3D Printing Advances Spine Surgery Planning and Training

Learn how preeminent spine specialists use 3D printing to visualize complex patient anatomy and bring down medical education costs.

Peer Reviewed

A novel spine surgery simulator developed using 3D printing technology at Mayo Clinic Florida provides a lifelike model for spine surgery training and presurgical planning at a much lower cost and greater accessibility than cadaver models.

“We have created 3D-printed life-like cadavers that mimic real tissue.  Our current Chief Resident, William Clifton, MD, developed this technique and we have been using it for resident and medical student training,” Dr. Nottmeier told SpineUniverse. “At this point, most of our 3D printing experience at Mayo Clinic Florida has been limited to education and teaching.”

“The advantage of this is that we can train residents and medical students on procedures similar to what we would do with cadavers, but without the regulations and disposal issues associated with cadavers,” explained Dr. Nottmeier, who is Professor of Neurosurgery at Mayo Clinic Florida.

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“From a clinical standpoint, we have 3D-printed actual patient spines from preoperative CT scans in cases with complicated anatomy to help the surgeon better prepare for the patient’s anticipated surgery or to better understand the pathology in patients who have symptoms related to a dynamic etiology.”

Advantages of 3D-Printing Spine Surgery Training

In addition to the much lower cost of 3D-printed spine surgery simulators compared with human cadavers, other advantages include:

  • Nonhazardous
  • Easily transportable and disposable
  • Can stimulate some aspects of human surgical anatomy that can’t be simulated in the cadaver (bleeding, CSF leak from the dural tube)

A Case in Point

The technology also can be used to better examine the etiology of a patient’s symptoms to inform surgical planning. Dr. Nottmeier displayed the new technology in a NASS2020 virtual session, in which he described a patient with left upper extremity pain and weakness as well as hypoglossal nerve palsy resulting from a calcified cyst at the craniocervical junction in the left aspect of the spinal canal. Notably, the vertebral artery ran through the cyst, further complicating the treatment plan.

“To develop a treatment algorithm for the patient, you really have to get a sense of what the pathology is and how it is causing the patient’s symptoms,” Dr. Nottmeier said.

3d printing screwsImages courtesy Eric Nottmeier, MD

The surgical team suspected a dynamic etiology of the patient’s symptoms and were able to confirm this using 3D modeling, which showed that the patient’s spinal cord and cervicomedullary junction become compressed only when she turns her head to the left.

Based on the findings from a 3D-printed simulator, the team decided to perform an occipitocervical fusion in a neutral position rather than perform the riskier surgical excision of the cyst.

Steps in Ex-Vivo 3D Modeling for Spine Surgery

Dr. Nottmeier outlined the 5 steps he and his colleagues use in ex-vivo 3D modeling:

  1. Image acquisition: First, upload CT scans into the 3D slicer, an open-access software program that creates STL files used in 3D printing. Review the axial, sagittal, and coronal views to ensure that all slices are present and show the pertinent anatomy. 
  2. Anatomic segmentation: The scan is segmented using a thresholding technique and allocated to the appropriate housefield units to incorporate the anatomic structures of interest. The technology allows joint segments to interact with each other as they would in a cadaver. A 3D-rendering is then created from these scans.
  3. STL file creation: Then, upload the STL files into Meshmixer, an open-access editing software, to QC each component and view the anatomy before 3D printing the model.
  4. Virtual dynamic assessment: the surgical area of interest is viewed with the normal anatomic movement of joints.
  5. Mechanical dynamic assessment: allows for verification of the virtual assessment. 

3D Printed Implants

Michael Hisey, MD, of Texas Back Institute and colleagues also are using 3D printing for presurgical planning, as well as for traditional and custom-made implants. Uses of 3D printed models for surgical planning at Texas Back Institute have included visualizing the intricacies of congenital scoliosis.

For example, “you might have a vertebra that is only half formed, and another half of it has attached itself to the vertebra above or below, which creates very strongly progressive deformities, and makes it very hard to understand anatomy,” Dr. Hisey explained.

Presurgical planning for congenital scoliosis surgery is especially helpful as the operation is typically performed in children, in whom the targets for pedicle screw insertion are small. “You have to be very exacting with your surgery,” he said.

Dr. Hisey also uses 3D printed models for adult degenerative cases. “I've had a case where there was a big bone spur that I couldn't convince myself was occluding a neuroforamen,” Dr. Hisey said. “When we 3D printed a model of the area, I could see very clearly that the spur was interfering with the nerve,” and the model allowed for visualization of the best way to remove the spur, he added.

Texas Back Institute outsources 3D printing to companies with rapid prototype printers. The presurgical simulators take approximately a week to produce and, thus, are typically used for elective rather than emergency surgeries. Dr. Hisey also has outsourced 3D printing to build instruments tailored to the needs of a specific case.

Additionally, other researchers are developing 3D printed intervertebral discs containing a spring technology that is patient-specific, meaning that the springs are tighter in a heavier person than in a lighter person, Dr. Hisey explained. 

While 3D printing using polymer for presurgical planning is within the reach and budget of most spine surgery offices, the technology used to produce titanium implants is more sophisticated and expensive, Dr. Hisey noted. Thus, spine surgery practices typically outsource their titanium 3D printing needs. 

What Is the Best Material for 3D Printing in Spine Surgery?

For teaching purposes, the 3D printed simulators are made from a polymer. Dr. Nottmeier and colleagues demonstrated the feasibility of using a 3D-printed simulator of trabecular bone using thermoplastic polymers with polymeric foam that replicates the same porosity, density, and feel when probing a pedicle and inserting a pedicle screw, in a study published in Scientific Reports.1 The simulator can be adjusted for different pathological conditions, such as osteoporosis, Dr. Nottmeier said. 

Current Limitations of 3D Printing in Spine Surgery

While the accuracy of 3D printing has improved over time, current printers can print at 25 microns, which will leave 3D-printed titanium implants with tiny imperfections on the surface because of the additive process used in production, Dr. Hisey explained. These imperfections may make it more difficult to engage implants with typical insertion instruments.

Thus, manufacturers may need to “do a bit of post-processing to tap and smooth out the threaded holes so that screws will engage correctly and fit closely,” Dr. Hisey said.

The Future of 3D Printing in Spine Surgery

Dr. Nottmeier sees considerable opportunity to use 3D printing simulators to improve outcomes in complicated spine surgeries, like corrective spinal deformity surgery and tumor resection.

“In patients with complicated anatomy (eg, scoliosis, tumor), their spines can be 3D-printed so the surgeon would be able to actually perform the surgery on the 3D model prior to the actual surgery,” Dr. Nottmeier said. “3D printed simulators also will revolutionize training in that cadavers may not be needed anymore.”

“Accordingly, this opens up the options for education and training to more institutions and makes training more accessible,” Dr. Nottmeier concluded. “For example, training labs could be held anywhere without the restrictions of having a cadaver lab because the ‘tissue’ has no biohazard characteristics and be worked on and disposed of anywhere.”

“Ten years down the road, I think we will all have 3D printers,” Dr. Hisey said. He also envisions that surgeons will be able to print the instruments and implants needed for each day rather than shipping trays across the country as is currently done. Materials that are not used can be recycled for future use, he noted.

In the near future, Dr. Hisey suggested that local hospitals should have a 3D printer that can make “just what you need just when you need it. I think will be the future of implants, not just in spine surgery, but in orthopedics in general,” he said.

Dr. Hisey concluded that there is a learning curve with using 3D printing in spine surgery and that a staff member needs to spearhead the process for each practice.

Updated on: 11/24/20
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Eric W. Nottmeier, MD
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