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Lasers in veterinary medicine—where have we been, and where are we going?
Vet Clin Small Anim 32 (2002) 495–515
Lasers in veterinary medicine—where have we been, and where are we going? Kenneth E. Bartels, DVM, MS Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, Oklahoma 74078, USA
The principles necessary for the concept of laser development were reported as early as the nineteenth century with Bohr’s theory of optical resonance. In 1917, Einstein proposed the concept of stimulated light emission. Finally in 1960, Theodore Maiman developed the first operational laser, which was a pulsed ruby laser [1]. His work was based on Albert Einstein’s explanation of stimulated emission of radiation, coupled with Townes’ and Schawlow’s 1958 work with optical masers [2]. Since then, much of the progress in laser technology followed weapons research or commercial applications in the communication and manufacturing industries. When the ‘‘Cold War’’ ended, increased initiatives by laser manufacturers, formerly dedicated to military applications, provided a tremendous stimulus for advancements in both industrial and medical laser technology. Since its medical use began, the laser has been and is still considered by many to be ‘‘a tool in search of an application.’’ Medical lasers of the past were cumbersome, expensive, and difficult to maintain. As biomedical laser technology merges with the economic reality of medicine, however, innovations and improvements in existing devices and development of new concepts will continue. New ideas and modifications of current laser technology will be essential to keep pace with changes in veterinary medicine. Relatively recent technologic introductions, including the development of diode lasers and fiberoptic delivery systems, as well as portable, affordable, and reliable carbon dioxide (CO2) lasers, are now available to veterinarians. Other types of lasers with different wavelengths and delivery parameters are also available, depending on clinical requirements. The vernacular or perceived concept of ‘‘using laser,’’ implying there is one perfect wavelength, or only one
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all-purpose system, is unacceptable. Because of specific properties, certain lasers may be uniquely suited to a given clinical task compared with other lasers or energy modalities. The use of lasers in veterinary medicine includes both nonsurgical and surgical applications. Surgical applications involve direct physical alteration or removal of target tissue. Laser surgical treatments are referred to as photothermal or photomechanical applications. Examples of commonly used photothermal applications include laser hyperthermia and laser tissue vaporization. An example of a photomechanical application is laser lithotripsy, in which laser light creates an acoustical shock wave used to break down urologic or biliary calculi. Nonsurgical applications include such techniques as laser biostimulation, diagnostic use including optical biopsy, and photodynamic (PDT) therapy. The potential for future development of additional applications, especially for noninvasive biological sensors, depends on the continued interest of all medical specialties, especially veterinary medicine. Although the recent development and use of biomedical lasers may be a significant step ahead of mechanical instrumentation, it falls short of what is needed to be considered as the optimal ‘‘light knife’’ for every surgical situation. Considering differences in laser–tissue interaction, it is still very uncertain whether an ‘‘ideal’’ laser wavelength will ever exist. The promised benefits, however, of lasers in general surgery are a combination of reduced morbidity, better overall clinical results, an eventual reduction in expense for patients/clients, more productive use of operating room facilities, and increased efficiency of the surgical staff. These advantages coupled with objective evaluations of current surgical instrumentation should place biomedical lasers at the forefront of twenty-first century medical technology. Discounting future use of free-electron lasers with multiwavelength variability, acceptance of biomedical use of lasers with a fixed-wavelength has depended more on cost, capability of fiberoptic delivery, portability, flexibility, ease of use, and dependability. Present-day medicine uses many different types of biomedical lasers. Each instrument is usually acquired for a specific purpose, such as dermatologic or endoscopic applications. Overall, laser energy can be an extremely precise method for tissue removal or cellular destruction. Medical lasers are expensive and require a dedication to proper use and objective evaluation. Lasers in common use today are the CO2, neodymium:yttrium aluminum garnet (Nd:YAG), argon (Ar), potassium titanyl phosphate (KTP) or frequencydoubled YAG, ruby, diode, holmium (Ho:YAG), erbium (Er:YAG), and dye lasers [3]. The following general description can be used as a guide to medical lasers. In no way should it be considered a complete discussion. Laser types, wavelength preference, energy parameters, and delivery devices are changed frequently, because they are closely aligned with changes in today’s technologic advancements in computer hardware and software.
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Medical lasers Carbon dioxide laser (10,600 nm) The CO2 laser was one of the first medical lasers used for tissue ablation. It was developed in 1964 by C.K.N. Patel at Bell Laboratories [4]. At 10,600 nm (10.6 lm), the wavelength is ideal for cutting and vaporization because it is highly absorbed by water. It can cut tissue cleanly when the beam is focused onto the target and can debulk tissue by photovaporization when the beam is defocused. It is considered a far-infrared (IR) wavelength even though it emits at the short wavelength end of the far-IR spectrum, and the beam is invisible to the eye. The CO2 laser produces light that does not transmit through quartz or glass fibers. Currently, two basic types of surgical CO2 lasers are available: sealed tube and freeflowing devices. Sealed-tube lasers have a ‘‘shelf life’’ (i.e., the expected life of the laser tube before it needs recharging with the gain medium). The anticipated shelf life of a sealed metal tube laser can be longer than 10 years, depending on the technology used by the manufacturer and the amount of clinical use. Sealed-tube technology uses either direct electrical current (DC) or radiofrequency (RF) to excite the gain medium in a CO2 laser. RF excitation sealed-tube CO2 lasers have been growing in popularity because they can generate more power and can work well at lower powers. Sealed-tube CO2 lasers are mechanically and electronically simpler, tend to be smaller, produce less noise than free-flowing lasers, and are capable of emitting up to 20 to 30 W. A free-flowing CO2 laser requires a replaceable external gas cylinder containing a special mixture of gases as the gain medium (Fig. 1). CO2 is the light emitter, nitrogen helps excite CO2, and helium is used as a buffer gas for heat transfer [5]. Free-flowing lasers have been commonly sold on the secondary market (used lasers from human hospitals) to veterinarians and usually require consideration of purchasing a maintenance contract in addition to the cost of the device. They are usually large, more complex devices that require periodic manufacturer’s maintenance for proper alignment and power output, but have capabilities of producing more than 100 W of CO2 laser power. CO2 lasers range in size and power from very large units (>100 W) to small devices delivering 20 W or less that are more compatible for small animal surgery. Knowledge of laser tissue–interaction and laser physics may deter the informed laser surgeon from purchasing high-power CO2 lasers (>20 W) for most procedures in small animal surgery. The ability to exceed 20 W when performing larger ablative procedures may be an advantage. Historically, CO2 lasers have been used as a continuous wave (CW) laser. Control of energy delivery and pulse characteristics (superpulse) or using temporal/spatial scanners is more important, however, for ablative procedures requiring extreme precision more than is laser power alone.
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Fig. 1. Articulated arm delivery system in a free-flowing, DC stimulated carbon dioxide laser (Sharplan Model 743, Sharplan Lasers, Inc., Allendale, NJ). Note: This laser is no longer manufactured. Sharplan Lasers, Inc. is now part of Lumenis, Inc., Santa Clara, CA.
Diode lasers Developed in 1972, semiconductor diode lasers have progressed tremendously in concert with other aspects of medicine. Engineering and commercial specifications have allowed advancement of devices with wavelengths varying from approximately 635 to 980 nm. The diode lasers with the most medical significance are gallium aluminum arsenide (GaAlAs) or indium aluminum arsenide devices (InGaAs) at 780 to 980 nm. Laser light output is generated when electric current is passed through the diode. Individual diodes emit light from the edge of a wafer or from their surface. Standard ‘‘single-emitter’’ diodes can be combined on the same semiconductor chip
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to achieve very high output power from a device that is still very small. High-power diode lasers generate laser emission with an electrical-to-optical conversion efficiency of 30% to 50%, making them the most efficient lasers available. These direct diode laser systems collect diode emission and channel it onto tissue using an appropriate delivery system [6]. Coupling diode laser energy directly to fiberoptic delivery advices is a tremendous advantage when considering endoscopic application and minimally invasive surgical techniques. Therapeutic devices that use semiconductor diode lasers were first approved for surgical use in this country in 1989. Diode lasers (1–4 W) are also used for photocoagulation of retinal and other ocular tissues and have been used for ophthalmologic applications since approximately 1984. The compact size and high efficiency offer significant ergonomic and economic advantages. High-power, semiconductor diode lasers appropriate for other surgical applications have been recently introduced for a variety of uses (Fig. 2). These lasers currently provide up to 15 to 60 W at 810 nm or 980 nm; wavelengths that can penetrate deeply into most types of soft tissue and can produce tissue interactions comparable to the Nd:YAG laser (1064 nm) [7,8]. Considering whether a diode laser emitting 810-nm wavelength is superior or inferior to a diode laser emitting 980 nm is an exercise in physics and laser-tissue interaction. Furthermore, the theoretical differences (980 nm is somewhat higher in its water absorption coefficient characteristic than 810 nm) on a tissue target should be negligible to most laser surgeons. More importantly, the delivery system, a fiber in contact or noncontact mode, and the laser energy parameters will provide a greater impact on perceived tissue interaction. Although diode lasers are especially useful for some surgical applications, the direct diode systems used today have some limitations. The beam quality
Fig. 2. Diode laser (Diomed, Inc., Andover, MA).
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of high-power diode lasers limits the amount of power that can be coupled efficiently through small core fibers. Most high-power (25–60 W) directdiode systems use 400 to 600 lm diameter fibers. Fiber core sizes in the 200 to 300 lm range may be needed in the near future for some applications using miniaturized endoscopes. Narrow wavelength coverage is another shortcoming of direct-diode laser systems. Currently, diode laser materials are practical for generating high-output powers at wavelengths in the 635 to 2000 nm range. High-power devices that can generate yellow, green, blue, and even near-ultraviolet (UV) and mid-IR wavelengths may be possible in the future. Diode lasers are continuous-wave devices with little energy-storage capacity. They cannot produce the high peak power needed for some medical applications, such as lithotripsy, where production of a photoacoustic effect is required or in some dermatologic applications where high peak powers are required for therapeutic results. To overcome this limitation and still preserve the advantages of using diode lasers (i.e., size, reliability, ruggedness), diodepumped solid-state lasers will become more important [6]. Diode lasers can be used with fiber delivery accessories in noncontact mode for tissue coagulation (power requirements of 5–25 W CW), and for non-contact tissue vaporization (power requirements of 25–60 or greater W CW). For precise incisional applications, ‘‘hot-tip’’ quartz, sapphire, or so-called ‘‘dual use’’ fibers can be used (power requirements of 5–20 W CW) in contact mode. As mentioned, diode lasers can be used for many of the same applications as 1064 nm CW Nd:YAG lasers [9]. Surgical diode lasers offer considerable advantages, however, compared with the Nd:YAG laser. They are smaller, lighter, require less maintenance, are extremely userfriendly, and can be more economical. Some predict that prices for diode lasers will eventually drop to the point where they may be competitive with high-end electrosurgery devices [10]. Other applications for diode laser energy include chromophore-enhanced tissue ablation and coagulation, tissue fusion or laser welding, and PDT therapy [1,11,12]. Diode laser wavelengths of 805 to 810 nm have been used for tissue welding because applications have been centered around the peak absorption spectrum of indocyanine green (780–820 nm), which is the selective chromophore used in a fibrinogen-based solder [11]. Finally, the small, convenient size coupled with reliability and user friendliness has also focused extensive diode laser development for applications in PDT therapy [8]. Nd:YAG laser (1064 nm) The Nd:YAG or ‘‘YAG laser’’ is a solid-state laser that differs from the CO2 laser because the wavelength allows transmittance though tissue, in addition to surface absorption. Powers of up to 100 W can be delivered through small-core optical fibers that can easily be inserted through the accessory channels of standard gastrointestinal (GI) endoscopes. The Nd:YAG laser was one of the first lasers to be used in veterinary medicine
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because of its fiber delivery and endoscopic accessibility. Transendoscopic use of the Nd:YAG laser in the horse has been and continues to be an effective method for treating upper airway obstruction and for some endoscopic urogenital procedures [1,13]. It has also been used for soft tissue procedures in small animals, including prostatic resection, perianal fistula ablation, vaporization of facial tumors, and even ablation of a brain tumor [13–17]. Advantages of approaching conditions with limited anatomic access, performance of upper respiratory and urogenital procedures with local rather than general anesthesia, and a decrease in morbidity time have provided the motivation for continuing this extremely successful effort. Because the Nd:YAG laser has less specific absorption by water and hemoglobin than the CO2 and Ar lasers, the depth of thermal injury can exceed 3 mm in most tissues, which can be useful for coagulation of large volumes of tissue [18]. Rapid tissue vaporization in noncontact (free-beam) mode is possible with a bare fiber; however, collateral thermal injury may be substantial. Soft tissue applications using noncontact or free-beam mode usually require power levels approaching 100 W. The use of various temporal emission modes, including CW and pulsed modes (free-running, Q-switched, and mode-locked), allows extreme versatility in power delivery when using the Nd:YAG laser for many clinical applications in soft tissue surgery, ophthalmology, and urology. Frequency-doubled Nd:YAG or KTP laser (532 nm) The frequency-doubled Nd:YAG laser, also known as KTP (potassium titanyl phosphate) lasers, emits a visible green light and is basically equivalent to the Ar lasers used in many surgical and dermatologic applications. Present-day clinical applications use photothermal reactions to coagulate, vaporize, or cut soft tissue. Absorption of the 532-nm wavelength is negligible in water. The visible green laser beam passes through water and saline with virtually no absorption, which is extremely important in a wet or flooded surgical field. Because the 532-nm wavelength is strongly absorbed by the oxyhemoglobin component of blood, it can be used efficiently and precisely to heat blood-perfused soft tissue. In whole blood, absorption depth at 532 nm is approximately 0.5 mm. This strong absorption can actually be a problem if the target tissue is covered with blood, because the laser energy will be severely attenuated before it reaches the tissue’s surface [19]. One currently manufactured frequency-doubled Nd:YAG laser (Model 800 Series Laserscope, Laser Scope, Inc., San Jose, CA) can provide either 532 nm or 1064 nm through the same optical fiber. The 532-nm wavelength is used for precise cutting and vaporization. By switching to the 1064-nm wavelength, the same laser and fiber delivery system can be used for deep tissue coagulation or rapid vaporization when only modest surgical precision is required. The laser is also used by the manufacturer as a dye laserpumping source to control PDT interactions.
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Ar laser (458 and 524 nm) The blue-green Ar laser is strongly absorbed by hemoglobin and is especially useful in nonbleeding vascular lesions when precision and minimal penetration (approximately 1 mm) is required. Although heavily absorbed by blood, Ar laser energy can be readily transmitted through water, gastric fluid, aqueous or vitreous humor, and urine. Consequently, this laser can be used to precisely cut, vaporize, and superficially coagulate soft tissue that is well perfused with blood. Treatment of hemoglobin-poor tissue generally relies on the production of carbonized tissue or ‘‘char’’ for efficient heating of tissue. Bare fibers can be used in contact or noncontact modes for cutting, vaporization, or coagulation. Although older versions often lacked enough power to vaporize target tissue, newer 15-W Ar lasers are more efficient for vaporization and cutting applications. Ruby laser (694 nm) Although first investigated for its medical potential by Maiman in 1961, the ruby laser has not received widespread use. It was resurrected in the late 1980s as a medical device for removing tattoos and birthmarks. The 694-nm wavelength is absorbed strongly by dark pigments, such as melanin, and the pigments used for making tattoos, but only weakly by hemoglobin. Therefore, the visible ruby laser wavelength can penetrate several millimeters into skin without being severely attenuated by blood. Because of this fact, the ruby laser is used in selective photothermolysis procedures for removing tattoos [19]. Holmium lasers (2100 nm) and erbium lasers (2900 nm) Clinical solid-state Ho:YAG lasers have appeared recently for arthroscopic surgery, general surgery, laser angioplasty, and thermal sclerostomy. Other applications include laser discectomy, removal of sessile polyps in the GI tract, and otorhinolaryngeal procedures. The main benefit of the Ho: YAG laser is its ability to cut and vaporize soft tissue somewhat like a CO2 laser, with the added advantage that holmium energy can be delivered through flexible, low OH, quartz, or polyamide optical fibers. Good surgical precision and control can be obtained with a bare optical fiber. Unlike visible wavelength lasers, and again like the CO2 laser, photothermal interactions with the Ho:YAG laser do not rely on hemoglobin or other pigments for efficient heating of tissue. The water component of tissue is responsible for absorbing Ho:YAG laser energy (2100 nm) and for converting it to heat. The depth of absorption is quite shallow at approximately 0.3 mm. When cutting or vaporizing tissue, actual zones of thermal injury vary from 0.1 to 1 mm, depending on exposure parameters and the type of tissue. These small thermal necrosis zones provide better surgical precision and adequate hemostasis. Current holmium instruments are flashlamp-pumped systems. The active laser medium consists of a chromium-sensitized YAG host crystal doped
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with holmium and thulium ions. This active medium is referred to as Tm, Ho, Cr:YAG, or THC:YAG and is common to all Ho:YAG laser medical devices. Unlike the CO2 laser, higher power Ho:YAG lasers cannot operate in a CW mode at room temperature. The relatively low pulse rates (5–20 Hz with 250–350 lsec pulses) available from most Ho:YAG lasers may be considered a disadvantage because cutting may be slow or may result in jagged tissue edges during incisional applications. In addition, at higher pulse energies (‡1 J), considerable amounts of acoustic or photomechanical energy are generated in tissue. An audible acoustical ‘‘pop’’ may be generated and actually heard during laser application. Acoustic energy may be considered an advantage, however, when using holmium energy for photodisruptive (photothermal/photomechanical phenomena) procedures, such as lithotripsy of gallstones or urologic calculi [20]. Another mid-IR, solid-state laser is the Er:YAG laser. Its wavelength (2900 nm) is more strongly absorbed in water. Dental applications, including hard tissue ablation (Food and Drug Administration [FDA]-approved) and incisional applications, are considered appropriate for this wavelength. Hemostatic ability is minimal, however, and lack of readily available delivery fibers has hindered its potential use. Although somewhat brittle, sapphire fibers have been used for Er:YAG energy delivery [19]. Dye laser (variable wavelength with dyes—400 to 1000 nm) Developed at the IBM Laboratories in 1966, dye lasers offer an advantage of ‘‘tunability’’ of wavelength over a considerable range to obtain absorption coefficients and tissue interaction characteristics applicable to multiple medical specialties, including oncology, ophthalmology, urology, and especially dermatology [5]. Pulsed and CW dye lasers use an active laser medium, consisting of an organic dye dissolved in an appropriate solvent. For the dye laser to work, the dye solution must be recirculated at high velocity through the laser resonator. Dye lasers are useful for medical applications because they can generate high-output powers and pulse energy at wavelengths throughout the visible wavelength spectrum (400–700 nm). They are usually pumped by Ar lasers, flashlamps, or a frequency-doubled Nd:YAG laser. Dye lasers have been used for lithotripsy of biliary and urologic calculi (pulsed), activating photosensitizers for PDT therapy (CW), ophthalmologic operations (pulsed or CW), and dermatologic applications (pulsed and CW) including treatment of birthmarks and removal of tattoos [19].
Laser delivery systems A delivery system is defined as the optical hardware needed to transfer energy from the laser to the treatment site. Devices for guiding laser beams to the patient include articulated arms with internal mirrors, hollow waveguides, and optical fibers. Articulated arms and hollow waveguides are used
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with laser wavelengths (2.8–10.6 lm) that cannot be transmitted through conventional glass or quartz fiber optics. The CO2 laser produces far-IR light and is included in this category. Currently, CO2 laser light must be delivered to the tissue through a hollow tube called an articulated arm or through a hollow waveguide. The articulated arm is a tube with mirrors aligned in joints that reflect light into a system of lenses that create a collimated and focused beam. The articulated arm can be somewhat fragile and difficult to manipulate in a small operating room; however, some manufacturers have worked diligently to minimize this limitation. Because the laser beam is invisible to the eye, a low-power red helium-neon (633 nm) or diode (635–660 nm) laser is usually used to provide an aiming beam. CO2 lasers using articulated arms for energy delivery must be realigned periodically during scheduled maintenance checks (Fig. 1). As mentioned, laser energy delivery through an articulated arm has inherent disadvantages because of size of the arm, portability, and its inability to be used for minimally invasive procedures through endoscopic visualization. The addition of attached semirigid hollow waveguides to some CO2 systems enhances their flexibility for performing many laser procedures. Waveguides can direct CO2 energy delivery economically and efficiently. Hollow waveguides are manufactured from high-quality stainless steel, flexible glass tubes, and other materials that have reflective interiors to direct energy. Although laser energy decays as it traverses the length of the hollow waveguide, software included in some CO2 devices will compensate for this energy loss to provide for consistent energy delivery at the termination. Hollow waveguides capable of transmitting CO2 energy from a compact, robust laser device have been one of the major influences to drive laser technology into veterinary medicine. Although this particular laser (AccuVet CO2 Laser, Lumenis, Inc., Santa Clara, CA) delivers a noncollimated beam at the tissue target site, various tapered tips (0.3–1.4 mm in diameter) concentrate and direct the energy, which increases or decreases the power density, depending on the tip diameter (AccuVet CO2 Laser, Lumenis, Inc., Santa Clara, CA) (Fig. 3). Although there are still drawbacks because currently available hollow waveguides are not readily endoscopically deliverable, improved waveguide development and advancing CO2 laser fiber technologies should overcome this limitation in the forseeable future. The availability of functional and inexpensive optical fibers for laser delivery has played a crucial part in the acceptance of lasers for medical applications. The fibers used in laser medical delivery are most often composed of quartz glass and have diameters ranging from 0.1 to 1 mm. Laser energy is contained within and follows the bends and curves (total internal reflection) of the fiber within certain limits (eg, numeric aperture), until it reaches the tip where it exits [21]. Although configurations of fiber tips (eg, flat, orb, chisel, conical) and their ability to transmit energy are a science in their own right, delivery parameters are primarily based on two factors: contact (hot-tipped) delivery or noncontact delivery (Fig. 4). In general,
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Fig. 3. Hollow waveguide delivered CO2 laser (AccuVetTM Carbon Dioxide Laser, Lumenis, Inc., Santa Clara, CA).
lower power (5–20 W) is delivered through contact tips. Contact tip fibers include sculpted quartz fibers, contact-tipped sapphire fibers, metal-capped fibers, temperature-controlled bare fibers, and dual effect (used both in contact and noncontact modes) fibers. Although the use of contact tips for endoscopic application is widely accepted, some tips are too large to insert through flexible endoscopes. Most sculpted or cleaved quartz fiber tips used in contact mode must be ‘‘carbonized’’ (i.e., contact application of fiber to target tissue or a sterile wooden tongue depressor to form a layer of carbonized particles so the ‘‘hot tip’’ becomes effective in tissue). Finally, the ability to guide laser energy, fiber thinness, flexibility, economy, and ruggedness makes quartz optical fibers essential for endoscopic applications. Laser scanners or pattern generators are available as accessories to CO2 and some solid-state lasers, such as the Er:YAG or KTP lasers. They were introduced to convert a CW or pulsed beam of laser energy into a scanned,
Fig. 4. Configurations of laser optical delivery fibers used with diode and Nd:YAG lasers.
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shuttered beam to reduce tissue carbonization and to enhance precise vaporization of tissue. Laser scanners can increase the speed of treatment and can provide better control of beam overlap. They provide a nonaligned treatment pattern and decrease laser beam ‘‘dwell time’’ on specific target areas, which allows the thermal energy in adjacent tissue to cool adequately between pulses [19]. Computerized or robotized scanning devices are used in some models of the larger lasers applied in aesthetic laser surgery. Smaller, less expensive flashscanners may scan a 100-lm focused beam in a spiral pattern on the target tissue with the aid of rapidly oscillating mirrors or a mechanical device that rotates the waveguide termination tip in the laser handpiece (Fig. 5). Biomedical lasers in veterinary medicine Early reports concerning the use of lasers for medical applications involved animals—either as experimental models or as clinical veterinary patients. In 1968, the removal of a vocal cord nodule in a dog demonstrated one of the first practical clinical applications of the CO2 laser as a precision surgical instrument [22,23]. Many other biomedical laser research teams have also relied on animal models for determining initial laser parameters and efficacy. At this stage of research, veterinarians can and should be the main catalyst for the advancement of biomedical laser technology and laser-based therapeutic techniques with potential human application. Often, objective protocols will prove some ideas and applications as impractical for veterinary clinical purposes. Although the idea or development of a new device may be inapplicable
Fig. 5. CO2 laser scanner used for char-free tissue ablation (NovaScanTM, Lumenis, Inc., Santa Clara, CA).
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to ‘‘mainstream veterinary medicine’’ because of the current economic limitations, our participation as an equal partner on the biomedical research team places us in a strategic position to be leaders in today’s total health care establishment. Since their introduction into veterinary medicine in the 1970s, highpowered surgical lasers have been used primarily for photothermal procedures to vaporize or ablate the target tissue. Techniques have often been described using a variety of device settings that may or may not take into account objective evaluation of the technology and give appropriate attention to laser tissue–interaction phenomenon. It is imperative that clinicians learn the fundamentals and the applied aspects of the technology for clinical practice. New procedures involving other aspects of laser energy interaction include laser lithotripsy and low-level laser photobiostimulation. Laser ablation of intervertebral discs has also become a procedure using minimally invasive techniques through fiberoptic energy. Low-level laser therapy (photobiostimulation) Biostimulation using light energy, which is usually considered a photochemical effect, has attracted interest in both the clinical and research arenas in both veterinary and human medicine. To many scientists and clinicians, the idea that low-intensity light energy (<500 mW average power) can promote and upgrade metabolic processes that result in tissue repair and pain relief is unbelievable and akin to ‘‘snake oil’’ practice [24]. At a minimum, it is on the fringes of accepted practice in the unconventional aspects of complementary and alternative veterinary medicine. Yet, reports from almost every region of the world indicate that low-intensity lasers promote the repair process of skin, tendons, ligaments, bone, and cartilage in experimental animals and wounds of various etiologies in humans. Reports that suggest the contrary complicate the matter, creating the present situation in which laser or low-energy photon therapy is viewed with extreme skepticism. Several manufacturers throughout the world have developed devices for using low-level laser energy in human medicine. Marketing efforts in the United States have been directed to veterinarians since the FDA has not yet approved this type of laser therapy for human use (Fig. 6) [25]. Biostimulation, or low-energy photon therapy, is defined as nonthermal interaction of monochromatic radiation with a target site [6]. Although the physiologic interaction for this type of energy application on tissue is still not understood, low-energy lasers have been reported to modulate various biologic processes, such as mitochondrial respiration or adenosine triphosphate synthesis, to accelerate wound and joint healing, and to promote muscle regeneration [26,27]. In addition, pain attenuation or pain removal has been reported using this type of low-energy photon therapy. Recommended veterinary applications include first aid treatment for traumatic and surgical wounds, strains; musculoskeletal pain and dysfunction; rheumatoid arthritis
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Fig. 6. Low-level laser energy biostimulation and therapy device (Veterinary Therapy Laser, Model PLLSD0009, American Veterinary Laser, Farmington Hills, MI).
and osteoarthritis; neurologic applications, such as neuralgia, and other nerve injuries; and sport injuries ranging from contusions to muscle tears [28]. Devices used for biostimulation vary in their optical properties. Often, laser wavelengths are in the visible or near-IR range. Laser power output is not sufficient to cut or vaporize tissue. Advocates have also reported similar results using less expensive light sources (wider wavelength bands, noncoherent light sources). More research is being conducted in this area as low-level laser or photon therapy becomes less controversial, and peerreviewed case reports or projects are being reported in the veterinary literature [29–31]. Methods are also being formulated to provide objective and replicable results to prove the efficacy of this therapy. It is incumbent on the manufacturers of these devices to support these types of investigations rather than promote the technology based solely on anecdotal reports. Laser lithotripsy Gastrointestinal and urologic applications have primarily involved uses in soft tissue surgery. Recently, however, the application of laser photothermal and photomechanical energy through endoscopically delivered optical fibers to break down urologic and biliary calculi has been approved and practiced in humans. Much of the preliminary work, including mechanism of action and early clinical reports, originated from veterinary medical applications. Currently, the flash-lamp pulsed dye laser and the Ho:YAG lasers are used for laser lithotripsy in veterinary medicine.
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The pulsed dye laser operates at a wavelength of 504 nm with 1.0 lsec pulses. Urolith fragmentation is accomplished by a photoacoustic and mechanical effect through formation of an energy plasma that consists of a rapidly expanding cloud of generated electrons and ions. The plasma absorbs additional laser energy and generates a symmetrically expanding cavitation bubble. Urolith fragmentation is caused by a rapid collapse of the cavitation bubble, creating a strong acoustic shockwave that exceeds the tensile strength of the urolith (Fig. 7). Because energy dissipates rapidly with increased distance, the pulse dye laser is associated with low risk of soft tissue damage. Energy absorption of this laser is related, however, to composition of the urolith, which decreases its effectiveness on light-colored calculi. Flash-lamp pulsed dye lasers are high maintenance devices and have virtually been replaced by the Ho:YAG laser as a laser lithotripter. The mechanism of Ho:YAG lithotripsy is mainly photothermal. Unlike the dye laser, the pulsed energy of the Ho:YAG laser is strongly absorbed by water. Direct contact of the fiber perpendicular to the calculus is required for effective use in liquid media because a cavitation bubble is formed by vaporization of water molecules. The bubble is pear shaped and undergoes asymmetric expansion and collapse. This results in an acoustic emission and shockwave generation. Coupled with an irregularly shaped cavitation bubble, a vapor channel is formed that effectively conducts laser energy to the stone (‘‘Moses Effect’’). Consequently, the surface of the calculus is ablated by direct laser radiation and a rapid increase in surface temperature. Because of a longer pulse duration (250–350 ms), vaporization of water molecules is continuous, and expansion of interstitial water and vapor results in surface ejection of fragments from the calculus (Fig. 8). Calculi composition does have an effect on Ho:YAG lithotripsy efficiency [20,32]. In addition, efficacy of lithotripsy for urologic calculi seems to vary among animal species, which also may result from differences in stone composition.
Fig. 7. Flashlamp pulsed dye laser lithotripsy of a urologic calculus (in vitro).
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Fig. 8. Ho:YAG laser lithotripsy of a biliary calculus (in vitro).
Considering current technologies, lithotripsy using the flash-lamp pulsed dye laser seems to be more successful in the horse than when using the Ho:YAG laser. The Ho:YAG laser has been successfully used, however, in most other animal species (i.e., dog, pig, cow, llama), and considering its solid-state characteristics, reliability, portability, and cost, it will most likely remain the primary device for laser lithotripsy in veterinary medicine. Laser intervertebral disc ablation A percutaneous approach for photothermal ablation or vaporization of the nucleus pulposus in lumbar discs using laser energy has been reported as a treatment of intervertebral disc disease in humans and dogs [33]. Although the Nd:YAG, KTP, and diode lasers have also been used for this minimally invasive procedure, the Ho:YAG laser has advantages over other approved lasers with different wavelengths. As mentioned previously,
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because the Ho:YAG wavelength is strongly absorbed by water, depth of tissue penetration is limited, and zones of necrosis and collateral thermal effects are minimized because of the high water content of the nucleus pulposus. Using the dog as a model, investigators have shown that acute and chronic histopathologic effects of percutaneous Ho:YAG laser disc ablation on neighboring tissue are minimal. Proponents of laser discectomy claim positive results in humans are related to a decrease in intradiscal pressure caused by a decrease in volume of the nucleus pulposus after ablation. Further disc extrusion also can be prevented; however, disc ablation is not effective when sequestration of a herniated fragment has occurred. Surgical fenestration of thoracolumbar intervertebral discs in dogs has been recommended primarily as a prophylactic procedure to prevent further herniation of nucleus pulposus from a partially herniated disc and exacerbation of associated clinical signs. The procedure should also reduce the chances of subsequent herniation of other discs in seven to eight statistically significant locations. Considered a major surgical procedure by most veterinary surgeons, disc fenestration has the potential for postoperative complications, including pneumothorax, spinal cord and nerve injury, and hemorrhage. Percutaneous Ho:YAG laser ablation of canine thoracolumbar intervertebral discs has the advantage of being a minimally invasive procedure, which can decrease postoperative complications, shorten recovery time, and reduce medical expenses (Fig. 9A, B). On the basis of efficacy and safety of a preliminary clinical study [34], an additional 250 cases involving dogs diagnosed with thoracolumbar intervertebral disc disease have undergone prophylactic percutaneous laser disc ablation from the tenth thoracic to the fourth lumbar vertebra (T10-11 to L3-4) [K.E. Bartels, et al; work in Progress]. A recurrence rate of less than 5% coupled with minimal postablation complications over an eight-year period has provided the incentive to continue using laser disc ablation as a viable alternative technique to surgical fenestration. In addition, laser disc ablation of the cervical and lumbosacral areas may also prove to be a viable technique for the future.
Future innovations The use of lasers in medicine is an exciting treatment modality that will continue to produce innovative and new methods for managing diseased tissue. Research focused on basic laser–tissue interaction and selective tissue destruction will become increasingly important. Orthopedic use of biomedical lasers in veterinary medicine has been somewhat limited. The CO2 laser has been used for ablation of methylmethacrylate and has the potential to be beneficial during revision of total hip prostheses [35]. As delivery methods improve and as devices with appropriate wavelengths (Ho:YAG—2.1 lm; Er:YAG—2.8 lm) become more economically available for veterinary use,
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Fig. 9. Ho:YAG laser ablation of thoracolumbar intervertebral discs. (A) Seven myelographic (20 gauge–21/2 in) needles are percutaneously inserted under fluoroscopic guidance into intervertebral disc spaces T10-11 to L3-4. (B) Ho:YAG laser fiber is inserted through each needle to the level of the nucleus pulposus, and the disc material is vaporized or undergoes coagulative change.
orthopedic applications will undoubtedly increase as they have in human medicine for cartilage reshaping and ablation through arthroscopic visualization. The CO2 laser also has potential for use in open surgical procedures where precise ablation is necessary, such as during joint exploration [36,37]. The use of lasers as diagnostic tools and sensors is one of the fastest growing branches of biomedical laser development. Clinical applications involving noninvasive recognition of malignant cells, abnormal tissue, or abnormal metabolites have tremendous potential. Use of available and future laser diagnostic technology could have a significant impact on the veterinary profession. Blood, urine, or tissue can be illuminated by a laser beam, and by analyzing the reflected or luminescent light collected and transmitted by a second fiber, information is obtained about that biologic fluid or tissue [33]. Because technical knowledge and instrumentation for laser surgery are expanding almost exponentially, the availability of equipment is also
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expanding as rapidly. With increased use, it is essential that our current clinical and research activities accurately reflect responsible medical and scientific use. Strategies or ‘‘gimmicks’’ in the form of new and unique equipment and miracle cures can attract individuals interested in offering special treatments that have no proven benefit. An objective and practical approach to laser surgical procedures in veterinary medicine is essential if the total beneficial potential is to be realized. ‘‘Zap and vaporize’’ techniques coupled with a ‘‘burn and learn’’ philosophy can do potential harm to patient and operator and can outweigh any beneficial effect. These concepts have no place in the objective use of lasers in veterinary medicine. A concerned effort must be made to evaluate the use of a laser for its potential patient benefit, rather than portraying it as a miracle device of the twenty-first century that is advertised on an illuminated billboard in front of a hospital. Although the use of biomedical lasers has created an entirely new definition for performing surgery, a surgeon’s knowledge of pathophysiology and technical expertise must be the primary factor to determine whether a laser should be used for a particular surgical procedure in lieu of more conventional approaches. Finally, the use of any new technology, including the application of biomedical lasers in veterinary medicine, does not replace the basic issues and essential rules of a surgical practice, such strict aseptic technique, appropriate use of instruments, and good surgical judgment.
Summary Future use of lasers in medicine depends on the active participation of veterinarians in the inception and development of new devices that meet the needs of the entire medical profession. The sensible clinical approach that must be taken every day in the practice of veterinary medicine equips the veterinarian with a unique ability to understand the practical applications of biomedical lasers. Veterinary medicine can and should be in the forefront during these exciting times, adding an essential dimension to development of this twenty-first century technology.
References [1] Tullners EP. Transendoscopic contact neodymium:yttrium aluminum garnet laser correction of epiglottic entrapment in standing horses. J Am Vet Med Assoc 1990;144:1971–80. [2] Itzkan I, Drake EH. History of lasers in medicine. In: Arndt KA, Dover JS, Olbricht SM, editors. Lasers in cutaneous and aesthetic surgery. Philadelphia: Lippincott-Raven; 1997. p. 3–10. [3] Katzir A. Medical lasers. In: Lasers and optical fibers in medicine. San Diego, CA: Academic Press; 1993. p. 180–209. [4] Patel CKN, McFarlane RA, Faust WL. Selective excitation through vibrational energy transfer and optical maser action in N2-CO2. Phys Rev 1964;13:617–9.
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[5] Hecht J. Carbon dioxide lasers. In: The laser guidebook. New York: McGraw-Hill; 1992. p. 159–83. [6] Manni JG. High-power diode-based lasers for medical applications. Biophotonics Int 1997; 1:40–6. [7] Judy MM, Matthews JL, Aronoff BL, et al. DF: soft tissue studies with 805 nm diode laser radiation: thermal effects with contact tips and comparison with effects of 1064 nm Nd:YAG laser radiation. Lasers Surg Med 1993;13:528–36. [8] Wieman TJ, Fingar VH. Photodynamic therapy. Surg Clin N Am 1992;72:609–22. [9] Taylor DL, Schafer SA, Nordquist RE, et al. Comparison of a high power diode laser with the Nd:YAG laser using in situ wound strength analysis of healing cutaneous incisions. Lasers Surg Med 1997;21:248–54. [10] Bartels KE, Zediker MS. Biomedical lasers at the cutting edge. Photonics Spectra 1993;27: 92–7. [11] Bartels KE, Morton RJ, Dickey DT, et al. Use of diode laser energy (808 nm) for selective photothermolysis of contaminated wounds. In: Prog Biomedical Optics (International Society for Optical Engineers Proceedings Series), Lasers in surgery: advanced characterization, therapeutics, and systems V, (2395). 1995. p. 602–6. [12] Treat MR, Oz MC, Bass LS. New technologies and future applications of surgical lasers. The right tool for the right job. Surg Clin N Am 1992;72:705–47. [13] Tate LP, Sweeney CL, Bowman KF, et al. Transendosocpic Nd:YAG laser surgery for treatment of epiglottal entrapment and dorsal displacement of the soft palate in the horse. Vet Surg 1990;19:356–63. [14] Ellison GW, Bellah JR, Stubbs WP, et al. Treatment of perianal fistulas with Nd:YAG laser. Results in twenty cases. Vet Surg 1995;24:140–7. [15] Feder BM, Fry TR, Kostolich M, et al. Nd:YAG laser cytoreduction of an invasive intracranial meningioma in a dog. Prog Vet Neurol 1993;4:3–9. [16] Hardie EM, Stone EA, Spaulding KA, et al. Subtotal canine prostatectomy with the neodymium:yttrium aluminum garnet laser. Vet Surg 1990;19:348–55. [17] Shelley BA, Bartels KE, Ely RW, et al. Use of the neodymium:yttrium aluminum garnet laser for treatment of squamous cell carcinoma of the nasal planum in a cat. J Am Vet Med Assoc 1992;201:756–8. [18] Swaim CP, Mills TN. A history of lasers. In: Krasner N, editor. Lasers in gastroenterology. New York: Wiley-Liss; 1991. p. 3–23. [19] Fuller TA, Intintoli A. Laser instrumentation. In: Arndt KA, Dover JS, Olbricht SM, editors. Lasers in cutaneous and aesthetic surgery. Philadelphia: Lippincott-Raven; 1997. pp. 52–64. [20] Spindel ML, Moslem A, Bhatia KS, et al. Comparison of holmium and flashlamp pumped dye lasers for use in lithotripsy of biliary calculi. Lasers Surg Med 1992;12: 482–9. [21] Katzir A. Fiberoptic diagnosis. In: Lasers and optical fibers in medicine. San Diego (CA): Academic Press; 1993. p. 180–209. [22] Jako GJ. Laser biomedical engineering: clinical applications in otolaryngology. In: Goldman L, editor. The biomedical laser. Technology and clinical applications. New York: Springer-Verlag; 1981. p. 175–98. [23] Jako GJ. Laser surgery of the vocal cords; an experimental study with the carbon dioxide laser on dogs. Laryngoscope 1972;82:2204. [24] Enwemeka CS. Quantum biology of laser photostimulation [editorial]. Laser Ther 1999; 11(2):52–4. [25] Felten RP. FDA status of biostimulation lasers. In: Abstracts of American Society for Laser Medicine and Surgery 15th Annual Meeting. Toronto, Ontario, Canada; 1994. [26] Collier M, Haugland LM, Bellamy J, et al. Effects of holmium:YAG laser on equine articular cartilage and subchondral bone adjacent to traumatic lesions: a histopathological assessment. J Arthrosc Rel Med 1997;9:536–45.
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[27] Shefer G, Halevy O, Cullen O, et al. Low level laser irradiation shows no histopatholgical effect on myogenic satellite cells in tissue culture. Laser Ther 11(3):114–7. [28] Po¨ntinen PJ. Low-energy photon therapy. In: Schoen AM, Wynn SG, editors. Complementary and alternative medicine—principles and practice. St. Louis (MO): Mosby; 1998. p. 247–74. [29] Ghamsari SM, Taguchi K, Abe N, et al. Evaluation of low level laser therapy on primary healing of experimentally induced full thickness teat wounds in dairy cattle. Vet Surg 1997; 26:114–20. [30] Hardie EM, Carlson CS, Richardson DC. Effect of Nd:YAG laser energy on articular cartilage healing in the dog. Lasers Surg Med 1989;9:595–601. [31] Lucroy MD, Edwards BF. Low-intensity laser light-induced closure of a chronic wound in a dog. Vet Surg 1999;28:292–5. [32] Teichman JM, Vassar GJ, Glickman RD. Holmium:yttrium-aluminum-garnet lithotripsy efficiency varies with stone composition. Urology 1998;52:392–7. [33] Choy DSJ, Case RB, Fielding W, et al. Percutaneous laser nucleolysis of lumbar discs. N Engl J Med 1987;317:771–2. [34] Dickey DT, Bartels KE, Henry, et al: Use of the holmium yttrium aluminum garnet laser for percutaneous thoracolumbar intervertebral disk ablation in the dog paper. J Am Vet Med Assoc 1996;208:1263–7. [35] Lange DN, Rochat MC, Bartels KE, et al. Comparison of carbon dioxide laser modalities for removal of polymethylmethacrylate cement. Vet Comp Orthop Trauma 1997;10:25–31. [36] Nixon AJ, Krook LP, Roth JE, et al. Pulsed carbon dioxide laser for cartilage vaporization and subchondral bone perforation in horses. Part II: morphologic and histochemical reactions. Vet Surg 1991;203(3):200–8. [37] Roth JE, Nixon AJ, Gantz VA. Pulsed carbon dioxide laser for cartilage vaporization and subchondral bone perforation in horse. Part I: technique and clinical results. Vet Surg 1991;203(3):190–9.
Lasers in veterinary medicine—where have we been, and where are we going? Kenneth E. Bartels, DVM, MS Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, Oklahoma 74078, USA
The principles necessary for the concept of laser development were reported as early as the nineteenth century with Bohr’s theory of optical resonance. In 1917, Einstein proposed the concept of stimulated light emission. Finally in 1960, Theodore Maiman developed the first operational laser, which was a pulsed ruby laser [1]. His work was based on Albert Einstein’s explanation of stimulated emission of radiation, coupled with Townes’ and Schawlow’s 1958 work with optical masers [2]. Since then, much of the progress in laser technology followed weapons research or commercial applications in the communication and manufacturing industries. When the ‘‘Cold War’’ ended, increased initiatives by laser manufacturers, formerly dedicated to military applications, provided a tremendous stimulus for advancements in both industrial and medical laser technology. Since its medical use began, the laser has been and is still considered by many to be ‘‘a tool in search of an application.’’ Medical lasers of the past were cumbersome, expensive, and difficult to maintain. As biomedical laser technology merges with the economic reality of medicine, however, innovations and improvements in existing devices and development of new concepts will continue. New ideas and modifications of current laser technology will be essential to keep pace with changes in veterinary medicine. Relatively recent technologic introductions, including the development of diode lasers and fiberoptic delivery systems, as well as portable, affordable, and reliable carbon dioxide (CO2) lasers, are now available to veterinarians. Other types of lasers with different wavelengths and delivery parameters are also available, depending on clinical requirements. The vernacular or perceived concept of ‘‘using laser,’’ implying there is one perfect wavelength, or only one
E-mail address: [email protected] (K.E. Bartels). 0195-5616/02/$ - see front matter 2002, Elsevier Science (USA). All rights reserved. PII: S 0 1 9 5 - 5 6 1 6 ( 0 2 ) 0 0 0 0 2 - 5
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all-purpose system, is unacceptable. Because of specific properties, certain lasers may be uniquely suited to a given clinical task compared with other lasers or energy modalities. The use of lasers in veterinary medicine includes both nonsurgical and surgical applications. Surgical applications involve direct physical alteration or removal of target tissue. Laser surgical treatments are referred to as photothermal or photomechanical applications. Examples of commonly used photothermal applications include laser hyperthermia and laser tissue vaporization. An example of a photomechanical application is laser lithotripsy, in which laser light creates an acoustical shock wave used to break down urologic or biliary calculi. Nonsurgical applications include such techniques as laser biostimulation, diagnostic use including optical biopsy, and photodynamic (PDT) therapy. The potential for future development of additional applications, especially for noninvasive biological sensors, depends on the continued interest of all medical specialties, especially veterinary medicine. Although the recent development and use of biomedical lasers may be a significant step ahead of mechanical instrumentation, it falls short of what is needed to be considered as the optimal ‘‘light knife’’ for every surgical situation. Considering differences in laser–tissue interaction, it is still very uncertain whether an ‘‘ideal’’ laser wavelength will ever exist. The promised benefits, however, of lasers in general surgery are a combination of reduced morbidity, better overall clinical results, an eventual reduction in expense for patients/clients, more productive use of operating room facilities, and increased efficiency of the surgical staff. These advantages coupled with objective evaluations of current surgical instrumentation should place biomedical lasers at the forefront of twenty-first century medical technology. Discounting future use of free-electron lasers with multiwavelength variability, acceptance of biomedical use of lasers with a fixed-wavelength has depended more on cost, capability of fiberoptic delivery, portability, flexibility, ease of use, and dependability. Present-day medicine uses many different types of biomedical lasers. Each instrument is usually acquired for a specific purpose, such as dermatologic or endoscopic applications. Overall, laser energy can be an extremely precise method for tissue removal or cellular destruction. Medical lasers are expensive and require a dedication to proper use and objective evaluation. Lasers in common use today are the CO2, neodymium:yttrium aluminum garnet (Nd:YAG), argon (Ar), potassium titanyl phosphate (KTP) or frequencydoubled YAG, ruby, diode, holmium (Ho:YAG), erbium (Er:YAG), and dye lasers [3]. The following general description can be used as a guide to medical lasers. In no way should it be considered a complete discussion. Laser types, wavelength preference, energy parameters, and delivery devices are changed frequently, because they are closely aligned with changes in today’s technologic advancements in computer hardware and software.
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Medical lasers Carbon dioxide laser (10,600 nm) The CO2 laser was one of the first medical lasers used for tissue ablation. It was developed in 1964 by C.K.N. Patel at Bell Laboratories [4]. At 10,600 nm (10.6 lm), the wavelength is ideal for cutting and vaporization because it is highly absorbed by water. It can cut tissue cleanly when the beam is focused onto the target and can debulk tissue by photovaporization when the beam is defocused. It is considered a far-infrared (IR) wavelength even though it emits at the short wavelength end of the far-IR spectrum, and the beam is invisible to the eye. The CO2 laser produces light that does not transmit through quartz or glass fibers. Currently, two basic types of surgical CO2 lasers are available: sealed tube and freeflowing devices. Sealed-tube lasers have a ‘‘shelf life’’ (i.e., the expected life of the laser tube before it needs recharging with the gain medium). The anticipated shelf life of a sealed metal tube laser can be longer than 10 years, depending on the technology used by the manufacturer and the amount of clinical use. Sealed-tube technology uses either direct electrical current (DC) or radiofrequency (RF) to excite the gain medium in a CO2 laser. RF excitation sealed-tube CO2 lasers have been growing in popularity because they can generate more power and can work well at lower powers. Sealed-tube CO2 lasers are mechanically and electronically simpler, tend to be smaller, produce less noise than free-flowing lasers, and are capable of emitting up to 20 to 30 W. A free-flowing CO2 laser requires a replaceable external gas cylinder containing a special mixture of gases as the gain medium (Fig. 1). CO2 is the light emitter, nitrogen helps excite CO2, and helium is used as a buffer gas for heat transfer [5]. Free-flowing lasers have been commonly sold on the secondary market (used lasers from human hospitals) to veterinarians and usually require consideration of purchasing a maintenance contract in addition to the cost of the device. They are usually large, more complex devices that require periodic manufacturer’s maintenance for proper alignment and power output, but have capabilities of producing more than 100 W of CO2 laser power. CO2 lasers range in size and power from very large units (>100 W) to small devices delivering 20 W or less that are more compatible for small animal surgery. Knowledge of laser tissue–interaction and laser physics may deter the informed laser surgeon from purchasing high-power CO2 lasers (>20 W) for most procedures in small animal surgery. The ability to exceed 20 W when performing larger ablative procedures may be an advantage. Historically, CO2 lasers have been used as a continuous wave (CW) laser. Control of energy delivery and pulse characteristics (superpulse) or using temporal/spatial scanners is more important, however, for ablative procedures requiring extreme precision more than is laser power alone.
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Fig. 1. Articulated arm delivery system in a free-flowing, DC stimulated carbon dioxide laser (Sharplan Model 743, Sharplan Lasers, Inc., Allendale, NJ). Note: This laser is no longer manufactured. Sharplan Lasers, Inc. is now part of Lumenis, Inc., Santa Clara, CA.
Diode lasers Developed in 1972, semiconductor diode lasers have progressed tremendously in concert with other aspects of medicine. Engineering and commercial specifications have allowed advancement of devices with wavelengths varying from approximately 635 to 980 nm. The diode lasers with the most medical significance are gallium aluminum arsenide (GaAlAs) or indium aluminum arsenide devices (InGaAs) at 780 to 980 nm. Laser light output is generated when electric current is passed through the diode. Individual diodes emit light from the edge of a wafer or from their surface. Standard ‘‘single-emitter’’ diodes can be combined on the same semiconductor chip
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to achieve very high output power from a device that is still very small. High-power diode lasers generate laser emission with an electrical-to-optical conversion efficiency of 30% to 50%, making them the most efficient lasers available. These direct diode laser systems collect diode emission and channel it onto tissue using an appropriate delivery system [6]. Coupling diode laser energy directly to fiberoptic delivery advices is a tremendous advantage when considering endoscopic application and minimally invasive surgical techniques. Therapeutic devices that use semiconductor diode lasers were first approved for surgical use in this country in 1989. Diode lasers (1–4 W) are also used for photocoagulation of retinal and other ocular tissues and have been used for ophthalmologic applications since approximately 1984. The compact size and high efficiency offer significant ergonomic and economic advantages. High-power, semiconductor diode lasers appropriate for other surgical applications have been recently introduced for a variety of uses (Fig. 2). These lasers currently provide up to 15 to 60 W at 810 nm or 980 nm; wavelengths that can penetrate deeply into most types of soft tissue and can produce tissue interactions comparable to the Nd:YAG laser (1064 nm) [7,8]. Considering whether a diode laser emitting 810-nm wavelength is superior or inferior to a diode laser emitting 980 nm is an exercise in physics and laser-tissue interaction. Furthermore, the theoretical differences (980 nm is somewhat higher in its water absorption coefficient characteristic than 810 nm) on a tissue target should be negligible to most laser surgeons. More importantly, the delivery system, a fiber in contact or noncontact mode, and the laser energy parameters will provide a greater impact on perceived tissue interaction. Although diode lasers are especially useful for some surgical applications, the direct diode systems used today have some limitations. The beam quality
Fig. 2. Diode laser (Diomed, Inc., Andover, MA).
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of high-power diode lasers limits the amount of power that can be coupled efficiently through small core fibers. Most high-power (25–60 W) directdiode systems use 400 to 600 lm diameter fibers. Fiber core sizes in the 200 to 300 lm range may be needed in the near future for some applications using miniaturized endoscopes. Narrow wavelength coverage is another shortcoming of direct-diode laser systems. Currently, diode laser materials are practical for generating high-output powers at wavelengths in the 635 to 2000 nm range. High-power devices that can generate yellow, green, blue, and even near-ultraviolet (UV) and mid-IR wavelengths may be possible in the future. Diode lasers are continuous-wave devices with little energy-storage capacity. They cannot produce the high peak power needed for some medical applications, such as lithotripsy, where production of a photoacoustic effect is required or in some dermatologic applications where high peak powers are required for therapeutic results. To overcome this limitation and still preserve the advantages of using diode lasers (i.e., size, reliability, ruggedness), diodepumped solid-state lasers will become more important [6]. Diode lasers can be used with fiber delivery accessories in noncontact mode for tissue coagulation (power requirements of 5–25 W CW), and for non-contact tissue vaporization (power requirements of 25–60 or greater W CW). For precise incisional applications, ‘‘hot-tip’’ quartz, sapphire, or so-called ‘‘dual use’’ fibers can be used (power requirements of 5–20 W CW) in contact mode. As mentioned, diode lasers can be used for many of the same applications as 1064 nm CW Nd:YAG lasers [9]. Surgical diode lasers offer considerable advantages, however, compared with the Nd:YAG laser. They are smaller, lighter, require less maintenance, are extremely userfriendly, and can be more economical. Some predict that prices for diode lasers will eventually drop to the point where they may be competitive with high-end electrosurgery devices [10]. Other applications for diode laser energy include chromophore-enhanced tissue ablation and coagulation, tissue fusion or laser welding, and PDT therapy [1,11,12]. Diode laser wavelengths of 805 to 810 nm have been used for tissue welding because applications have been centered around the peak absorption spectrum of indocyanine green (780–820 nm), which is the selective chromophore used in a fibrinogen-based solder [11]. Finally, the small, convenient size coupled with reliability and user friendliness has also focused extensive diode laser development for applications in PDT therapy [8]. Nd:YAG laser (1064 nm) The Nd:YAG or ‘‘YAG laser’’ is a solid-state laser that differs from the CO2 laser because the wavelength allows transmittance though tissue, in addition to surface absorption. Powers of up to 100 W can be delivered through small-core optical fibers that can easily be inserted through the accessory channels of standard gastrointestinal (GI) endoscopes. The Nd:YAG laser was one of the first lasers to be used in veterinary medicine
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because of its fiber delivery and endoscopic accessibility. Transendoscopic use of the Nd:YAG laser in the horse has been and continues to be an effective method for treating upper airway obstruction and for some endoscopic urogenital procedures [1,13]. It has also been used for soft tissue procedures in small animals, including prostatic resection, perianal fistula ablation, vaporization of facial tumors, and even ablation of a brain tumor [13–17]. Advantages of approaching conditions with limited anatomic access, performance of upper respiratory and urogenital procedures with local rather than general anesthesia, and a decrease in morbidity time have provided the motivation for continuing this extremely successful effort. Because the Nd:YAG laser has less specific absorption by water and hemoglobin than the CO2 and Ar lasers, the depth of thermal injury can exceed 3 mm in most tissues, which can be useful for coagulation of large volumes of tissue [18]. Rapid tissue vaporization in noncontact (free-beam) mode is possible with a bare fiber; however, collateral thermal injury may be substantial. Soft tissue applications using noncontact or free-beam mode usually require power levels approaching 100 W. The use of various temporal emission modes, including CW and pulsed modes (free-running, Q-switched, and mode-locked), allows extreme versatility in power delivery when using the Nd:YAG laser for many clinical applications in soft tissue surgery, ophthalmology, and urology. Frequency-doubled Nd:YAG or KTP laser (532 nm) The frequency-doubled Nd:YAG laser, also known as KTP (potassium titanyl phosphate) lasers, emits a visible green light and is basically equivalent to the Ar lasers used in many surgical and dermatologic applications. Present-day clinical applications use photothermal reactions to coagulate, vaporize, or cut soft tissue. Absorption of the 532-nm wavelength is negligible in water. The visible green laser beam passes through water and saline with virtually no absorption, which is extremely important in a wet or flooded surgical field. Because the 532-nm wavelength is strongly absorbed by the oxyhemoglobin component of blood, it can be used efficiently and precisely to heat blood-perfused soft tissue. In whole blood, absorption depth at 532 nm is approximately 0.5 mm. This strong absorption can actually be a problem if the target tissue is covered with blood, because the laser energy will be severely attenuated before it reaches the tissue’s surface [19]. One currently manufactured frequency-doubled Nd:YAG laser (Model 800 Series Laserscope, Laser Scope, Inc., San Jose, CA) can provide either 532 nm or 1064 nm through the same optical fiber. The 532-nm wavelength is used for precise cutting and vaporization. By switching to the 1064-nm wavelength, the same laser and fiber delivery system can be used for deep tissue coagulation or rapid vaporization when only modest surgical precision is required. The laser is also used by the manufacturer as a dye laserpumping source to control PDT interactions.
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Ar laser (458 and 524 nm) The blue-green Ar laser is strongly absorbed by hemoglobin and is especially useful in nonbleeding vascular lesions when precision and minimal penetration (approximately 1 mm) is required. Although heavily absorbed by blood, Ar laser energy can be readily transmitted through water, gastric fluid, aqueous or vitreous humor, and urine. Consequently, this laser can be used to precisely cut, vaporize, and superficially coagulate soft tissue that is well perfused with blood. Treatment of hemoglobin-poor tissue generally relies on the production of carbonized tissue or ‘‘char’’ for efficient heating of tissue. Bare fibers can be used in contact or noncontact modes for cutting, vaporization, or coagulation. Although older versions often lacked enough power to vaporize target tissue, newer 15-W Ar lasers are more efficient for vaporization and cutting applications. Ruby laser (694 nm) Although first investigated for its medical potential by Maiman in 1961, the ruby laser has not received widespread use. It was resurrected in the late 1980s as a medical device for removing tattoos and birthmarks. The 694-nm wavelength is absorbed strongly by dark pigments, such as melanin, and the pigments used for making tattoos, but only weakly by hemoglobin. Therefore, the visible ruby laser wavelength can penetrate several millimeters into skin without being severely attenuated by blood. Because of this fact, the ruby laser is used in selective photothermolysis procedures for removing tattoos [19]. Holmium lasers (2100 nm) and erbium lasers (2900 nm) Clinical solid-state Ho:YAG lasers have appeared recently for arthroscopic surgery, general surgery, laser angioplasty, and thermal sclerostomy. Other applications include laser discectomy, removal of sessile polyps in the GI tract, and otorhinolaryngeal procedures. The main benefit of the Ho: YAG laser is its ability to cut and vaporize soft tissue somewhat like a CO2 laser, with the added advantage that holmium energy can be delivered through flexible, low OH, quartz, or polyamide optical fibers. Good surgical precision and control can be obtained with a bare optical fiber. Unlike visible wavelength lasers, and again like the CO2 laser, photothermal interactions with the Ho:YAG laser do not rely on hemoglobin or other pigments for efficient heating of tissue. The water component of tissue is responsible for absorbing Ho:YAG laser energy (2100 nm) and for converting it to heat. The depth of absorption is quite shallow at approximately 0.3 mm. When cutting or vaporizing tissue, actual zones of thermal injury vary from 0.1 to 1 mm, depending on exposure parameters and the type of tissue. These small thermal necrosis zones provide better surgical precision and adequate hemostasis. Current holmium instruments are flashlamp-pumped systems. The active laser medium consists of a chromium-sensitized YAG host crystal doped
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with holmium and thulium ions. This active medium is referred to as Tm, Ho, Cr:YAG, or THC:YAG and is common to all Ho:YAG laser medical devices. Unlike the CO2 laser, higher power Ho:YAG lasers cannot operate in a CW mode at room temperature. The relatively low pulse rates (5–20 Hz with 250–350 lsec pulses) available from most Ho:YAG lasers may be considered a disadvantage because cutting may be slow or may result in jagged tissue edges during incisional applications. In addition, at higher pulse energies (‡1 J), considerable amounts of acoustic or photomechanical energy are generated in tissue. An audible acoustical ‘‘pop’’ may be generated and actually heard during laser application. Acoustic energy may be considered an advantage, however, when using holmium energy for photodisruptive (photothermal/photomechanical phenomena) procedures, such as lithotripsy of gallstones or urologic calculi [20]. Another mid-IR, solid-state laser is the Er:YAG laser. Its wavelength (2900 nm) is more strongly absorbed in water. Dental applications, including hard tissue ablation (Food and Drug Administration [FDA]-approved) and incisional applications, are considered appropriate for this wavelength. Hemostatic ability is minimal, however, and lack of readily available delivery fibers has hindered its potential use. Although somewhat brittle, sapphire fibers have been used for Er:YAG energy delivery [19]. Dye laser (variable wavelength with dyes—400 to 1000 nm) Developed at the IBM Laboratories in 1966, dye lasers offer an advantage of ‘‘tunability’’ of wavelength over a considerable range to obtain absorption coefficients and tissue interaction characteristics applicable to multiple medical specialties, including oncology, ophthalmology, urology, and especially dermatology [5]. Pulsed and CW dye lasers use an active laser medium, consisting of an organic dye dissolved in an appropriate solvent. For the dye laser to work, the dye solution must be recirculated at high velocity through the laser resonator. Dye lasers are useful for medical applications because they can generate high-output powers and pulse energy at wavelengths throughout the visible wavelength spectrum (400–700 nm). They are usually pumped by Ar lasers, flashlamps, or a frequency-doubled Nd:YAG laser. Dye lasers have been used for lithotripsy of biliary and urologic calculi (pulsed), activating photosensitizers for PDT therapy (CW), ophthalmologic operations (pulsed or CW), and dermatologic applications (pulsed and CW) including treatment of birthmarks and removal of tattoos [19].
Laser delivery systems A delivery system is defined as the optical hardware needed to transfer energy from the laser to the treatment site. Devices for guiding laser beams to the patient include articulated arms with internal mirrors, hollow waveguides, and optical fibers. Articulated arms and hollow waveguides are used
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with laser wavelengths (2.8–10.6 lm) that cannot be transmitted through conventional glass or quartz fiber optics. The CO2 laser produces far-IR light and is included in this category. Currently, CO2 laser light must be delivered to the tissue through a hollow tube called an articulated arm or through a hollow waveguide. The articulated arm is a tube with mirrors aligned in joints that reflect light into a system of lenses that create a collimated and focused beam. The articulated arm can be somewhat fragile and difficult to manipulate in a small operating room; however, some manufacturers have worked diligently to minimize this limitation. Because the laser beam is invisible to the eye, a low-power red helium-neon (633 nm) or diode (635–660 nm) laser is usually used to provide an aiming beam. CO2 lasers using articulated arms for energy delivery must be realigned periodically during scheduled maintenance checks (Fig. 1). As mentioned, laser energy delivery through an articulated arm has inherent disadvantages because of size of the arm, portability, and its inability to be used for minimally invasive procedures through endoscopic visualization. The addition of attached semirigid hollow waveguides to some CO2 systems enhances their flexibility for performing many laser procedures. Waveguides can direct CO2 energy delivery economically and efficiently. Hollow waveguides are manufactured from high-quality stainless steel, flexible glass tubes, and other materials that have reflective interiors to direct energy. Although laser energy decays as it traverses the length of the hollow waveguide, software included in some CO2 devices will compensate for this energy loss to provide for consistent energy delivery at the termination. Hollow waveguides capable of transmitting CO2 energy from a compact, robust laser device have been one of the major influences to drive laser technology into veterinary medicine. Although this particular laser (AccuVet CO2 Laser, Lumenis, Inc., Santa Clara, CA) delivers a noncollimated beam at the tissue target site, various tapered tips (0.3–1.4 mm in diameter) concentrate and direct the energy, which increases or decreases the power density, depending on the tip diameter (AccuVet CO2 Laser, Lumenis, Inc., Santa Clara, CA) (Fig. 3). Although there are still drawbacks because currently available hollow waveguides are not readily endoscopically deliverable, improved waveguide development and advancing CO2 laser fiber technologies should overcome this limitation in the forseeable future. The availability of functional and inexpensive optical fibers for laser delivery has played a crucial part in the acceptance of lasers for medical applications. The fibers used in laser medical delivery are most often composed of quartz glass and have diameters ranging from 0.1 to 1 mm. Laser energy is contained within and follows the bends and curves (total internal reflection) of the fiber within certain limits (eg, numeric aperture), until it reaches the tip where it exits [21]. Although configurations of fiber tips (eg, flat, orb, chisel, conical) and their ability to transmit energy are a science in their own right, delivery parameters are primarily based on two factors: contact (hot-tipped) delivery or noncontact delivery (Fig. 4). In general,
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Fig. 3. Hollow waveguide delivered CO2 laser (AccuVetTM Carbon Dioxide Laser, Lumenis, Inc., Santa Clara, CA).
lower power (5–20 W) is delivered through contact tips. Contact tip fibers include sculpted quartz fibers, contact-tipped sapphire fibers, metal-capped fibers, temperature-controlled bare fibers, and dual effect (used both in contact and noncontact modes) fibers. Although the use of contact tips for endoscopic application is widely accepted, some tips are too large to insert through flexible endoscopes. Most sculpted or cleaved quartz fiber tips used in contact mode must be ‘‘carbonized’’ (i.e., contact application of fiber to target tissue or a sterile wooden tongue depressor to form a layer of carbonized particles so the ‘‘hot tip’’ becomes effective in tissue). Finally, the ability to guide laser energy, fiber thinness, flexibility, economy, and ruggedness makes quartz optical fibers essential for endoscopic applications. Laser scanners or pattern generators are available as accessories to CO2 and some solid-state lasers, such as the Er:YAG or KTP lasers. They were introduced to convert a CW or pulsed beam of laser energy into a scanned,
Fig. 4. Configurations of laser optical delivery fibers used with diode and Nd:YAG lasers.
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shuttered beam to reduce tissue carbonization and to enhance precise vaporization of tissue. Laser scanners can increase the speed of treatment and can provide better control of beam overlap. They provide a nonaligned treatment pattern and decrease laser beam ‘‘dwell time’’ on specific target areas, which allows the thermal energy in adjacent tissue to cool adequately between pulses [19]. Computerized or robotized scanning devices are used in some models of the larger lasers applied in aesthetic laser surgery. Smaller, less expensive flashscanners may scan a 100-lm focused beam in a spiral pattern on the target tissue with the aid of rapidly oscillating mirrors or a mechanical device that rotates the waveguide termination tip in the laser handpiece (Fig. 5). Biomedical lasers in veterinary medicine Early reports concerning the use of lasers for medical applications involved animals—either as experimental models or as clinical veterinary patients. In 1968, the removal of a vocal cord nodule in a dog demonstrated one of the first practical clinical applications of the CO2 laser as a precision surgical instrument [22,23]. Many other biomedical laser research teams have also relied on animal models for determining initial laser parameters and efficacy. At this stage of research, veterinarians can and should be the main catalyst for the advancement of biomedical laser technology and laser-based therapeutic techniques with potential human application. Often, objective protocols will prove some ideas and applications as impractical for veterinary clinical purposes. Although the idea or development of a new device may be inapplicable
Fig. 5. CO2 laser scanner used for char-free tissue ablation (NovaScanTM, Lumenis, Inc., Santa Clara, CA).
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to ‘‘mainstream veterinary medicine’’ because of the current economic limitations, our participation as an equal partner on the biomedical research team places us in a strategic position to be leaders in today’s total health care establishment. Since their introduction into veterinary medicine in the 1970s, highpowered surgical lasers have been used primarily for photothermal procedures to vaporize or ablate the target tissue. Techniques have often been described using a variety of device settings that may or may not take into account objective evaluation of the technology and give appropriate attention to laser tissue–interaction phenomenon. It is imperative that clinicians learn the fundamentals and the applied aspects of the technology for clinical practice. New procedures involving other aspects of laser energy interaction include laser lithotripsy and low-level laser photobiostimulation. Laser ablation of intervertebral discs has also become a procedure using minimally invasive techniques through fiberoptic energy. Low-level laser therapy (photobiostimulation) Biostimulation using light energy, which is usually considered a photochemical effect, has attracted interest in both the clinical and research arenas in both veterinary and human medicine. To many scientists and clinicians, the idea that low-intensity light energy (<500 mW average power) can promote and upgrade metabolic processes that result in tissue repair and pain relief is unbelievable and akin to ‘‘snake oil’’ practice [24]. At a minimum, it is on the fringes of accepted practice in the unconventional aspects of complementary and alternative veterinary medicine. Yet, reports from almost every region of the world indicate that low-intensity lasers promote the repair process of skin, tendons, ligaments, bone, and cartilage in experimental animals and wounds of various etiologies in humans. Reports that suggest the contrary complicate the matter, creating the present situation in which laser or low-energy photon therapy is viewed with extreme skepticism. Several manufacturers throughout the world have developed devices for using low-level laser energy in human medicine. Marketing efforts in the United States have been directed to veterinarians since the FDA has not yet approved this type of laser therapy for human use (Fig. 6) [25]. Biostimulation, or low-energy photon therapy, is defined as nonthermal interaction of monochromatic radiation with a target site [6]. Although the physiologic interaction for this type of energy application on tissue is still not understood, low-energy lasers have been reported to modulate various biologic processes, such as mitochondrial respiration or adenosine triphosphate synthesis, to accelerate wound and joint healing, and to promote muscle regeneration [26,27]. In addition, pain attenuation or pain removal has been reported using this type of low-energy photon therapy. Recommended veterinary applications include first aid treatment for traumatic and surgical wounds, strains; musculoskeletal pain and dysfunction; rheumatoid arthritis
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Fig. 6. Low-level laser energy biostimulation and therapy device (Veterinary Therapy Laser, Model PLLSD0009, American Veterinary Laser, Farmington Hills, MI).
and osteoarthritis; neurologic applications, such as neuralgia, and other nerve injuries; and sport injuries ranging from contusions to muscle tears [28]. Devices used for biostimulation vary in their optical properties. Often, laser wavelengths are in the visible or near-IR range. Laser power output is not sufficient to cut or vaporize tissue. Advocates have also reported similar results using less expensive light sources (wider wavelength bands, noncoherent light sources). More research is being conducted in this area as low-level laser or photon therapy becomes less controversial, and peerreviewed case reports or projects are being reported in the veterinary literature [29–31]. Methods are also being formulated to provide objective and replicable results to prove the efficacy of this therapy. It is incumbent on the manufacturers of these devices to support these types of investigations rather than promote the technology based solely on anecdotal reports. Laser lithotripsy Gastrointestinal and urologic applications have primarily involved uses in soft tissue surgery. Recently, however, the application of laser photothermal and photomechanical energy through endoscopically delivered optical fibers to break down urologic and biliary calculi has been approved and practiced in humans. Much of the preliminary work, including mechanism of action and early clinical reports, originated from veterinary medical applications. Currently, the flash-lamp pulsed dye laser and the Ho:YAG lasers are used for laser lithotripsy in veterinary medicine.
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The pulsed dye laser operates at a wavelength of 504 nm with 1.0 lsec pulses. Urolith fragmentation is accomplished by a photoacoustic and mechanical effect through formation of an energy plasma that consists of a rapidly expanding cloud of generated electrons and ions. The plasma absorbs additional laser energy and generates a symmetrically expanding cavitation bubble. Urolith fragmentation is caused by a rapid collapse of the cavitation bubble, creating a strong acoustic shockwave that exceeds the tensile strength of the urolith (Fig. 7). Because energy dissipates rapidly with increased distance, the pulse dye laser is associated with low risk of soft tissue damage. Energy absorption of this laser is related, however, to composition of the urolith, which decreases its effectiveness on light-colored calculi. Flash-lamp pulsed dye lasers are high maintenance devices and have virtually been replaced by the Ho:YAG laser as a laser lithotripter. The mechanism of Ho:YAG lithotripsy is mainly photothermal. Unlike the dye laser, the pulsed energy of the Ho:YAG laser is strongly absorbed by water. Direct contact of the fiber perpendicular to the calculus is required for effective use in liquid media because a cavitation bubble is formed by vaporization of water molecules. The bubble is pear shaped and undergoes asymmetric expansion and collapse. This results in an acoustic emission and shockwave generation. Coupled with an irregularly shaped cavitation bubble, a vapor channel is formed that effectively conducts laser energy to the stone (‘‘Moses Effect’’). Consequently, the surface of the calculus is ablated by direct laser radiation and a rapid increase in surface temperature. Because of a longer pulse duration (250–350 ms), vaporization of water molecules is continuous, and expansion of interstitial water and vapor results in surface ejection of fragments from the calculus (Fig. 8). Calculi composition does have an effect on Ho:YAG lithotripsy efficiency [20,32]. In addition, efficacy of lithotripsy for urologic calculi seems to vary among animal species, which also may result from differences in stone composition.
Fig. 7. Flashlamp pulsed dye laser lithotripsy of a urologic calculus (in vitro).
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Fig. 8. Ho:YAG laser lithotripsy of a biliary calculus (in vitro).
Considering current technologies, lithotripsy using the flash-lamp pulsed dye laser seems to be more successful in the horse than when using the Ho:YAG laser. The Ho:YAG laser has been successfully used, however, in most other animal species (i.e., dog, pig, cow, llama), and considering its solid-state characteristics, reliability, portability, and cost, it will most likely remain the primary device for laser lithotripsy in veterinary medicine. Laser intervertebral disc ablation A percutaneous approach for photothermal ablation or vaporization of the nucleus pulposus in lumbar discs using laser energy has been reported as a treatment of intervertebral disc disease in humans and dogs [33]. Although the Nd:YAG, KTP, and diode lasers have also been used for this minimally invasive procedure, the Ho:YAG laser has advantages over other approved lasers with different wavelengths. As mentioned previously,
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because the Ho:YAG wavelength is strongly absorbed by water, depth of tissue penetration is limited, and zones of necrosis and collateral thermal effects are minimized because of the high water content of the nucleus pulposus. Using the dog as a model, investigators have shown that acute and chronic histopathologic effects of percutaneous Ho:YAG laser disc ablation on neighboring tissue are minimal. Proponents of laser discectomy claim positive results in humans are related to a decrease in intradiscal pressure caused by a decrease in volume of the nucleus pulposus after ablation. Further disc extrusion also can be prevented; however, disc ablation is not effective when sequestration of a herniated fragment has occurred. Surgical fenestration of thoracolumbar intervertebral discs in dogs has been recommended primarily as a prophylactic procedure to prevent further herniation of nucleus pulposus from a partially herniated disc and exacerbation of associated clinical signs. The procedure should also reduce the chances of subsequent herniation of other discs in seven to eight statistically significant locations. Considered a major surgical procedure by most veterinary surgeons, disc fenestration has the potential for postoperative complications, including pneumothorax, spinal cord and nerve injury, and hemorrhage. Percutaneous Ho:YAG laser ablation of canine thoracolumbar intervertebral discs has the advantage of being a minimally invasive procedure, which can decrease postoperative complications, shorten recovery time, and reduce medical expenses (Fig. 9A, B). On the basis of efficacy and safety of a preliminary clinical study [34], an additional 250 cases involving dogs diagnosed with thoracolumbar intervertebral disc disease have undergone prophylactic percutaneous laser disc ablation from the tenth thoracic to the fourth lumbar vertebra (T10-11 to L3-4) [K.E. Bartels, et al; work in Progress]. A recurrence rate of less than 5% coupled with minimal postablation complications over an eight-year period has provided the incentive to continue using laser disc ablation as a viable alternative technique to surgical fenestration. In addition, laser disc ablation of the cervical and lumbosacral areas may also prove to be a viable technique for the future.
Future innovations The use of lasers in medicine is an exciting treatment modality that will continue to produce innovative and new methods for managing diseased tissue. Research focused on basic laser–tissue interaction and selective tissue destruction will become increasingly important. Orthopedic use of biomedical lasers in veterinary medicine has been somewhat limited. The CO2 laser has been used for ablation of methylmethacrylate and has the potential to be beneficial during revision of total hip prostheses [35]. As delivery methods improve and as devices with appropriate wavelengths (Ho:YAG—2.1 lm; Er:YAG—2.8 lm) become more economically available for veterinary use,
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Fig. 9. Ho:YAG laser ablation of thoracolumbar intervertebral discs. (A) Seven myelographic (20 gauge–21/2 in) needles are percutaneously inserted under fluoroscopic guidance into intervertebral disc spaces T10-11 to L3-4. (B) Ho:YAG laser fiber is inserted through each needle to the level of the nucleus pulposus, and the disc material is vaporized or undergoes coagulative change.
orthopedic applications will undoubtedly increase as they have in human medicine for cartilage reshaping and ablation through arthroscopic visualization. The CO2 laser also has potential for use in open surgical procedures where precise ablation is necessary, such as during joint exploration [36,37]. The use of lasers as diagnostic tools and sensors is one of the fastest growing branches of biomedical laser development. Clinical applications involving noninvasive recognition of malignant cells, abnormal tissue, or abnormal metabolites have tremendous potential. Use of available and future laser diagnostic technology could have a significant impact on the veterinary profession. Blood, urine, or tissue can be illuminated by a laser beam, and by analyzing the reflected or luminescent light collected and transmitted by a second fiber, information is obtained about that biologic fluid or tissue [33]. Because technical knowledge and instrumentation for laser surgery are expanding almost exponentially, the availability of equipment is also
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expanding as rapidly. With increased use, it is essential that our current clinical and research activities accurately reflect responsible medical and scientific use. Strategies or ‘‘gimmicks’’ in the form of new and unique equipment and miracle cures can attract individuals interested in offering special treatments that have no proven benefit. An objective and practical approach to laser surgical procedures in veterinary medicine is essential if the total beneficial potential is to be realized. ‘‘Zap and vaporize’’ techniques coupled with a ‘‘burn and learn’’ philosophy can do potential harm to patient and operator and can outweigh any beneficial effect. These concepts have no place in the objective use of lasers in veterinary medicine. A concerned effort must be made to evaluate the use of a laser for its potential patient benefit, rather than portraying it as a miracle device of the twenty-first century that is advertised on an illuminated billboard in front of a hospital. Although the use of biomedical lasers has created an entirely new definition for performing surgery, a surgeon’s knowledge of pathophysiology and technical expertise must be the primary factor to determine whether a laser should be used for a particular surgical procedure in lieu of more conventional approaches. Finally, the use of any new technology, including the application of biomedical lasers in veterinary medicine, does not replace the basic issues and essential rules of a surgical practice, such strict aseptic technique, appropriate use of instruments, and good surgical judgment.
Summary Future use of lasers in medicine depends on the active participation of veterinarians in the inception and development of new devices that meet the needs of the entire medical profession. The sensible clinical approach that must be taken every day in the practice of veterinary medicine equips the veterinarian with a unique ability to understand the practical applications of biomedical lasers. Veterinary medicine can and should be in the forefront during these exciting times, adding an essential dimension to development of this twenty-first century technology.
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