## 4 Discussion

Table 11: One-way analysis of variance (ANOVA) test for statistical significance between all examined implant materials and associated average infection rate. Polyetheretherketone (PEEK) is a biocompatible linear thermoplastic which is finding increased acceptance in the medical industry as an alternative to metal components. The core driving factor to the development of PEEK as a biomaterial was the need to balance strength and rigidity: to develop an implant system which met the structural strength requirements of the body, whilst minimizing rigidity in an attempt to reduce stress shielding risk [33; 81]. Traditionally PEEK found use in spinal treatment applications, as well as cranial and maxillofacial applications [54; 56; 82]. It must be noted that PEEK is a hydrophobic material. As such, it does not actively bond to tissue, creating potential issues in regards to anatomical integration as bone cells will not bond to the implants [59]. Whilst there are a number of coatings which can alleviate these concerns [53; 58], un-coated PEEK remains at a disadvantage in regards to achieving stable fixation in a short timeframe. Polyethylene is a thermoplastic material which finds use in numerous areas of the medical industry. This is due to its variability and adaptability, as its method of manufacture and formation can give it various physical properties tailoring it for a broad range of applications, from medical packaging to implants [83]. It can be manufactured as a porous structure, commonly used in cranioplasty applications, allowing for the creation of semi-rigid structures which are both strong and flexible and allow for alteration with minimal effort [60; 84; 85]. It must be noted that for certain applications such porous structures this material may not have the mechanical properties to act in load-bearing structures [86; 87]. Calcium phosphate bone cements are finding acceptance as methods of cranial reconstruction given their strong biocompatibility and osteoconductive nature [66]. In addition, ready availability and ease of use present calcium phosphate cements as appealing alternatives to current reconstruction materials [67]. Based on the findings from the systematic review, the following can be inferred from the results:  
* Implant material/manufacturing method has an effect on infection risk. Whilst the results of the one-way ANOVA (\(p\) = 0.12) show no statistical significance when analyzing all implant materials as a whole, analysis of means highlights variation between individual implant materials and manufacturing methods; which, dependent on the materials compared directly, can be considered significant. Further literature analysis highlights the following factors as prime sources of increased infection and complication risk for cranial and craniofacial implants. * Latency Period between Tissue Removal and Implant [1; 8]
* The literature suggests that a necessary latency period, from initial flap removal to implantation of the desired implant, may lead to a lower infection risk. * Previous Multiple Operations/Recent Operation [1; 9]
* The literature suggests that a higher number of operations can lead to a higher risk of infection. Additionally, short timeframes between surgeries are considered to further increase infection risk. * Increased Surgical Time [6; 10; 11]
* The literature suggests that increased surgical time dramatically increases the risk of infection. Longer surgical times (greater than 4 h) were associated with a higher risk of infection. Additional factors, including gender, pre-existing infection, surgical errors, surgical complexity and poor outcomes, were also considered to potentially lead to an increase in infection potential. Infection risk can be further impacted through factors such as implant and defect size, fixation position and fixation stability. However, like the inherent infection resistance of implant materials, these factors were considered to have a lesser impact on potential risk than the aforementioned prime sources.