Incision properties and thermal effects of three C02 lasers in soft tissue - medical fibers treatment

Keywords:laser fibers, medical fiber,  Time:17-02-2016

The CO2 laser emits light energy that is strongly absorbed by water and therefore also by tissues with a high water content, such as the oral soft tissues. The absorbed energy causes vaporization of the intra- and extracellular fluid and destruction of the cell mem- branes.6, 7 The exact nature and extent of the laser effect on soft tissue is governed by several factors. These include the total amount of energy delivered to the tissues over the entire period of irradiation (measured in Joules) as well as power levels (Watts), that is, en- ergy delivered per second. A unit of energy delivered over a very short period of time will have a greater effect than that same unit of energy delivered over a long time period. Spot size of the light beam used is also important. A unit of power or energy delivered over a large area by a large spot size will have milder effects than the same power or energy focused into a small spot size.

Structures directly adjacent to the area of vapor- ization demonstrate a range of thermal effects, de- pending on their proximity to the irradiation site and their optical properties. These marginal thermal interactions can range from mere transient heating to protein denaturation, water evaporation, or even car- bonization and burning. One characteristic difference between a laser and a scalpel cut is the generation of a coagulated tissue layer along the walls of the laser incision. This zone of thermal damage to adjacent structures should ide- ally be kept to a minimum, as it may impede wound healing, graft take, and reduce tensile strength, espe- cially if it is extensive. Furthermore, deeply penetrat- ing laser-induced temperature increases can threaten the vitality of adjoining structures such as teeth, pulp, or periodontium. In the CO2 lasers traditionally available to clinical dentistry, light at 10.6 #m is delivered by means of an articulated arm and a handpiece to the surgical site. As the articulated arm configuration consisting of hollow rigid tubes linked by joints can be cumbersome when working intraorally, various alternative delivery systems, usually in the form of hollow waveguides, are now becoming available. Hollow waveguides, flexible or semiflexible fibers used to conduct the laser fibers, often provide better maneuverability of the delivery system.

Recently, C02 lasers that deliver light in the 9.3 #m region of the infrared spectrum have also been developed for clinical use; 9.3 ~zm better matches the absorption characteristics of hydroxyapatite, provid- ing improved ablation characteristics in hard tissues and consequently greater protection for pulpal tissues. Technologic advances have now allowed manufacture of a coherent beam delivery system for this wave- length. It was the aim of this investigation to determine thermal events, incision characteristics, and soft tis- sue damage resulting from standardized laser incision using three different CO2 lasers: one emitting light at 9.3 #m via a hollow waveguide delivery system, the second emitting light at 10.6 #m with an articulated arm delivery system, the third also emitting light at 10.6 #m and fitted with a hollow waveguide delivery system.

MATERIAL AND METHODS

In this investigation, nine fresh pig's mandibles were used not more than 6 hours after the animal's death. The mandibles were cooled until 1 hour before use, then returned to room temperature. Standardized incisions 3 cm in length were made with a laser in the oral mucosa parallel to the border of the mandible and 5 mm below the gingival margin. To standardize the incision length, a template was positioned 3 mm below the planned incision site dur- ing the performance of each incision.

A total of 30 in- cisions were made. A minimum of three per param- eter were made with each laser type; one of these in-cisions was performed in the anterior third of the mandible, the second in the middle third, and the fi- nal incision in the posterior third. Three different CO2 lasers were used; one emitted at 9.3 #m, the other two emitted at 10.6 #m. Before laser irradiation, a copper-constant thermo- couple Philips Type K (Omega Engineering, Inc., Stamford, Conn.) with 0.25 mm diameter and a 63% response time of 7 ms was inserted into the soft tissue, directly below its surface and halfway (1.5 cm) along the length of the incision. Laterally, the thermocou- ple was positioned at a distance of 1 mm plus one half of the spot size from the incision line. Thus, for laser A, the thermocouple was located 1.12 mm lateral to the line of incision; for laser B it was positioned 1.11 mm laterally and for laser C 1.15 mm laterally.

Laser parameters

Laser A (Medical Optics Inc., Carlsbad, Calif.) emitted light at 9.3 urn; the light was delivered via a coherent hollow wave-guide and a focusing hand- piece. Spot size measured 250 ~tm. Laser B (Sharplan Lasers, Inc., Allendale, N.J.) emitted light at 10.6 #m via an articulated arm beam delivery system and a focusing handpiece. Spot size measured 220 #m. La- ser C (Luxar Corp., Bothell, Wash.) also emitted light at 10.6 #m; the light was delivered through an inco- herent hollow fiber waveguide to produce a spot size of 300/zm. All lasers were set to the continuous wave mode. Beam characteristics for each laser were cali- brated by one laser engineer to conform to manufac- turer's specifications directly before this investiga- tion. Photographic paper was used to measure and document spot sizes before each irradiation. Distance from the point of emission of laser light to the tissues was standardized by using a jig.

Duration of irradiation for each incision measured 4 seconds and was timed with a stop watch. Actual power levels emitted were 1, 4, and 12 W. A PRJ-M power meter (Gentec) was used to deter- mine actual values directly before each laser incision. These specific power levels were selected as they rep- resent the range of minimum to maximum available in many clinical devices.