Current Concepts – Laser-assisted Shoulder Surgery

Light amplification by stimulated emission of radiation (laser) is a unique type of light energy produced by man. Laser light is different from visible light in its characteristics of collimation (all emitted light is almost perfectly parallel), coherent (light waves are all in phase in both time and space) and monochromatic (one specific wavelength). Maiman developed the laser in 1960, not for medical use but for aerospace. The carbon dioxide (CO₂) laser was first applied in arthroscopy in the early 1980s, which led to considerable controversy in terms of both efficacy and benefit beyond the normal mechanical techniques, while concerns with air embolism and air extravasation limited its use. The holmium (Ho) 2.1 nanometer laser was introduced in 1987 as the first fibre-opticdelivered free-energy laser beam for arthroscopic application in a water medium. The Food and Drug Administration (FDA) approved the Ho 2.1 laser in 1989 for all peripheral joint applications.

Physics and Properties
A laser light wave is created in a lasing cavity containing a lasing medium, such as a Ho-doped crystal rod of yttrium, aluminium and garnet (Ho:YAG). The crystal rod is excited by a high-intensity flash lamp (commonly krypton), causing the release of photons, which become trapped in the lasing cavity. This ‘optical resonator’ is composed of an internal rod and a precisely aligned parallel mirror at each end. One mirror is 100% reflective of the wavelength, while the opposite mirror reflects a pre-determined amount of photons (light energy), allowing a percentage of the impinging photons to pass through the mirror and become the usable output or ‘laser beam’.

The principle of lasing phenomena is the ability of photons to stimulate the emission of other photons, each having the same wave length and direction of travel. When a photon passes close to an excited electron, the electron will become stimulated to emit a photon that is identical in wavelength, phase and spatial coherence to the impinging photon. This process can be amplified between the two mirrors of the optical resonator. Changing the lasing medium will change the characteristics of the laser light by altering the wavelength produced, with each wavelength having specific properties and tissue effects that are generally determined by experimentation. The common laser pointer is prepared using a helium neon 630nm wavelength laser (visible). The neodymium:YAG is 1,064nm in the infrared spectrum, which commonly penetrates 4–6mm in avascular tissue. The Ho:YAG at 2,100nm is in the near-infrared (invisible) spectrum, with a depth of penetration of 0.5mm in avascular tissue, and CO₂ at 10,600nm in the far infrared spectrum penetrates approximately 0.2mm in tissue.

The effect of a laser beam on tissue can be varied by adjusting the laser energy, the spot size and the exposure time (dwell). The combination of spot size (beam diameter) and laser energy is expressed as Joules/cm2, or ‘energy density’. The energy density varies directly with the energy level and adversely with the spot size or will vary inversely with the square of the beam diameter. Energy density is one of the most important operating parameters to understand at a given wavelength and reflects the amount of energy actually delivered per unit area. The development of pulsed as opposed to continuous laser application of energy allows optimal specific tissue effects and minimal thermal damage. The pulse frequency can vary from 150 to 350 nanoseconds, which optimises tissue absorption and minimises charring or burning by eliminating the amount of heat delivered at any one time. The delivery of laser energy can be via non-contact (free beam) or direct contact (hot tip). Often, contact tips will accumulate debris upon them that block the laser energy and ultimately lead to a cautery tip effect.

Applications
Laser light, when absorbed by tissue, is converted to heat energy, so the effect is thermal, not anything unique to the specific wavelength utilised. Laser–tissue interaction may cause reflection, scattering, transmission, conduction or absorption. Therefore, the visible effect we see of the laser is to cut, coagulate or vaporise, identical to the thermal effect of current radiofrequency (RF) units.

The Ho 2.1 laser is the wavelength used in orthopaedics today, which is an invisible beam of light in the infrared zone of radiation directed by a helium neon visible aiming beam aligned with the treatment beam. The Ho 2.1 laser is well-absorbed by water; because of this absorption, when the beam is fired a small amount of laser energy at the tip of the free beam will boil water immediatelly adjacent to it (in an aqueous medium) creating a vapour bubble, which allows the laser energy to pass through this bubble and reach the tissues to be absorbed. Although the Ho 2.1 laser is used in a contact mode, it is actually a free beam spaced slightly back from the tip of the probe, to operate as described. Application of thermal energy from the Ho:YAG laser will commonly produce thermal damage in an area of 25–50 microns, with an area of adjacent thermal change of 250–300 microns and a normal depth of penetration of approximately 0.5mm.