Page 107 - Clinical relevance of current materials for cranial implants

INTRODUCTION
Cranioplasty aims to repair a defect in the cranium and is one of the oldest neurosurgical procedures. Archeological evidence dates back to 3000 BC and suggests that the Incas performed skull reconstruction using gold plates1. In the 16th century Fallopius also recommended repair with gold plates2 and one century later, in 1668, the Dutch surgeon van Meekeren reported on the repair of a cranial defect in a Russian soldier with bone derived from a dog skull2.
Cranioplasty provides protection to the underlying brain and is performed for both
functional and esthetic reasons. It aspires neurologic recovery, as described with reconstruction for the sinking scalp flap or syndrome of the trephined 3-8. Disadvantages 5 to delayed cranioplasty involve a temporarily unprotected brain as well as an aesthetic
deformity9. Timing seems to be important in the neurological outcome of patients
but also in avoiding complications10. Cranioplasty is most commonly performed
after previous craniectomy for traumatic brain injury, stroke, after intracranial tumor
surgery and intracranial infections11-21.
Material choice for cranioplasty is still controversial, which brings complexity to this seemingly straightforward procedure10,22-24. Harvest sites for autologous bone grafts include iliac crest, rib, sternum, scapula and the skull25. At present, autologous bone flap replacement using the previously removed bone flap is the most common practice. Autologous bone does not exert immune rejection and is effective as a substrate for bone ingrowth and revascularization. Besides this autologous bone reconstruction has relatively low costs26. However, there is a risk of infection, resorption and in this case its strength gradually reduces. This has led to a search for synthetic materials8,10,24,27-29. At present, there are primarily 3 classes of allografts: metal, ceramic and polymer30. Titanium is the only metal still in use. It is a biocompatible material with a low infection rate 31. Nonetheless titanium has certain disadvantages: the material is expensive and leads to artifacts on imaging27,32. Furthermore, it is a very strong material that shows no deflection in cases of traumatic stress and consequently it has no protective energy-absorbing properties31. Hydroxyapatite is a ceramic, which is known to be a good scaffolding material for bony ingrowth30. Unfortunately, it is rather limited for use in larger defects because of its brittleness and low tensile strength33,34. Poly(methyl methacrylate) (PMMA), a polymer, has been widely used because of its low cost, radiolucency and lack of thermoconduction. Nonetheless it is associated with complications such as infection, fragmentation and a lack of incorporation27,35.
PEEK cranioplasty
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