In-house dental lab-based cranial prosthesis fabrication: a technical note
Article information
Abstract
Cranial prostheses are frequently required for patients with cranial defects secondary to trauma, decompressive craniectomy, or other pathologies. When the resected or craniotomized bone cannot be reused, cranioplasty with artificial materials offers both aesthetic and protective benefits. However, high-end custom-made options, like polyether ether ketone or titanium prostheses, are expensive and not widely available. Heat-cured polymethyl methacrylate (PMMA) prostheses are generally preferred over their cold-cured counterparts. In-house dental laboratories can provide a cost-effective and practical solution by employing a lost-wax technique akin to denture fabrication, utilizing a three-dimensional printed custom open mold. Fabricating large heatcured PMMA cranioplasts presents certain challenges, such as the need for large flasks and potential porosity. These can be overcome by using a large stainless steel container (a tiffin box) and M-Seal epoxy to ensure an airtight curing process. This method can be easily adopted by standard dental laboratories. At our center, four patients have successfully fitted with cranioplasty prostheses produced using this technique. Even though the patients are outside of the scope of this technical note all of them indicated high satisfaction, and no complications were reported. This straightforward approach demonstrates that in-house, heat-cured PMMA cranioplasts can represent a viable, cost-effective option for cranial reconstruction.
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.
Customized three-dimensional printed skull model featuring an integrated mold at the defect site.
Wax pattern for cranioplasty.
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.
Heavy-gauge, two-piece stainless steel tiffin box used as a flask.
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.
Finalized acrylic cranioplasty prosthesis.
Prosthesis fixed to the skull model at three points using mini titanium plates and screws.
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.
Notes
Conflict of interest
No potential conflict of interest relevant to this article was reported.
Funding
None.
Author contributions
Conceptualization: Gunjan Chouksey, Babu Lal. Data curation: Gunjan Chouksey. Formal analysis: Prateek Shakti, Vineeta Yadav, Jitendra Kumar, Zenish R Bhatti. Methodology: Gunjan Chouksey, Babu Lal. Project administration: Gunjan Chouksey, Babu Lal. Writing - original draft: Gunjan Chouksey, Babu Lal, Jitendra Kumar. Writing - review & editing: Prateek Shakti, Vineeta Yadav, Jitendra Kumar, Zenish R Bhatti, Babu Lal. Validation: Gunjan Chouksey, Jitendra Kumar, Zenish R Bhatti.
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.
Abbreviations
PEEK
polyether ether ketone
PMMA
polymethyl methacrylate
3D
three-dimensional