Surgery – Instruments – Light application
Reexamination Certificate
2000-08-02
2002-11-19
Peffley, Michael (Department: 3739)
Surgery
Instruments
Light application
C606S013000, C606S002000
Reexamination Certificate
active
06482199
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally related to the field of pulsed electromagnetic energy source systems suitable for material and biological tissue modification processing and removal and is more particularly related to a material removal and modification method and apparatus in which pulsed electromagnetic sources of high ablation-to-deposition depth ratios are operable at pulse repetition rates ranging up to approximately several hundreds of thousands of pulses per second so as to efficiently and precisely remove substantial material volumes while substantially eliminating collateral damage.
BACKGROUND OF THE INVENTION
The past three decades have brought increased interest in the use of lasers in material processing applications. Early procedures for material processing and cutting involved optical drilling using continuous wave or relatively long pulse (e.g., 50 to 350 &mgr;s) lasers such as CO2, ruby and ND:YAG (Neodymium doped Yittrium Aluminum Garnet). These systems, however, required relatively high radiant exposure and resulted in significant alterations to surrounding tissue. As a consequence, lasers could become an effective cutting tool only in areas which did not require high degree of precision or control.
Optical drilling with ER:YAG (Erbium doped YAG) lasers yielded encouraging results in the late 1980s, and has demonstrated its capability to perform as an efficient drill while incurring only relatively low levels of collateral damage to surrounding tissue, provided that no more than one to three pulses per second were applied to the target material. The success of ER:YAG systems, operating in the microsecond pulse duration regime and minimizing thermal damage has also been observed in several areas of applications in material processing and medicine, and can be attributed to the high absorption coefficient of these materials at the particular wavelengths characteristic of the Er:YAG system (2900 nm), when used in combination with the relatively short pulse durations and at low pulse repetition rates.
Laser systems adapted to hard tissue processing, such as dentin and enamel removal in dental applications are disclosed in: 1. Hibst R, Kelly U. Experimental studies of the application of the Er:YAG laser on dental hard substances: I. Measurement of the Ablation Rate.
Laser Surgery and Medicine
1989, 9:352-7; and, 2. Keller U, Hibst R. Experimental studies of the application of the Er:YAG laser on dental hard substances: II. Light microscopy and SEM investigations.
Lasers in Surgery and Medicine
1989; 9:345-351.)
Both pulsed CO2 and Er:YAG are disclosed in: Walsh, J. T., Flotte, T. J., Anderson, R. R., Deutsch, T. F., “Pulsed CO
2
Laser Tissue Ablation: Effect of Tissue Type and Pulse Duration on Thermal Damage,”
Lasers in Surgery and Medicine,
Vol. 8, pp. 108-118, 1988; Walsh, J. T., Flotte, T. J., Deutsch, T. F., “Er:YAG Laser Ablation of Tissue: Effect of Pulse Duration and Tissue Type on Thermal Damage,”
Lasers in Surgery and Medicine,
Vol. 9, No. 4, pp. 314, 1989; and Walsh, J. T., Deutsch, T. F., “Er:YAG Laser Ablation of Tissue: Measurement of Ablation Rates,”
Lasers in Surgery and Medicine,
Vol. 9 No. 4, pp. 327, 1989.
A Ho:YSGG laser system is disclosed in Joseph Neev, Kevin Pham, Jon P. Lee, Joel M. White, “Dentin Ablation with Three Infrared Lasers,”
Lasers in Surgery and Medicine,
18:121-128 (1996).
The laser systems disclosed (Er:YSGG, HO:YSGG, and Pulsed CO2) all operate in the IR region of the electromagnetic spectrum and are pulsed in two different regimes: about 250 microsecond pulse durations for the ER:YSGG and HO:YSGG lasers, and about 150 microsecond pulse durations for the CO2 system.
While the disclosed removal rate is in the range of approximately tens of micrometers per pulse, the disclosed laser systems exhibit wavelength dependent absorption and result in high removal rates by operating at pulse energies in excess of 30 millijoules per pulse and often on the order of a few hundreds of &mgr;J per pulse. Enhancing material removal by increasing laser power is, however, accompanied by increased photothermal and photomechanical effects which causes collateral damage in adjacent material. In addition, increasing power leads to plasma de coupling of the beam, e.g., incident laser energy is wasted in heating the ambient in front of the target. High intensity pulses additionally cause very loud acoustic snaps, when the laser pulse interacts with tissue. These snaps or pops include a large high frequency component which is very objectionable to a user or, in the case of a medical application, to a patient. In addition to the psychological impact of such noise, these high frequency snaps are able to cause hearing loss in clinicians when repeated over a period of time.
U.S. Pat. No. 5,342,198, to Vassiliadis, et al. discloses an ER:YAG IR laser system suitable for the removal of dentin in dental applications. The laser produces a pulsed output having a beam with a pulse duration in the range of several tens of picoseconds to about several milliseconds. Although disclosed as being efficient in the removal of dentin and dental enamel, the mechanism by which material removal is effected is not understood. Significantly, however, the only laser systems disclosed as suitable for the process are those which operate at wavelengths (1.5 to 3.5 microns) that have proven to be generally effective for enamel interaction. Thus, the absorption characteristics of the material target are of primary concern to the removal rate. In addition, high energy levels are required to remove enamel and dentin, leading to the problem of thermal damage and acoustic noise.
Additional possibilities for the application of lasers to the field of dentistry in particular, and to hard tissue ablation in general, have been proposed by the use of excimer lasers that emit high intensity pulses of ultraviolet (UV) light.
Several such pulsed UV excimer laser systems, typically with pulse durations in the approximately 1 to 125 nanosecond range are disclosed in:
1. Neev J, Stabholz A., Liaw L. L, Torabinejad M, Fujishige J. T., Ho P. H, Berns M. W., “Scanning Electron Microscopy and Thermal characteristics of Dentin ablated by a short-pulse XeCl Laser”, Lasers in Surgery and Medicine;
2. Neev J, Liaw L, Raney D, Fujishige J, Ho P, Berns M. Selectivity and efficiency in the ablation of hard Dental tissue with ArF pulsed excimer lasers. Lasers Surgery and Medicine 1991; 11:499-510;
3. Neev J, Raney D, Whalen W, Fujishige J, Ho P, McGrann J, Berns M. Ablation of hard dental tissue with 193 nm pulsed laser radiation: A photophysical study. Spie proceedings, January 1991; and
4. Neev J, Raney D, Whalen W, Fujishige J, Ho P, McGrann J, Berns M. Dentin ablation with two excimer lasers: A comparative study of physical characteristics. Lasers Life Sci 1992; 4(3):1-25. Both the short wavelengths and nanosecond range pulse durations used by excimer lasers contribute to define a different regime of laser-tissue-interaction. Short wavelength ultraviolet photons are energetic enough to directly break chemical bonds in organic molecules. As a consequence, UV excimer lasers can often vaporize a material target with minimal thermal energy transfer to adjacent tissue. The resultant gas (the vaporization product) is ejected away from the target surface, leaving the target relatively free from melt, recast, or other evidence of thermal damage.
Another important characteristic of UV excimer lasers is that materials which are transparent to light in the visible or near infra-red portions of the electromagnetic spectrum often begin to exhibit strong absorption in the UV region of the spectrum. It is well established that the stronger a materials absorption at a particular wavelength, the shallower the penetration achieved by a laser pulse having that wavelength. Thus, in many types of materials, a pulse typically only penetrates to a depth in the range of from about 10 to about 100 micrometers. By simply counting pulses, great precision can be achieved in defining removal dep
Peffley Michael
Price and Gess
Vrettakos Pete J
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