Securing cochlear implants to the skull: Two alternate methods.
In view of the various problems encountered with the traditional methods of securing cochlear implants-including dural tear and suture dissolution following infection-we devised two alternate methods of performing this procedure. We use a titanium mesh or a Gore-Tex patch secured with two 4-mm screws to fix the receiver to the skull. No patient who has undergone either of these procedures at our institution has experienced any of the complications that are associated with the older silk, nylon, and Dacron sutures. Moreover, our two alternate methods are less technically difficult and can be performed in a shorter period of time.
The incidence of electrode slippage in cochlear implants was described by Webb et al in their review of implant complications worldwide. They reviewed the outcomes of 4,969 nucleus implants in adults and children and identified complications in 70 (1.4%); 61 of these patients (1.2%) experienced electrode migration and nine (0.2%) experienced receiver migration. In a followup study, Hoffman and Cohen reported that 42 of these 70 patients (60%) required revision surgery. In our experience, the traditional methods of securing the implant receiver with silk ties or nylon sutures are not optimal. In two of 93 implant cases (2%) reviewed at our institution, wound infection led to an erosion of the ties and movement of the receiver under the scalp flap, necessitating reimplantation. In response to this problem, we devised two alternate methods-using a titanium mesh or a GoreTex patch-of securing the cochlear implant receiver.
Titanium mesh. A hockey stick-shaped incision is made in the postauricular area, and a mastoidectomy via a facial recess approach is performed. The cochlear implant bed is then drilled in the skull by egg-shelling the middle fossa periosteum. One hole each is made on the anterior and posterior aspects of the implant receiver with a 1.7-mm diameter cuffed 4-mm twist drill bit. A cochleostomy is then performed, and the electrode is threaded into the cochlea. Two 2.0 Leibinger 4-mm screws are used to hold a 5 x 5-cm, 0.3-mm-thick Leibinger titanium mesh over the cochlear implant and secure it in the depression made in the skull. The temporalis fascia and skin are then closed in two layers (figure 1).
Gore-Tex patch. The implant is placed in a manner similar to the one described above. After preparing the implant bed in the skull, two monocortical holes are drilled-one anterior and one posterior to the implant- with a 2.0-mm-diameter cuffed straight 4-mm drill bit. A 3 x 6-cm, 0.4-mm-thick cardiovascular Gore-Tex patch is placed over the implant. While the patch is being stretched, holes are made in it over the screw sites with a Rosen needle. The patch is then secured to the skull with two 4-mm screws placed through the holes made with the needle. The stretching of the patch in effect holds the implant securely against the skull (figure 2).
The traditional methods of securing cochlear implants are associated with complications. Graham et al described a method of using silk or Dacron ties to secure implants. But in our experience, silk sutures eroded following infection in two patients. Furthermore, Dacron ties have been associated with necrosis of the overlying flap, stitch abscess, and granulation tissue, which have caused implant exposure and external meatal skin erosion.
These complications prompted efforts to develop a simpler method of securing implants that would cause fewer complications. Some of these methods concengery difficult.  Cohen and Kuzma reported on the use of a titanium clip that is fixed to the incus bar to secure the distal electrode.  Balkany and Telischi described the split-bridge technique, in which a slot is drilled through the incus bar.  In this procedure, the electrode is placed deep into the short process of the incus, which fixes it.  In a modification of this technique, a silicone-sheet sleeve has been used to protect the electrode. One potential complication associated with this modification is that the electrode can slide inside the sheath. 
Migration of the receiver also presents a problem. Different methods of preparing the receiver bed and securing the receiver have been described. Hoffman and Cohen have advocated creating a deep well down to the dura and leaving an "island" of bone to prevent receiver migration in some children.  However, this method can increase the risk of dural tear, especially when drilling holes in the surrounding diploic bone to allow for the passing of the securing sutures. Also, nearby bleeding could cause an epidural hematoma.
The two methods that we have described are different from those described by others. We have found that electrode migration can be prevented by securing the receiver and part of the proximal electrode with a titanium mesh or Gore-Tex patch. At our institution, 170 patients have undergone cochlear implantation in which the device was secured with a Gore-Tex patch, and 10 have received the mesh. During a mean followup of 12 months, we have found that electrode migration has not been a problem. Covering the implant with the titanium mesh or the Gore-Tex patch has not caused any difficulties in signal transmission to the receiver. Titanium and Gore-Tex have a long track record of successful use in humans by facial plastic surgeons. [10,11] Complications have been minimal. These materials are well tolerated by tissues, do not migrate or fail, and are not often infected or extruded. [10,11]
Another advantage of these two methods is that they carry a lower risk of dural tear and cerebrospinal fluid leakage. Using the cuffed drill bits and limiting penetration with the 4-mm screws, we have not experienced any cases of dural tears, subdural or epidural hematomas, or cerebrospinal fluid leakage that were caused by the use of this technique. In addition, screw extrusion has not been a problem. Finally, the surgically simpler methods we have described require less time to perform than do the island-and-suture method. With the silk sutures, the average length of time from the completion of the implant-bed drilling to the securing of the device was 3 minutes and 35 seconds. With the Gore-Tex patch, the average time was 2 minutes. Overall, we prefer the Gore-Tex patch to the titanium mesh because it is easier to use and less expensive.
From the Department of Otolaryngology, University of Minnesota Hospital and Clinics, Minneapolis.
Reprint requests: Samuel C. Levine, MD, Department of Otolaryngology, Box 396, UMHC, 420 S.E. Delaware St., Minneapolis, MN 55455. Phone: (6i2) 626-0486; fax: (612) 625-2101; e-mail: levin001 @tc.umn.edu
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