Page 21 - Clinical relevance of current materials for cranial implants

General introduction and outline of this thesis
PMMA is still one of the most frequently used alloplastic materials for cranioplasties18. 1 It is inexpensive, easy to use, and radiolucent. PMMA does not interact with the surrounding tissue, during the hardening process PMMA induces an exothermic
reaction, consequently heating the adjacent tissues, is associated with an high infection
rate, and controversies exist in literature about its toxicity14,18,26. In the last years, a transformation of PMMA cranioplasties is observed. With the use of the CT-scan of the cranial defect of the patient, a mold of the cranial defect can be printed with additional manufacturing. During the cranioplasty procedure, the PMMA particles and liquid are mixed and placed into the mold. After a while, when the cranioplasty has cooled down, it is taken out of the mold. Adjustments can be made and is subsequently used for cranial reconstruction. This is an indirect method for additive manufacturing. Recent technology enables direct printing of an implant with higher accuracy; this is called a Patient Specific Implant (PSI).27
Hydroxyapatite
Hydroxyapatite is a substance made out of two different calcium phosphates, mixed with water. It has a hexagonal structure and is similar to human bone in composition and morphology28,29. In 1952, Ray and Ward used synthetic hydroxyapatite crystals for the reconstruction of hips and legs of monkeys, dogs and cats30. They discovered that the crystals used were transformed into new bone. Hence, they concluded that hydroxyapatite has the property to function as a matrix for bone generation30. Hydroxyapatite was further developed by the American Dental Association in 1986 and became available for cranial reconstructions in 199631,32. It has a good osteoconductivity, biocompatibility and it is easy to use14,16,32. Hydroxyapatite allows the expansion of the growing skull and results in incorporation into the surrounding bone16. The process of the conversion of hydroxyapatite into bone takes time, which means that the material is brittle and may not sufficiently protect the brain16,17,33.
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