Functional Aspects of Cranial Implants : Mechanical and Regenerative Properties

Abstract: In several neurosurgical procedures, the skull must be temporarily opened. The resulting bone defect can subsequently be reconstructed with a cranial implant. However, the complication rate of this surgical procedure is high (~20%). The most common complication for cranial implants is infection. Currently, the most frequently used implant materials are titanium alloys, PMMA or PEEK. An improved clinical outcome – in terms of increased bone regeneration, vascularization and soft tissue compatibility – could possibly be obtained through the use of bioactive and osteoconductive materials such as calcium phosphates (CaP).This thesis focuses on CaP–titanium composite (CaP–Ti) implants. This recently developed implant type is increasingly used with a promising outcome. However, a thorough understanding of its functional properties is lacking, something that is of high importance for their clinical use, but also for future biomaterial development. The overall aim of this thesis is to increase the knowledge of the in vivo functional aspects of CaP–Ti composite implants, with a specific focus on the mechanical and regenerative properties.The mechanical properties of the implant were investigated experimentally and numerically at quasi-static and impact loading rates. An important finding was that the titanium structure makes the CaP–Ti implant capable of cerebral protection in impact situations comparable to the one that was tested. Moreover, the mechanical response of the CaP–Ti implants could be predicted by the developed numerical models at both quasi-static and impact loading rates. The developed numerical framework makes an important contribution to future evaluations of patient-specific CaP–Ti cranial implant designs in various loading scenarios. A comparison of two additive manufacturing (3D-printing) processes demonstrated that lower geometrical accuracy and higher surface roughness made electron beam produced implants inferior in terms of mechanical strength, as compared to laser melted implants.In order to assess the regenerative properties, the volumetric balance of the implant was investigated by CT in ten patients. After one year, the total volume of the implant had decreased – mainly at the outside of the implants in the direction of the scalp. However, all patients had a volumetric increase at the interface between the implant and the bone defect. In a histological analysis of a retrieval specimen from one of the patients, the volumetric increase could be confirmed as bone regeneration, and the decrease as CaP degradation. Remodeling of the CaP into bone was also observed, but was not detected in the clinical CT. In retrieval specimens from an animal study, it was found that correlation of some µCT cross-sections to histology can result in improved and more robust quantitative µCT evaluations.