  We propose a quantum version of the quadratic volume simplicity constraint
for the EPRL spin foam model. It relies on a formula for the volume of
4-dimensional polyhedra, depending on its bivectors and the knotting class of
its boundary graph. While this leads to no further condition for the 4-simplex,
the constraint becomes non-trivial for more complicated boundary graphs. We
show that, in the semi-classical limit of the hypercuboidal graph, the
constraint turns into the geometricity condition observed recently by several
authors.
