- Email: [email protected]
Heat generation in hydroxyapatite-coated implants as a result of CO2 laser application
Vol. 79 No. 4
April 1995
1 ORAL A N D MAXILLOFACIAL SURGERY
Editor." Larry J. Peterson
Heat generation in hydroxyapatite-coated implants as a result of CO2 laser application James Q. Swift, DDS, a Jason E. Jenny, b and Kenneth M. Hargreaves, DDS, PhD, c Minneapolis, Minn. UNIVERSITY OF MINNESOTA SCHOOL OF DENTISTRY Previous studies have demonstrated that heat may induce bone resorption and minimize the regenerative capacity of bone. This finding is of potential clinical importance because the carbon dioxide laser may often be used to surgically expose dental implants. However, little is known about the actual amount of heat generated at the implant-bone interface. This experiment measured heat generation on the surface of dental implants exposed to the carbon dioxide laser. A total of 90 trials were performed. A complete factorial (3 x 3 x 2) experimental design was used to evaluate the interactions among laser wattage output (4, 8, and 15 watts), duration of exposure time (1, 5, or 15 seconds) and variations in emission conditions (pulsed or continuous laser mode). Linear increases in temperature to temperatures greater than 50 ~ C were observed with increases in wattage output or duration of exposure time. The pulse mode generated significantly less heat. The results of this study suggest that caution should be used when using the carbon dioxide laser for second stage dental implant surgery. (ORAt SURGORALMED ORALPATHOLORALRADIOLENDOD1995;79:410-5)
It is estimated that the most widely used dental implants today are endosseous root form implants that depend on an intact bone to implant interface for retention. The criteria for the success of these dental implants in man has been described previously.1 Constant cooling of the bone at the implant recipient site is recommended to avoid necrosis and subsequent failure of integration. Elevations of tissue temperature above 53 o C causes cessation of blood flow, denaturation of proteins, and bone necrosis. 2 Duration of the temperature increase is an additional factor; maintaining a 47 ~ C stimulus applied to bone for 1 minute results in deleterious effects on bone regeneration. 3 Implant uncovery surgery, often termed stage II surgery, should be carefully executed to avoid disturbance of the bone-implant healing established in the interval between stage I placement and stage II uncovery. Exposure of implant coverscrews oftentimes is accomplished by means of an incision with aAssistant Professor, Director, Division of Oral and Maxillofacial Surgery, Department of Diagnostic and Surgical Sciences. bpredoctoral dental student. CAssociate Professor, Division of Endodontics, Department of Restorative Sciences, and Department of Pharmacology. Copyright 9 1995 by Mosby-Year Book, Inc. 1079-2104/95/$3.00 + 0 7/12/62000
410
elevation of a flap or through use of a soft tissue punch. 4 The use of dental lasers to ablate the soft tissues covering the implants is an alternate method of implant exposure that has been gaining popularity recently. The role of lasers in the dental and surgical fields has expanded greatly since the late 1970s. 5 The carbon dioxide (CO2) laser offers the advantages of precise cutting of tissues and excellent coagulation of small blood vessels that leads to a better visualized surgical site. 68 In addition, there are anecdotal reports of decreased postoperative pain with the laser as compared with standard techniques. However, comparatively little is known about the effect of the CO2 laser energy on dental implants or the surrounding tissues when used for the uncovering process. One potential complication of laser use is extensive heat generation in peri-implant tissues including bone. Studies have demonstrated that the surfaces of plasma-sprayed hydroxyapatite (HA)-coated titanium dental implants are altered by irradiation with another type of dental laser, the neodymium:yttriumaluminum-garnet (Nd:YAG) laser. 9 Furthermore, a recent investigation has demonstrated that the use of the Nd: YAG laser can cause substantial surface temperature increases on titanium implants. 1~ Although CO2 lasers have been recommended over Nd:
ORAL SURGERYORAL MEDICINE ORAL PATHOLOGY Volume 79, Number 4 YAG in second stage surgery because of the possibility that CO2 will cause less heat generation when applied to implant surfaces, 11, 12 no study to date has directly tested this hypothesis. Thermally induced cell lysis occurs when the laser is applied to soft tissues. Within soft tissue, up to 98% of laser energy is converted to heat and absorbed with little scatter or penetration. Detrimental effects of heat on implant surfaces, surrounding bone, gingiva, or mucosa are possible with such conversion. Thus heat generation must be evaluated as a potential adverse side effect. Therefore the purpose of this experiment was to measure concurrent heat generation on the surface of a dental implant after administration of varying wattages, exposure times, and modes of steady versus pulsating beams of CO2 laser photons to the implant coverscrew.
MATERIAL AND METHODS Implants In this experiment, the Sustain implant system (LifeCore Biomedical Corporation, Chaska, Minn.) was t~sed. Thirty HA-coated implants (4.0 mm X 13 mm) ~ t h 90 coverscrews were used. Each implant was used\in three experimental trials with a minimum period of 1~00hours at room temperature between trials. Each coverscrew was only used one time per trial.
Dental laser The CO2 laser used was the LX-20 produced by the Luxar Corporation (Bothell, Wash.). The CO2 laser emits a beam of monochromatic light q~ith a wavelength of 10.6 #m, which falls into the far infrared spectrum. The LX-20 has a power output range from 2 to 20 watts ( • 2 watts) and can be operated in either a continuous or pulsed mode of laser beam delivery. The power, wattage, and exposure time were varied according to the experimental design; all other conditions of the laser were set to standard recommendations from Luxar. A Minarik MicroMaster electronic timer (Los Angeles, Calif.) accurate to 1/100th of a second was coupled with the laser to measure the duration of the laser photon emission time. A laser tip of 0.8 mm diameter was selected, and the tip of the laser was positioned 2.0 mm from the surface of the coverscrew as depicted in Fig. 1. The laser's power was verified with the fiber verification procedure, which insures accuracy of _ 20% of the demonstrated wattage output. The distance from the sheath end to the laser tip was measured (8.0 mm) before the start of each session, and the same tip and sheath was used during the en~ tire project. As shown in Fig. 1, a retainer for a thermistor temperature probe was constructed. The retainer allowed contact between the implant surface
Swift, Jenny, and Hargreaves
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Fig. 1. Experimental set-up used to standardize the distance and orientation between CO2 laser and implant coverscrew. and the probe. It also assured consistency in the placement of the probe 1.3 mm from the top of the implant body.
Laser treatment The experiment was performed with the use of two pulse conditions. Half of the trials were performed with the continuous laser mode and half of the trials were performed with a pulsed mode (Luxar pulsed mode # 7). This pulsed mode provides an intermittent emission duration of 0.02 seconds (that is, the duty cycle = 40%). The pulsing provides repeated and distinct bursts of photon energy designed for the purpose of lowering overall heat generation.
Experimental design A complete factorial (3 • 3 X 2) experimental design was used to evaluate the interactions among laser wattage output (4, 8, and 15 watts), duration of exposure time (1, 5, or 15 seconds), and variations in emission conditions (pulsed or continuous laser mode). Each of the 18 treatment combinations were replicated with an N = 5, giving a total of 90 experimental trials. Because of the fact that more than 70% of biolog-
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April 1995 7O * *
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coverscrew temperature as a function of variations in wattage, emission time, and recovery time after exposure to a continuous beam of CO2 laser photons. **p < 0.01 versus all other treatments.
ical tissue consists of water, the implant was positioned in a 37 ~ C water bath (Lab-Line instruments, Melrose Park, Ill.) during the course of the experiment (Fig. 1). The water bath allowed for an in vitro simulation of thermal conductivity and diffusivity of heat similar to that which would occur in vivo. This method also allowed the baseline temperature of the implant to approximate physiologic temperature. The implants were bathed within a plastic bag in the water bath for a minimum of 6 hours before the trials. The t e m p e r a t u r e was measured before treatment to assure a baseline temperature of 37 ~ C. Immediately before each specific trial, the implant coverscrew was tightened, placed in the retainer with the probe in position and situated in the water bath. A single chann e l Thermistemp telethermometer (YSI 42sc) coupled to a 2-channel chart recorder (LKB 2210, Bromma, Sweden) was used to record the temperature change on the surface of the implant from the temperature probe. The telethermometer was set at a range of 20 ~ C to 80 ~ C, which gives an accuracy of +0.5 ~ C. To ensure a constant coverscrew/laser distance of 2.0 mm, a calibrated metallic strip of 2.0 m m in width was used t o verify the distance between the coverscrew and the laser. To achieve greater consistency in all trials, the laser tip was placed to allow the beam to strike an area just outside of the hexagonal cover screw hole in an area located approximately 90 degrees from the temperature probe. The implants with cover screws were then exposed to the laser beam, and temperatures were recorded.
Statistics Data were analyzed by a three-way analysis of variance ( A N O V A ) for repeated measures. Differences between groups were evaluated by Duncan's post-hoc multiple comparison test. The area under the time-response curve was calculated with the use of the trapeziodal rule (with the B M D P statistical package). Differences were considered significant if the probability that they occurred as a result of chance alone was less than 5% (p < 0.05).
RESULTS The effects of a continuous laser beam emission on implant surface temperature is depicted in the time response curve in Fig. 2. A three-way repeated measures A N O V A indicated significant interactions between the variables of wattage and duration over time (F108,972 = 7.28; p < 0.0001). Considerable surface temperature increases were observed; the greatest recorded heat generation occurred with a treatment of 15 watts for 15 seconds. The mean peak surface temperatures recorded for the laser when operated in a continual mode were 47.2 + 3.3 ~ C, 55.9 + 3.1~ C, and 67.0 _+ 9.3 ~ C for the 4, 8, and 15 watts, respectively. However, for all treatment combinations, peak surface temperatures were not sustained over time. To simplify direct comparisons between treatment combinations of the continuous laser beam, the area under the time-response curve (0 to 170 seconds) was calculated (Fig: 3). The area under the curve (AUC) provides a cumulative estimate of the total heat exposure and m a y serve as a better predictor for potential
Swift, Jenny, and Hargreaves
ORAL SURGERY ORAL MEDICINEORAL PATHOLOGY Volume 79, Number 4
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thermal-induced injury. A two-way A N O V A of the A U C results indicated significant interactions between the variables of wattage and duration (F4,36 = 7.78; p < 0.0001). The time-related effects of a pulsed laser beam emission on implant surface temperature is illustrated in Fig. 4. A N O V A indicated significant interactions between the variables of wattage and duration over time (F108,972 = 10.78;p < 0.0001). Considerablesurface temperature increases were observed; the greatest recorded heat generation occurred with a treatment of 15 watts for 15 seconds. However, similar to
the results from the continuous mode, peak surface temperatures after pulsed beam delivery were not sustained over time. The mean peak surface temperatures recorded for the laser when operated in a pulsed mode were 41.6 ~ C • 1.32 ~ C, 46.0 • 1.33 ~ C, and 62.4 • 3.11 o C for 4, 8, and 15 watts, respectively. The peak increases in the pulsed mode were about 7 ~ to 14 ~ C lower than peak temperatures observed in the continuous mode (Fig. 2 versus Fig. 4). The area under the time-response curve (0 to 70 seconds) following application of the pulsed laser beam is presented in Fig. 5. Similar to the results ob-
414
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ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY
April 1995 1000"
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Fig. 5. Net effect of a pulsed laser beam on implant temperature. The area under the time-response curves is presented as a function of duration of beam emission. The AUC was taken from Fig. 4 and calculated with the trapeziodal method. Error bars -- SEM. served with the continuous beam delivery, increases in either beam wattage or duration resulted in linear increases in surface temperature. A two-way A N O V A of the AUC results indicated significant interactions between the variables of wattage and duration (F3,36 = 15.97; p < 0.0001). In addition, the total responses in the pulsed mode were about 100 to 200 ~ C seconds lower than corresponding responses observed in the continuous mode (Fig. 3 versus Fig. 5).
DISCUSSION Successful integration of dental implants is dependent upon numerous factors that collectively contribute to clinical success or failure. This study evaluated one such factor: heat. Heat generation at the implantbone interface should be minimized at all stages of the implant surgical procedures because it could ultimately lead to implant failure. This study demonstrates the in vitro capability of the CO2 laser to generate heat on the Surface of an implant body when the coverscrew is subjected to laser irradiation. The heat generated would appear to be transferred from the energy of the laser photons at the coverscrew surface and then conducted throughout the implant body until finally being transferred to the water that acted as a heat sink. Speculation could be made that the highest implant body temperatures would be produced along the body of the implant at an area of coverscrew-implant body contact. The laws of thermal conductivity and diffusivity would then allow for heat dissipation away from the implant body as the distance from the implant coverscrew increased. By placing the temperature probe 1.3 mm down the implant body shaft, the recorded temperatures could be
estimated to be from the area of the implant body with the highest surface temperature. Thus, if pathologic responses would occur at the bone/implant interface as a result of increased bone temperature, some areas along the interface could be affected whereas other areas remain unchanged. As depicted in Figs. 2 to 5, operating parameters including laser operating Wattage, exposure time, and mode of beam delivery all alter amounts of heat generation. Operating wattage and exposure times were directly in proportion to implant surface temperatures. This correlation can be expected as wattage and exposure time has been previously demonstrated to be proportional to the depth of laser incision. 13Therefore an increase in these two variables may be indicative of increased bone/implant surface temperatures and subsequent pathologic responses of the bone to such treatment. In this study, the greatest temperature rise was seen when applying 15 watts of laser energy continuously for 15 seconds. This resulted in a peak temperature greater than 15~ C over that temperature that has been determined to be physiologically tolerable (50 ~ C). Using 15 watts for 5 seconds or 8 watts for 15 seconds also generated temperatures of greater than 50 ~ C on the implant surface. All other applications failed to generate 50 ~ C on the implant surface. By minimizing laser wattage and exposure time, heat generation in a clinical situation may be able to be decreased. As demonstrated in Figs. 2 through 5, the implant temperatures when the laser was operating in a pulsed beam delivery were significantly lower than when the laser was operated in its continuous mode. The only application of the pulse mode that generated greater
ORALSURGERYORALMEDICINEORALPATHOLOGY Volume 79, Number 4 than 50 ~ C was 15 watts of laser energy applied for 15 seconds. All other applications failed to generate greater than 50 ~ C. Thus the pulse mode of beam delivery appears to generate lower amounts of heat at the implant body surface than corresponding continuous laser applications. These results can be explained by the fact that during the pulsed laser mode, intermittent periods of beam emission occurred. The results of lower heat generation suggests that the pulsed laser mode could supply an effective method of controlling heat generation. The disadvantage of the laser's pulsed mode of beam delivery is that the ablation time of the tissue may be prolonged causing an increase in the time duration of the clinical procedure. Because time of beam duration relates directly to implant surface temperature, the lessened temperature increase as a result of pulsed mode of beam delivery may not be as effective in minimizing temperature because of the increased procedure time required. Although these results may not be directly comparable with a clinical situation, this study does suggest the capability of heat generation in levels exceeding those shown previously to produce tissue injury. Thus excessive heat generation may be of clinical significance when CO2 dental lasers are used during stage II implant surgery. These results would suggest that animal studies should be performed that would determine the parameters of wattage and exposure time that may be used safely without risk of detrimental heat generation. A proper protocol of laser usage in stage II implant uncovering should be established to avoid undesired bone damage and to increase the success of endosseous impalnts. CONCLUSIONS This study shows that significant heat (greater than 50 ~ C) can be generated on the lateral surface of an HA-coated titanium dental implant when laser energy is applied to the coverscrew. It is possible that in vivo heat transmitted to bone adjacent to the implant may have a detrimental effect on the bone-implant interface. It is conceivable that excessive heat could
Swift, Jenny, and Hargreaves
415
contribute to bone or soft tissue damage or necrosis. The greatest heat generation was shown when using the laser in continuous mode at higher wattage levels for longer duration. When using the CO2 laser for second stage implant surgery, pulsed mode with short exposure time (less than 15 seconds) and lower wattage levels (8 watts or less) may limit heat generation on the implant surface to levels that will not cause tissue damage and disrupt the implant-bone interface. REFERENCES 1. Albrektsson T, Brfinemark P-I, Hansson HA, Lindstrom J. Osseointegrated titanium implants: requirements for ensuring long-lasting, direct bone to implant anchorage in man. Acta Orthop Scand 1981;52:155-70. 2. Eriksson AR, Albrektsson T, Grane B, McQueen D. Thermal injury to bone. Int J Oral Surg 1982;11:115-21. 3. Eriksson AR, Albrektsson T. Temperature threshold levels for heat-induced bone tissue injury: a vital microscopic study in the rabbit. J Prosthet Dent 1983;50:101-7. 4. Babbush CA. Dental implants: principles and practice. Philadelphia: WB Saunders, 1991:112, 5. Lenz H, Eichler J, Schaffer G, et al. Production of a nasoantral window with argon laser. J Oral Maxillofac Surg 1977;5:314-8. 6. Myers TD. Lasers in dentistry. J Am Dent Assoc 1991; 122:4750. 7. Pecaro BC, Garehime WJ. The CO2 laser in oral and maxillofacial surgery. J Oral Maxillofac Surg 1983;41:725-8. 8. Myers TD, McDanial DJ. Pulsed Nd:YAG dental laser: review of clinical applications. J Can Dent Assoc 1991;19:25-30. 9. Block CM, Mayo JA, Evans GH. Effects of the Nd:YAG dental laser on plasma sprayed and hyroxyapatite coated titanium dental implants: surface alteration and attempted sterilization. Int J Oral Maxillofac Implants 1992;4:441-9. 10. Chu RT, Watanabe L, White JM, Marshall GW, Marshall S J, Hutton JE. Temperature rise and surface modification of lased titanium cylinders [Abstract]. J Dent Res 1992;71:312. 11. Pick R. Using lasers in clinical practice. J Am Dent Assoc 1993;124:37-47. 12. Walsh WJ. The use of lasers in implantology: an overview. J Oral Implantology 1992;18:335-40. 13. Schuller DE. Use of the laser in the oral cavity. Otolaryngol Clin North Am 1990;23:31-42.
Reprint requests: James Q. Swift, DDS University of Minnesota Division of Oral and Maxillofacial Surgery 7-147 Moos Tower Minneapolis, MN 55455
April 1995
1 ORAL A N D MAXILLOFACIAL SURGERY
Editor." Larry J. Peterson
Heat generation in hydroxyapatite-coated implants as a result of CO2 laser application James Q. Swift, DDS, a Jason E. Jenny, b and Kenneth M. Hargreaves, DDS, PhD, c Minneapolis, Minn. UNIVERSITY OF MINNESOTA SCHOOL OF DENTISTRY Previous studies have demonstrated that heat may induce bone resorption and minimize the regenerative capacity of bone. This finding is of potential clinical importance because the carbon dioxide laser may often be used to surgically expose dental implants. However, little is known about the actual amount of heat generated at the implant-bone interface. This experiment measured heat generation on the surface of dental implants exposed to the carbon dioxide laser. A total of 90 trials were performed. A complete factorial (3 x 3 x 2) experimental design was used to evaluate the interactions among laser wattage output (4, 8, and 15 watts), duration of exposure time (1, 5, or 15 seconds) and variations in emission conditions (pulsed or continuous laser mode). Linear increases in temperature to temperatures greater than 50 ~ C were observed with increases in wattage output or duration of exposure time. The pulse mode generated significantly less heat. The results of this study suggest that caution should be used when using the carbon dioxide laser for second stage dental implant surgery. (ORAt SURGORALMED ORALPATHOLORALRADIOLENDOD1995;79:410-5)
It is estimated that the most widely used dental implants today are endosseous root form implants that depend on an intact bone to implant interface for retention. The criteria for the success of these dental implants in man has been described previously.1 Constant cooling of the bone at the implant recipient site is recommended to avoid necrosis and subsequent failure of integration. Elevations of tissue temperature above 53 o C causes cessation of blood flow, denaturation of proteins, and bone necrosis. 2 Duration of the temperature increase is an additional factor; maintaining a 47 ~ C stimulus applied to bone for 1 minute results in deleterious effects on bone regeneration. 3 Implant uncovery surgery, often termed stage II surgery, should be carefully executed to avoid disturbance of the bone-implant healing established in the interval between stage I placement and stage II uncovery. Exposure of implant coverscrews oftentimes is accomplished by means of an incision with aAssistant Professor, Director, Division of Oral and Maxillofacial Surgery, Department of Diagnostic and Surgical Sciences. bpredoctoral dental student. CAssociate Professor, Division of Endodontics, Department of Restorative Sciences, and Department of Pharmacology. Copyright 9 1995 by Mosby-Year Book, Inc. 1079-2104/95/$3.00 + 0 7/12/62000
410
elevation of a flap or through use of a soft tissue punch. 4 The use of dental lasers to ablate the soft tissues covering the implants is an alternate method of implant exposure that has been gaining popularity recently. The role of lasers in the dental and surgical fields has expanded greatly since the late 1970s. 5 The carbon dioxide (CO2) laser offers the advantages of precise cutting of tissues and excellent coagulation of small blood vessels that leads to a better visualized surgical site. 68 In addition, there are anecdotal reports of decreased postoperative pain with the laser as compared with standard techniques. However, comparatively little is known about the effect of the CO2 laser energy on dental implants or the surrounding tissues when used for the uncovering process. One potential complication of laser use is extensive heat generation in peri-implant tissues including bone. Studies have demonstrated that the surfaces of plasma-sprayed hydroxyapatite (HA)-coated titanium dental implants are altered by irradiation with another type of dental laser, the neodymium:yttriumaluminum-garnet (Nd:YAG) laser. 9 Furthermore, a recent investigation has demonstrated that the use of the Nd: YAG laser can cause substantial surface temperature increases on titanium implants. 1~ Although CO2 lasers have been recommended over Nd:
ORAL SURGERYORAL MEDICINE ORAL PATHOLOGY Volume 79, Number 4 YAG in second stage surgery because of the possibility that CO2 will cause less heat generation when applied to implant surfaces, 11, 12 no study to date has directly tested this hypothesis. Thermally induced cell lysis occurs when the laser is applied to soft tissues. Within soft tissue, up to 98% of laser energy is converted to heat and absorbed with little scatter or penetration. Detrimental effects of heat on implant surfaces, surrounding bone, gingiva, or mucosa are possible with such conversion. Thus heat generation must be evaluated as a potential adverse side effect. Therefore the purpose of this experiment was to measure concurrent heat generation on the surface of a dental implant after administration of varying wattages, exposure times, and modes of steady versus pulsating beams of CO2 laser photons to the implant coverscrew.
MATERIAL AND METHODS Implants In this experiment, the Sustain implant system (LifeCore Biomedical Corporation, Chaska, Minn.) was t~sed. Thirty HA-coated implants (4.0 mm X 13 mm) ~ t h 90 coverscrews were used. Each implant was used\in three experimental trials with a minimum period of 1~00hours at room temperature between trials. Each coverscrew was only used one time per trial.
Dental laser The CO2 laser used was the LX-20 produced by the Luxar Corporation (Bothell, Wash.). The CO2 laser emits a beam of monochromatic light q~ith a wavelength of 10.6 #m, which falls into the far infrared spectrum. The LX-20 has a power output range from 2 to 20 watts ( • 2 watts) and can be operated in either a continuous or pulsed mode of laser beam delivery. The power, wattage, and exposure time were varied according to the experimental design; all other conditions of the laser were set to standard recommendations from Luxar. A Minarik MicroMaster electronic timer (Los Angeles, Calif.) accurate to 1/100th of a second was coupled with the laser to measure the duration of the laser photon emission time. A laser tip of 0.8 mm diameter was selected, and the tip of the laser was positioned 2.0 mm from the surface of the coverscrew as depicted in Fig. 1. The laser's power was verified with the fiber verification procedure, which insures accuracy of _ 20% of the demonstrated wattage output. The distance from the sheath end to the laser tip was measured (8.0 mm) before the start of each session, and the same tip and sheath was used during the en~ tire project. As shown in Fig. 1, a retainer for a thermistor temperature probe was constructed. The retainer allowed contact between the implant surface
Swift, Jenny, and Hargreaves
411
/ / C02Laser
J
p--I I I I I I
Thermocouple~~ Implant
Fig. 1. Experimental set-up used to standardize the distance and orientation between CO2 laser and implant coverscrew. and the probe. It also assured consistency in the placement of the probe 1.3 mm from the top of the implant body.
Laser treatment The experiment was performed with the use of two pulse conditions. Half of the trials were performed with the continuous laser mode and half of the trials were performed with a pulsed mode (Luxar pulsed mode # 7). This pulsed mode provides an intermittent emission duration of 0.02 seconds (that is, the duty cycle = 40%). The pulsing provides repeated and distinct bursts of photon energy designed for the purpose of lowering overall heat generation.
Experimental design A complete factorial (3 • 3 X 2) experimental design was used to evaluate the interactions among laser wattage output (4, 8, and 15 watts), duration of exposure time (1, 5, or 15 seconds), and variations in emission conditions (pulsed or continuous laser mode). Each of the 18 treatment combinations were replicated with an N = 5, giving a total of 90 experimental trials. Because of the fact that more than 70% of biolog-
412
Swift, Jenny, and Hargreaves
ORAL SURGERY ORAL MEDICINEORAL PATHOLOGY
April 1995 7O * *
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,
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,
,
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Fig. 2. Effect of continuous laser beam on implant temperature. Time-response curve for the increase in
coverscrew temperature as a function of variations in wattage, emission time, and recovery time after exposure to a continuous beam of CO2 laser photons. **p < 0.01 versus all other treatments.
ical tissue consists of water, the implant was positioned in a 37 ~ C water bath (Lab-Line instruments, Melrose Park, Ill.) during the course of the experiment (Fig. 1). The water bath allowed for an in vitro simulation of thermal conductivity and diffusivity of heat similar to that which would occur in vivo. This method also allowed the baseline temperature of the implant to approximate physiologic temperature. The implants were bathed within a plastic bag in the water bath for a minimum of 6 hours before the trials. The t e m p e r a t u r e was measured before treatment to assure a baseline temperature of 37 ~ C. Immediately before each specific trial, the implant coverscrew was tightened, placed in the retainer with the probe in position and situated in the water bath. A single chann e l Thermistemp telethermometer (YSI 42sc) coupled to a 2-channel chart recorder (LKB 2210, Bromma, Sweden) was used to record the temperature change on the surface of the implant from the temperature probe. The telethermometer was set at a range of 20 ~ C to 80 ~ C, which gives an accuracy of +0.5 ~ C. To ensure a constant coverscrew/laser distance of 2.0 mm, a calibrated metallic strip of 2.0 m m in width was used t o verify the distance between the coverscrew and the laser. To achieve greater consistency in all trials, the laser tip was placed to allow the beam to strike an area just outside of the hexagonal cover screw hole in an area located approximately 90 degrees from the temperature probe. The implants with cover screws were then exposed to the laser beam, and temperatures were recorded.
Statistics Data were analyzed by a three-way analysis of variance ( A N O V A ) for repeated measures. Differences between groups were evaluated by Duncan's post-hoc multiple comparison test. The area under the time-response curve was calculated with the use of the trapeziodal rule (with the B M D P statistical package). Differences were considered significant if the probability that they occurred as a result of chance alone was less than 5% (p < 0.05).
RESULTS The effects of a continuous laser beam emission on implant surface temperature is depicted in the time response curve in Fig. 2. A three-way repeated measures A N O V A indicated significant interactions between the variables of wattage and duration over time (F108,972 = 7.28; p < 0.0001). Considerable surface temperature increases were observed; the greatest recorded heat generation occurred with a treatment of 15 watts for 15 seconds. The mean peak surface temperatures recorded for the laser when operated in a continual mode were 47.2 + 3.3 ~ C, 55.9 + 3.1~ C, and 67.0 _+ 9.3 ~ C for the 4, 8, and 15 watts, respectively. However, for all treatment combinations, peak surface temperatures were not sustained over time. To simplify direct comparisons between treatment combinations of the continuous laser beam, the area under the time-response curve (0 to 170 seconds) was calculated (Fig: 3). The area under the curve (AUC) provides a cumulative estimate of the total heat exposure and m a y serve as a better predictor for potential
Swift, Jenny, and Hargreaves
ORAL SURGERY ORAL MEDICINEORAL PATHOLOGY Volume 79, Number 4
413
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thermal-induced injury. A two-way A N O V A of the A U C results indicated significant interactions between the variables of wattage and duration (F4,36 = 7.78; p < 0.0001). The time-related effects of a pulsed laser beam emission on implant surface temperature is illustrated in Fig. 4. A N O V A indicated significant interactions between the variables of wattage and duration over time (F108,972 = 10.78;p < 0.0001). Considerablesurface temperature increases were observed; the greatest recorded heat generation occurred with a treatment of 15 watts for 15 seconds. However, similar to
the results from the continuous mode, peak surface temperatures after pulsed beam delivery were not sustained over time. The mean peak surface temperatures recorded for the laser when operated in a pulsed mode were 41.6 ~ C • 1.32 ~ C, 46.0 • 1.33 ~ C, and 62.4 • 3.11 o C for 4, 8, and 15 watts, respectively. The peak increases in the pulsed mode were about 7 ~ to 14 ~ C lower than peak temperatures observed in the continuous mode (Fig. 2 versus Fig. 4). The area under the time-response curve (0 to 70 seconds) following application of the pulsed laser beam is presented in Fig. 5. Similar to the results ob-
414
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Fig. 5. Net effect of a pulsed laser beam on implant temperature. The area under the time-response curves is presented as a function of duration of beam emission. The AUC was taken from Fig. 4 and calculated with the trapeziodal method. Error bars -- SEM. served with the continuous beam delivery, increases in either beam wattage or duration resulted in linear increases in surface temperature. A two-way A N O V A of the AUC results indicated significant interactions between the variables of wattage and duration (F3,36 = 15.97; p < 0.0001). In addition, the total responses in the pulsed mode were about 100 to 200 ~ C seconds lower than corresponding responses observed in the continuous mode (Fig. 3 versus Fig. 5).
DISCUSSION Successful integration of dental implants is dependent upon numerous factors that collectively contribute to clinical success or failure. This study evaluated one such factor: heat. Heat generation at the implantbone interface should be minimized at all stages of the implant surgical procedures because it could ultimately lead to implant failure. This study demonstrates the in vitro capability of the CO2 laser to generate heat on the Surface of an implant body when the coverscrew is subjected to laser irradiation. The heat generated would appear to be transferred from the energy of the laser photons at the coverscrew surface and then conducted throughout the implant body until finally being transferred to the water that acted as a heat sink. Speculation could be made that the highest implant body temperatures would be produced along the body of the implant at an area of coverscrew-implant body contact. The laws of thermal conductivity and diffusivity would then allow for heat dissipation away from the implant body as the distance from the implant coverscrew increased. By placing the temperature probe 1.3 mm down the implant body shaft, the recorded temperatures could be
estimated to be from the area of the implant body with the highest surface temperature. Thus, if pathologic responses would occur at the bone/implant interface as a result of increased bone temperature, some areas along the interface could be affected whereas other areas remain unchanged. As depicted in Figs. 2 to 5, operating parameters including laser operating Wattage, exposure time, and mode of beam delivery all alter amounts of heat generation. Operating wattage and exposure times were directly in proportion to implant surface temperatures. This correlation can be expected as wattage and exposure time has been previously demonstrated to be proportional to the depth of laser incision. 13Therefore an increase in these two variables may be indicative of increased bone/implant surface temperatures and subsequent pathologic responses of the bone to such treatment. In this study, the greatest temperature rise was seen when applying 15 watts of laser energy continuously for 15 seconds. This resulted in a peak temperature greater than 15~ C over that temperature that has been determined to be physiologically tolerable (50 ~ C). Using 15 watts for 5 seconds or 8 watts for 15 seconds also generated temperatures of greater than 50 ~ C on the implant surface. All other applications failed to generate 50 ~ C on the implant surface. By minimizing laser wattage and exposure time, heat generation in a clinical situation may be able to be decreased. As demonstrated in Figs. 2 through 5, the implant temperatures when the laser was operating in a pulsed beam delivery were significantly lower than when the laser was operated in its continuous mode. The only application of the pulse mode that generated greater
ORALSURGERYORALMEDICINEORALPATHOLOGY Volume 79, Number 4 than 50 ~ C was 15 watts of laser energy applied for 15 seconds. All other applications failed to generate greater than 50 ~ C. Thus the pulse mode of beam delivery appears to generate lower amounts of heat at the implant body surface than corresponding continuous laser applications. These results can be explained by the fact that during the pulsed laser mode, intermittent periods of beam emission occurred. The results of lower heat generation suggests that the pulsed laser mode could supply an effective method of controlling heat generation. The disadvantage of the laser's pulsed mode of beam delivery is that the ablation time of the tissue may be prolonged causing an increase in the time duration of the clinical procedure. Because time of beam duration relates directly to implant surface temperature, the lessened temperature increase as a result of pulsed mode of beam delivery may not be as effective in minimizing temperature because of the increased procedure time required. Although these results may not be directly comparable with a clinical situation, this study does suggest the capability of heat generation in levels exceeding those shown previously to produce tissue injury. Thus excessive heat generation may be of clinical significance when CO2 dental lasers are used during stage II implant surgery. These results would suggest that animal studies should be performed that would determine the parameters of wattage and exposure time that may be used safely without risk of detrimental heat generation. A proper protocol of laser usage in stage II implant uncovering should be established to avoid undesired bone damage and to increase the success of endosseous impalnts. CONCLUSIONS This study shows that significant heat (greater than 50 ~ C) can be generated on the lateral surface of an HA-coated titanium dental implant when laser energy is applied to the coverscrew. It is possible that in vivo heat transmitted to bone adjacent to the implant may have a detrimental effect on the bone-implant interface. It is conceivable that excessive heat could
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contribute to bone or soft tissue damage or necrosis. The greatest heat generation was shown when using the laser in continuous mode at higher wattage levels for longer duration. When using the CO2 laser for second stage implant surgery, pulsed mode with short exposure time (less than 15 seconds) and lower wattage levels (8 watts or less) may limit heat generation on the implant surface to levels that will not cause tissue damage and disrupt the implant-bone interface. REFERENCES 1. Albrektsson T, Brfinemark P-I, Hansson HA, Lindstrom J. Osseointegrated titanium implants: requirements for ensuring long-lasting, direct bone to implant anchorage in man. Acta Orthop Scand 1981;52:155-70. 2. Eriksson AR, Albrektsson T, Grane B, McQueen D. Thermal injury to bone. Int J Oral Surg 1982;11:115-21. 3. Eriksson AR, Albrektsson T. Temperature threshold levels for heat-induced bone tissue injury: a vital microscopic study in the rabbit. J Prosthet Dent 1983;50:101-7. 4. Babbush CA. Dental implants: principles and practice. Philadelphia: WB Saunders, 1991:112, 5. Lenz H, Eichler J, Schaffer G, et al. Production of a nasoantral window with argon laser. J Oral Maxillofac Surg 1977;5:314-8. 6. Myers TD. Lasers in dentistry. J Am Dent Assoc 1991; 122:4750. 7. Pecaro BC, Garehime WJ. The CO2 laser in oral and maxillofacial surgery. J Oral Maxillofac Surg 1983;41:725-8. 8. Myers TD, McDanial DJ. Pulsed Nd:YAG dental laser: review of clinical applications. J Can Dent Assoc 1991;19:25-30. 9. Block CM, Mayo JA, Evans GH. Effects of the Nd:YAG dental laser on plasma sprayed and hyroxyapatite coated titanium dental implants: surface alteration and attempted sterilization. Int J Oral Maxillofac Implants 1992;4:441-9. 10. Chu RT, Watanabe L, White JM, Marshall GW, Marshall S J, Hutton JE. Temperature rise and surface modification of lased titanium cylinders [Abstract]. J Dent Res 1992;71:312. 11. Pick R. Using lasers in clinical practice. J Am Dent Assoc 1993;124:37-47. 12. Walsh WJ. The use of lasers in implantology: an overview. J Oral Implantology 1992;18:335-40. 13. Schuller DE. Use of the laser in the oral cavity. Otolaryngol Clin North Am 1990;23:31-42.
Reprint requests: James Q. Swift, DDS University of Minnesota Division of Oral and Maxillofacial Surgery 7-147 Moos Tower Minneapolis, MN 55455