Receiver Bed and Electrode Array Trough

Preparation of the receiver bed begins by selection and design of the placement. It is important to place the receiver bed posterior enough to accommodate the ear level hook or behind-the-ear speech processor. Placement of the receiver bed in a position that is too far anterior is a common mistake made by inexperienced cochlear implant surgeons. The consequence of this misplacement is extrusion of the receiver following skin erosion due to the ear hook or behind-the-ear speech processor placing pressure on the skin overlying the receiver. Another consideration in designing the position of the receiver bed is the anticipated position of a hat band, should the patient frequently wear hats or wish to wear hats. Each device manufacturer provides templates that are used in designing the shape of the receiver bed and these accurately represent the size of the receiver. The CII Bionic Ear device also utilizes a plastic template that is of the size and shape of the internal device.

There are specific differences in how the receiver bed is designed and these will be described in separate subsections of this portion of the chapter; however, general techniques are described first. Although it is possible to inset the receiver in some adult patients, virtually all young children and the majority of older children and adults require a craniotomy that extends down to the dura and allows the insetting of the receiver. The technique for accomplishing this includes completing a cra-niectomy of the dimensions necessary for accommodating the receiver. The outer cortex and diploic layers are removed and the inner cortex of the skull is thinned until the dura is visible through the bone. Next, the craniotomy is performed using a 2-mm coarse diamond bur and is carried down to the dura without violating this layer. The surrounding bony edges are then smoothed to remove any rough edges, and conformation of the template to the craniotomy is completed to be certain that the receiver fits in the

Mastoidectomy

FIGURE 28-1 Common design for skin incision used during cochlear implantation.

created space. It is estimated that 6 months are required for the bone to regrow across this gap and provide stable bone beneath the cochlear implant receiver; however, this important issue has not been systematically studied using CT imaging.1 This is an important consideration when making a decision for a patient with a cochlear implant in place to undergo an MRi, and consequently should be performed prior to MRI scanning. For young children a small (usually 2.5 or 3 mm in diameter) cutting bur is used to remove the outer cortex and diploic layer, whereas for older children and adults a 5-mm cylinder burr is helpful because it allows perpendicular walls to be created around the receiver site while simultaneously dissecting bone with the cutting flukes located on the tip of this cylinder bur.

Figure 28-2 shows the two methods available to create the tie-down suture holes through the bone. Four sites for suture tie-downs are created so that two separate 3-0 nylon sutures can be used to secure the cochlear implant receiver.

Bone wax is useful in controlling hemostasis, and if there is epidermal bleeding, placement of strips of Surgicel (Ethicon Inc., Somerville, NJ) are placed into the epidural space to achieve hemostasis and prevent postoperative epidural hematoma formation. Specific details necessary to create the receiver beds for the Nucleus Contour device, the Clarion CII and HiRes 90K devices, and the Med-El C40H device are described below.

Nucleus Contour

The Nucleus Contour device has two options regarding creation of the receiver bed. The first is to create the cylindrical well to receive the deepest portion of this cochlear implant. It is common in very young children to require a craniotomy down to the dura and creation of the bony island as described above. For the majority of adults and older children, this is not necessary and a craniectomy is all that is required. The remaining portion of the internal

Tympanic Membrane Implant
FIGURE 28-2 Securing the internal receiver-stimulator to the skull requires (A) four holes drilled through the skull into the bone bed (Med-El C40H or CII Bionic Ear), or (B) eight channels drilled in pairs (Nucleus Contour or HiRes 90K Bionic Ear).

device is placed lateral to the skull, and, in particular, the magnet and loop antenna have a very low profile and are barely palpable beneath the scalp flap. The remaining rectangular portion of the internal device/receiver is palpable, and for this reason it is especially important for this device to be placed well posterior to the position that will be occupied by the behind-the-ear speech processor. Another option for this device is to create a receiver bed that will accommodate most of the volume of the Nucleus Contour receiver. This is done in the same manner as described above and the advantage of this is that it allows a more integrated placement of the internal device, but it does require creation of a complex bony island and craniotomy, which requires additional surgical time.

The Nucleus Contour device has a separate ground electrode, which is placed beneath the temporalis muscle that is located lateral to the principal electrode array. A trough is created between the internal receiver device and the area of the mastoidectomy. A 2-mm cutting bur is used to create this trough, and an important point is to complete the dissection such that one of the bony margins is cantilevered over the tract created in the bone. This further helps in securing and protecting the electrode array.

Clarion CII and HiRes 90K Bionic Ear

The internal device for the CII Bionic Ear is the largest of the internal devices made by the three companies presently in the United States marketplace. The depth of this device is 6 mm; consequently, virtually all patients receiving this device require a craniotomy to inset it. The special consideration in preparing the receiver bed for this device is principally related to the fan-tail that allows the electrode array to leave the internal receiver device. Accommodation for this structure is critical to avoid excessive pressure or bending of the electrode array. It is important to dissect the bone underlying the fan-tail down to the same level that the floor of the receiver bed will be placed into. A 2mm cutting bur is used to create the trough that accommodates the electrode array. The Clarion devices have the ground electrode built into the primary electrode array, and therefore a second electrode carrier containing the ground electrode is not used with these devices. The trough utilizes a cantilevered segment of the bone on either the superior or inferior aspect of this trough, and is helpful in keeping the electrode array from becoming exteriorized on the lateral aspect of the skull (Fig. 28-3).

FIGURE 28-3 The trough created between the bone bed and the mastoidectomy site is cantilevered to facilitate retention of the electrode array beneath the surface of the skull.

The HiRes 90K Bionic Ear has a Silastic carrier surrounding the components and has a position for a replaceable internal magnet within the center of the loop antenna. The issues specific to the HiRes 90K Bionic Ear receiver bed are similar to those described in the Nucleus Contour device. The design of the HiRes 90K Bionic Ear differs from the Nucleus Contour device principally in the midportion of the internal device between the magnet and loop antenna and the electrode array. The design allows for a more tapered device, which theoretically results in fewer problems with extrusion and skin erosion than are experienced with the Nucleus Contour device. Thus, it is possible to create the smaller internal well, relative to the CII Bionic Ear device, to accommodate the most medial aspects of this device while allowing the remainder of the device to be positioned lateral to the skull. It is also possible to create a larger complex receiver bed that would accommodate this entire component, as has been described in the Nucleus Contour device subsection. The trough to accommodate the electrode array is the same as is necessary to accommodate the CII Bionic Ear electrode array.

The biggest difference in the receiver bed for the Med-El C40H is that the electrode array and ground electrode emanate from the side of the cochlear implant instead of from the proximal end of the cochlear implant, as is the case with the Nucleus Contour, CII Bionic Ear, and HiRes 90K Bionic Ear devices. Accommodation for this design feature requires that the bony trough begins at the side of the implant internal device and must be positioned so that one of the holes necessary to secure the implant can be placed.

Because the depth of the Med-El C40 +/ is 3.9 mm, some of the adult patients do not require a cranio-tomy down to the level of the dura, and creation of a bone island. This is necessary, however, in virtually all children and in many adults. The same principles described in earlier sections apply with this device, both in terms of creation of the bony island and the trough necessary to accommodate the electrode array. The ground electrode is separate and is placed beneath the temporalis muscle, as is the case with the Nucleus Contour device.

During the creation of the craniotomy and bone island necessary to accommodate the receiver-stimulator, inadvertent dural injury may occur. If this violation is small, placement of fascia is done through the opening, so that tissue remains both medial to and lateral to the dura in a dumbbell-shaped manner, and the hole is easily sealed. It is also important to determine that no parenchymal injury has occurred in this clinical setting.

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Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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