During setting and hardening, the hybrid bone substitute ionomeric cement (lonocem®) achieves a stable and durable bond with the apatite of the adjacent bone without interpository soft tissue. Fluid contact during setting results in the release of aluminium ions which may reach critical levels as high as 3000 μg/l. On epidural application it is, therefore, essential to prevent cement constituents from gaining access to the intradural space. After the cement has hardened, the presence of aluminium is demonstrable in the adjacent bone to a maximum depth of 20 μm (EDX microanalysis). In rabbits, epiduralplacement of freshly mixed cement causes slight thickening of the dura. There is reason to believe that human dura, with a thickness 10 times greater, is impermeableto components of the cement. After epidural application of the freshly mixed cement in the frontobasal and laterobasal regions and at the skull cap and petrous apex, 76 patients in all have been followed for up to 6.5 years. During this period no complications have arisen and functional (and cosmetic) results are promising. The availability of preformed implants (lonoroc®, lonocast®) permitted the peridural placement of minimal quantities of freshly mixed cement. These implants were fixed to localized sites on the adjacent calvarial bone by use of lonocem®. Notwithstanding the stringent manufacturer guidelines, there have been reports in the literature that during the vulnerable stage of setting neurotoxic aluminium ions were released into the dural space with a fatal outcome intwo cases. In view of potential intradural complications, such as may occur in case of dural leaks, it was considered that further application of the material adjacent to the dura was no longer warranted. The production of lonocem® was discontinued in May 1995.