# How Does the Biocompatibility of Molybdenum Compare to the Gold Standard Titanium?--An In Vivo Rat
## 3 Results  
We enrolled 96 Wistar rats in the study and operated on them. The 96 animals were divided into 2 approximately equal cohorts: 49 animals (51.0%) received a molybdenum implant and 47 (49.0%) a titanium implant, as shown in Figure 4a. Both implant types were produced by the selective laser melting process. For the exact compositions of the plates, see Section 2. As shown in Figure 4b, 10 animals (10.4%) died immediately postoperatively (6 animals (60.0%) with molybdenum and 4 animals (40.0%) with titanium implants due to unspecific complications. Dissections of the animals performed immediately after death by a veterinarian did not reveal any pathological abnormalities. Another animal (with a titanium implant) died three days postoperatively from acute kidney failure. In the necropsy of the animal, the veterinarian could not establish a connection to the inserted titanium implant. The acute kidney failure was most likely a complication of the general anesthesia. Figure 5 shows the clinical inflammatory reactions in the period immediately postoperatively and up to a maximum of postoperative week 12 (T1). If an inflammatory reaction occurred in one of the animals during this time window, it was immediately recorded, and the animal was marked as positive for inflammation. Postoperative infections were present in 11 of 85 animals (12.9%). Within this subset, a molybdenum implant was implanted in 4 animals (36.4%) and a titanium implant in 7 animals (63.6%). No statistically significant dependence of the implanted material on the inflammatory response could be demonstrated (\(\chi\)2(1) = 0.861, \(p\) = 0.354, Cramer's V = 0.101). According to clinical assessment, all local inflammatory reactions were scratch marks in response to the shaved area. One animal had to be euthanized for welfare reasons after the seventh postoperative week due to an excessively large sore area. This animal had been implanted with a titanium specimen. Thus, from week 8 onwards, a total of 84 animals were included in the trial. Consequently, 28 animals were euthanized after 12 weeks (T2), 24 weeks (T3), or 52 weeks (T4). After 12 weeks (T2), infection occurred in 4 of 84 animals (4.8%) (Figure 6). A molybdenum implant was used in 25.0% (\(n\) = 1) and a titanium implant in 75.0% (\(n\) = 3). With \(p\) = 0.329, no significant dependence of the implant choice on the inflammatory response was demonstrated (\(\chi\)2(1) = 0.953, \(p\) = 0.329, Cramer's V = 0.107). No infections occurred from week 13 onward. At T3, the total collective consisted of a total of 55 animals, 45.5% (\(n\) = 25) of which had received molybdenum and 54.5% (\(n\) = 30) of which had received titanium implants. At T3, after 52 weeks, the distribution was as follows in \(n\) = 27 animals: 40.7% (\(n\) = 11) molybdenum and 59.3% (\(n\) = 16) titanium implants. Figure 4: **(a)** Wistar rats were implanted with either a molybdenum (above) or titanium implant (below); scale bar represents 10 mm; **(b)** 10 animals (10.4%) died immediately postoperatively (6 animals (60.0%) with molybdenum and 4 animals (40.0%) with titanium implants. Figure 5: Clinical inflammatory response within the first 12 weeks postoperatively (T1). In summary, the molybdenum samples did not induce a significant inflammatory response at any time compared to titanium. Irrespective of the materials used, the infections that could be clinically detected were identified as scratch marks resulting from shaving and the reactions of the animals to the shaved areas (Supplementary Figure S1). The animals had an average body weight at T1 of 457 \(\pm\) 38.4 g (molybdenum implants) and 454 \(\pm\) 39.2 g (titanium implants). During the course, the average weight was 554 \(\pm\) 57.7 vs. 544 \(\pm\) 55.0 (T2), 605 \(\pm\) 40.7 vs. 604 \(\pm\) 69.9 (T3), and 656 \(\pm\) 51.6 vs. 689 \(\pm\) 74.0 (T4). The differences between the two groups were not significant at any of the time points.