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London-The structural integrity of the cornea is compromised by refractive surgeries, and the deeper the surgery is the more compromise takes place, according to John Marshall, PhD, FRCOphth(Hon).
Dr. Marshall, the Frost professor of ophthalmology, St. Thomas' Hospital, London, described how he uses optical engineering to determine the fabric of the cornea and how and why that fabric is weakened by surgery.
Designing the cornea would be a nightmare, according to Dr. Marshall, because of the properties needed for its proper functioning.
All of these characteristics have different physical requirements. To allow light through, the cornea must be thin, but conversely, to contain IOP it must be thick. "At the same time, because it is a dynamic system it must be able to cope with the forward and inward movement of the ciliary processes during accommodation, and it must contain the variants of blood pressure," he said.
Considering all these factors, the cornea is constantly under stress. Also considering hysteresis, Dr. Marshall pointed out, if any biologic system is loaded, it will slowly increase in size and return to baseline if the load factor is removed.
"However, if the biologic system is overstressed, it may not return to its normal size," he said.
While Barraquer defined ectasia as dilation or expansion, Dr. Marshall prefers to express the dictionary definition as fatigue.
"We are not necessarily looking at a gross loss of corneal elasticity, but we are looking at a fatigue in the system," he stated.
When the surface of the cornea is subjected to surgery, complications include epithelial hyperplasia. If the surgery is deep, it is thought that there are no disturbances. However, Dr. Marshall pointed out that among the eyes on which he or George O. Waring III, MD, FACS, FRCOphth, of InView Vision, Atlanta, have performed LASIK, about 200 eyes have had the corneas removed.
"In my study, all the perturbations occurred in the flap, not in the stromal bed. Considering the structure of the cornea, the surface lamellae are small and highly interrelated, making for a surface with high mechanical strength. Deep in the cornea the mechanical strength becomes progressively weaker," Dr. Marshall explained.
Dr. Marshall cited a study in which x-ray diffraction showed that the intersections of fibers vary in the peripheral cornea.
"At the center the fibers cross roughly at right angles, and more peripherally the angle increases," he explained. Dr. Marshall likened this to the structure of a piece of fabric in which the weft and warp cross at right angles. In the cornea, the fibers are crossed at 120° to produce a strong structure.
Considering this structural design, the current refractive surgeries seem to be less than beneficial for the corneal structure.
"Unfortunately, our current surgeries are performed in the strong area in depth and in the strong area peripherally. During surface or deep ablations, clearly progressively more fibers are cut the deeper the refractive procedure is," Dr. Marshall said.
In their study of the corneal structure, he and his colleagues have been using optical engineering measurements that have been used to evaluate railroad bridges and tires following the crash in Paris of the Concorde.
"We started with electronic speckle pattern interferometry. This allows us to measure the displacement of the cornea but also the strain map set up within the cornea. We found that after cutting and replacing a flap during LASIK, the corneal integrity is not replaced," Dr. Marshall emphasized.
Later, using shearing interferometry, Dr. Marshall and his colleagues have been able to obtain a very good analysis of the precise movements in the corneal system.