## 2 Materials and Methods
### Indentation Testings  
The description of the biomechanical measurement methods concerning the articular cartilage is based on [PERSON] et al. [25]. The indentation test (DIN EN ISO 14577) was carried out first, followed by thickness measurement using the needle measuring method [28].  
Biomechanical testing of the menisci and cartilage was carried out by means of indentation tests (DIN EN ISO 14577-1:2015) [29] (see Figure 1). A Mach-1 Model V500 css test machine (Biomomentum Inc., Laval, Canada), a multiaxial load cell with 17 N MA 233 (ATI industrial Automation, Apex, NC, USA) for the menisci and a multiaxial load cell with 70 N Model FTIFPS1 (ATI industrial Automation, Apex, NC, USA) for the articular cartilage [25], and a Newport Motion Controller ESP 301 (Newport, Irvine, CA, USA) were used. These materials allow for the examination of soft tissue as well as the complete mapping of the tissue [25, 26, 30]. The menisci were fixed to the MA646 specimen holder (Biomomentum Inc., Laval, QC, Canada) by Loctide (Henkel AG and CoAG KGaA, Dusseldorf-Holthausen, Germany) according to physiological conditions and were then anchored in the testing device. For biomechanical testing of the articular cartilage of the tibial plateau, we first marked the meniscus-covered areas with tissue markers [25]. The samples were then fixed in the testing machine.  
After that, the measuring points had to be determined. For this purpose, the glued menisci and tibial plateaus were recorded with a camera. In the Mach-1 Analysis Software Version 4.1.0.17 (Biomomentum, Montreal, QC, Canada), a position grid could then be projected on these images with which the measurements were taken [25].  
The position grid was created mechanically for the arthritis-free meniscus and all tibial plateaus and manually adapted to the shape and size of the arthritis meniscus. The indentation test (DIN EN ISO 14577) was then carried out using a spherical indenter MA680 (Biomomentum Inc., Laval, QC, Canada) with a diameter of 1 mm [25]. In this way, the entire tissue can be tested. The maximum applied load was defined as the highest load registered by the load cell per measuring point. An average of 45 measuring points was distributed over the meniscus. When the articular cartilage was tested in the same way as the meniscus, an average of 35 measuring points was distributed over the entire tibial plateau.  
Since meniscus and articular cartilage differ in their biomechanical properties, the testing criteria differ. The testing criteria chosen are in the range of previous studies [25, 26]. The meniscus is less stiff than the cartilage. Therefore, a 0.1 mm/s faster indentation speed was chosen in comparison to the indentation speed of the cartilage [25]. The articular cartilage lies flat on the subchondral bone. Therefore a 0.1 mm higher indentation depth was chosen. Since the meniscus was glued on the metal, we selected a smaller depth. Otherwise, the indenter could touch the metal before reaching the predefined indentation depth. Thus, following testing criteria were used:  
Figure 1: (**A**) Mach-1 thickness measurement by needle method (joint cartilage); (**B**) Mach-1 Indentation (meniscus); (**C**) Mach-1 thickness measurement by needle method (meniscus).  
The following criteria were used to test the meniscus: Poisson ratio: 0.5, contact criterion: 0.1 N, predefined indentation amplitude: 0.2 mm, indentation speed: 0.2 mm/s, and relaxation time: 10 s.  
The following criteria were used to test the joint cartilage [25]: Poisson ratio: 0.5 contact criterion: 0.1 N, predefined indentation amplitude: 0.3 mm, indentation velocity: 0.1 mm/s, and relaxation time: 10 s.  
In order to prevent dehydration of the tissue, we added NaCl 0.9% to the tissue repeatedly using a disposable pipette during the measurement.