All depends on the study's aim. If this is a static model with the knee in extension and vertical loading then the mentioned concept of non separation can be considered. If on the other hand there will be joint motion within the analysis then due to the "presence of articular fluid" can be considered frictionless, as the presence of friction can be considered "negligible". In this case though this has to be mentioned as a limitation to the study.

Here is a reference that may help you;

J Appl Biomech. 2010 Nov;26(4):432-43.

The effect of the frontal plane tibiofemoral angle and varus knee moment on the contact stress and strain at the knee cartilage.

Yang NH, Canavan PK, Nayeb-Hashemi H.

Thanks

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