In the original LASIK, the corneal flap is cut with a microkeratome, a device similar to a carpenter’s plane. Any metallic shards or irregularities in the blade can easily damage the delicate cornea. To avoid this problem, a team of physicists, engineers, and ophthalmologists at the University of Michigan developed a procedure to make this cut with the intense pulses of the femtosecond laser.

To avoid the side-effects associated with a cut made by a microkeratome blade, the ophthamologist cuts the flap with the laser itself (image courtesy of Intralase).

Flap (right) and disk of corneal material (left) removed from a pig’s eye in a procedure performed entirely with an excimer laser (image courtesy of Center for Ultrafast Optical Science, University of Michigan. http://www.eecs.umich.edu/USL/).

The pulses of the femtosecond laser last only 10^-13 seconds, as opposed to 10^-8 seconds for the excimer, and these ultrashort pulses are made much more intense by a technique called chirped pulse amplification. In general, a laser pulse is amplified by passing it through additional matched lasers, where stimulated emission can vastly increase the number of photons. At very high power, the amplified pulse can have so much energy that it could destroy these lasers. To sidestep this effect, a group at Michigan’s Center for Ultrafast Optical Science developed a way to spread out the different frequencies in the pulse with diffraction gratings to produce a much longer and less intense pulse that can be amplified, as shown in the drawing. Then, with amplification complete, the femtosecond pulse is reconstituted.

Getting back to LASIK, to avoid the damage caused by the microtome, surgeons cut out the underside of the flap with femtosecond laser pulses. These pulses are focused inside the cornea and vaporize tissue at the focal point. The result is a short-lived bubble of gas, which dissolves into the water in the cornea. A rapid sweep of the focus creates a surface of bubbles that define the underside of the flap, and a second cut, which is cylindrical, enables the surgeon to fold back the flap. At this point, the excimer laser performs the LASIK surgery, essentially as before.

Beyond cutting the flap, the Michigan team is developing a way to perform LASIK entirely with the femtosecond laser. In this surgery, the laser ablates two curved surfaces, which define the material to be removed. The upper surface is also the underside of the flap. The surgeon then folds back the flap and removes the “lenticle” of corneal tissue underneath. The image shows the flap, and the disk-shaped tissue that was removed, when this experimental procedure was performed on a pig’s eye.

A further possible procedure avoids the flap altogether. Here the laser would ablate a lenticle-shaped tissue within the cornea, the bubbles would be absorbed in the watery tissue, and the upper and lower surfaces of the ablated lenticle would grow together, providing a reshaped cornea.

This kind of research, not to mention the development of LASIK itself, is made possible by collaboration among physicists, engineers, and MDs. The story is the same for numerous other advances in high-tech medicine, such as the PET scan, the pacemaker/defibrillator, and the endoscope, to name but a few. The corresponding fields of physics range from elementary particles to electricity and magnetism to optics, and more.