History Of Laser Eye Surgery – Part Two

In this, the second installment in the History of Laser Eye Surgery series, we take you from the point that the excimer laser was used in the process Barraquer termed keratomileusis – sculpting the cornea – to the establishment of PRK criteria which ultimately paved the way for LASIK laser eye surgery.

The History Of The Excimer Laser – 1917

In the early 1990s, in-situ keratomileusis was combined with the emerging technology of excimer lasers for corneal tissue ablation to finally become laser in-situ keratomileusis – with the birth of LASIK as we know it today. (11)

But the excimer laser has a history of its own starting in 1917. After Max Plank had described the quantum theory, Albert Einstein predicted that the energy released by an electron moving from an outer orbital to a lower energy inner orbital could be initiated by an external source, which he called stimulated emission. (12)

It was not until 1952 that Einstein’s theory was made a reality when microwaves were used as the external source to produce microwave amplification of stimulated emission of radiation – or a MASER.

Later, the microwave was replaced by light, and MASER became LASER as first defined by Gordon Gould in 1957. The breakthrough ideas were the use of optical pumping to kick-start the process, and to enable amplification of the emission by using a pair of parallel semi-reflective mirrors to partially reflect photons back and forth through the active medium.

In 1970, the term excimer laser was introduced to describe a laser built using a xenon dimer gas, the name excimer coming from an abbreviation of “excited dimer”.

It was not until 1981 that an Argon-Fluoride excimer laser was fired onto organic tissue when researchers at IBM made an incision in the leftover cartilage of a Thanksgiving dinner and found no evidence of damage to the surrounding tissue unlike the charring seen around an incision made with a solid-state laser with a wavelength of 532 nm.

They demonstrated that complex patterns could be made at a micronic level with each pulse removing a fraction of a micron. This research culminated in excimer lasers being used for etching microchips.

At the same time, Taboada, also found no thermal damage to the remaining tissue after a 248-nanometer excimer laser pulse onto corneal epithelium. (13)

Thus it was established that excimer laser-tissue interaction was effectively non-thermal, but rather one of direct splitting of molecular bonds with minimal adjacent heating. This process was coined in the term “photo-ablation”.

Trokel Connects Lasers With Eye Surgery – 1983

Having seen Taboada’s work, Trokel contacted Srinivasan at IBM in order to investigate the potential of using their excimer laser to improve the accuracy of radial keratotomy incisions. (14,15) Trokel later began working with Marshall to study the ultrastructural aspects of corneal photoablation. (16)

They compared the quality of the wounds made by an excimer laser at 193 nm with one at 248 nm as well as made by steel and diamond blades. The quality of the wounds was best with 193 nm. (17) This finding was in agreement with similar studies by other groups18-21 at that time. The wound quality suggested to Marshall that large area ablation could be performed in the central cornea, rather than just for peripheral linear incisions. This was described as photorefractive keratectomy (PRK).

For PRK to become a feasible procedure in human eyes, the following criteria needed to be satisfied:

  • First, the depth of tissue removal required for a given refraction change must be known. Munnerlyn proposed an algorithm adapted from Barraquer’s earlier formulae to calculate the ablation profile as a function of refractive error and optical zone diameter. (22)
  • Second, the quality and the clarity of the ablated surface must be preserved. Earlier studies in rabbit corneas demonstrated only limited haze after a large area ablation. (23) Myopic ablation on a donor eye also showed that the ablated surface was clean and smooth. (24)
  • Third, the wound healing process must not result in scarring. It was already known that there is no scarring after radial incisions. (25,26) Then, Marshall demonstrated no changes in corneal transparency 8 months after PRK in 12 monkey corneas,(27) and McDonald reported stable dioptric change in a primate cornea with good healing and long-term corneal clarity up to one year after PRK. (28)

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