The front of the kneecap (patella) is rounded, but the back is angulated. The flat surfaces at the back are falled 'facets'.
Although one talks mostly of a medial and a lateral facet, the patella has in fact seven facets - the medial and lateral facets are each divided into three parts and there is a seventh facet (the odd facet) medial to the others.
The different facets are not all in contact with the sides of the underlying trochlear groove at the same time. As the knee goes from extension to flexion (straight to bent) the load shifts from facet to facet and the forces experienced by each during this excursion are different.
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