My thesis work required a lot of identical proximal femur segments for a mock surgery that treats femoroacetabular impingement, where a bump forms on the femoral neck and shreds the cartilage of the acetabulum, leading to osteoarthritis. Since we were in the preliminary stages of the project, we didn’t have any surgical navigation showing how much bone to remove, so we needed a clear visual guide to assist the subject during the milling.
Amber Simpson, a Post-Doc at Vanderbilt posted a guide on her website on how to make plastic bone models that she used for her thesis. It was pretty decent and I decided to adapt it for my research. I’ve since moved away from using whole bones but didn’t want my efforts to go unpublished, so I decided to post it up here for longevity.
1. Get bone model.
You can go about this a variety of ways. Ms. Simpson CT scanned human bones and 3D printed them. I didn’t like the surface finish of the 3D printer (I later needed to laser scan the bones), so I used a commercially available femur from a certain manufacturer that everyone knows.
2. Create a mould from the model.
Mimicking the guide, I made a wooden frame with hinges and clasps that fit the bone I was casting, leveled one half of the bone in one half of the frame, and poured the mould using Oomoo 30 silicone rubber. Once this cured, I sprayed the surface with mould release, closed the frame and carefully filled the remaining half through a hole in the top.
3. Cast your bones.
I cast the bones using Smooth-Cast 300, a two-part resin with a 10 minute cure time and a 70D hardness, similar to bone (85-95D). One study I performed had an average femur weight of 233.62 +/- 2.77g, which was an acceptably low deviation.
4. Create your Cam Lesion.
Now I mentioned before that we needed a clear visual guide to help with our bone removal, so we need to cast the cam lesion onto the surface of the bone. I created one using plasticine (not pictured) inspired by clinical data in our lab.
5. Create the Cam Lesion Mould.
This step is identical to Step 2, after you’ve made the first half of your mould. Given the anterosuperior location of the cam lesion, the majority of cases do not cross over to where the first half of the mould is, so you just need to pour a second half-mould using a bone that has a cam-lesion on it. This way you can swap half of the mould in or out depending on if you’re casting bones. Once this mould has cured, drill a hole into the highest point on the side so you can pour in additional casting material.
6. Pour the Cam Lesion.
Place one of the cast bones into the mould. It should be a tight fit, preventing the extra casting material from running everywhere. I mixed and poured in Alumilite Black resin (shore hardness of 75D) into the hole I made in the side of the cam lesion mould, shown above, and let it cure. This allows the cam lesion to be perfectly cast to the surface of the bone, shown at right. You can then remove this black lesion, trim the edges (essentially flashing), and superglue it to your bone.
7. Inspect the final product.
As you can see above, there is a bit of a learning curve to this casting method but I promise it works. The final weight of the lesions was 5.31 +/- 0.20g.