Over the last decade, the use of pedicle screw based spinal instrumentation in the lumbar and thoracic spine has increased tremendously thanks to their superior biomechanical properties and reduction possibilities. Nevertheless, despite the perception that cervical pedicle fixation can provide superior holding strength, it has not been routinely implemented due to large anatomical variation of cervical pedicles between individuals. Moreover, the small bone size of the pedicles and the proximity to the vertebral artery and the spinal cord increase the need for very precise screw placement. Several methods have been explored so far, most prominently computer-assisted screw insertion. Now a research team has looked into using the Mimics Innovation Suite to create a patient-specific and safe solution!

Designing a Patient-Specific Template based on a model created with the Mimics Innovation Suite

The idea Sheng Lu and her research team had, was to use patient-specific drill templates with a preplanned trajectory for cervical pedicle screw placement. To get started, the team selected 25 patients with indications for pedicle screw placement and took a spiral 3D CT-scan of each of them. They imported the scanner images in the Mimics Innovation Suite and generated an accurate 3D reconstruction of the desired vertebra. Based on these 3D models they determined the optimal screw size and orientation for each model separately. A virtual screw was placed on both sides of the 3D model, making it possible to start designing a navigational template. To create the template, they inversed the vertebral surface, which assured them of a perfect fit. As such, these templates were engineered on each patient’s specific anatomy.
3D model of the cervical vertebra generated in Mimics
The navigational template surface was created as the inverse of the vertebra

The resulting patient-specific navigational template

Verifying the Template Pre-Operatively using 3D Printed Models

Navigational template fits AM model of the vertebra perfectly
Even though the virtual model showed that there was a perfect match between the anatomy and the template, the team wanted to verify the stability and fit with their own hands prior to the surgery. In order to do so, the vertebra and its corresponding navigational template were 3D printed. By placing the template on the model of the vertebra, the researchers reassured themselves of the unique fit of the template. They also checked the screw trajectory by drilling K-wires through the navigation holes; here again the results were within the limits of the predefined precision needs.
 

Putting the Templates to Use During Surgery

Intraoperative usage of the manufactured template
After sterilization, the templates were used intra-operatively for navigation. The surgeon performing the surgeries found it really easy to find the best fit for the template manually, as there was no significant free motion of the template when it was placed in position and pressed slightly against the vertebra. As such, the navigational template fulfilled its purpose for use as an in situ drill guide.

Confirming the Success of the Technique

Fluoroscopy shows good positioning of pedicle screw
After surgery, the positions of the pedicle screws were evaluated using X-ray and CT scans. The results were positive as there were no pedicle perforations and the two surgeons who evaluated the results, independent from one another, also noted that the insertion caused no injury to the vertebral artery or to the spinal cord.
Although the technique requires a very clean preparation of the bone surface, it has been reported to be the best technique yet. There are in fact several distinct advantages in using these patient-specific templates.
  • Firstly, the surgeon can decide on the location, orientation, and size of each screw based on the unique morphology of the patient.
  • Secondly, this technique is relatively simple to use, so it does not require specific additional expertise on the surgeon’s part.
  • Thirdly, it has the potential to eliminate the need for complex equipment and time-consuming procedures in the operating theatre, which can result in a reduction in operating time.
  • Fourthly, screws can be accurately placed while reducing the risk of spinal canal or blood vessel perforation.
  • Finally, the need for fluoroscopy during screw insertion may be eliminated, which considerably reduces the radiation exposure to the patient and the members of the surgical team.

With its wide applicability, high accuracy, proven safety, and cost-effectiveness, this design will likely enjoy widespread use in the future… and become the backbone of cervical pedicle screw placement.

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