Spontaneous fractures in custom-made porous hydroxyapatite cranioplasty implants: is fragility the only culprit?

BACKGROUND: Although the porous hydroxyapatite (PHA) used in custom-made cranioplasty implants is a material appreciated for its biomimetic properties, before osteointegration it is initially very fragile. Nevertheless, we wondered whether this primary fragility is entirely due to brittleness or whether the surgeon's actions may influence the behavior of the material. METHODS: To study the influence of the surgeon's behavior, we made a virtual model of a custom-made PHA cranioplasty implant and submitted it to three implant procedural variables using finite element methods. In the first test, a scenario in which the surgeon's design, validation, and positioning techniques are impeccable, the edges of the implant adhered well to the craniectomy margins. In the second test, a discrepancy between a portion of the perimeter of the craniectomy and the profile of the prosthesis was modeled, and in the third test, several gaps were simulated between the implant and the craniectomy margins. RESULTS: Our mathematical model showed that when local and general discontinuities were included in the test scenarios, there was an increase in the load coming to bear on the cranioplasty implant, which amounted to 80 and 50 %, respectively. CONCLUSIONS: The fragility of custom-made PHA cranioplasty implants increases if the surgeon fails to achieve a precise design and validation, and/or an accurate surgical procedure. Nevertheless, careful attention during these phases helps to maintain the strength of the implant, given the more favorable mechanical conditions, without interfering with its biomimetic capacity.