R - Add symbols in labels from a plot3D

Question

I have tried inserting text3D but the rgl package masks it, below an example:

plot3d(x1, y1, z1, cex=1.5, size=4, type="l",col="red" )
text3D(2, -5, 0, labels = expression(theta[1]), add = TRUE, adj = 1)

How can I do it?