3D printing and scanning technology has improved since its introduction in 1986, when the first stereolithographic (SLA) systems were introduced [1] and are embraced by both surgical and forensic departments across the world [2]. Despite the applications of 3D printed prosthetics in both medicine and various disciplines of forensic science to date, limited studies can be found on the use of the application in Forensic Medicine, specifically, during post mortem reconstruction. The aim of this article is to explore current reconstruction techniques in forensic medicine to improve aesthetics and bony structure stability in situations requiring repair of skull and facial bone damage due to trauma or the tissue retrieval process in post mortem procedures.

Skull Clips For Repair of Adult Crania After Brain Retrieval

The skull clip drawing (figure 1.1) was created from a 3D model using Onshape (Boston, MA) cloud-based CAD software.

One male adult patient (deceased) and one female adult patient (deceased) between the ages of 60 and 80 were selected for this pilot, which was conducted between April and July 2022. Once the post mortem examination had taken place and the brain retrieval was completed the reconstruction of the skull commenced. Using a 2.5mm hex drill bit, holes in the calvarium were drilled into the bone where the skull clip would be positioned (figure 1.2A). The pre-drilled holes were drilled at the left and right superior temporal region, and posterior occipital region in the area where the clips will be fixed. One side of pre-drilled holes were located on the ‘skull cap’ and the other hole was drilled on the fixed cranium in preparation to fix the skull clip in place. 

While still in a pilot phase, there are positive indications that the new method is faster than the current method of suturing the skull cap back on using muscular anchor points. Importantly, the clips ensure no movement of the skull cap when it is positioned back on the skull, suggesting that the new reconstruction technique is stable and as the skull cap cannot be displaced, ensures no opportunity for disfigurement post reconstruction or prior to family viewing. After completion of the scalp reconstruction took place, an assessment of the forehead region showed no visual resemblance of the skull clips underneath the skin (figure 1.3).

Development and Application of 3D Printed Scaffolds for Paediatric Cranial Post Mortem Reconstruction

The design and prototype phase of the paediatric scaffold began with the use of raw CT imaging data sets from a 27-day old male and 7-month-old female infant. Each DICOM (Digital Imaging and Communications in Medicine) format CT scan, comprising a set of trans axial slices, was imported into the freeware package Meshmixer (Autodesk, Inc, San Francisco, CA) for conversion into a 3D model (OBJ format, Wavefront Technologies, Santa Barbara, CA).  The resulting 3D skull model was then imported into the freeware Computer Aided Design (CAD) package Onshape (Boston, MA) where it was used in the development of a parametric ‘Boolean’ cranial cavity model (figure 2.1).  

The ‘Boolean’ Digital Model

Based on the Boolean operation of union, which takes 2 objects and merges the interiors of the object creating a new object or shape, [3] it was realised that the cranial cavity could be suitably represented by a series of five spheres located in the sagittal plane to infill the internal space (figure 2.2).

The diameters and sagittal plane locations of the spheres could be easily adjusted to fill the internal space in CAD depending upon the dimensions of the OBJ (Wavefront OBJect) skull model.

Once the scaffold model was finalised in CAD it was then exported in Standard Tessellation Language (STL) format for conversion into a 3D-printable file.  

3D Printed Skull Replica

Prior to printing the Boolean scaffold a replica skull was required to test the prototype. The 7-month-old female data set was selected for the first replica skull model as access to the cranial cavity was required in order to place the scaffold inside the cavity. 

The 7-month-old female skull replica was printed in PLA plastic (figure 2.3). The scaffold would then be placed inside this replica and the skull cap would be placed back on top of the scaffold to assess size and suitability of infilling the cranial cavity.

The Boolean System Scaffold

Two types of plastic were selected for use: i) Poly Lactic Acid (PLA) or “cornstarch” and ii) Acrylonitrile Butadiene Styrene (ABS). The stability and flexibility/rigidity of each plastic type were compared in each prototype. The print time for the Boolean scaffold was 8 hours for each prototype. 

Results – Boolean System

Prototype 1

Prototype 1 was printed in PLA plastic using the following parameters: 210C extrusion temperature, 60C build plate temperature and 0.25mm layer thickness. 

The resulting 3D printed model (figure 2.4) was inflexible and the fit inside the cranial cavity was too tight. These factors would not allow for the calvarium to be replaced neatly, with the scaffold pushing the calvarium away from the cranial base. This would likely create deformities, as the 2 separate cranial pieces could not be united during the reconstruction process.

Prototype 2

The goal for prototype 2 was to create a scaffold that had more flexibility than the first prototype. It was decided to proceed with ABS plastic for all remaining Boolean prototypes, with the hope that more flexible plastic will facilitate better reconstruction once the calvarium was replaced. 

This second prototype was printed with the following parameters: 230C extrusion temperature, 110C build plate temperature and 0.25mm layer thickness. These printing parameters would remain consistent for all subsequent ABS prototypes.

The resulting scaffolding was still too rigid to permit a seamless union of the 2 cranial pieces during reconstruction. This would likely pose a problem with anatomical asymmetry in an actual postmortem case, as the pressure of the scaffold inside the cranial cavity on the infant cranial bones could cause the cranial bones to deform. 

Prototypes 3 – 5

Significant changes were made to prototypes 3-5. Rather than having a solid structure (Boolean shaped) that formed the scaffold, various engineered sections were cut and removed in CAD to increase the compliance and flexibility of the mould in the cranial cavity and in turn, create a better fit to infill the cavity, resulting in a seamless union of the calvarium to the cranial base. 

Prototype 5 was the final Boolean inspired scaffold produced. The engineered ‘fronds’ from the previous prototype 3 and 4 were cut closer together to reduce the over-expansion of the scaffold (see figure 2.7 a) and 2.7 b)). 

Prototype 5, however was still too inflexible and tight for the printed skull replica. Its rigidity continued to cause the printed skull replica to push outwards and the calvarium could not be placed with anatomical symmetry.

Band System

The band system was a more generic approach and much easier to create than the Boolean system, the band system incorporates a 3D printed band with a self-locking sawtooth rachet-type mechanism (figures 2.8a, 2.8b, 2.8c). The 27-day old male skull replica was used for testing the band scaffold. The print time for each single band was 4 hours.

Prototype 1

Prototype 1 of the band system comprised of a single band scaffold conforming to the coronal plane of the internal cranial cavity 3D CAD model (figure 2.9a and 2.9b). 

This protype was printed using PLA. Although the geometry of the prototype 1 band scaffold was similar to the shape of the internal skull cavity, in practice the PLA material proved to be too difficult to manipulate when being placed in the skull replica and adjusting the size the of the band and the saw-toothed mechanised failed to lock in to one another. This resulted in the band pushing against the petals of the infants’ skull from the inside, resulting in large gaps between the individual cranial bones (figure 2.9b) and would likely cause deformities in skull shape during reconstruction. 

Prototype 2

Minor adjustments were made for prototype 2 to enhance the built-in saw-toothed ratchet mechanisms and improve flexibility of the band. To improve flexibility, PLA was replaced with ABS plastic.

Changes to the ratchet mechanism were made, making them bigger and longer to enable a larger range of diameter to be accommodated and the self-locking mechanism to be more functional (figure 2.10). This prototype generally worked well in the coronal plane, with no observable pressure on the cranial petals, but the sagittal plane skull components were not supported.  In practice, this could lead to collapse of deformity of the cranial bones in the midline plane of the skull. It was evident that a band system in multiple anatomical planes would be needed to improve the stability of the petals from inside the cranial cavity.

Prototype 3

To support the sagittal skull components an additional band was created to enable a more complete scaffold system for the cranial cavity. It also involved the printing of an additional base plate with a stabilising clip that the sagittal band could slide through creating the coronal and sagittal scaffold band in one piece (figure 2.11).

Prototype 3 was again printed in ABS plastic for flexibility, had a total print time of 8 hours which included both bands and the base plate and clip. This prototype incorporated ratchet-band system in both the sagittal and coronal planes, enabling a more complete support of the cranial bones from within. The multiaxial bands were key to providing support without creating unnecessary deformations of the fragile infant cranial bones and this design became the final prototype for this project. 

Discussion

The use of 3D printed skulls clips would improve the efficiency of the reconstruction method and stability of the calvarium for adult post mortem reconstruction. This is a low cost, time efficient novel technique that has strong potential to become standard practice nationally in the field of post mortem cranial reconstruction.

Due to the fragility of infant cranial bones, this research sought to design and implement a stable scaffold that would improve cranial reconstruction outcomes in paediatric postmortems.

Whilst the Boolean system proved to be time consuming to print and difficult to fit the band system however, specifically the double band which provided structural integrity in both the coronal and sagittal planes, showed more promise, as the ratchet system design means that the band size can be modified to suit the size of the infants’ cranial cavity at the time of reconstruction. 

There are no current guidelines as to what materials can be used during reconstruction. It is therefore recommended that the band system be considered as a new method of infant reconstruction and use of this reconstruction method should be included in the training of future forensic postmortem technicians. 

The results of this study demonstrate that 3D printing has potential to improve post mortem reconstruction outcomes in the discipline of Forensic Medicine and should be considered as a standard approach to cranial reconstruction, particularly in infants. 

About the Author:

Lisa Bilton is a Forensic Medicine Researcher with a drive to enhance and optimise professional standards in post mortem reconstruction using innovative technology such as 3D printing, supporting partnerships across medico-legal teams and driving interdisciplinary collaboration with forensic scientists/pathologists and physicians.

Having completed more than 500 post mortem reconstructions as a Forensic Scientist/Technician within Australian Health Pathology, Lisa is exploring opportunities to extend her Forensics research in 3D printing in post mortem reconstruction. 

She firmly believes that with the technology available today, can improve the standards in industry and – most importantly – provide better outcomes for families of deceased, particularly paediatric patients.

Alongside her forensic medicine practice and research, Lisa has also lectured mortuary practise subjects with Western Sydney University.

References:

Aimar, A., Palermo, A., & Innocenti, B. (2019). The role of 3D printing in medical applications: A State of the Art. Journal of Healthcare Engineering. 1:5340616.

Carew, R., Morgan, R., Phil, D., & Rando, C. (2019). A Preliminary Investigation into the accuracy of 3D modelling and 3D printing in forensic anthropology evidence reconstruction.  Journal of Forensic Sciences, (64), 342–352.

Charton, J., Laurentjoye,M., & Youngjun, K. (2017). 3D Boolean operations in virtual surgical planning. International Journal of CARS, (12),1697–1709