Corneal lenticular formation using a femtosecond laser for hyperopia and mixed astigmatism correction
An ophthalmic laser system and related method for performing corneal lenticule incision and extraction for treating hyperopia and mixed astigmatism of the eye are described. Various techniques are used to optimize the procedure for concave lenticule incisions. One technique employs a fast-scan-slow-sweep scanning scheme to form the lenticule incisions, where the sweep angle increment is set to ensure at least double-pass cut for the entire lenticule. Another technique allows for desired distribution of refractive powers between the top and bottom lenticule incisions. Other techniques configure the lenticule such that its edge thickness is a constant; or such that the highest point of the top lenticule incision, located near the lenticule periphery, is at a predefined depth below the anterior corneal surface; or to maximize lenticule thickness for refractive treatment while ensuring that minimum anterior depth and minimum residual bed thickness in the peripheral region of the lenticule are maintained.
1 . A method implemented in an ophthalmic surgical laser system for forming a lenticule in a cornea of a patient's eye to treat hyperopia or mixed astigmatism, the method comprising:
by a laser source, generating a pulsed laser beam comprising a plurality of laser pulses;
by a high frequency scanner, scanning the pulsed laser beam back and forth at a predefined frequency to form a laser beam scanline, the scanline being a straight line having a defined length;
by a scanline rotator, rotating a direction of the scanline around an optical axis of the laser beam;
by a scanning system including an XY-scanner and a Z-scanner, delivering the scanline to the cornea and moving the scanline within the cornea in a depth direction along the optical axis of the laser beam and in two lateral directions perpendicular to the optical axis; and
by a controller, controlling the laser source, the high frequency scanner, the scanline rotator, and the scanning system to successively form a first plurality of scanline sweeps which collectively form a first lenticule incision of the lenticule in the cornea, wherein each sweep is formed by placing the scanline perpendicular to a meridian of the first lenticule incision and moving the scanline along the meridian from one edge of the lenticule incision to an opposite edge of the lenticule incision, wherein a sweep angle increment between the meridians of adjacent sweeps is equal to or smaller than a value θ 0 =sin −1 (L/D), wherein L is the length of the scanline and D is a diameter of the lenticule incision.
2 . The method of claim 1 , wherein the lenticule is configured to treat mixed astigmatism of the eye, and wherein the meridian of one of the sweeps is aligned with an astigmatism axis of the eye.
3 . The method of claim 1 , further comprising, by the controller, receiving a user input specifying the diameter D of the first lenticule incision, and calculating the value θ 0 based on the diameter D and the scanline length L.
4 . The method of claim 1 , further comprising, by the controller, controlling the laser source, the high frequency scanner, the scanline rotator, and the scanning system to successively form a second plurality of scanline sweeps which collectively form a second lenticule incision of the lenticule in the cornea, wherein each of the second plurality of scanline sweeps is formed by placing the scanline perpendicular to a meridian of the second lenticule incision and moving the scanline along the meridian from one edge of the second lenticule incision to an opposite edge of the second lenticule incision, wherein a sweep angle increment between the meridians of adjacent sweeps is equal to or smaller than the value θ 0 .
5 . The method of claim 4 , wherein the meridians of the first plurality of sweeps forming the first lenticule incision and the meridians of the second plurality of sweeps forming the second lenticule incision are offset from each other by one half of the sweep angle increment.
6 . The method of claim 4 , wherein the first lenticule incision is a top lenticule incision and has a concave shape that provides a first refractive power, the second lenticule incision is a bottom lenticule incision and has a concave shape that provides a second refractive power higher than the first refractive power, and wherein a combined refractive power of the top and bottom lenticule incisions is equal to a defined total refractive power.
7 . The method of claim 6 , further comprising, before forming the top and bottom lenticule incisions:
receiving an input that defines total lower order refractive powers, higher order refractive powers, and a refractive power distribution ratio between the top and bottom lenticule incisions;
calculating lower order refractive powers of the top and bottom lenticule incisions based on the total lower order refractive powers and the refractive power distribution ratio; and
adding the higher order refractive powers to the bottom lenticule incision.
8 . The method of claim 4 , wherein the first lenticule incision is a top lenticule incision and the second lenticule incision is a bottom lenticule incision, the method further comprising forming a ring cut in the cornea, wherein the ring cut extends along an entire periphery of the lenticule and intersect both the top and the bottom lenticule incisions to form an isolated volume of the lenticule, wherein at least one of the top and bottom lenticule incisions is a concave shape along at least one meridian and has different curvatures along two different meridians, and wherein an edge thickness of the lenticule, define as a vertical distance between an intersection of the ring cut with the top lenticule incision and an intersection of the ring cut with the bottom lenticule incision, is a constant value around an entire circumference of the lenticule.
9 . The method of claim 8 , wherein along at least some meridians, the top lenticule incision curves upwardly and then bends downwardly as it extends toward an edge of the lenticule, and the bottom lenticule incision curves downwardly and then bends upwardly as it extends toward the edge of the lenticule.
10 . The method of claim 8 , further comprising, before scanning a focus of the laser beam in the cornea:
receiving one or more refractive powers and a lenticule diameter as input from a user;
receiving the constant value of the edge thickness as an input from the user; and
calculating shapes of the top and bottom lenticule incisions based at least in part on the one or more refractive powers, the lenticule diameter, and the constant value of the edge thickness.
11 . The method of claim 4 , wherein the first lenticule incision is a top lenticule incision and the second lenticule incision is a bottom lenticule incision, the method further comprising, before forming the top and bottom lenticule incisions:
calculating a shape of the top lenticule incision based on a refractive power and a diameter of the top lenticule incision, the shape being a concave shape along at least some meridians, with a highest point located near a periphery of the top lenticule incision;
calculating a depth of the top lenticule incision within the cornea by placing the highest point at a predefined depth below an anterior corneal surface; and
calculating a shape and a depth of the bottom lenticule incision based on a refractive power and a diameter of the bottom lenticule incision;
wherein the top lenticule incision and the bottom lenticule incision are formed according to the calculated shapes and depths.
12 . The method of claim 11 , further comprising, before calculating a depth of the top lenticule incision, receiving the predefined depth as an input from a user.
13 . The method of claim 4 , wherein the first lenticule incision is a top lenticule incision and the second lenticule incision is a bottom lenticule incision, the method further comprising, before forming the top and bottom lenticule incisions:
determining a corneal thickness in an outer area of the cornea near an edge of the lenticule;
calculating a maximum lenticule thickness by subtracting a minimum anterior depth and a minimum residual bed thickness from the corneal thickness; and
calculating shapes of a top lenticule incision and a bottom lenticule incision, wherein along at least one meridian, both the top lenticule incision and the bottom lenticule incision have a concave shape, and wherein a vertical distance between a highest point of the top lenticule incision and a lowest point of the bottom lenticule incision is smaller than or equal to the maximum lenticule thickness;
wherein the top lenticule incision and the bottom lenticule incision are formed according to the calculated shapes.
14 . The method of claim 13 , wherein the minimum anterior depth is from 90 to 150 μm and the minimum residual bed thickness is from 200 to 300 μm.
15 . An ophthalmic surgical laser system for forming a lenticule in a cornea of a patient's eye to treat hyperopia or mixed astigmatism, comprising:
a laser source configured to generate a pulsed laser beam comprising a plurality of laser pulses;
a high frequency scanner configured to scan the pulsed laser beam back and forth at a predefined frequency to form a laser beam scanline, the scanline being a straight line having a defined length;
a scanline rotator configured to rotate a direction of the scanline around an optical axis of the laser beam;
a scanning system including an XY-scanner and a Z-scanner, configured to deliver the scanline to the cornea and move the scanline within the cornea in a depth direction along the optical axis of the laser beam and in two lateral directions perpendicular to the optical axis; and
a controller configured to control the laser source, the high frequency scanner, the scanline rotator, and the scanning system to successively form a first plurality of scanline sweeps which collectively form a first lenticule incision of the lenticule in the cornea, including forming each sweep by placing the scanline perpendicular to a meridian of the first lenticule incision and moving the scanline along the meridian from one edge of the lenticule incision to an opposite edge of the lenticule incision, wherein a sweep angle increment between the meridians of adjacent sweeps is equal to or smaller than a value θ 0 =sin −1 (L/D), wherein L is the length of the scanline and D is a diameter of the lenticule incision.
16 . The system of claim 15 , wherein the controller is further configured to receive a user input specifying the diameter D of the first lenticule incision, and to calculate the value θ 0 based on the diameter D and the scanline length L.
17 . The system of claim 15 , wherein the controller is further configured to control the laser source, the high frequency scanner, the scanline rotator, and the scanning system to successively form a second plurality of scanline sweeps which collectively form a second lenticule incision of the lenticule in the cornea, including forming each of the second plurality of scanline sweeps by placing the scanline perpendicular to a meridian of the second lenticule incision and moving the scanline along the meridian from one edge of the second lenticule incision to an opposite edge of the second lenticule incision, wherein a sweep angle increment between the meridians of adjacent sweeps is equal to or smaller than the value θ 0 .
18 . The system of claim 17 , wherein the meridians of the first plurality of sweeps forming the first lenticule incision and the meridians of the second plurality of sweeps forming the second lenticule incision are offset from each other by one half of the sweep angle increment.
19 . The system of claim 15 , wherein the lenticule is configured to treat mixed astigmatism of the eye, and wherein the meridian of one of the sweeps is aligned with an astigmatism axis of the eye.
20 . A method implemented in an ophthalmic surgical laser system for forming a lenticule in a cornea of a patient's eye to treat hyperopia or mixed astigmatism, the method comprising:
operating the ophthalmic surgical laser system to generate a pulsed laser beam; and
scanning a focus of the laser beam in the cornea to form a top lenticule incision, a bottom lenticule incision, and a ring cut in the cornea, wherein the ring cut extends along an entire periphery of the lenticule and intersect both the top and the bottom lenticule incisions to form an isolated volume of the lenticule, wherein at least one of the top and bottom lenticule incisions is a concave shape along at least one meridian and has different curvatures along two different meridians, and wherein an edge thickness of the lenticule, define as a vertical distance between an intersection of the ring cut with the top lenticule incision and an intersection of the ring cut with the bottom lenticule incision, is a constant value around an entire circumference of the lenticule.