###### Abstract  
In severe cases of total knee arthroplasty, where off-the-shelf implants are not suitable or available anymore (i.e., in cases with extended bone defects or periproshetic fractures), custom-made knee implants represent one of the few remaining treatment options. Design verification and validation of such custom-made implants is very challenging. The aim of this study is to support surgeons and engineers in their decision on whether a developed design is suitable for the specific case. A novel method for the pre-clinical testing of custom-made knee implants is suggested, which relies on the biomechanical test and finite element analysis (FEA) of a comparable reference implant. The method comprises six steps: (1) identification of the main potential failure mechanism and its corresponding FEA quantity of interest, (2) reproduction of the biomechanical test of the reference implant via FEA, (3) identification of the maximum value of the corresponding FEA quantity of interest at the required load level, (4) definition of this value as the acceptance criterion for the FEA of the custom-made implant, (5) reproduction of the biomechanical test with the custom-made implant via FEA, (6) conclusion, whether the acceptance criterion is fulfilled or not. Two exemplary cases of custom-made knee implants were evaluated with this method. The FEA acceptance criterion derived from the reference implants was fulfilled in both custom-made implants. Subsequent biomechanical tests verified the FEA results. The suggested method allows a quantitative evaluation of the biomechanical properties of a custom-made knee implant without performing a biomechanical test with it. This represents an important contribution in the pre-clinical testing of custom-made implants in order to achieve a sustainable treatment of complex revision total knee arthroplasty patients in a timely manner. finite element analysis; custom-made implants; pre-clinical testing +
Footnote †: journal: Applied Sciences  
the patient's anatomy through an improved implant design and implant fit [5], clinically important improvements compared to conventional off-the-shelf implants could not be shown for \"uncomplicated\" primary TKA [6]. In more \"complicated\" TKAs, such as trauma injuries or revision cases, which are often associated with severe bone loss, personalized knee implants provide a substantial benefit for the patients [7,8]. In these cases, when off-the-shelf implants are not suitable or available anymore, personalized knee implants could serve as one of the few remaining options to restore the joint function or even prevent limb amputation [9]. The costs for a custom-made knee implant are higher compared to off-the-shelf implants and highly depend on the complexity of the implant and whether additional instruments are required. It was reported that the costs vary between EUR 4321.53 and EUR 19,900.82 [9]. The overall healthcare costs in terms of projected lifetime healthcare cost has been reported to be three times lower comparing reconstruction with amputation of a limb-threatening injury (USD 163,282 compared to USD 509,275) [10]. Personalized knee implants are distinguished in patient-matched implants and custom-made implants [11,12]. Patient-matched implants are adjusted to the patient's anatomy within a specific design envelope and are designed and produced under the responsibility of the implant manufacturer. Custom-made implants are intended to address the specific condition of one single patient with specific design characteristics in a single-unit production [12,13]. The responsible surgeon and the corresponding engineer work in close collaboration, whereby, in very simplified terms, the surgeon is responsible for the medical evaluation and the engineer for its proper and careful realization [11,14]. The manufacturing of a custom-made implant is based on established and validated processes under a quality management system [15,16,17].