## 4 Discussion
### Properties of 3D-Printed Materials for Maxillofacial Prosthesis
[12] provided no clear follow-up timeline, merely noting the absence of complications up to the date of publication. The materials used in these cases also reveal variations in biocompatibility certification. The ACEO silicone employed in three of the clinical cases [10; 13; 15] was USP Class VI certified, meeting ISO 10993 standards [51] for biocompatibility [13]. This certification supports its suitability for use in direct 3D printing of medical devices, including maxillofacial prostheses. Conversely, the other two clinical cases utilized PolyJet resins, namely TangoPlus [16] and its newer version VeroFlex [12]. While these materials are certified as non-toxic by the manufacturer, they lack formal biocompatibility certification, raising concerns about their safety for prolonged or direct contact with human tissue. This limitation highlights a critical gap in understanding regarding the current materials used for 3D-printed maxillofacial prostheses. Material properties and their implications for clinical use are summarized in Table 4. The ACEO silicone exhibited favorable mechanical properties, including tear resistance and flexibility, aligning with the requirements for soft maxillofacial prostheses. However, PolyJet resins demonstrated limitations in mechanical performance and long-term durability. These findings suggest that while direct 3D printing technologies are advancing, significant improvements in material design and biocompatibility are needed to ensure optimal clinical outcomes. In the in vitro study published by [PERSON] et al. [36], the influence of material composition on the mechanical properties of 3D-printed maxillofacial silicones was clearly demonstrated. Adjustments to the silicone composition, such as varying the proportions of long, medium, and short polydimethylsiloxane (PDMS) chains and incorporating additives like fillers, thixotropic agents, and moderators, were shown to directly affect tensile strength, tear strength, and viscosity. For example, increasing the filler content enhanced tear and tensile strength, resulting in greater durability and improved edge strength for the printed prosthesis. Thixotropic agents improved viscosity and ensured shape retention during printing by preventing layer slumping. However, excessive use of thixotropic agents could weaken mechanical properties by interfering with the cross-linking process. Similarly, while moderators extended the working time by slowing polymerization, they also led to a decrease in the tear and tensile strength of the silicone [36]. Ensuring strong adhesion for the anchorage of the retention system is critical for the stability and longevity of maxillofacial prostheses. Additively manufactured silicones, such as those used in direct 3D printing, demonstrate potential for integrating retention elements like titanium magnets directly into the silicone bulk. Studies suggest that while these materials can provide clinically acceptable adhesion when combined with suitable primers (e.g., \"A 304\" general purpose primer, Factor II, Lakeside, AZ, USA or Platinum Primer \"G611\", Principality Medical, Newport, UK), the bond strength is generally lower compared to conventional silicones [47]. Among the included clinical reports, all except the auricular prosthesis presented by [PERSON] et al. [13] used adhesives for maxillofacial prosthesis retention. In the case of the auricular prosthesis, the retention system was integrated into the prosthesis using a digitally designed substructure. This substructure was virtually modeled around the retention magnets, which were then physically secured within the printed prosthesis bulk using a silicone material (Multisi Epithetik Country; Bredent GmbH) after applying a primer (Multisi-Primer; Bredent GmbH) to ensure secure adhesion and stability. Further research is needed to optimize the adhesion properties of 3D-printed silicones, particularly under complex angulated forces, to improve their reliability in clinical applications.