Laser Revision Stapedectomy

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Chapter 24 Laser Revision Stapedectomy

Larry B. Lundy

Videos corresponding to this chapter are available online at www.expertconsult.com.

In recent years, the safety and efficacy of revision stapedectomy have come under scrutiny. Experienced surgeons report that the results of revision stapedectomy are often worse than results of primary stapedectomy, and that the risks of sensorineural hearing loss, tinnitus, and vertigo are increased. With the application of laser technology to revision stapes surgery, less traumatic and more precise techniques can be applied, allowing better results and diminished risks compared with revision stapedectomy without lasers. This chapter reviews the clinically relevant principles of laser technology, compares results of revision stapedectomy with and without laser application, defines candidates for surgery, and reviews surgical technique.

LASER PHYSICS AND PRINCIPLES

Laser energy is derived from the release of energy (photons) occurring when stimulated electrons return to their resting orbital. As proposed by Einstein in 1917, when photons of the appropriate wavelength strike excited atoms, a second additional photon is released as the electron returns to its ground state (Fig. 24-1). In this stimulated emission situation, the photon that is emitted from the excited atom has exactly the same frequency, direction, and phase as the incident photon, providing laser energy that is collimated, coherent, and monochromatic.

FIGURE 24-1 According to the Bohr atomic model, an electron can absorb a photon, and the electron makes a transition to a higher energy level from its normal ground state. The electron eventually returns to the ground state by the spontaneous emission of a photon. In the condition of stimulated emission, a photon of appropriate energy interacts with the electron already in its excited state, causing the release of two photons.

(From Weisberger EC: Lasers in Head and Neck Surgery. New York, Igaku-Shoin, 1991.)

Lasers are typically named by the active medium, or the source of atoms that are excited and undergo stimulated emission of photons. The active medium can be a liquid, solid, or gas. Common gas lasers include CO₂, argon, and helium-neon. An example of solid state lasers are the neodymium:yttrium-aluminum-garnet (Nd:YAG) and the potassium titanyl phosphate crystal (KTP). The KTP laser is simply a Nd:YAG laser beam that passes through a KTP crystal, which halves the wavelength and doubles the frequency of the laser beam (Table 24-1).

TABLE 24-1 Characteristics of Laser Types

The wavelength of the emitted photons, or laser beam, has important characteristics for tissue interaction. Lasers whose wavelengths fall into visible (380 to 700 nm) and infrared (700 nm to 1 mm) portions of the electromagnetic spectrum are considered thermal lasers. Interaction of these lasers with normal biologic materials is mediated by a photothermal process. On contact with tissue, the laser energy is converted to thermal energy, resulting in a rapid increase in tissue temperature. The laser-tissue interaction depends as much on the tissue type and its composition (e.g., bone, muscle, cartilage, or nerve) as it does on the laser energy.

Visible-spectrum laser (argon at 514.5 nm and KTP at 532 nm wavelengths) energy absorption by tissue depends partly on tissue color. For soft tissue work, chromophores of hemoglobin and melanin absorb most of the energy. Lighter color tissues reflect most of the laser energy. Energy absorption from the invisible CO₂ laser (10,600 nm wavelength) is primarily by intracellular and extracellular water, which is instantaneously converted to steam.

For any laser, the magnitude of the laser-tissue interaction can be regulated by the laser’s power output, the power density at the point of impact, and the energy fluence. Every surgeon who uses a laser should thoroughly understand these fundamental concepts. Power is the time rate at which energy is emitted, and is expressed as watts. The power output is directly adjusted by the control panel on the laser console. Laser energy is delivered through a focusing lens. Power density is a measure of the intensity, or concentration, of the laser beam spot size (Fig. 24-2). It is the ratio of power to surface area of the spot size, and is expressed in terms of watts per square centimeter:

FIGURE 24-2 Planes of focus with spot size and power density.

where area of spot size is πr², and where r = spot size radius in centimeters.

Power density is inversely proportional to the square of the radius of the spot size. Consequently, for any specific power output, changes in the spot size can have a tremendous effect on power density (Fig. 24-3).

FIGURE 24-3 Relationship between spot size and power density.

The third fundamental, practical concept is that of fluence, which is simply the power density × time, expressed in joules (J). This is the total amount of energy delivered to the tissues:

As fluence increases, the volume of affected tissue also increases. The thermal energy having an impact on tissue increases dramatically as the time of exposure increases. If power density (W/cm²) is held constant, and exposure time is doubled, the energy delivered is doubled. The thermal effect of the tissue increases significantly, however, because the increase in temperature is continuous. For example, assume the laser power is set at 2 W, the spot size is 0.2 mm², and exposure time is 0.2 second. Is this the same as delivering two separate impulses at 0.1 second each? Yes and no. Yes, it is the same in terms of energy delivered from the laser. But no, it is not the same in terms of thermal energy imparted to the tissue because during the time between the two separate 0.1 second pulses, no matter how brief, the tissue is cooling (Fig. 24-4). Other factors come into play, such as the absorption characteristics of the damaged tissue in the center of the laser spot and dissipation of heat, but the important concept is that of the increase and decrease of the temperature.

FIGURE 24-4 A and B, Temperature and time of exposure relationship for a single pulse (A) versus two separate pulses (B).

HISTORY OF REVISION STAPEDECTOMY

Background

After the introduction of the stapedectomy procedure by Shea ¹ in 1958, the surgical treatment of otosclerosis was revolutionized. During the 1960s and 1970s, otologic surgeons performed hundreds of thousands of stapedectomies. An otologic surgeon commonly performed thousands of stapes procedures during his or her peak professional years. The accepted success rate (as defined by closure of the air-bone gap to ≤10 dB) was 90% or greater, with a 1% or less incidence of significant sensorineural hearing loss, including deafness. The most common technique during this time was the total stapedectomy, with removal of the entire stapes footplate. The small fenestra technique began gaining some acceptance, although not universal, in the early 1980s.

As with any surgical procedure, the success rate of stapedectomy was not 100%; with even a small percentage of failures (i.e., air-bone gap closure of >10 dB) in such a large pool of patients, there was a significant number of patients who were candidates for revision stapes procedures. Stapes surgeons soon discovered two important facts: The success rate of revision stapedectomy was not nearly as high as that of primary stapedectomy, and the incidence of significant sensorineural hearing loss, including dead ears, was significantly higher than the incidence associated with primary stapedectomy. Prominent otologists obtained air-bone gap closure within 10 dB in 50% or less of revision cases.²^–⁷ The incidence of significant postoperative sensorineural hearing loss ranged from 3% to 20%, with 14% having profound loss.^2,^3,^6,⁸

Glasscock ² and Sheehy³ and their associates and Lippy and Schuring^8,⁹ advocated leaving the oval window neomembrane intact and undisturbed, if possible, in revision cases to reduce the risk of severe sensorineural hearing loss, even though it may result in fewer patients with postoperative hearing improvement. Feldman and Schuknecht,⁴ Pearman and Dawes,¹⁰ and Derlacki⁵ reported opening the neomembrane to identify the vestibule and ensure correct prosthesis placement.

To diminish the risk of significant sensorineural hearing loss, the surgeon often would not open the oval window and vestibule, and would place a prosthesis on the existing oval window membrane. By doing so, the prosthesis would often rest on a thick fibrous oval window membrane or perhaps a residual bony footplate or new bone growth. Although this technique protected against sensorineural hearing loss, the lower incidence of closure of the air-bone gap to within 10 dB remained. The stark contrast of results of revision stapedectomy compared with primary stapedectomy, plus technologic advances in hearing aids, diminished the enthusiasm for revision stapes surgery in all but the most experienced hands.

Laser

Concomitant with the realization that revision stapedectomy surgery was not as successful as primary stapedectomy was the introduction of lasers in temporal bone surgery. The use of the laser in temporal bone surgery was the object of experiments in 1967 by Sataloff ¹¹ and 1972 by Stahle and colleagues.¹² The evolution of laser otologic surgery has been based on a mixture of clinical, experimental animal, and laboratory observations. In 1977, Wilpizeski¹³ examined argon and CO₂ lasers on monkeys by performing myringotomy, ossicular amputation, stapes fenestrations, lysis of stapedial tendon, and crurotomy. He noted damage to the organ of Corti in monkeys after using “excessive power.”

Escudero and associates ¹⁴ were the first to use a laser in human otologic surgery in 1977. They used the argon laser with a fiberoptic handpiece to tack temporalis fascia to tympanic membrane perforations. In 1979, Perkins¹⁵ presented a preliminary report of argon laser stapedotomy with excellent initial results in 11 patients. In 1980, DiBartolomeo and Ellis¹⁶ expanded argon laser applications in 30 patients for middle ear and external ear soft tissue and bony problems. In 10 patients, otosclerosis was corrected, including one revision case. In 1983, McGee¹⁷ reported on the use of argon laser in more than 500 otologic cases, 100 of which were primary stapedectomies. There were no laser-related complications in his study. In 1989, McGee¹⁸ reported an update on 2500 tympanomastoid procedures, of which 510 were primary stapedectomies. By comparing 100 consecutive laser stapedectomies with a previous 139 small fenestra stapedectomies using instruments, McGee found that the laser technique permitted much shorter hospital stay, less vertigo, and excellent hearing results (93% air-bone gap closure ≤10 dB at 6 months). This large study indicated the safety of argon laser use for stapedectomy, and yielded comparable hearing results and less vertigo. This clinical evidence is inconsistent with experimental animal studies, in which temporary changes of cochlear microphonics and saccular perforations were noted.^17,^19,²⁰ Clinical experience from several centers has illustrated the safe use of lasers in ear surgery; however, arguments and opinions persist regarding the best type of laser.²¹^–²⁵

Comparison of Lasers

The visible spectrum lasers were used initially because they were the only ones that were precise and accurate enough for stapes surgery. Similar to any tool or instrument, each type of laser has advantages and disadvantages. The visible-wavelength lasers (argon and KTP) have the practical advantage of precision because the aiming beam and the working beam are the same. The blue-green (argon) or green (KTP) aiming beam has clear, crisp margins, which allows extreme precision. Several years ago, some manufacturers of the KTP laser incorporated the use of a separate helium-neon laser aiming beam. This helium-neon aiming beam produces a red spot, and the margins are not as clear and crisp as the blue-green argon laser or green KTP laser. The alignment of the two beams (the helium-neon aiming beam and the visible laser beam) remains very precise because the wavelengths are similar (helium-neon is 632.8 nm wavelength).

The CO₂ laser beam is invisible; a separate laser aiming beam (helium-neon) is required. This aiming beam is coaxial and focuses in a different plane from the CO₂ laser beam because of the differences in wavelength of the helium-neon and CO₂ lasers. With these lasers, there is more potential for misalignment and a greater margin of error than with the KTP and argon lasers. It is crucial that the CO₂ laser and its helium-neon aiming beam be calibrated precisely and checked frequently during the procedure to ensure maximum accuracy.

The tissue absorption of the visible-laser energy is color dependent, and for the argon and KTP lasers, peak absorption is dark red. For lightly colored tissues, such as a neomembrane or bone, a significant amount of laser energy is reflected rather than absorbed. A practical solution involves placing a minute quantity of blood in the field, or applying several bursts (not in rapid succession) to get a dark char, increasing the laser energy absorption. With the CO₂ laser, this absorption is not a problem because the laser beam is invisible, and absorption by water and tissue is not color dependent.

The visible-wavelength laser beam can be carried by thin fiberoptic cables, which allow two modes of delivery: a micromanipulator attached to the microscope or a hand-held probe (Fig. 24-5). The hand-held probe provides greater angle of divergence of the laser beam (i.e., rapid deterioration of power density) and must be placed very close to the tissue. This instrument is directly in the operative field and can obstruct a portion of the visual operative field. With the micromanipulator, the angle of the laser beam is less divergent. It does not require an instrument in the field (except a suction for smoke plume removal), but does require the use of a “joystick” control mechanism to direct the beam. The CO₂

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