# Using Statistical Shape Models to Optimize TKA Implant Design
## 4 Discussion  
Statistical Shape Models are very interesting tools when analyzing the morphology in the population. They can be used for example to predict tibiofemoral kinematics [26], knee joint instability [27], or to quantify bony defects in hip revision surgery [28; 29; 30]. To our knowledge, there is no guideline on how to best use these models in the process of implant design. The aim of this study was to assess how to generate a valid SSM for implant design and to provide examples of how to use SSMs to evaluate the anatomic fit of TKA components. Those examples may contribute to the definition of guidelines on how to validate the anatomical fit of TKA components. When generating a SSM for implant validation, it is crucial to analyze which input population should be used. For each application, a good compromise between homogeneity and size of the input population must be found. Gender and ethnicity have an important  
Figure 10: (**A**): Underhang of the femoral component in the frontal view. The underhang is most important on the lateral side of the anterior shield. (**B**): Underhang of the femoral component in the axial view. The underhang is most important on the medial side of the distal cut. Figure 9: Statistical Shape Model of Patella resected at 6 mm and implanted with a universal patella (Aesculap, Tuttlingen, Germany). The implant peak (red) is 1.6 mm lateralized with respect to the native patella peak (blue). impact on the SSM and thus are distinguishable features in the populations. Therefore, it would be beneficial to examine the fit of the implant to SSMs built on different gender and ethnicities. In the present approach, it was decided to build two SSMs, one representing the Asian and the other the Caucasian population. The use of 120 Caucasian and 112 Asian anatomies, as well as the use of reference systems based on relevant anatomical landmarks, enabled robust SSMs. The fact that clinically relevant landmarks were used to define the reference systems greatly facilitated the use of the SSMs for design input and validation. The crucial step to practically use the SSM for implant design was to generate a database of bone models out of the average SSMs. The seven Caucasian and five Asian bone models generated represented the average population well. Their AP/ML dimensions were located in the center of the cloud of points representing the AP/ML dimensions of the different patients. An additional 10 statistical models were computed to represent not only the average but also the patients with AP/ML dimensions at the borderline. Hence, a database of 22 bone models statistically well represented the Caucasian and Asian populations. Various analyses can be performed on the database of bones to design and validate TKA implants. For design input, the outer shape of the proximal tibia is, for example, a key element to define the contour of the tibia plateau. Different studies compare the implant design with the anatomy of the population [16, 17] by using a large number of CT scans of the input population. Algorithms are used to automatically place the components and evaluate their fit on all these anatomies. With the approach presented in the current study, the number of bone models included in the analysis could be drastically reduced by using bone models that statistically represent the population. For the design of the anterior shield of the femoral component, the 3D geometry of the trochlea is an important input parameter which has been described in literature [31, 32, 33]. However, differences in the reference systems used make it difficult to compare these studies. [PERSON] [34] used 42 bone models to compare different trochlea designs. Our proposal to use a limited number of statistically relevant bone models is again a major advantage, as more anatomies are covered and less analyses are required. This also applies to the analysis of the overhang/underhang of the femoral component.