Electrode array design has continuously evolved to reduce insertion trauma and improve hearing conservation. However, one downside of some delicate (smaller diameter, more flexible) electrodes has been a tendency for tip fold-over. Tip fold-over may occur during insertion when the electrode array tip impinges the modiolar wall (or other structure) and is temporarily held stationary while the more proximal electrode advances past it. The phenomenon has also been called “tripping” and may be more common in perimodiolar electrodes.(1,2) Tip fold-over may result in a variety of negative consequences, ranging from the need to program-out electrode contacts all the way to removal and replacement of the entire electrode array. Fold-over is also associated with cochlear insertion trauma.
Prof. Angel Ramos and colleagues at Las Palmas University have recently analyzed insertion techniques for a newer delicate perimodiolar electrode array using both a plastic model of the cochlea and human cadaver temporal bones.(1) Dynamic insertion was studied with fluoroscopy while final electrode position was studied with cone beam CT. The authors analyzed the outcomes of the manufacturer’s recommended insertion technique and the effects of three common errors that could be made in surgery:
1. Improper alignment (rotation) of the pre-curved electrode at the cochleostomy (should be toward the modiolus);
2. Over-insertion of the electrode ‘sheath’;
3. Pre-extrusion of the array from the sheath prior to proper positioning of the sheath.
Using the recommended technique, the authors report good results with no tip fold-overs. However, in each of the three technique-error conditions, tip fold-over was common: improper alignment--100%; over-insertion—60%; pre-extrusion—40%. Imaging also demonstrated dislocation of the electrode into Scala vestibuli in most bones with tip fold-over. It should be noted that, typical of anatomic studies, the number of insertions in this study are limited.
As electrodes become more delicate, the potential for tip fold-over becomes more substantial. This may be especially true with perimodiolar electrodes. Ramos et al. demonstrate the need meticulously to follow good surgical techniques and to be aware of common errors.
It is important to remember that initial studies of new electrodes are performed by surgeons who were highly involved in the experimental development of the electrode over a period of months or years. These surgeons are highly aware of the characteristics of the new array as well as what can go wrong during insertion. Subsequent FDA pre-market approval/CE Marking studies are usually carried out by a small group of highly selected senior CI surgeons who receive more training on a new electrode than other practicing CI surgeons usually do. As a result, sometimes surgical issues are not apparent until the new electrodes are released for widespread use.
For these reasons it is important to obtain specific training on any new device prior to performing the procedure. Manufacturers will provide this on request in a number of ways. Reading the surgeon’s manual and having a rep standing by in the O.R. for a first case are helpful but may not be sufficient.
1. Ramos-Macias A, De Miguel AR, Falcon-Gonzalez JC. Mechanisms of electrode fold-over in cochlear implant surgery when using a flexible and slim perimodiolar electrode array. Acta Oto-Laryngol 2017. PAP DOI:10.1080/00016489.2016.127149.
2. Briggs RJ, Tykocinski M, Xu J, et al. Comparison of round window and cochleostomy approaches with a prototype hearing preservation electrode. Audiol Neurotol. 2006;11:42–48.