Page 99 - Clinical relevance of current materials for cranial implants

The effects on molecular weight and mechanical properties for PMMA bone cements
used for fixation of artificial joints is reported on in the literature. Hughes et al.27
reported a 12% drop in molecular weight of Palacos® (Smith & Nephew Inc., London,
United Kingdom) used for fixation of the hip following 15 years in vivo. The molecular
weight of Simplex® (Stryker Howmedica Osteonics, Inc., Mahwah, New Jersey, USA),
retrieved from hip fixation, was reduced by 46% after 23 years. After 16 years no
significant reduction in molecular weight was found for Simplex® used for knee
fixation27. CMW1 cement, retrieved from hip fixation, was stable in vivo and did not
show a reduction in molecular weight, even after more than 20 years. In our study the 4 molecular weight of CMW-3 did not significantly change during 15 years in the human
cranium.
By manually manufacturing the cranioplasty during surgery, the local thickness and shape are difficult to control. The fractured PMMA cranioplasty in our study was manually manufactured and had a thickness of less than 3 mm at several locations, and point defects even up to 1.5 mm, carrying an inherent higher risk of fracture. Nowadays, this risk can be mitigated by the use of a 3D-printed nylon mold. After the mold is manufactured, it can be sterilized and used for reconstruction. During surgery PMMA can be cured following manufacturer’s instructions and put into the mold when it is moldable. Because of the mold, the PMMA can be evenly distributed with a consistent thickness, which results in fewer weak points in the implant and should therefore make it more resistant to fracture. By using a mold the high temperatures reached during polymerization do not need to be suppressed to prevent damage of the underlying tissues, leading to a final implant with a higher degree of polymerization.13–16 Preoperative, ex vivo manufacturing of a PMMA cranioplasty would allow for an even better control of the environmental conditions, especially increased pressure, during polymerization, which leads to a reduced PDI and improved mechanical and biocompatibility properties.
In vivo fractured PMMA cranioplasty
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