Universal Guide for Skull Extraction and Custom-Fitting of Implants to Continuous and Discontinuous Skulls

Overview of presented implant types and their individual custom-fitting approaches. A, 3-dimensional (3D) skull surface in NURBS format reconstructed from the extracted 3D skull surface acquired from a CT scan. B, Virtual bending (orange), After creating a 2-dimensional (2D) reference surface the to-be-matched implant part is designed and extruded. It is then virtually bent before completion, which implies the thickness of headpost “legs” (perforated metal strips) is maintained while fitting them to skull curvature. C, Virtual cutting (blue), The lower end of large-scale “chamber” (enclosure with lid) together with the placed eyelets are fitted to the skull curvature such that remaining height matches desired specification. The example shows a wireless recording chamber with additional interior elements to hold a circuit board and multiple electrode array connectors (Berger et al., 2020). D, Hybrid (green), Example of a standard chamber to access single brain ROI with legs for mounting; this design combines virtual bending (orange) and cutting (blue).

Overview of 3D skull reconstruction for discontinuous or uneven skull surfaces. A, Discontinuous skull with holes reconstructed from CT scan. B, A fine mesh is created out of the originally extracted 3D skull model. All mesh points, which represent the discontinuity, are manually removed. C, Afterwards the mesh is reconverted into a 3D (NURBS) surface.

Example of custom-fitted implant for an animal with discontinuous skull surface. Left, The skull contains three holes from previous craniotomies and implants. Right, A headpost was designed and custom-fitted taking the anterior hole into account by designing the most posterior leg as a cover for the hole. An extended chamber with inlays for the use with wireless headstages was matched to the skull curvature around two large preexisting craniotomies. Black rectangles on the cortical surface mark the planned microelectrode array positions.

Versatility of our design process. A, Our approach is suitable for intact and discontinuous skulls. B, Arrows indicate the presented combination of skull condition and implant type. Lines indicate possible combination of skull condition and implant type, which were not presented in this article. The process and guide are adapted to users without any prior knowledge in CAD programming. Single-piece designs were achieved within ∼5-h designing time (blue), more complex implant systems within ∼8 h (black). C, Three types of implant fitting methods are covered: virtual bending, virtual cutting, and the combination of both (hybrid), which can be saved in different file formats (e.g. Stereolithography (.STL) or STandard for Exchange of Product model data (.STEP)). D, The resulting designs are producible in various (bio-compatible) implant materials (e.g. titanium, polyetheretherketon (PEEK), polylactide (PLA), Nylon).

Examples of close-fitting implants. Left, Titanium headpost on a discontinuous skull with holes, which was designed by virtually bending. Middle, Extended chamber for array recordings with its inlay on the same discontinuous skull. Right, Standard chamber created by virtually bending the legs and virtually cutting the top part (hybrid).

Summary of implant duration in days. Light brown bars indicate days since headpost implantation with still lasting functionality by the time of submission of this manuscript. Dark brown, Duration of two headposts, which lost their functionality. Light green, Extended chamber implanted on a discontinuous skull. Dark green, Days since implantation of the standard chamber-both still intact.