###### Abstract
Due to the inherent unique design characteristic, the design process is one of the most critical parts during the workflow of a custom-made implant [15]. In particular, the validation and verification for critical mechanical endurance properties is a very challenging task. Pre-clinical biomechanical tests, as routinely performed for off-the-shelf implants, are impractical for a single-unit custom-made implant due to the unique design and, depending on the diagnosis, the limited available supply time. It has been recommended that a custom-made implant should be based on a underlying medical device or reference implant, which allows certain variants or modifications [14,16]. Validation and verification of the intended purpose of the underlying reference implant should be performed and documented in advance. When a specific custom-made implant is requested by a surgeon, it must be proven that the custom-made implant is covered by the underlying medical device [16]. In a few cases, a patient's diagnosis requires a custom-made implant with a specific design feature, which is not covered by an underlying reference implant. This feature in the custom-made implant needs to be evaluated in advance to the clinical use within the mandatory benefit-risk analysis. Since prolonged supply time often negatively affects the patient's condition, the shortest possible evaluation time for the benefit-risk analysis is desirable. For mechanical considerations, a finite element analysis (FEA) is beneficial and has been commonly used in the pre-clinical testing of orthopedic devices [18]. Using FEA in pre-clinical testing, it is recommended to define the \"question of interest\" and the \"context of use\" [19] at the beginning. Then, the model risk needs to be identified (i.e., the severeness of the consequences caused by incorrect FEA results). Based on this, the model credibility could be obtained (i.e., the usefulness of the model by verification and validation), which includes the evaluation against an adequate comparator. An adequate comparator could be an in vitro study (e.g., biomechanical test) or an in vivo study (e.g., a clinical trial). Only in cases when the model credibility in the defined \"context of use\" is acceptable, could the \"question of interest\" be answered by the model. To predict the mechanical properties of custom-made knee implants using an FEA, the potential failure mechanism and its corresponding quantity of interest need to be identified [20]. Then, an adequate comparator needs to be defined, which could be the corresponding reference implant and its biomechanical results. However, this requires that the custom-made implant is comparable to the reference implant in terms of intended use, design, material, manufacturing and loading, and that the reference implant has been successfully tested according to given standards. The aim of this study is to provide surgeons and engineers with a method to evaluate the mechanical properties of a custom-made knee implant, in a fast and quantitative way, to support their decision on whether the design is suitable for this specific case. Within this method, the biomechanical test of a reference implant is reproduced by FEA to obtain the value of the crucial quantity of interest at the required load level. The biomechanical test is then also reproduced with the custom-made implant and the obtained value is used as the acceptance criterion. It is assumed that, when the reference implant fulfills the biomechanical acceptance criterion and the custom-made implant is exposed to equal or less mechanical burden, the custom-made implant would fulfill the acceptance criterion. Under the above-described requirements, it is hypothesized that the mechanical performance of a custom-made implant can be evaluated solely by FEA. The hypothesis was tested in two exemplary custom-made knee implants. In custom-made implant 1, a geometrical modification of the reference implant was evaluated.