Temporal bone investigations on landmarks for conventional or endosteal insertion of cochlear electrodes

Our anatomical findings place special emphasis on the requirement to follow an infero-anterior approach to the round window, to expose the scala tympani safely for 'normal' cochlear implantation. It is also known how easily the basilar membrane may be accidentally damaged, despite exercising considerable caution in the approach used. With regard to an 'endosteal electrode' it can be stated that there are no really specific indicators to locate the spiral ligament, or each of the scalae, on the lateral aspect of the tissue layer encasing the cochlea. For the concept of an endosteal electrode, however, the soft tissue layer of the lateral aspect of the cochlea is considered to be sufficiently thick to serve as a physical barrier between the electrode and the inner ear fluid.

To re-evaluate surgical techniques of gaining access to the scala tympani for cochlear implantation (cochleostomy, 'fenestration'). There are two reasons for this study. First, recent publications show that in a significant number of patients the electrode array was unintentionally inserted into the 'wrong' scala (sc. vestibuli). Second, dealing with an alternative concept proposed by Lehnhardt for patients with residual hearing ('endosteal electrode'), the anatomical site of the spiral ligament should be known. In a study on human temporal bones the topography of the middle and inner ear is revised with regard to the presence of anatomical or surgical landmarks that may guide the surgeon.

Anatomical examinations were performed on 10 temporal bones (5 fresh specimens and 5 fixed in formalin), in which the bone of the promontory was carefully milled. The consistency of identification and the relative location of specific surgical indicators or landmarks such as 'blue lines' and 'gray lines' were evaluated for 10 temporal bones. Furthermore, the projection of the lateral attachment of the basilar membrane on the promontory was determined with regard to round window anatomy.

In all cases, a major blue line indicated the lateral aspect of the basal cochlear turn while milling the promontorial bone. In a limited number of cases (20%), an additional gray line potentially indicated the spiral ligament before the last shell of bone was removed. In 80% of the cases it was possible to remove the bony layer and leave the endosteum intact as a precondition for a potential endosteal electrode insertion. In addition, through the examination of these models, the relative anatomical location of structures, such as the scala vestibuli, scala tympani, spiral ligament, and basilar membrane, is reviewed.