Assembling the knee is one of the more difficult pieces of the puzzle to convey because so many holes must be drilled before you can begin knitting everything together. Given the complexity of it, I invite you to begin with a video above where I walk through a finished knee so that you have a clearer idea of where this is going.
At it's heart, we are trying to duplicate the pull of the major structures that hold the knee together:
- Patellar Tendon and Quadriceps
- LCL and MCL
- ACL and PCL
Let's get started...
Step 1: Assemble the pieces that you need. These include the:
You'll also want a healthy amount of 1/8'' bungee cord and some twine or a zip-tie.
Step 2: Drill four holes in the patella
These should be distributed evenly across the width of the patella without exiting or entering along the articulating surface of the bone. This is a delicate operation and should be done with care.
These holes will hold the bungee cords that imitate the patellar tendon inferiorly and the pull of the quadriceps superiorly.
Step 3: Drill holes to anchor the patella
Drill into both the femur and tibia as shown. These will match up with the bungees that connect to the patella. Note at the tibia: these will insert into the same bony prominence that forms the attachment site of the patellar tendon.
Step 4: Prepare for the "Collateral Ligaments"
Four drill holes are necessary to secure the knee against varus and valgus forces similar to the MCL and LCL. On both the femur and the tibia, drill into the lateral and medial condyles, aiming for targets highlighted in the second picture below.
Just as a side note, this is a rough approximation of these ligaments. For starters, the LCL attaches to the head of the fibula in reality and the lateral tibia in the skeletal model.
Step 5: Drill for the cruciate ligaments
The drill holes for this next part are some of the more difficult to explain, and I think it helps to simply think of putting some cords where the ligaments are in real life. This will require boring four holes (two in both the femur and tibia)
Drill into the bones as shown in this photo, creating a path where these bungee cords go. Keep in mind that only one continuous cord will be used for the actual model. On the tibia, the path will go through the intercondylar tubercles.
Step 7: Bind together all the pieces of the knee
Now that all the holes are drilled, thread the cord through them and adjust the tightness to the desired level. This will take some iterations of adjustment, particularly for the patella. It's probably easiest to assemble the pieces in the following order:
- MCL + LCL
- Patellar attachments
Step 8: Prevent hyperextension
It's time to protect the knee from hyperextending. Use a piece of twine or a zip-tie (as shown below) to tension the two ends of the ACL/PCL bungee as shown in the photo.
Reflecting back on the kind of thing you're likely to see on most skeletons, I hope you'll agree this is an improvement. And I must underscore that this knee design comes not from me, but John Chester, MD.
The next stage of this project will jump down to the foot so it'll be ready for the last step of the lower extremity: the ankle.
Please ask any questions in the comments below!