The biharmonic problem with Dirichlet and Neumann conditions concerning unique sol

The function $\displaystyle u \in H_0^2(U) $ (where $\displaystyle U\subset \mathbb{R}^n$ is bounded with smooth boundary $\displaystyle \partial U$) is a weak solution of the biharmonic equation

$\displaystyle \Delta(\Delta u)=f \mbox{ in }U$

and

$\displaystyle u=\frac{\partial u}{\partial\nu}=0\mbox{ on }\partial U$

provided $\displaystyle \int_U \Delta u\Delta v dx=\int_U f v dx$ for all $\displaystyle v\in H_0^2(U)$.

I want to show that if $\displaystyle f\in L^2 (U)$ then the weak solution $\displaystyle u$ is unique. I imagine this problem satisfies the conditions of the Lax Milgram theorem when the integral is considered as a bilinear operator. However, I am wondering why one can't show that $\displaystyle B[u,v]=\int_U \Delta u\Delta v dx$ defines an inner product on the $H_0^2(U)$ and apply the Riesz Representation Theorem directly to the problem? Am I missing something simple here?