Pediatric orthopaedists find it helpful for patient education too
If the word “printer” calls to mind an ink-stained man and a large wooden press, or even a fancy copy machine, you’re behind the times. Today, printers aren’t limited to ink and paper, or even two-dimensional products.
In fact, for more than 30 years, three-dimensional (3-D) printers have been around and able to create custom solid objects. As technology has advanced, prices have dropped, making 3-D printing an option for rapid prototyping of designs—even bones, joints, and implants.
Recently, AAOS Now editorial board member Howard R. Epps, MD, of Texas Children’s Hospital, spoke with two other pediatric orthopaedists—Kevin S. Horowitz, MD, and Scott B. Rosenfeld, MD—about how they use 3-D printing in their practices.
Dr. Epps: My first question for each of you is, how have 3-D models helped you in your practice? And what applications have there been?
Dr. Horowitz: We have both used 3-D models for complex hip deformities; a 3-D model certainly helps provide a better understanding of the complex deformity. Also, we can actually do some preoperative planning on the models. And we can show the models to the patients, side by side with some normal models. That gives them a better appreciation of the anatomy.
Dr. Rosenfeld: We do more hip deformity surgery than we do any other type of deformity, and we find it useful for these procedures. Some of our partners have used 3-D printed models to assess a foot deformity like a tarsal coalition.
The first time I used a 3-D printed model was with a patient who had Perthes disease. The femoral head had a funny shape in multiple planes. Just looking at radiographs and the computed tomography (CT) scan on the computer screen, I couldn’t get a sense of where the femoral head was round.
We decided to print a 3-D model based on the CT imaging data. Once it was done, I could take the femoral head in one hand and the socket in the other hand and figure out which part of the femoral head fit nicely into the socket. That enabled me to template a surgery that would allow me to put the round part of the femoral head in the socket. We actually made the cuts on the model to determine the best way to correct the deformity.
Dr. Epps: How long does it take to get a model and what is involved in getting one?
Dr. Horowitz: We start with a 3-D CT scan. Then we send it to the 3-D printing unit, and, depending on the size of the bone we want to print, we can get the model in 24 hours—sometimes even less time.
Dr. Rosenfeld: The time it takes to get the model is very dependent on the printer. Our hospital has a basic 3-D printer that costs around $2,500. With this basic printer, production of a full pelvis 3-D model probably takes about 30 hours of printer time. About 10 to 12 hours of data input by the technician is required prior to the actual printing. Ours is a relatively slow printer. The higher-end printers are much faster and don’t require as much maintenance through the printing process.
Dr. Epps: So for people who aren’t familiar with this process, how does it work, step by step? You see a patient in the clinic and you think this might be a benefit; what happens from there?
Dr. Rosenfeld: When we see a patient in clinic who has a deformity that we want to learn more about, we explain that we need to obtain a CT scan. Our current 3-D printer software only allows us to print from a CT scan instead of an MRI. So we put in a request for a CT scan, and it is scheduled, sometimes on the same day, but usually within a couple of days.
Then we have to put in an order for a 3-D rendering of the CT scan, and another order for printing of the 3-D model. Once that order is in, it generally takes 24 to 48 hours to turn around and produce the model. The dataset from the CT scan has to be manually entered by a technician into a software program that converts it to the data that the printer requires.
The printer lays down layers of material, 0.1 mm to 0.3 mm thick. It uses strands of plastic material that look like thick fishing line, which are fed into the machine. Because the shapes of the bones that we are printing do not lend themselves to standing upright, the models are produced in the middle of a honeycomb-like cube (Fig. 1). When it’s done, you have this 3-D cube, with the model in the middle. The model is then cut out of the surrounding cube.
When we get the model, we meet with the family to review the deformity, using the model as an example. I always let the patient or family hold and manipulate the model because I think that gives them a better appreciation of the anatomy.
If I am using the model to template the surgery, I can make the osteotomy cuts on the model to show the family where the cuts will be and how the bone will be manipulated to correct the deformity (Fig. 2).
Finally, I’ve even brought the model into the operating room (OR) and used it as an intraoperative guide for certain osteotomies such as a periacetabular osteotomy.
Dr. Epps: What is the model made of and does it come out true to size or is it smaller?
Dr. Horowitz: The model is made of a plastic polymer called acrylonitrile butadiene styrene, which comes in long strands on a spool like fishing line.
Dr. Rosenfeld: Most of the pelvic models that I have used were made to size. We’ve taken radiographs of the model and of the patient’s pelvis and laid them on top of each other. They’re pretty close.
Dr. Epps: But you can practice doing cuts on it? Can you practice an osteotomy on a model?
Dr. Horowitz: You can, but the material can be difficult to cut. Our 3-D printer basically produces a shell. So when you cut through it, the inside is just fine strands of plastic.
Dr. Rosenfeld: Yes, the cortical part of the model is nice and thick. You can put a screw through it and get pretty good purchase on the bone. But what would represent the metaphyseal portion is quite hollow and in a honeycomb configuration.
They can change the settings on the printer and make it print more densely throughout. But to save time and material, they generally don’t do that. And we don’t need it; for us, the main purpose of the model is to get a better understanding of the anatomy, not necessarily to be able to obtain good purchase implants on the model or to get a good sense of what it is going to feel like to cut it.
Dr. Epps: So if you needed to, could you get a representation of the internal architecture of the bone as well if that were going to be helpful?
Dr. Horowitz: That is my understanding, although we have not done this. I know that the cardiac models are made of a substance that is more flexible and gelatinous. So certain printers can work with various materials.
Dr. Rosenfeld: There are all sorts of printers—some that print using stainless steel or titanium, some that use a more gelatinous material, and some, like ours, that use a plastic polymer.
Dr. Epps: One question people always have is about costs and billing. Do you have any ideas?
Dr. Rosenfeld: The costs are mostly associated with the staff time spent producing the models. The material that we use is relatively inexpensive. A 1 kg spool of polymer costs around $30. Printing a full pelvis requires about half a spool. Our radiology department is working on a system to bill for the production of the models.
Dr. Epps: Do you think it saves time in the OR?
Dr. Rosenfeld: Yes, I do think it saves time. For example, in the case I described with the Perthes hip, I knew what angle I wanted to achieve ahead of time because I had already modeled it.
Dr. Horowitz: In certain cases, you may be able to recognize that a particular implant won’t work before taking the patient to the OR.
Dr. Epps: Finally, what about the application of 3-D printers in trauma or other areas? Do you see them playing a role there?
Dr. Rosenfeld: I think they can be used in this setting as well. The issue would be timing. If it takes 24 to 48 hours to produce the model, I don’t think that’s fast enough to be used in a trauma setting. A faster printer would be more useful for a trauma case.
But for a difficult deformity—a pelvic or acetabular fracture, a bad tibia fracture, a spine injury—where you want to understand the deformity a little bit better, it could certainly be useful.
Dr. Horowitz: Although we haven’t used it here, 3-D printing has been used for very complex spine deformities. It may lend itself to preoperative planning and a better understanding of the deformity.
Disclosure information: Dr. Epps—AAOS Now, Pediatric Orthopaedic Society of North America, Texas Orthopaedic Association. Dr. Horowitz—No information available. Dr. Rosenfeld—OrthoPediatrics, Wolters Kluwer Health - Lippincott Williams & Wilkins UpToDate