Page 151 - Clinical relevance of current materials for cranial implants
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                                Virtual surgical planning for skincancer with cranial bone invasion
Consequently, most authors advise a form of cranioplasty alongside microvascular soft
tissue coverage. Biocompatible autologous bone, which has long time been considered
the golden standard in neurosurgical and craniofacial literature12,34,46–50, has a high complication rate including infection (0-26%) and resorption (1-50%) with a high
removal rate51. Especially, in case of tumor invasion into the skull, the use of autologous
bone is limited by donor sites and finite number and size. Alloplastic materials may
overcome these shortcomings and offer a solution for optimal protection of the brain
and satisfactory aesthetic outcome. Different materials are developed for cranioplasty,
each with their own advantages. The ideal material is biocompatible, radiolucent,
resistant to ionizing radiation and heat, MRI compatible, easy to use, and allows a low
cost preoperative design and manufacturing to achieve an aesthetic satisfying result51. Unfortunately, this material does not exist yet. Titanium, poly(methyl methacrylate)
(PMMA), poly(ether ether ketone) (PEEK) and hydroxyapatite are the most mentioned
materials. Titanium is radiopaque and appears to conduct heat and cold which makes
a full cranioplasty of titanium not a good option for cranioplasty47. It is well known
that a titanium mesh may cause artefacts on a CT or MRI, which could impede follow-
up of cranioplasty patients after oncological resection52. Also, when inserted directly
on the dura it may cause scalp thinning and penetrate the overlying tissue53. PMMA 7 is a radiolucent, relatively cheap and easy to use material. However, this material is manufactured using liquid MMA in combination with PMMA particles. Different
studies describe the potentially toxic and adverse effects of MMA54. Hydroxyapatite is similar to the mineral phase of human bone and can stimulate bone formation. However, the material itself is very brittle until replaced by bone, the exact time interval is unknown and depends on patient specific factors47. The question also remains if hydroxyapatite has any bone formation capability when used in prior irradiated tissues. PEEK is a more modern plastic, resistant to high temperatures, has a good biocompatibility and mechanical characteristics comparable to cortical bone. However, the material itself is expensive and without bioactivity55. The reported overall complication rates for simultaneous cranioplasty and microvascular free tissue transfer are high (21.0%-57.9%)40,41,43. The main shortcoming and serious complication of using alloplastic materials are the potential for infection and exposure, which might require removal of the cranioplasty.

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