Each human bone has such unique shape, perfectly designed by nature for its function. Alteration of normal bone shapes can lead to unpleasant deformities, loss of function and pain. Most commonly, abnormally shaped bones are the consequence of fractures or problems during development. When bones are fractured and displaced, they may heal or unite with a different shape: the healed bone may be angled, shortened, rotated, and thickened… This condition is called malunion (“mal” – bad; “union” – healing). In other individuals, the abnormal bone shape occurs during development: something happens before or after birth that leads to bone misshape. This condition is called dysplasia (“dys” – abnormal; “plasia” during growth and development).
As we get deeper and deeper to the 21st century, technology permeates many aspects of our life. Some technological advances, which were considered a distant dream a few years ago, are now applied in surgery: robots are used to perform surgery, virtual and mixed/augmented reality are used for training and surgery, artificial intelligence is used for diagnosis and planning… And yes, one of the cool technologies used today to improve the outcome of orthopedic surgery is already here: 3D printing of your own bones!
What is three-dimensional printing?
Three-dimensional printing (3D-P) creates an object by joining or solidifying material layer by layer under computer control. Since material is progressively added in layers to create the object, some call this process additive manufacturing. The steps for 3D-P are summarized below.
The object to be printed may be created virtually in a computer or it may be copied from a real object with a 3D scanner or three-dimensional rendering using multiple digital photographs. In orthopedics, bones are scanned using radiation (computed tomography) and the computer file that stores the scanned bones is called DICOM (Digital Imaging and Communications in Medicine) file. Any of these inputs is converted into a special file format (STL/AMF) that is then sliced in layers into a G-code file. The G-code file is loaded into the computer that controls the printer. The material to be used (polymer, metal or other) is loaded into the printer. The printer then creates layers of material that are joined or solidified to form the object. The object created by the printer may require additional post-processing or finishing to remove irregularities or modify the object. Currently, it is possible to print objects using different materials, different colors…, and in addition the object to be printed may be modified virtually before printing.
Why is 3D printing so useful for correction of bone deformity?
Abnormal bone shapes are typically quite complex: the misshapen bone may be shortened, angled and twisted all at the same time. Correction of deformed bones requires cutting the bone in surgery and realigning the cut parts so that the shape is restored to normal. This procedure is called corrective osteotomy (“osteo” – bone; “tomy” – to cut). Understanding where and how to cut the deformed bone and how to get the correction perfect is difficult, as you can imagine.
Since the arms and legs have right and left symmetrical bones, the classic way to plan osteotomies was to obtain radiographs of the normal and abnormal side (for example, the right and the left collarbones), trace the outline of the bones on transparent paper, overlie the outlines of the normal and abnormal bones (flipping the normal side over so that the right tracing looks like a normal left tracing, for example), and cutting the paper tracings with scissors to plan where and how to make the cut. This process is somewhat inaccurate, trying to plan in 2 dimensions (a flat piece of paper) how to change the shape of a tridimensional object.
A major advancement in understanding how to correct the shape of a bone came with computed tomography (CT). Multiple radiographic slices of bones are obtained in three planes. The information obtained is stored in files (DICOM files) that can then be loaded in a computer. Segmentation is the process of identifying and marking the outline of the bones of interest in each of the slices. Computer software can then be used to create a three-dimensional virtual rendering of the bones of interest that can be visualized and rotated on a computer screen.
But now we can do even more! The DICOM files can be used to model the bone for 3D-P. CT scans of the right and left bones (normal and abnormal) are obtained, the 3D rendering of the normal bone is virtually flipped or mirrored so that it looks like the abnormal side should. The renderings may be virtually overlapped, and cuts may be planned on the screen, virtually moving the pieces until the abnormal bone looks like the normal opposite side mirrored counterpart. And not only that: computer aided design can be used to create a guide for the cut specific for that patient, and even fixation devices such as plates. Then, all is 3D printed! The surgeon receives a bunch of polymer models: the normal side mirrored, the abnormal side before and after the cut and correction, the guide, and the fixation devices. “Surgery” can be then performed in these models before the real surgery happens, which allows the surgeon to practice. Finally, the printed guides, bones and fixation devices are used as well the day of surgery under sterile conditions. Amazing!!!
A real life example
We will share with you the surgical procedure to correct an abnormal left collarbone that healed in a malunited position after fracture. Collarbone malunions change the position of the shoulder blade in space, and all the muscles around the shoulder become pretty painful. Normal movement of the shoulder is more difficult. In addition, the nerves and vessels under the collarbone may be compressed. Finally, extreme malunions make the shoulder region look bad, clothes do not fit nicely, and the strap of a bra, tank-top, purse, or backpack will just fall off. For these reasons, patients benefit from surgery to perform a corrective osteotomy.
The radiographs of both collarbones shown above start to show the problem. As we discussed, three-dimensional rendering of the CT scan is more useful, as the virtual rendering may be rotated and analyzed from every angle. Then it is time for the surgeon and the engineer to sit together in front of a computer modelling station and play on the screen with the renderings of the normal and abnormal side. The final products? 3D printed replicas of the normal mirrored bone, the abnormal bone with the planned cut, the guide to perform the cut, and in selected cases the hardware to fix the cut bone until it heals.
The day of surgery, the cutting guide and the bone replicas are sterilized and used during surgery. The cutting guide fits perfectly the surface of the malunited bone and facilitates performing a perfect cut, just as planned. The replicas of the abnormal bone and mirrored normal bone are used to make sure that the geometry of the abnormal bone is restored to normal. The bone is then stabilized in its new position with a plate and screws.
If you want to watch a video of the surgery, click below.
The best is yet to come…
Used of advanced technology in the operating room will transform orthopedic surgery over the next few years. Three-dimensional printing is an incredible tool, but as mentioned previously, many other technological advances continue to revolutionize shoulder and elbow surgery: decision making guided by artificial intelligence, surgeons using virtual or augmented reality to practice or perform surgeries, robotics incorporated into the surgical suite,… The future is here!