New 3D printed cranioplasty template enables posterior fossa craniotomy in preclinical validation

In a development that could reshape how neurosurgeons approach one of the most delicate corridors in the human skull, researchers at the University of Leipzig have unveiled a 3D printed template-and-implant system that dramatically tightens the fit of bone reconstruction after retrosigmoid craniotomy. The study, published in the journal 3D Printing in Medicine, reports that the new patient-specific concept reduced the average gap between the reconstructed implant and the surrounding skull to just 2.11 millimeters, compared with 5.52 millimeters for the current standard approach, while completing the entire procedure in an average of thirteen minutes and twenty seconds.

The retrosigmoid approach is a workhorse of modern neurosurgery, providing access to tumors and vascular lesions in the cerebellopontine angle, a crowded region near the brainstem where acoustic neuromas, meningiomas and vascular compressions are commonly treated. Surgeons reach this area by drilling through the occipital bone behind the ear, creating a window over the posterior cranial fossa. But closing the resulting defect has long been an unsolved nuisance. Because the removed bone fragment is always smaller than the opening created by the trephine, and because surgeons frequently enlarge the craniotomy intraoperatively to improve visibility, gaps of up to seventeen millimeters can remain between the reinserted bone and the skull edge. These gaps are implicated in postoperative complications, including cerebrospinal fluid leaks, and often require additional closure with bone cement, a material whose mixing, handling and curing can add roughly fifteen minutes to the operation.

The Leipzig team, led by Svenja Jung and colleagues in the Department of Neurosurgery together with the Institute of Forensic Medicine and the Fraunhofer Institute for Machine Tools and Forming Technology, designed a two-part solution that pairs a surgical marking template with a matching patient-specific cranioplasty implant. The concept unfolds across four stages, only the last of which takes place in the operating room. Preoperatively, the template and implant are designed from segmented CT data and printed in advance. In surgery, the template is placed directly on the skull and its contours, an inner boundary that must not be undercut and an outer boundary that defines the maximum extent of the craniotomy, are traced onto the bone with a surgical marker. The surgeon then performs the D-shaped craniotomy, with a vertical cut along the mastoid, following these lines. Finally, the implant, which mirrors the template’s outer contour, is trimmed if necessary and fixed into place with plates and screws.

The geometry of the system is deliberately forgiving. Because the implant corresponds to the maximum allowable craniotomy rather than a fixed minimum, any opening smaller than the outer template contour can still be completely covered: an auxiliary guide, itself 3D printed, transfers the template’s inner contour onto the implant so that it can be accurately shortened at the operating table. Only a craniotomy exceeding the outer contour would defeat complete coverage, and the template’s markings are designed to discourage that outcome. Implant thickness was set at five to eight millimeters, matching comparable commercial implants and providing stability in a region where skull thickness varies from a few millimeters to as much as two centimeters.

Manufacturing the components required three distinct additive processes. The templates were produced by stereolithography, an inverted vat polymerization technique, using a biocompatible resin from Formlabs on a Formlabs 3BL printer, with build orientation chosen to keep support structures off the skull-contacting surface. The implants were fabricated by fused deposition modeling on an UltiMaker S7, oriented so that support structures and post-processing fell on the outer surface, preserving a smooth internal face for contact with the brain. The auxiliary trimming guide was printed in polyamide 12 by HP’s Multi Jet Fusion powder bed fusion process. Although the clinically established material polyetheretherketone, or PEEK, is the intended end point, the team used polylactic acid for the preclinical tests to control costs, noting that patient-specific PEEK implants currently run between six thousand and nine thousand euros per patient including the template.

Testing was conducted on a sophisticated 3D printed phantom replicating the posterior cranial fossa on both sides, complete with an integrated cerebellum for orientation and interchangeable skull modules that could be swapped between participants. The phantom itself combined four materials, three printing techniques and one casting process, and was mounted on a camera tripod for mobility. Nine senior and attending neurosurgeons from University Hospital Leipzig each performed bilateral craniotomies on the phantom, producing eighteen procedures in total, one of which could not be completed. After each craniotomy, the implant was trimmed, inserted and fixed, and the modules were scanned by CT at 0.625 millimeter slice thickness for gap analysis in the open-source software 3D Slicer, with nine standardized measurement points defined on each implant.

The results were striking. Across 154 recorded gap measurements, the new concept achieved a mean gap of 2.11 millimeters, roughly 3.5 millimeters smaller than the standard of care, with markedly lower variability. The worst-case gap shrank from 17.59 millimeters under the conventional approach to 9.87 millimeters with the template-implant system, and the best-case measurement fell from 0.72 to 0.07 millimeters. Timing data proved equally encouraging: positioning the template and performing the craniotomy took an average of five minutes and forty-five seconds, while trimming, placement and fixation of the implant added seven minutes and thirty-five seconds, for a total of thirteen minutes and twenty seconds. The researchers calculate that, after accounting for time that would be spent on craniotomy and closure regardless of method, the concept imposes only about five minutes of additional procedural effort, substantially less than bone cement reconstruction.

A comparison of printing accuracy reinforced the reliability of the workflow. Digitization of the printed templates, implants and phantom modules with a Keyence scanner and overlay against the original CAD models in GOM Inspect software revealed deviations consistently below one millimeter, attributable mainly to scan merging, support removal and minor resin residues rather than to systematic printing error. The surgical sets themselves were drawn from standard neurosurgical instrumentation, including a craniotomy set from Aesculag and a spiral bone drill from adeor medical, underscoring that the concept requires no exotic hardware beyond the printed components.

The study is candid about its limitations, which are considerable. The template functioned only as a marking guide, not a fixed cutting edge, leaving the final cut to each surgeon’s judgment, and several participants extended the craniotomy beyond what the implant could cover or inadvertently worked on the skull instead of the hand-held implant, creating unintended additional defects. Positioning relied on externally visible anatomical landmarks because no MRI data or neuronavigation were available for the reduced test models, whereas clinical practice would combine CT and MRI with neuronavigation to place the burr hole precisely over the sinus angle. The comparison dataset also came from 58 real patients with craniotomies of varying size and position, which naturally inflates the variability of the historical control group. A clinically meaningful assessment, the authors stress, will require validation on cadaveric specimens, which are planned next, followed by clinical studies addressing reproducibility, efficiency and safety.

Even so, the preclinical feasibility results represent a compelling proof of concept for bringing the template-based planning methods already routine in orthopedic, maxillofacial and vascular surgery, where guides are used to pre-cut stents, plan tumor resections and position screws, into an area of neurosurgery where they have been conspicuously absent. Smaller, occipitally located cranial defects have been covered with 3D printed cranioplasties before, but a guiding template for defining the surgical approach itself had not previously been reported. By defining the craniotomy preoperatively and manufacturing the implant to match its maximum extent, the Leipzig system converts an improvised closure problem into a planned, standardized procedure. If the upcoming cadaveric and clinical validations hold up, patients undergoing retrosigmoid surgery could benefit from tighter reconstruction, fewer gaps for cerebrospinal fluid to escape through, and more predictable outcomes, all delivered by technology that in many hospitals already sits one imaging study and one print job away.