# Different ISO Standards' Wear Kinematic Profiles Change the TKA Inlay Load
## 4 Discussion  
The main important finding of this study is that the force-controlled norms of 2002 and 2009 are different in the resulting femorotibial kinematics and von Mises stress behaviour within the inlay over the gait cycle. Moreover, the displacement-controlled standard from 2014 generated up to four times more von Mises stress in all three tested inlay designs. Figure 4: Stress in the cruciate-retaining (CR) inlay for different ISO norms: (**a**) first peak, (**b**) second peak, (**c**) third peak. There were differences between FC-2002 and FC-2009 regarding the kinematic results within the CR inlay. FC-2009 led to more internal rotation in all inlays than FC-2002. Furthermore, at about 75% of the gait cycle, increased anterior movement was observed with the FC-2009 norm compared to FC-2002. The ISO standard changed from 2002 to 2009 by modifying the anterior/posterior and torsional springs (30 to 44 N/mm and 600 to 360 \(N\)mm/\({}^{\circ}\)) and adding a slack condition for \(\pm\)2.5 mm (a/p spring) and \(\pm\)6\({}^{\circ}\) (torsional spring). As a result, the movement experiences less resistance. This might explain the increased anterior movement and internal rotation of the tibia in the model with the ISO 2009 standard for all inlays. The adapted force-controlled ISO from 2009 showed higher mechanical stress during the gait cycle in the CR and DD designs, which might also lead to higher wear rates. In general, the changes in the standard from 2002 to 2009 show an increase in the range of motion. This can have a negative effect on the wear contact area of both components in the form of increased wear. Studies by [PERSON] et al. and [PERSON] et al. were also able to confirm this observation with an increased amount of abrasion particles found across different ISO norms [8; 15; 16]. [PERSON] et al. compared the kinematics of posterior-stabilised inlays produced according to the standard protocol ISO 14243-1 versions from 2002 and 2009. In agreement with our results, they found a change in anterior/posterior (AP) translation and internal/external rotation, which resulted in a 3.2 times higher wear rate. They came to the conclusion that a change in the kinematics has a strong influence on the wear behaviour in TKA [16]. Compared to FC-2009, the displacement-controlled ISO standard (2014) can lead to a higher mechanical load due to displacement-controlled movement. In the FC-2009 norm, an additional peak at about 70% of the gait cycle occurred for CR and DD designs compared to the older version. The maximum contact pressure occurred as expected with each inlay on the medial side. However, DC-2014 showed a more dorsal distribution of the pressure, resulting in a smaller surface. This can be explained by the fact that displacement control dictates anterior movement of the inlay, whereas force control is directed both anteriorly and posteriorly. The DC-2014 standard is controlled by a defined path. This control mechanism does not account for possible resistance in the joint. Therefore, the standard automatically leads to more movement in the prosthetic joint and creates more von Mises stress. Increased stress in the inlay can also lead to increased wear of the prosthesis. This fact leads to the problem that if the prostheses are tested with different standards, higher wear cannot automatically be attributed to the type of prosthesis. The different results of the tested standards are extremely limited in terms of comparability. In a comparison of different inlay designs, increased movement in the posterior direction with a maximum at the beginning of the swing phase was only seen in the UC inlay for FC-2009. The more paradoxical posterior displacement of the UC inlay can be explained by the different definition of the restraint spring system (9.3 \(N\)/mm vs. 44 \(N\)/mm). The maximum total rotation increased from 7.0\({}^{\circ}\) to 10.1\({}^{\circ}\) for the UC inlay.