Page 100 - Clinical relevance of current materials for cranial implants

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Chapter 4
Macroporosity can occur when PMMA is prepared by hand due to air getting entrapped in the material. Using a vacuum system leads to a lower macro-porosity in PMMA. Similar trends are observed when reducing the speed of mixing and decreasing the amount of strokes. However, when PMMA is mixed under vacuum conditions the micro-porosity is increased as the boiling point of the liquid MMA component is lowered.12 Our study shows reduced porosity in the CMW-3 cured at atmospheric pressure with the use of a 3D printed mold compared to the fractured implant and a further reduction in micro-porosity when cured at 2.2 bar. As reported in the literature, the flexural strength in our study is influenced by the porosity.28 With increased porosity, the flexural strength is reduced. The location and distribution of the pores is important, as pores near the midline will experience lower stresses.
To understand why this PMMA cranioplasty fractured, FEA was performed. The FEA confirmed that the stresses increased from the center to the point defect under a load of 100 N. The load of fracture of 503 N will most probably not be reached in daily life as patients are assumed to be more vigilant and careful. However, hitting the head on a hard surface at a speed of 0.42 m/s might easily occur. The patient might not even remember such a minor incident, as the patient in this case could also not recall a trauma involving the cranioplasty. The FEA model is a simplified model, modelled only with the defect at the suspected weak spot and without underlying tissues. Only the bone surrounding the implant is included. The stresses and consequently the displacements are very concentrated and of such a low level away from the weak spot that detailing of the environment is not of significant influence (Figure 7). The pressure in the head in a normal healthy human ranges from 70 mm to 180 mm water column29. This pressure results into a upward load distributed over the entire implant, lowering the stresses but is hardly of influence on a concentrated load of 503 N.
These combined findings suggest that fracture of a PMMA cranioplasty is more likely to occur when features such as a reduced thickness, high porosity, and pores located near the surface are situated closely together, and an impact occurs at that specific location.