The Recent Revolution in the Design and Manufacture of Cranial Implants: Modern Advancements and Future Directions

Abstract

Large format (i.e., >25 cm) cranioplasty is a challenging procedure not only from a cosmesis standpoint, but also in terms of ensuring that the patient's brain will be well-protected from direct trauma. Until recently, when a patient's own cranial flap was unavailable, these goals were unattainable. Recent advances in implant computer-aided design and 3-dimensional (3-D) printing are leveraging other advances in regenerative medicine. It is now possible to 3-D-print patient-specific implants from a variety of polymer, ceramic, or metal components. A skull template may be used to design the external shape of an implant that will become well integrated in the skull, while also providing beneficial distribution of mechanical force in the event of trauma. Furthermore, an internal pore geometry can be utilized to facilitate the seeding of banked allograft cells. Implants may be cultured in a bioreactor along with recombinant growth factors to produce implants coated with bone progenitor cells and extracellular matrix that appear to the body as a graft, albeit a tissue-engineered graft. The growth factors would be left behind in the bioreactor and the graft would resorb as new host bone invades the space and is remodeled into strong bone. As we describe in this review, such advancements will lead to optimal replacement of cranial defects that are both patient-specific and regenerative.

Figure 1

Titanium mesh cranial implant (courtesy: Medtronic, Minneapolis, MN).

Figure 2

PMMA implant (courtesy: Stryker, Kalamazoo, MI).

Figure 3

MEDPOR^® implant (courtesy: Stryker).

Figure 4

PEEK implant (courtesy: A: Stryker; B: KLS Martin, (City, St of HQ); C: KLS Martin).

Figure 4

PEEK implant (courtesy: A: Stryker; B: KLS Martin, (City, St of HQ); C: KLS Martin).

Figure 4

PEEK implant (courtesy: A: Stryker; B: KLS Martin, (City, St of HQ); C: KLS Martin).

Figure 5

Cranial template used to design five (a-e) patient-specific implants, all with tapered edge fit. Red surface on taper is in contact with surrounding skull (Figure 4 from: Kyoung-june Min and David Dean. Highly Accurate CAD Tools for Cranial Implants. In (R.E. Ellis and T.M. Peters, Eds.): MICCAI 2003, LNCS 2878, pp. 99-107, 2003. © Springer-Verlag Berlin Heidelberg 2003).

Figure 6

3-D printed Poly(propylene fumarate) (resorbable polymer) scaffolds seeded with human mesenchymal stem cells at (A) 6 hours, (B) 18 hours, (C) 30 hours, and (D) 48 hours. (Figure 8 from: Wallace, Jonathan, Martha O. Wang, Paul Thompson, Mallory Busso, Vaijayantee Belle, Nicole Mammoser, Kyobum Kim, John P. Fisher, Ali Siblani, Yueshuo Xu, Jean F Welter, Donald P. Lennon, Jiayang Sun, Arnold I Caplan, and David Dean. “Validating continuous digital light processing (cDLP) additive manufacturing accuracy and tissue engineering utility of a dye-initiator package.” Biofabrication 6, no. 1 (2014): 015003).

Figure 7

(A) Mesh cranioplasty after removal of large infiltrative space-occupying intracranial lesions may lead to accumulation of subgaleal/epidural hematoma through the pores of the mesh graft (white arrow) – one disadvantage of using mesh-cranioplasty in such cases. (B) Trauma to the cranioplasty graft in such conditions can precipitate subdural hemorrhage (red arrow) as well as impact brain injury leading to parenchymal bleeds. (C, D) In patients with abnormally thick skulls or hyperostosis, a synthetic graft that matches the skull thickness can produce catastrophic intracranial injury upon subsequent direct impact over the graft. In such cases, a thicker plate with embedded re-engineered bone would be superior. (E, F) Post-operative cranial osteomyelitis involving a craniotomy flap across the superior sagittal sinus. (G) CT head non-contrast axial section following cranioplasty removal and wound wash-out. (H) 3-D reconstruction of cranial defect crossing superior sagittal sinus (indicated in blue) showing the extent of proposed customized delayed non-mesh cranioplasty.