Low-cost 3D Modeling Software for Generating Patient-specific Drill Guide Templates for Cervical Pedicle Screw Insertion: An In Vitro Study

Abstract

Background. Instrumented posterior cervical spine surgery (IPCSS) can be conducted using screws inserted through the pedicles of the vertebra. A safe IPCSS method uses 3D-printing to produce templates that will serve as drill guides for screw placement.

Objectives. This study describes the generation of 3D-printed drill guides using low-cost general purpose 3D modeling software and the comparison of screw insertion accuracy scores against the traditional landmark method and guides created using commercial grade software.

Methods. Twenty-five (25) subaxial pedicles of five cadaveric spines were selected and scanned using computed
tomography (CT). A digital reconstruction of the five cadaveric spines were created based on the CT DICOM data. A low-cost 3D modeling software, Rhinoceros 3D, was utilized for trajectory planning and generation of a patientspecific drill template using the digital reconstruction. The templates were then fabricated in ABS plastic using a fused deposition modeling (FDM) 3D printer. Insertion of cervical pedicle screws on the cadaveric spines was done by an orthopedic resident using the 3D printed guides. Postoperative CT scans were obtained, and placement accuracy of the screws were scored by two assessors utilizing a four-point rating system. Screws in correct placement were scored Grade 0 while misplaced screws with neurovascular damage were given a score of Grade 3.

Results. Accuracy scores for the 3D-printed drill guides were 52% for assessor 1 and 44% for assessor 2. For
assessor 1, screw placement in C3, C6, and C7 received the highest scores. For assessor 2, the highest scores were achieved in C3 and C7. The hybrid method of Bundoc et al. achieved scores of 94% while 3D printed guides utilizing commercial software like Materialise Mimics, Geomagic Freeform, or UG Imageware achieved scores of 80-100%. The traditional landmark method had scores ranging from 12% to 94% depending on the skill of the surgeon.

Conclusion. Commercial medical 3D image-based engineering software has high acquisition costs that might be beyond the reach of most institutions. A sub-$1000 general purpose 3D modeling software can be used to create drill templates. Several factors were identified in the design and fabrication of the template that can be addressed to increase accuracy. Trajectory planning can also be improved by automating the process. The researchers recommend further studies in these areas specially in the context of developing 3D printing as a support service for surgical operations in the Philippines.