INTRODUCTION
A cranial prosthesis [
1] is an artificial replacement for a natural cranial structure in patients who have undergone decompressive craniectomy or craniotomy [
2,
3] to address cranial defects caused by head injuries, cerebral tumors, ischemia, infections, or other pathologies. This approach is particularly useful when autologous bone cannot be restored following craniotomy because the relevant portion of the skull was crushed or damaged by trauma or disease. The prosthesis aids in restoring head symmetry, offers protection to the brain, and improves the patient’s quality of life [
4].
Cranial prostheses can be constructed from a variety of materials, including titanium, polyether ether ketone (PEEK), hydroxyapatite, polyethylene, silicones, and polymethyl methacrylate (PMMA) [
5]. Among these, PMMA is the most popular and cost-effective alloplastic material for the creation of cranial prostheses. Its popularity stems from its chemical inertness, ease of fabrication, and affordability [
6]. However, if cost is not a factor, materials such as PEEK or titanium are considered superior choices.
Traditionally, neurosurgeons fabricate self-cured PMMA-based cranioplasts during the surgical procedure. Self-cured materials harden through a chemical reaction without the need for applied heat. However, this method has several disadvantages, including difficulty controlling the thickness and contour to achieve the desired shape, comparatively low strength, and the risk of potentially cytotoxic monomer leakage. To address the shortcomings of the self-cured PMMA cranioplast, some practitioners have attempted to create heat-cured PMMA-based prostheses by taking an impression of the clinical skull defect and then forming a stone mold. In heat curing, the material hardens through the application of heat [
1]. Nevertheless, this technique does not solve all issues; for instance, the surgeon still may not achieve the desired thickness and contour of the cranioplast. Moreover, it increases the number of patient visits and requires intraoperative modification. All of these techniques are cumbersome and challenging to perform.
The fabrication of PMMA-based cranioplasts presents several laboratory challenges. For large cranial defects, the processing of the wax pattern necessitates the use of a specialized, larger flask, such as the Hanau recon Flask (Whip Mix Corp.) [
1]. These flasks are expensive and not readily available in India.
To address the aforementioned challenges, we propose a novel, straightforward, and repeatable method for fabricating PMMA-based cranioplasts using a three-dimensional (3D) printed mold in an in-house dental laboratory.
IDEA
The process began with the use of a pre-existing, patient-specific 3D printed skull model (3D printer: Ultimaker 2+, UltiMaker; technology: fused deposition modeling; material: acrylonitrile butadiene styrene) equipped with an integrated mold at the defect site (
Fig. 1). The depth of the mold was uniformly set to 4 mm. To establish a separating layer, thin aluminum foil was shaped to fit within the mold. Standard wax sheets (DPI modeling wax; Dental Products of India) were folded repeatedly to achieve a total thickness of 4–5 mm and then firmly pressed into the mold. Any excess wax was carefully trimmed and made level with the adjacent skull surface (
Fig. 2). Subsequently, the outer surface was smoothed, and the entire wax pattern was carefully removed for heat curing.
A durable two-piece stainless steel tiffin box, which could be secured in dental clamps, was selected (
Fig. 3). Prior to investing the wax pattern, Vaseline (petroleum jelly) was applied to the inner surface of the box to prevent the investment material from adhering. The wax pattern was then invested in the lower portion of the box with dental stone. Care was taken to ensure that the convex side of the pattern faced upwards, with half of it submerged in the lower stone portion and the remainder contained within the box.
To prevent adhesion during the casting of the upper lid, a separating medium (cold mold seal) was applied. The box, now filled with dental stone, was sealed with the upper lid, which had two large holes for the introduction of the dental stone liquid. The box was securely clamped and left undisturbed for 60 minutes to allow the dental stone to set completely. Afterward, the clamped box was subjected to de-waxing in boiling water (100 °C) for 5 minutes. Following de-waxing, the box was carefully opened, and any residual softened wax was rinsed away with hot water. Both parts of the box were then dried. A separating medium was applied to the de-waxed mold cavity and left to dry thoroughly. Heat-cured clear acrylic resin polymer (PMMA) and monomer (DPI-Heat Cure, Dental Products of India) were mixed and packed into the mold cavity for curing. To prevent air entrapment and porosities in the final prosthesis, a multipurpose sealant was used at the junction of the two box halves. The box was then placed in hot water at 60–70 °C for a 9-hour curing cycle. After cooling, the box was opened, and the prosthesis was gently removed. Finishing touches were applied, ensuring that the prosthesis formed a butt joint with the skull margin. Multiple 2-mm-diameter holes were randomly drilled while avoiding a 1-cm margin around the edges (
Fig. 4). A fitting trial was conducted using a 3D printed mold (
Fig. 5). The entire procedure was performed according to the standard protocol of the dental prosthetics laboratory.
The final prosthesis was sterilized using plasma sterilization; then, it was fixed at three points with mini titanium plates and screws by the neurosurgery team. Notably, no modifications to the prosthesis were necessary during fixation, and the postoperative results were deemed satisfactory. To date, our center has treated four patients with this type of cranioplasty prosthesis. The prostheses fabricated using this technique have been well-received by the patients, and no complications have been reported.
DISCUSSION
The present study presents a novel approach for the fabrication of cranial prostheses in a dental laboratory setting. This technique involves the use of heat-cured PMMA with a customized 3D-printed skull mold. Using a 3D-printed mold has several benefits over traditional stone moulage methods. For instance, it ensures consistent thickness and precise contouring for cranioplasty procedures. Moreover, this method streamlines the fabrication process, resulting in faster and more precise manufacturing of prostheses [
7].
Heat-cured PMMA has become a preferred material due to its widespread availability and ease of fabrication [
6]. Furthermore, it represents a cost-effective alternative to customized prostheses made from materials such as PEEK and titanium [
7].
Given the constraints often faced in routine dental laboratory settings, such as the unavailability of large flasks, alternative methods must be developed. For this study, thick-gauge metal tiffin boxes were employed as a readily accessible substitute. To address concerns regarding porosity in the final product, a sealing method using M-Seal epoxy was implemented. This provided an airtight seal during the curing process, minimizing the risk of porosity. Furthermore, adherence to standard curing protocols—including packing at the dough stage, applying the appropriate bench pressure, and using a long, low-temperature curing cycle—further optimized the fabrication process and supported the quality and reliability of the cranial prostheses.
Overall, this comprehensive approach represents a notable advancement in the techniques used to fabricate cranial prostheses, offering better outcomes and improved patient care in the field of cranial reconstruction surgery.
ACKNOWLEDGEMENTS
We would like to acknowledge Dr. Pradeep Chouksey (Assistant Professor, Department of Neurosurgery, All India Institute of Medical Sciences, Bhopal, India) and Dr. Anshul Rai (Additional Professor, Department of Dentistry, All India Institute of Medical Sciences, Bhopal, India) for their support and motivation.
Fig. 1.
Customized three-dimensional printed skull model featuring an integrated mold at the defect site.
Fig. 2.
Wax pattern for cranioplasty.
Fig. 3.
Heavy-gauge, two-piece stainless steel tiffin box used as a flask.
Fig. 4.
Finalized acrylic cranioplasty prosthesis.
Fig. 5.
Prosthesis fixed to the skull model at three points using mini titanium plates and screws.
REFERENCES
1. Kharade P, Dholam K, Gorakh A. A technique for fabrication of cranial prostheses using high-temperature vulcanizing silicone material. J Prosthet Dent 2017;118:113-5.
2. Goh RC, Chang CN, Lin CL, Lo LJ. Customised fabricated implants after previous failed cranioplasty. J Plast Reconstr Aesthet Surg 2010;63:1479-84.
3. Hutchinson P, Timofeev I, Kirkpatrick P. Surgery for brain edema. Neurosurg Focus 2007;22:E14.
5. Segall BW. The construction and implantation of a silicone rubber cranial prosthesis. J Prosthet Dent 1974;31:194-7.
6. Zanotti B, Zingaretti N, Verlicchi A, Robiony M, Alfieri A, Parodi PC. Cranioplasty: review of materials. J Craniofac Surg 2016;27:2061-72.
7. Lal B, Ghosh M, Agarwal B, Gupta D, Roychoudhury A. A novel economically viable solution for 3D printing-assisted cranioplast fabrication. Br J Neurosurg 2020;34:280-3.