Biomechanical Evaluation of Patient-Specific Polymethylmethacrylate Cranial Implants for Virtual Surgical Planning: An In-Vitro Study

Abstract

Cranioplasty with freehand-molded polymethylmethacrylate implants is based on decades of experience and is still frequently used in clinical practice. However, data confirming the fracture toughness and standard biomechanical tests are rare. This study aimed to determine the amount of force that could be applied to virtually planned, template-molded, patient-specific implants (n = 10) with an implant thickness of 3 mm, used in the treatment of a temporoparietal skull defect (91.87 cm2), until the implant cracks and finally breaks. Furthermore, the influence of the weight and porosity of the implant on its force resistance was investigated. The primary outcome showed that a high force was required to break the implant (mean and standard deviation 1484.6 ± 167.7 N), and this was very strongly correlated with implant weight (Pearson’s correlation coefficient 0.97; p < 0.001). Secondary outcomes were force application at the implant's first, second, and third crack. Only a moderate correlation could be found between fracture force and the volume of porosities (Pearson’s correlation coefficient 0.59; p = 0.073). The present study demonstrates that an implant thickness of 3 mm for a temporoparietal skull defect can withstand sufficient force to protect the brain. Greater implant weight and, thus, higher material content increases thickness, resulting in more resistance. Porosities that occur during the described workflow do not seem to reduce resistance. Therefore, precise knowledge of the fracture force of polymethylmethacrylate cranial implants provides insight into brain injury prevention and serves as a reference for the virtual design process.

Virtual Surgical Planning and Customized CAD/CAM Cranial Implants: Preoperative and Intraoperative Strategies for Temporal Intraosseous Meningioma Resection

“…6 While PSIs can provide smother cosmetic results than freehand PMMA implants, their process of manufacturing is time-consuming and expensive. 8 The demand for cost-effective patient-specific treatment led to a hybrid manufacturing process, with in-house silicon mold production used to produce accurate intraoperative PMMA implants. 9 Since August 2023, both implant types can now be manufactured at the point-of-care using the patient's preoperative CT or MRI imaging data for virtual planning.…”

“…Polymethylmethacrylate (PMMA) is mainly used in intraoperative freehand techniques, whereas industrially manufactured PSIs are often made from titanium or polyetheretherketone (PEEK) 6 . While PSIs can provide smother cosmetic results than freehand PMMA implants, their process of manufacturing is time-consuming and expensive 8 . The demand for cost-effective patient-specific treatment led to a hybrid manufacturing process, with in-house silicon mold production used to produce accurate intraoperative PMMA implants 9 .…”

Evaluation of the Dimensional Accuracy of Robot-Guided Laser Osteotomy in Reconstruction with Patient-Specific Implants—An Accuracy Study of Digital High-Tech Procedures

“…Previous studies have introduced workflows for implant fabrication to facilitate facial reconstruction, considering feasibility, cost-effectiveness, force resistance, and accuracy [ 27 , 32 , 37 , 38 ]. While patient-specific implants provide significant time-saving benefits during surgery by eliminating the need for manual adjustments such as bending, it is essential to consider the potential for inaccuracies in their fabrication.…”

Optimizing cranial implant and fixture design using different materials in cranioplasty

“…Their study illustrated that utilising a 3D reconstruction method and PEEK material would minimise time-consuming alterations while also improving the implant's fit, stability, and strength. Msallem et al 27 demonstrated that an implant thickness of 3 mm for a temporoparietal skull defect can withstand sufficient force to protect the brain. Greater implant weight and, therefore higher material content increases thickness, resulting in greater resistance.…”