Reconstruction of Full-Thickness Frontocranial Defects

Hydroxyapatite/Methylmethacrylate

Methylmethacrylate is an alloplastic substance that has long been used to reconstruct small to moderate cranial defects. It is mixed prior to use to form a moldable putty, and has a substantial exothermic reaction while hardening which must be taken into account when applying the material to tissue in situ . It has substantial tensile strength and compression resistance, is cheap, and relatively easy to use. However, as a foreign substance, it provides a nidus for infection, and even those surgeons with extensive experience using MMA for cranioplasty procedures recommend against its use in proximity to the frontal sinus due to unacceptably high risk for infection.

Hydroxyapatite is an inorganic phosphate-based compound that forms a hard cement after it is cured, similar in that with regard to MMA. This material, too, has limitations with regard to the size of the defect it is reliably able to reconstruct. It demonstrates superior osseointegration relative to MMA, a minimal inflammatory response, as well as some degree of osseoconduction that would theoretically mitigate some of the infection risk associated with alloplastic materials. A distinct advantage is that there is no associated exothermic reaction, and associated risk of thermal damage to surrounding tissue. However, as with MMA, there is compelling evidence that hydroxyapatite should never be used in close proximity to the frontal sinus due to significantly increased risks of bacterial seeding and subsequent infection.

In 2003 Moriera-Gonzalez et al. conducted a large study analyzing the clinical outcomes in long-term follow-up between autogenous bone, methylmethacrylate, and hydroxyapatite. Infection and implant exposure were the most common complications seen in those patients treated with methylmethacrylate and hydroxyapatite, and occurred in 13.3% and 22.4% of these cases respectively. In comparison, the rate of infection and exposure seen with autologous bone was significantly lower, at 7.1% and 1.3% respectively.

Titanium Mesh

Titanium is a commonly used material in the fixation of bony fractures and reconstructions, and is reasonably well tolerated by the body. Typically, for cranioplasty procedures, it is used in the form of a moldable mesh, which can be cut, bent, and fitted to cover the relevant defect. In this form it has been used successfully (particularly in conjunction with free tissue transfer for improved soft tissue coverage) for reconstruction of large frontocranial defects. It is, however, still a foreign substance, and thus prone to the complications inherent to such materials. Infection risk remains higher than that of autogenous bone, there is an ever-present risk of attenuation of the overlying native soft tissue envelope (especially in large reconstructions), and when used in context of irradiated tissue the results will be predictable and poor.

Polyetheretherketone (PEEK)

Polyetheretherketone (PEEK) is biocompatible, inert, and reported to have similar mechanical properties to bone. Importantly, it can be formed to precisely fit frontocranial defects preoperatively based off 3-dimensional CT scans. For these reasons PEEK has been growing in popularity, based on the arguments that these implants save operative time, prevent donor site morbidity, and result in improved aesthetic outcomes. While early results seem to indicate better outcomes relative to other alloplastic materials, clinical series are limited in scope (between 5 and 38 patients), and follow-up times are probably inadequate at this point (from 6 to 24 months) to provide an accurate representation of the potential for long-term success. There does appear to be an implant failure rate of up to 27% reported with PEEK, typically due to exposure and/or infection. A few direct comparison studies with PEEK cranioplasties have suggested a lower complication rate relative to autogenous bone, however these were small series with short follow-up times, and this was clearly not consistent with our experience.

Autogenous Nonvascularized Bone

Through our experience over the past 41 years, autogenous bone grafts are generally the most appropriate material with which to reconstruct the frontocranial region. Grafts can be readily harvested from split calvarium, split rib, or iliac crest cortical bone. The ability of these grafts to revascularize and consolidate after reconstruction allows them to become resistant to infection over the long term. While other published experiences vary in their quality and results, we have experienced zero graft failures as a result of infection or extrusion. The notion is commonly put forth that these nonvascularized grafts are prone to significant resorption, but again, this has not been the case in our experience.

The most comprehensive recent investigation to date regarding the use of autogenous bone for cranioplasties was published by Fearon et al. Their 14-year series of 96 patients had zero incidences of postoperative infections resulting in loss of the cranioplasty graft, all of which were performed with split calvarium. Only one patient experienced clinically significant graft resorption that required operative revision. Additionally, their extensive review of the literature concluded that alloplastic cranioplasty grafts demonstrated a nearly fivefold higher infection rate relative to autogenous bone grafts. They also noted that should infection occur with an autogenous graft, it can often be treated with drainage alone, as opposed to the near universal need to remove an alloplastic implant under similar circumstances.