Knots are only possible in
3D-space, i.e. 4D-spacetime.
Extended fermions must be solved mathematically,
that is linear, with open B(oundary)C(onditions).
This characteristic allows interaction of fermions in
all directions. As a result they must have mass, i.e.
interact with the spin2 gravitational field.
The speeds < lightspeed c allows knots in the oscillating paths
and this is why fermions can only be described in 4D-spacetime.
All extended bosons must be solved with closed BC, so only
interact in the direction of motion (worldline). As a direct
result only bosons have the possibility to be massless. The
fact that the solution of the 2D-extendedness is open or
closed explains the statistical behavior of fermions and
bosons in interactions completely.
explained in the following PDF: