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Cochlear implant soft surgery: Fact or fantasy?
Cochlear implant soft surgery: Fact or fantasy? NOEL L. COHEN, MD, New York, New York
A basic surgical principle is to be as gentle as possible to accomplish the goals of the operation. The concept of "soft surgery" for cochlear implants consisted of a small, localized cochleostomy and gentle electrode insertion, the hope being that by limiting damage to the inner ear, superior hearing results might be obtained. The technique includes deferring the cochleostomy until immediately before electrode insertion, use of a large burr to flatten the promontory, followed by a smaller burr to expose the endosteum, preservation of the endosteum of the scala tympani, smoothing of the bony edges with burrs and dissectors, limited opening of the scala tympani, no suctioning of perilymph, gentle electrode insertion, and potential use of a lubricant to facilitate insertion. Although this technique has a theoretic basis, is esthetically satisfying, and has been used in many cases involving the Nucleus device at multiple centers, no data are available that demonstrate its advantages. Furthermore, the Clarion device, the results of which seem to be comparable to those of the Nucleus device, requires much more extensive and potentially damaging surgery. The pros and cons of soft surgery will be discussed. Although soft surgery seems desirable to limit trauma within the cochlea, other factors such as full electrode insertion, stimulation strategy, and survival of ganglion cells may be more important predictors of successful results. (Otolaryngol H e a d Neck Surg 1997;1 17:214-6.)
I n 1993 Lehnhardt 1 first formulated the concept of what he termed "soft surgery" for cochlear implants. He did this in the hope of being able to preserve hearing while inserting a long intracochlear electrode into the proximal scala tympani. Previously, he had abandoned the old direct approach to the scala tympani through the round window, correctly reasoning that the bone removal required to straighten the most proximal "hook" area was unnecessarily traumatic to the cochlea. He then went on to perform a cochleostomy, by drilling through the promontory into the scala tympani anterior to the round window, thereby bypassing the hook area completely. After a decade of experience and hundreds of cases, he detailed, refined, and elaborated the technique of "soft surgery," the aim of which was to limit even potential trauma to the cochlea and the goal of
From the Department of Otolaryngology, New York University School of Medicine. Supported in part by the Oberkotter Foundation. Presented at the Sixth Symposiumon Cochlear Implants in Children. Miami Beach. Fla.. Feb. 2-3, 1996. Reprint requests: Noel L. Cohen. MD. Department of Otolaryngology, NYU Medical Center. 530 First St.. New York. NY 10016. Copyright © 1997 by the American Academy of OtolaryngologyHead and Neck SurgeryFoundation. Inc. 0194-5998/97/$5.00 + 0 2311175096
214
which was to permit the preservation of whatever preimplant hearing was present m a cochlear implant candidate.
METHOD The basic principle of soft surgery is to limit trauma to the inner ear. This is accomplished by performing a careful cochleostomy, paying meticulous attention to the avoidance of damage within the inner ear. In addition, the cochlea is opened as late as possible and kept open only briefly. Intracochlear suctioning ~s avoided. The electrode is inserted with the use of a lubricant such as Healon (hyaluronic acidL and no attempt is made to insert the stiffening rings. Drilling on the ossicular chain is also avoided. The technique for the cochleostomy is as follows: after all preliminary steps have been competed (e.g,, drilling of the well for the body of the implant and placement of tie-down holes and sutures), the surface of the promontory is flattened with a 1.5 mm diamond burr until the white circle representing the fibrous layer of the endosteum of the scala tympani is seen. Then 1.0 and 0.6 mm burrs are used to smooth the edges of the bone. Fine dissectors are then used to elevate the endosteum circumferentially off the bone, and a small slit is made with a fine triangular blade. After this, a drop of Healon is deposited on the cochleostomy, and the
OtolaryngologyHead and Neck Surgery
COHEN 21S
Volume 117 Number 3 Part 1
Healon-coated electrode is inserted until all electrodes are in. Care is taken not to suction within the cochlea at any time, and soft tissue is immediately placed around the electrode to seal the cochleostomy.
trade is then inserted and soft tissue is placed within the bony defect. Avoidance of Intracochlear Suctioning
As can be appreciated from the above, soft surgery has a theoretic basis: to do no harm, which is in accordance with generally accepted surgical practice, and consists of several subparts, some of which are better founded than others: avoidance of drilling on the incus, a meticulous cochleostomy, only brief opening of the scala tympani, no suctioning within the scala tympani, use of a lubricant to facilitate electrode insertion, and 17.5 mm electrode insertion. Let us consider each facet of this technique individually.
Again, avoidance of intracochlear suctioning is a good general principle, because there is usually little need for it. The suction tip itself can be traumatic to the scala tympani, the basilar membrane, and even the osseus spiral lamina. Large negative pressures in the scala tympani are capable of rupturing the basilar membrane, disrupting the scala media, and causing loss of endolymph. Fortunately, any blood or debris present in the proximal scala tympani can usually be removed with picks and footplate instruments. The final two aspects of "soft surgery" are the most controversial.
No Drilling on the Incus
Useof a Lubricant Such as Nyaluronic Acid
No drilling on the incus is adhered to as a general principle of good otologic surgery. Although it may not be of any consequence in dealing with a profoundly deaf ear, or even in the presence of minimal residual hearing, vestibular trauma may well occur.
Although Lehnhardt 1 uses hyaluronic acid to facilitate nontraumatic electrode insertion, others, such as Roland et al., 2 have investigated a number of potential lubricants to achieve deep electrode placement well into the middle turn. This is being done in the hope of obtaining superior low-frequency responses. This group found a profound loss of electrocochleographic response on injecting hyaluronic acid, methylcellulose, and glycerin into the guinea pig cochlea, with only partial recovery. Dendrite, spiral ganglion counts, and axon histology, however, appear well preserved. Using scanning electron microscopy after soft surgery in the guinea pig, Rogowski et al. 3 found "only minor changes in the outer hair cells." Thus although these substances are toxic when injected into the cochlea, they may well not interfere with the spiral ganglion or axon population and therefore not adversely affect patient performance. If either less traumatic or deeper insertion should ultimately appear to provide better access to speech information, this aspect of the technique may also be beneficial.
DISCUSSION
Delaying the Cochleostomy Until All Other Work Has Been Done and Closing It Immediately After Insertion Delaying the cochleostomy until all other work has been done and closing it immediately after insertion seems logical, because it avoids not only the possibility of unnecessary perilymph leak but also the intracochlear deposition of bone debris and blood clot. Furthermore, prompt closure of the cochleostomy may diminish may tendency for the electrode to slip out of the cochlea during the remainder of the operation. We frequently will lift up the packing before skin closure to be sure that the electrode has not partially extruded. Creation of a Meticulous Cochleostomy Creation of a meticulous cochleostomy is good surgical technique, which has evolved at many implant centers including our own. It is also taught at the surgeon's training courses organized by the Cochlear Corporation (Englewood, Colo.). We use a 1.6 mm diamond burr to make an excavation down to the white endosteum, followed by a 1 mm diamond burr to create an opening in the bone that is approximately 1.2 to 1.4 mm in diameter. Loose bone is then removed and the edges smoothed with the large McGee footplate rasp (Storz, St. Louis, Mo.). The endosteum is then incised widely around the circumference and either removed or elevated in trap-door fashion. The elec-
No Insertion of Stiffening Rings Most surgeons attempt to insert stiffening rings, as well as electrodes, and some are attempting to insert all rings past the cochleostomy (>25 mm insertion). This is possible with the Nucleus electrode (Cochlear Corp.) and may give better access to low-frequency information, but there are no data proving that this leads to superior performance. SUMMARY The general concept of soft surgery seems reasonable. However, when one considers published results of
OtolaryngoiogyHead and Neck Surgery September 1997
216 COHEN
various devices, questions arise. A study by the Veterans Administration 4 showed that results with the Ineraid device (Richards, Inc., Bartlett, Tenn:) (requiring a large cochleostomy to accomodate the surfacemounted ball electrodes) were the same as those for the Nucleus device (and its more limited cochleostomy) until a change in speech-processing software was introduced in the latter system. Similarly, although the cochleostomy required for the Clarion implant (Advanced Bionics Corp., Sylmar, Calif.) is extensive, necessitating the drilling of several millimeters of the proximal basal turn, the placement of a Teflon insertion tool at the junction of the pars inferior and the pars ascendans, and the mechanical extrusion of a preformed, stiff electrode, all of which are diametrically opposed to the Nucleus technique, the results in postlingually deafened adults appear equivalent. That cochlear implantation routinely produces loss of any residual hearing is commonly known. In the Severely Hearing Impaired Protocol, presented to the Food and Drug Administration by Cochlear Corporation, most patients lost all residual hearing, and the remainder had further loss (personal communication, Ms. Patti Arndt, Cochlear Corp., Severely Hearing Impaired Protocol presentation to Food and Drug Administration, April 20, 1995). Loss of hearing does not indicate loss of ganglion cells, nor do cochlear
implants stimulate surviving hair cells. Finally, in this population there was no correlation between preoperative hearing and postoperative performance with the implant CONCLUSIONS
Soft surgery is elegant, contains many useful techniques, and is particularly well suited to the Nucleus device, with its thin. flexible electrode. There is, however, no evidence that this technique is associated with improved patient performance, even in patients with some residual hearing. The topic should be pursued further and studies undertaken to evaluate the effects, if any, of atranmatic cochleostomy, use of lubricants, and both relatively shallow and deep electrode insertion. REFERENCES 1. Lehnhardt E. IntracochlearePtazierung der Cochlear-ImplantElektroden in soft surgery technique. HNO 1993;41:356-9. 2. Roland JT, Magardino TM, Go JT. Effects of glycerine, hyaluronic acid and hydroxypropylmethylcelluloseon the spiral ganglion of the guinea pig cochlea: Ann Otol Rhinol Laryngol Suppl 1995;9:64-8. 3. RogowskiM, Reiss G, Lehnhardt E. Morphologicalstudy of the guinea pig cochlea after cochlear implantation using the soft surgery technique. Ann Otol Rhinol Laryngol Suppl 1995;9: 433-6. 4. CohenNL, WaltzananSB, Fischer SG, et al. A prospective,randomized study of cochlear implants. N Engl ~ Med 1993;328: 233-7.
A basic surgical principle is to be as gentle as possible to accomplish the goals of the operation. The concept of "soft surgery" for cochlear implants consisted of a small, localized cochleostomy and gentle electrode insertion, the hope being that by limiting damage to the inner ear, superior hearing results might be obtained. The technique includes deferring the cochleostomy until immediately before electrode insertion, use of a large burr to flatten the promontory, followed by a smaller burr to expose the endosteum, preservation of the endosteum of the scala tympani, smoothing of the bony edges with burrs and dissectors, limited opening of the scala tympani, no suctioning of perilymph, gentle electrode insertion, and potential use of a lubricant to facilitate insertion. Although this technique has a theoretic basis, is esthetically satisfying, and has been used in many cases involving the Nucleus device at multiple centers, no data are available that demonstrate its advantages. Furthermore, the Clarion device, the results of which seem to be comparable to those of the Nucleus device, requires much more extensive and potentially damaging surgery. The pros and cons of soft surgery will be discussed. Although soft surgery seems desirable to limit trauma within the cochlea, other factors such as full electrode insertion, stimulation strategy, and survival of ganglion cells may be more important predictors of successful results. (Otolaryngol H e a d Neck Surg 1997;1 17:214-6.)
I n 1993 Lehnhardt 1 first formulated the concept of what he termed "soft surgery" for cochlear implants. He did this in the hope of being able to preserve hearing while inserting a long intracochlear electrode into the proximal scala tympani. Previously, he had abandoned the old direct approach to the scala tympani through the round window, correctly reasoning that the bone removal required to straighten the most proximal "hook" area was unnecessarily traumatic to the cochlea. He then went on to perform a cochleostomy, by drilling through the promontory into the scala tympani anterior to the round window, thereby bypassing the hook area completely. After a decade of experience and hundreds of cases, he detailed, refined, and elaborated the technique of "soft surgery," the aim of which was to limit even potential trauma to the cochlea and the goal of
From the Department of Otolaryngology, New York University School of Medicine. Supported in part by the Oberkotter Foundation. Presented at the Sixth Symposiumon Cochlear Implants in Children. Miami Beach. Fla.. Feb. 2-3, 1996. Reprint requests: Noel L. Cohen. MD. Department of Otolaryngology, NYU Medical Center. 530 First St.. New York. NY 10016. Copyright © 1997 by the American Academy of OtolaryngologyHead and Neck SurgeryFoundation. Inc. 0194-5998/97/$5.00 + 0 2311175096
214
which was to permit the preservation of whatever preimplant hearing was present m a cochlear implant candidate.
METHOD The basic principle of soft surgery is to limit trauma to the inner ear. This is accomplished by performing a careful cochleostomy, paying meticulous attention to the avoidance of damage within the inner ear. In addition, the cochlea is opened as late as possible and kept open only briefly. Intracochlear suctioning ~s avoided. The electrode is inserted with the use of a lubricant such as Healon (hyaluronic acidL and no attempt is made to insert the stiffening rings. Drilling on the ossicular chain is also avoided. The technique for the cochleostomy is as follows: after all preliminary steps have been competed (e.g,, drilling of the well for the body of the implant and placement of tie-down holes and sutures), the surface of the promontory is flattened with a 1.5 mm diamond burr until the white circle representing the fibrous layer of the endosteum of the scala tympani is seen. Then 1.0 and 0.6 mm burrs are used to smooth the edges of the bone. Fine dissectors are then used to elevate the endosteum circumferentially off the bone, and a small slit is made with a fine triangular blade. After this, a drop of Healon is deposited on the cochleostomy, and the
OtolaryngologyHead and Neck Surgery
COHEN 21S
Volume 117 Number 3 Part 1
Healon-coated electrode is inserted until all electrodes are in. Care is taken not to suction within the cochlea at any time, and soft tissue is immediately placed around the electrode to seal the cochleostomy.
trade is then inserted and soft tissue is placed within the bony defect. Avoidance of Intracochlear Suctioning
As can be appreciated from the above, soft surgery has a theoretic basis: to do no harm, which is in accordance with generally accepted surgical practice, and consists of several subparts, some of which are better founded than others: avoidance of drilling on the incus, a meticulous cochleostomy, only brief opening of the scala tympani, no suctioning within the scala tympani, use of a lubricant to facilitate electrode insertion, and 17.5 mm electrode insertion. Let us consider each facet of this technique individually.
Again, avoidance of intracochlear suctioning is a good general principle, because there is usually little need for it. The suction tip itself can be traumatic to the scala tympani, the basilar membrane, and even the osseus spiral lamina. Large negative pressures in the scala tympani are capable of rupturing the basilar membrane, disrupting the scala media, and causing loss of endolymph. Fortunately, any blood or debris present in the proximal scala tympani can usually be removed with picks and footplate instruments. The final two aspects of "soft surgery" are the most controversial.
No Drilling on the Incus
Useof a Lubricant Such as Nyaluronic Acid
No drilling on the incus is adhered to as a general principle of good otologic surgery. Although it may not be of any consequence in dealing with a profoundly deaf ear, or even in the presence of minimal residual hearing, vestibular trauma may well occur.
Although Lehnhardt 1 uses hyaluronic acid to facilitate nontraumatic electrode insertion, others, such as Roland et al., 2 have investigated a number of potential lubricants to achieve deep electrode placement well into the middle turn. This is being done in the hope of obtaining superior low-frequency responses. This group found a profound loss of electrocochleographic response on injecting hyaluronic acid, methylcellulose, and glycerin into the guinea pig cochlea, with only partial recovery. Dendrite, spiral ganglion counts, and axon histology, however, appear well preserved. Using scanning electron microscopy after soft surgery in the guinea pig, Rogowski et al. 3 found "only minor changes in the outer hair cells." Thus although these substances are toxic when injected into the cochlea, they may well not interfere with the spiral ganglion or axon population and therefore not adversely affect patient performance. If either less traumatic or deeper insertion should ultimately appear to provide better access to speech information, this aspect of the technique may also be beneficial.
DISCUSSION
Delaying the Cochleostomy Until All Other Work Has Been Done and Closing It Immediately After Insertion Delaying the cochleostomy until all other work has been done and closing it immediately after insertion seems logical, because it avoids not only the possibility of unnecessary perilymph leak but also the intracochlear deposition of bone debris and blood clot. Furthermore, prompt closure of the cochleostomy may diminish may tendency for the electrode to slip out of the cochlea during the remainder of the operation. We frequently will lift up the packing before skin closure to be sure that the electrode has not partially extruded. Creation of a Meticulous Cochleostomy Creation of a meticulous cochleostomy is good surgical technique, which has evolved at many implant centers including our own. It is also taught at the surgeon's training courses organized by the Cochlear Corporation (Englewood, Colo.). We use a 1.6 mm diamond burr to make an excavation down to the white endosteum, followed by a 1 mm diamond burr to create an opening in the bone that is approximately 1.2 to 1.4 mm in diameter. Loose bone is then removed and the edges smoothed with the large McGee footplate rasp (Storz, St. Louis, Mo.). The endosteum is then incised widely around the circumference and either removed or elevated in trap-door fashion. The elec-
No Insertion of Stiffening Rings Most surgeons attempt to insert stiffening rings, as well as electrodes, and some are attempting to insert all rings past the cochleostomy (>25 mm insertion). This is possible with the Nucleus electrode (Cochlear Corp.) and may give better access to low-frequency information, but there are no data proving that this leads to superior performance. SUMMARY The general concept of soft surgery seems reasonable. However, when one considers published results of
OtolaryngoiogyHead and Neck Surgery September 1997
216 COHEN
various devices, questions arise. A study by the Veterans Administration 4 showed that results with the Ineraid device (Richards, Inc., Bartlett, Tenn:) (requiring a large cochleostomy to accomodate the surfacemounted ball electrodes) were the same as those for the Nucleus device (and its more limited cochleostomy) until a change in speech-processing software was introduced in the latter system. Similarly, although the cochleostomy required for the Clarion implant (Advanced Bionics Corp., Sylmar, Calif.) is extensive, necessitating the drilling of several millimeters of the proximal basal turn, the placement of a Teflon insertion tool at the junction of the pars inferior and the pars ascendans, and the mechanical extrusion of a preformed, stiff electrode, all of which are diametrically opposed to the Nucleus technique, the results in postlingually deafened adults appear equivalent. That cochlear implantation routinely produces loss of any residual hearing is commonly known. In the Severely Hearing Impaired Protocol, presented to the Food and Drug Administration by Cochlear Corporation, most patients lost all residual hearing, and the remainder had further loss (personal communication, Ms. Patti Arndt, Cochlear Corp., Severely Hearing Impaired Protocol presentation to Food and Drug Administration, April 20, 1995). Loss of hearing does not indicate loss of ganglion cells, nor do cochlear
implants stimulate surviving hair cells. Finally, in this population there was no correlation between preoperative hearing and postoperative performance with the implant CONCLUSIONS
Soft surgery is elegant, contains many useful techniques, and is particularly well suited to the Nucleus device, with its thin. flexible electrode. There is, however, no evidence that this technique is associated with improved patient performance, even in patients with some residual hearing. The topic should be pursued further and studies undertaken to evaluate the effects, if any, of atranmatic cochleostomy, use of lubricants, and both relatively shallow and deep electrode insertion. REFERENCES 1. Lehnhardt E. IntracochlearePtazierung der Cochlear-ImplantElektroden in soft surgery technique. HNO 1993;41:356-9. 2. Roland JT, Magardino TM, Go JT. Effects of glycerine, hyaluronic acid and hydroxypropylmethylcelluloseon the spiral ganglion of the guinea pig cochlea: Ann Otol Rhinol Laryngol Suppl 1995;9:64-8. 3. RogowskiM, Reiss G, Lehnhardt E. Morphologicalstudy of the guinea pig cochlea after cochlear implantation using the soft surgery technique. Ann Otol Rhinol Laryngol Suppl 1995;9: 433-6. 4. CohenNL, WaltzananSB, Fischer SG, et al. A prospective,randomized study of cochlear implants. N Engl ~ Med 1993;328: 233-7.