The porous matrix may be cross-linked. [0032] Other benefits of a cross-linked matrix include greater implant residence time and shape retention (no fragmentation of the implant). [0033] Other cross-linking methods and agents may be used, such as formaldehyde, chromium salts, di-isocyanates, carbodiimides, difunctional acid chlorides, difunctional anhydrides, difunctional succinimides, dibromoisop