Commutator Estimates in von Neumann Algebras

Let $\mathcal{M}$ be a von Neumann algebra. For every self-adjoint locally measurable operator $a$, there exists a central self-adjoint locally measurable operator $c_0$ such that, given any $\varepsilon>0$, $|[a,u_\varepsilon]|\ge(1-\varepsilon)|a-c_0|$ for some unitary operator $u_\varepsilon\in\mathcal{M}$. In particular, every derivation $\delta\colon\mathcal{M}\to\mathcal{I}$ (where $\mathcal{I}$ is an ideal in $\mathcal{M}$) is inner, and $\delta=\delta_a$ for $a\in\mathcal{I}$.