Surgery – Instruments – Light application
Reexamination Certificate
2002-01-29
2003-12-23
Peffley, Michael (Department: 3739)
Surgery
Instruments
Light application
C606S005000, C606S011000
Reexamination Certificate
active
06666857
ABSTRACT:
BACKGROUND OF THE INVENTION
A review of the prior art reveals several US patents that define the status of scanning spot laser ablation and or eye-tracking systems. Lin, in U.S. Pat. No. 5,520,679, describes a scanning laser system and method of beam placement to produce smooth ablated surfaces; no compensation for eye motion, saccadic or other, is disclosed nor is there any feedback mechanism for controlling the location of the beam. Frey et. al, in U.S. Pat. Nos. 5,632,742, 5,980,513 puts forth a LADAR based eye-tracking apparatus in conjunction with a scanning excimer beam ablation system. Knopp et. al, presents in U.S. Pat. No. 5,865,832 a 2-axis servo-controlled mirror for tracking eye movement. Hohla in U.S. Pat. No. 5,645,550 uses a semi-rigid marking structure placed on the periphery of the eye.
These referenced patents are all implemented in analog form. The Frey and Knopp patents utilize contrast differences of eye tissue alone without the use of any external marking means; Frey selects the iris and pupil boundary while Knopp uses the iris and sclera boundary as the reference means for determining lateral (transverse or radial) eye position variations. In Frey's approach, the pupil must be maintained in the dilated state. Knopp, in utilizing PSD's (position sensing devices), averages the contrast of several portions of the iris/sclera boundary. While both systems under ideal conditions are capable of high eye tracking accuracy, the ablation procedure can produce changes in contrast across the stromal surface and thereby degrade the tracking precision. The Hohla patent presents an externally applied aiming assistance means for reducing such contrast variations, but does not specifically disclose means for achieving adequate speed in the tracking of saccadic eye motions.
Recent work in the field of evaluating overall optical performance of the eye using wavefront techniques (for example Williams et. al. U.S. Pat. No. 6,199,986) have presented means for determining the distortions of corneal surface and the other optical elements of the eye in terms of mathematical functions know as Zernike polynomials. PRK has been performed using the topography information provided by such wavefront techniques with considerable success in patients having slight to moderate myopia. A high degree of precision is required in locating the laser beam on the cornea and in determining the duration of ablation to produce the desired customized corneal surface. Real time topography measurement would greatly facilitate such customized ablations by insuring that the desired corneal topography is achieved.
A prospective system for performing PRK is one where the desired anterior corneal surface is sculpted with such a minimum of trauma that the surface topography immediately after ablation remains permanent thereafter, that postoperative discomfort is negligible and that after the regrowth of the epithelium, the wound healing response results in no corneal haze and the acuity of vision is retina and diffraction limited only. It is intuitive that for each laser-ablating pulse, minimizing the amount of energy delivered will also minimize the amount of phononic shock, heating and other traumatic effects on the cornea. The intensity of the pulse, i.e. the energy per unit area, must be above a threshold value, typically 50 mJ/cm
2
for 193 nm, in order to break the molecular bonds of the stromal cellular structure. At an intensity of about 150 to 200 mJ/cm
2
, the per pulse ablation depth for stromal tissue averages 0.25 &mgr;m. In prior art, this relatively constant per-pulse ablation depth provides an a priori means for predicting the resultant ablation depth, such a procedure being necessary in the absence of a real time topographic measurement apparatus. Lower intensities that could minimize trauma would require a longer operation time and would result in a very uncertain prediction of tissue removal. Minimizing the cross-sectional area of the ablating laser beam also can result in reduced trauma and also reduce the cost of the laser because the per pulse energy can be lowered thereby requiring a smaller laser that may take the form of a solid state frequency-multiplied laser or a small excimer laser. The minimization of intensity/fluence and beam crosssectional area is limited by the need to perform the desired ablation quickly enough to avoid excessive stromal dehydration and patient stress. A solution to minimizing operating time would involve maximizing the laser pulse rate. Such an approach places an increasing demand on the bandwidth of the laser beam scanning system and the real time topography system.
In most existing PRK procedures, the corneal epithelium is removed by mechanical and/or chemical means in order to expose the stroma for laser ablation. Swelling of the cornea has been observed using such techniques. In Hohla (U.S. Pat. No. 6,090,100), a method for removal of the corneal epithelium via excimer laser is presented wherein a dye, which is absorbed by the epithelium and not by the stroma, fluoresces in the presence of excimer radiation to guide selective epithelium removal. However, the use of a broad laser beam subjects the overall cornea to the same shock trauma encountered in general broad beam PRK. A narrow scanning laser beam is desirable to minimize such trauma along with removing the need for the fluorescent dye for the procedure of epithelial ablation.
The present invention addresses the foregoing items and proposes to meet the goals outlined within the state of the art of existing technology.
SUMMARY OF THE INVENTION
The system and method of the present invention generally comprise:
A pulsed laser operating at a high pulse rate producing a narrow beam of ablating radiation having a wavelength in the region of 193 nm; this pulsed beam combined collinearly with a similarly narrow continuous laser beam, the combined beam then directed to a two-axis electromechanically-controlled tiltable mirror whereupon it is reflected to a parabolic mirror (paraboloid) which collects the combined beam paths reflected from the tiltable mirror and collimates them after which they are separated by a wavelength selective means—the pulsed laser radiation scanning beam being directed normally to the surface of the ablatable object (anterior cornea of the eye) and the continuous laser beam directed to a two-dimensional photodetection device which generates feedback voltage signals to control the tiltable mirror,
an annular scleral mask with inscribed reference markings which is fitted over the eye, the mask leaving the cornea exposed to the collimated rays of pulsed radiation, the reference markings imaged by an objective lens onto a photodetector array of linear pixel elements, the photodetection signals used to control the tiltable mirror to compensate for translational and rotation eye movement; the mask being attached prior at the outset so that a wavefront means can be used to measure the optical distortions of the eye whereupon a corneal surface is calculated to correct the distortions;
a raster videokeratography topography system utilizing a portion of the excimer/ablating laser radiation to project a raster pattern on the semi-diffuse surface of the cornea undergoing photoablation, the diffusely reflected pattern then optically imaged onto a two dimensional photodetector whereupon it is digitized and the surface topography calculated;
the wavefront means utilizing the same components of the beam scanning system to produce the requisite collimated rays for retinal image mapping and optical analysis;
a control system performing the following functions: Sensing tiltable mirror position and through an analog high frequency loop and a digital mid/low-frequency loop, positioning the mirror to achieve beam positioning precision exceeding that of prior art; sensing the position of the cornea and adjusting the beam position to the desired precision within 0.0005 second; monitoring the topography a minimum of 5 maps per second to provide feedback control of the ablating beam;
the contro
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