# How Does the Biocompatibility of Molybdenum Compare to the Gold Standard Titanium?--An In Vivo Rat
For example, the Resorb-X(r) system ([PERSON], Tuttlingen, Germany) is an established osteosynthesis material for the surgical treatment in pediatric craniofacial surgery [21]. However, the systems developed so far based on polylactic acids have not provided completely satisfactory results. Due to the low mechanical strength, biodegradable polymers are unsuitable for implants that are subjected to high mechanical loads [4]. The use of polymers for such applications would lead to unacceptably large dimensions and introduce a high amount of degradable material into the body. In addition, possible foreign body reactions and long-term inflammation due to persistent degradation products have been described in various studies [22; 23]. Radiopacity and consequently visibility in X-ray based imaging for biodegradable polymers is even lower than for magnesium implants. Thus, despite the constant further development and increasing experience of surgeons with these osteosynthesis systems, disadvantages are still evident, even when using established systems. For these reasons, scientists continue to search for suitable materials that provide the required stability and biocompatibility for optimal bioresorbable osteosynthesis materials. In this context, molybdenum has proven to be a promising alternative. In the human body, it is found as a trace element and co-substrate of some specific enzymes [24]. The degradation products can be excreted renally [25]. In initial publications, molybdenum showed sufficient mechanical properties as a material for osteosynthesis [26; 27]. In our latest work on molybdenum, we were able to show that, in addition to being a potential material for cardiac stents [27], it is also a very promising biocompatible material for osteosyntheses [28]. We were able to show that molybdenum has the necessary biocompatibility and excellent mechanical properties for use as an osteosynthesis material, even in the larger dimensions which are required. Specifically, we were able to demonstrate that molybdenum shows a very uniform degradation in vitro, with a complete loss of stability expected after 8-9 months. Furthermore, molybdenum of high purity showed no significant cytotoxicity in mouse or human fibroblasts, nor in human osteoblasts. Despite these positive results, testing the material in a complex organism is an imperative next step. Adverse effects such as renal damage, which has already been described due to overly high amounts of molybdenum intake [29, 30], must be excluded. The aim of our experiments was, therefore, to investigate the influence of molybdenum on the organism of a mammal--in this case, the rat. We investigated whether molybdenum samples produced by the selective laser melting process (SLM), also called laser powder bed fusion (LPBF), behave in a complex biological organism in a similarly biocompatible way as the gold standard titanium. We placed particular focus on ensuring that both titanium and molybdenum were manufactured in such a way that they could potentially serve as computer-aided and computer-manufactured (CAD-CAM) patient-specific implants (PSI). For this purpose, a total of 96 rats were implanted with test specimens made of molybdenum and titanium subcutaneously (s.c.) in the fold of the neck. Local inflammation parameters, as described in Section 4, were recorded daily. Then, 12, 24, and 52 weeks later, the specimens from one-third of the total number of animals, respectively, were explanted, and the surrounding tissues were pathologically processed. The spleens, livers, and kidneys were also explanted and microscopically examined. To measure the degradation of the molybdenum samples, metallographic cross-sections of the samples were prepared and analyzed.