The “springform” technique in cranioplasty: custom made 3D-printed templates for intraoperative modelling of polymethylmethacrylate cranial implants

Computed tomography (CT) images with a slice thickness of 1 mm from patients prior to, and after craniectomies, were fused using Brainlab iPlan 3.0 (Brainlab AG, Munich, Germany) software. Semiautomatic threshold segmentation excluding soft tissue was performed. This way, 3D objects of the bony skull before and after craniectomy were created and loaded as stereolithography (STL) files into the Materialise Mimics inPrint 3.0 software (Materialise Inc., Leuven, Belgium) (Fig. 1a). Both the Brainlab iPlan and Materialise Mimics software are registered as CE-certified class I medical devices in cranio-facial surgery. Extracranial structures with high density (such as wound staples) that were part of the 3D model could be eliminated using the isolation function of the software. The precraniectomy model (PreCE) was then hollowed with a layer thickness of 5 mm (Fig. 1b–c). The resulting hollow (Fig. 1c) and the postcraniectomy model (PosCE, Fig. 1d) were cut along the craniectomy defect, leaving a rim of approximately 10 mm around the defect (Fig. 1e–g). Afterwards, the inner hollow was deleted leaving the outer layer as a mould of the PreCE skull and the rim as a representation of the PosCE which fits like a ring precisely into the outer hollow (Fig. 1h–j). Both templates were imported as STL files into the printer software PreForm 3.12.2 (Formlabs Inc., Somerville, MA, USA) and support structures for printing were added (Fig. 2).

Both templates (mould and ring) were printed separately on a Formlabs Form 3B or Form 2 stereolithography printer using biocompatible photopolymer resins (Formlabs Dental SG Resin, Formlabs Surgical Guide Resin). A layer resolution of 0.1 mm was applied. Both resins are CE certified as medical devices class I for printing of surgical guides in dentistry and can be sterilised. Afterwards, printed support structures were removed. Wash and cure requirements were completed, followed by sterilisation with chemical disinfection (70% isopropyl alcohol for 5 min) and steam sterilisation (autoclave at 134 °C for at least 20 min) in our in-hospital sterilisation unit, as prescribed by the company (Formlabs Material Data Sheet).

Prior to the first surgical application of the method, a trial implant was produced in the described manner. The heating of PMMA during polymerisation process did not affect the shape or structure of the cured resin templates. A CT scan of the PMMA implant was performed. Virtual implantation, via fusion with the postcraniectomy CT of the same patient in Brainlab iPlan 3.0 software, confirmed excellent fit and shape. Thus, major deviations in size and shaping of the implant throughout the design and printing process could be ruled out.

Intraoperative production of the cranial implant was performed in most cases directly before skin incision under sterile circumstances in the operating room. This is demonstrated in Fig. 3 and Video 1:

The implantation surgery was performed in accordance with institutional standards. Skin opening was performed along the previous incision, followed by preparation of bony margins, (neo-)dura and a temporalis muscle flap. Implants were fixed with four four-hole plates and self-tapping titanium screws (Fig. 4c–d). The dura was attached to the implant via multiple resorbable retention sutures to prevent epidural fluid collection. One or two subgaleal drains with suction were inserted, followed by wound closure. A cranial CT scan was regularly performed the following day. A postoperative follow-up after discharge was routinely conducted after 2 to 4 weeks at our outpatient clinic.

In this case series, fourteen consecutive patients (9 males, 5 females; median age 51 years, range 21–80 years) with large craniectomy defects for whom no autologous implant was available were treated from June 2020 to April 2021 at our institution with the above described cranioplasty technique. All analysed preoperative and postoperative clinical data were acquired retrospectively from electronic patient records.

In three (21.4%) patients, a postoperative intracranial haemorrhage, requiring revision surgery, occurred. In two of those patients, an epidural haematoma was revealed on routine CT scans at first postoperative day (IDs 7 and 11). Both patients had significant clinical risk factors for the development of intracranial bleeding. Patient 11 had a strict indication for anticoagulation because of a mechanical mitral valve implant. Preoperative bridging therapy was performed with low molecular weight heparin, which was paused on the day of the surgery. The patient received continuous intravenous heparin infusion therapy 4 h postoperatively, only after a CT scan had ruled out intracranial bleeding. Patient 7 received a ventriculo-peritoneal shunt because of posthaemorrhagic hydrocephalus in the same anaesthesia directly prior to cranioplasty. Even though both patients showed no specific clinical symptoms, revision surgery was performed due to the space-occupying effect (> 10 mm) of the haematoma. The implants were not exchanged. In follow-up CT scans no relevant epidural or subdural bleedings were observed. In another patient (ID 6) with a bilateral cranioplasty, a symptomatic epidural haematoma occurred on the left side on the second postoperative day, after a routine CT scan on the first day had shown no signs of intracranial bleeding. Further clinical workup revealed reduced Factor XIII (fibrin stabilizing factor) levels of only 55%, which provide a clinical explanation for the delayed postoperative bleeding. Revision surgery was performed and Factor XIII substituted. No further complications occurred in clinical follow-up. In one patient (ID 13), a subcutaneous fluid collection occurred, most probably because of a cerebrospinal fluid leakage. The fluid had to be drained with transcutaneous puncture, but no signs of infection were revealed in laboratory workup and no revision surgery was needed. No surgery-related mortality or new and permanent neurologic deficits were recorded. No postoperative infections or wound healing deficits were recorded.

In postoperative clinical follow-up, all patients/legal representatives were satisfied with skull shape and cosmetic appearance.

A reasonable aim of improving cranioplasty surgery is to reduce the net operating time and thus, time under anaesthesia and wound exposure time. The mean overall skin-to-skin time in our series was 92 min. Operating times reported by other authors are higher than in our cohort, ranging from 121, 126 and 135 min respectively [2, 14, 23], to 184 min [10] in comparable case series.

All software applications as well as the used printing material must be viewed — according to European Union law — as medical devices. For this reason, we limited ourselves to exclusively using software applications (Materialise Mimics InPrint/ Brainlab iPlan software) and printing material (Formlabs Surgical Guide and Formlabs Dental SG resins) that are already registered as CE-certified class I medical devices in the field of cranio-facial surgery. To use registered medical device software and hardware is more expensive than the free, open-source software but seems to be essential for patient safety and ethical approval of future study concepts.

Elective cranioplasty is — in general — associated with higher complication rates than other elective neurosurgical procedures [24]. In a comparable 3D printing-guided cranioplasty case series, Schön et al. also reported a relatively high early reoperation rate in three out of 16 patients (18.75%) because of epidural or subdural postoperative haematomas [23]. As in our series, most of the patients with postoperative haematomas had distinct clinical risk factors for intracranial bleeding after cranioplasty. Ventriculo-peritoneal shunt placement (patient 7) has previously been described as a risk factor [8]. Coagulation disorders like a Factor XIII deficiency (patient 6) or the vital need for early anticoagulation (patient 11) certainly played a crucial role in the postoperative intracranial haematomas that occurred in our series.

The 3D printing and implant modelling workflow we have developed reduce costs of cranioplastic procedures compared to CAD implants and cryoconserved autologous implants. The cost effectiveness of our method is striking, with approximated expenses of around 300 € (= 360 US $) per implant. In contrast, cryoconservation storage of autologous implants can be approximated for our medium size neurosurgical institution, with costs of around 500 € (= 600 US $) per reimplanted bone flap, taking into account several cost drivers (i.e. acquisition costs for freezers, certified packaging material, validation of the freezers at the time of purchase, maintenance and calibration (once a year), and inspection by national health authorities (every 2 years), and costs for microbiological testing) as well as a relatively high rate of autologous bone flaps which must be discarded due to contamination or patient death. CAD implants are available at prices ranging from around 5000 € to 10,000 € (6000–12,000 US $) per implant, signifying a cost reduction with 3D printing-aided cranioplasty of around 95%. This is in line with other studies [14, 23]. The technique could be especially interesting for low-income countries, if CAD implants might not be affordable or available and proper cryoconservation might be difficult.

These economic advantages have triggered the establishment of a 3D printing laboratory at our neurosurgical department. Clinical implementation of the described method has led to a noticeable improvement of implant shaping, operating time and patient satisfaction in our institution. One drawback is that 3D printing is time consuming for the surgeon performing the template design process. However, after a learning phase, we were able to reduce the 3D printing preparation time for a single cranial implant to about 30 min. A further 15 min is needed for postprinting requirements before sterilisation. Printing itself, which takes 3 to 5 h for the template ring and 5 to 8 h for the template mould, can be done any time without the need for constant surveillance.

The feasibility of the described cranioplasty technique could be demonstrated in a small retrospective cohort. To create more robust data, a prospective, randomised trial with larger cohorts is needed, especially to compare our approach with established implants like CAD implants or autologous bone implants. For now, only straightforward craniectomy defects of the skull have been reconstructed with the “springform” technique. For more complex procedures like skull base reconstructions, the technique does not seem to be suitable for now. Future concepts will aim on 3D printing implants itself without the need for moulding templates.