Designing cranial fixture shapes and topologies for optimizing PEEK implant thickness in cranioplasty

Abstract

Cranial defects are often caused by trauma, diseases or malignancy. To repair such defects, skull reconstruction or cranial implants are required for protecting intracranial structures and normalizing cerebral hemodynamics. To relieve Intracranial Pressure (ICP) surgeons restore cranial defects using the preserved bone of a patient. In case of non-availability of natural bone cranial implants are fabricated using biocompatible materials including Titanium Alloys (Ti-6Al-4V) and polyether-ether-ketone (PEEK). Patient-specific implants (PSI) manufactured using 3D printing, 3D imaging technologies and 3D modeling software, are preferred over conventional manufacturing methods. During cranial fixation bone fragments, grafts and bone flaps are fixed to the cranium to offer stable closure in surgery. Effective attachment of an implant to a defective skull is influenced by the joints and fixture arrangements at the interface. These fixtures can have various designs, materials and joining procedures. PEEK behaves similarly to bone during deformation, which means it is receptive to titanium fixations and supports soft tissue adhesion due to its inert nature. This study considers PEEK as an implant material for reconstructing a bone-defected skull with Ti-6Al-4V fixtures attached at the skull-implant interface. This work compares Ti-6Al-4V fixture shapes (Linear, Square and Burrhole) with different orientation angles and topologies (‘+’ and ‘X’) attached to PEEK cranial implants of variable thicknesses (1–7 mm). Using Finite Element Method (FEM)/Finite Element Analysis (FEA), von Mises stress analysis was analyzed to identify the implant thicknesses that would bear an external load of 1780 N and ICP of 15 mmHg without failure. With a yielding limit of 100 MPa for PEEK, all proposed design arrangements for the fixture plates in the skull-implant assembly were well within these limits exhibiting a maximum von Mises stress of 51.985 MPa for the least implant thickness of 1 mm and weight 12.90 g. Considering all the designs, a Linear Fixture shape with 45° angular orientations placed in ‘X’ topology exhibited least stress of 18.477 MPa and was therefore proposed as the most optimized design for the skull-implant fixture assembly. Selection of an optimized design arrangement is also dependent upon other clinical factors, such as number of screws used, the Additive Manufacturing (AM) technique for fabricating/cutting of precise slots within the implant. The skull and flexibility of the fixture plates are also considered in relation to the required operating theatre time and skills of the surgical team.