- the IT band functions to limit medial rotation of the tibia when the knee is flexed
- the IT band could be injured by excessive medial rotation of the tibia when the knee is flexed
- if the IT band is injured, longitudinal rotation of the tibia would be painful or destabilized when the knee is flexed
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 IT band knocked out
Since the range of motion is normal, you should have two checkmarks on your worksheet.
Screenshot of worksheet filled with all 6 rows for IT band.
From this, you can conclude that:
- the IT band does not function to limit varus-valgus rotation of the knee at near full extension or at 90º of flexion
Reattaching your IT tract
HINT
The MCL functions to stabilize the knee in many ways. You should have abnormal motion results (i.e., not checkmarks) in at least four of the rows of the table for the MCL column. You could have abnormal motion results in up to five rows if you simulate anterior-posterior translation combined with tibial longitudinal rotation.
Posterior cruciate ligament (PCL)
Reattach the MCL and knock out the posterior cruciate ligament (PCL) to complete the next column.
Anterior cruciate ligament (ACL)
Reattach the PCL and knock out the anterior cruciate ligament (ACL) to complete the next column.
Lateral collateral ligament (LCL)
Reattach the ACL and knock out the lateral collateral ligament (LCL) to complete the next column.
Reattaching all of the ligaments and the femur access door
Once you've finished running all of your simulations, reattach the LCL 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
Knocking out your knee's IT tract
Knock out the IT tract by simply unhooking it from its attachment on the tibia. Using the forceps can help in grabbing and pulling the "tail" of the cord.
Video of unhooking the IT tract from its attachment on the tibia by pulling on the tail of the cord.
Anterior-posterior translation
With your IT tract 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 motions should look like the video below.
Video of simulated AP translation at near full extension and 90º flexion with IT band knocked out
Since the range of motion is normal, you should have two checkmarks on your worksheet.
Screenshot of worksheet filled with the first 2 rows for IT band.
From this result, you can conclude that:
- the IT band does not function to limit anterior-posterior translation of the tibia at near full extension or at 90º of flexion
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 IT band knocked out
And you should have a checkmark for longitudinal rotation at near full extension and "MED" for longitudinal rotation at 90º of flexion.
Screenshot of worksheet filled with first 4 rows for IT band.
Knocking out the IT band causes excess (abnormal) medial rotation of the tibia, visible as the contact point between the femur and tibia moving beyond the edge of the articular cartilage. From this result, you can conclude that:
- the IT band functions to limit medial rotation of the tibia when the knee is flexed
- the IT band could be injured by excessive medial rotation of the tibia when the knee is flexed
- if the IT band is injured, longitudinal rotation of the tibia would be painful or destabilized when the knee is flexed
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 IT band knocked out
Since the range of motion is normal, you should have two checkmarks on your worksheet.
Screenshot of worksheet filled with all 6 rows for IT band.
From this, you can conclude that:
- the IT band does not function to limit varus-valgus rotation of the knee at near full extension or at 90º of flexion
Reattaching your IT tract
Before proceeding to knock out the other ligaments, be sure to rehook the IT tract into the tibia. Use the video below as a guide
Video of attaching the IT tract to the tibia
or follow these steps:
- Use the forceps to push the knot of the IT tract cord into the hole labeled "ITT-B."
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- Once the knot is fully within the hole, pull on the upper portion of the IT tract until the knot is secured under the internal hook inside the hole.
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If you don't reattach the IT tract, the rest of your tests will be the combined effect of knocking out the IT tract plus another ligament.
Do the ligaments have overlapping functions?
Take a look at your completed table. You now have a mostly complete picture of all the independent functions of the ligaments of the knee. Your results are a powerful tool to be able to ask deeper questions about knee and ligament function. For example, is there any overlap in the functions of the ligaments? That is, are there are any functions that are performed by two or more ligaments? If yes, why or if no, why not? Write your answer at the top of page 2 of your worksheet. If you're unsure how to determine this, check the hint below.
Your table is mostly complete because you didn't test the function of each ligament for all possible motions of the knee. For example, you only tested for abnormal motion at near full extension and at 90º of flexion; if a ligament knock were to cause abnormal motion at different angle of flexion-extension, you wouldn't have detected it. Also, your results only capture the independent functions of each ligament, not potential functions of a ligament in combination with another ligament; to test this, you would have to knock out two (or more) ligaments at the same time.
HINT
To determine if there is overlap in the functions among the ligaments of the knee, look across each row and see whether you obtained the same result (i.e., observed the same abnormal motion) for more than one ligament. If so, this would tell you that those ligaments have the same function.