THE EXCIMER LASER - BEAM SHAPING MODULES

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There are three beam shaping modules on the laser rail. The first module that the cool, ultraviolet laser beam encounters is the myopic, or nearsightedness, module.The strategy for treating myopia is to make the cornea flatter (a little less convex). Flattening of the cornea is achieved by removing more tissue from the center than the periphery.

The flattening is controlled by an iris diaphragm shutter that opens to control the size and shape of the laser beam. The shutter is controlled by the computer program. The iris diaphragm shutter opens more slowly for higher amounts of myopia. In this way, more flattening is achieved in the center of the cornea to correct higher amounts of myopia. This module is activated by the computer only if the surgeon programs in a myopic correction.

The second module that the laser beam can encounter is the astigmatic module. The strategy for treating astigmatism is to flatten the steep meridian of the cornea. Recall that astigmatism occurs when the cornea is steeper in one meridian and is shaped like a football or back surface of a teaspoon. Also note that the meridian 90 degrees away is flatter. Preferential flattening of the steeper meridian is achieved by opening a slit shutter to control the pattern of the laser beam.

This opening slit beam pattern is directed on to the center of the cornea, resulting in an ovoid pattern of reshaping. The opening slit shutter is controlled by the computer program. This module is activated by the computer only if the surgeon programs in an astigmatic correction. The combination of myopia and astigmatism is corrected by combining the opening slit and opening iris diaphragm together, creating an oval pattern of the center of the cornea.

The third module that the laser beam encounters is the hyperopic module. The strategy for treating hyperopia is to steepen the cornea. Steepening of the cornea is achieved by removing more tissue from the mid-periphery than the center of the cornea. The steepening is controlled by the computer directing the laser beam previously shaped by an opening slit to be focused on the mid-periphery of the cornea.

The computer directs the opening slit to move 360 degrees around the corneal surface, thus removing the mid-peripheral corneal tissue symmetrically. Directing the slit to mid-periphery of the cornea is achieved optically by an off-set lens placed in the path of the laser beam (Click here for illustration). The opening slit opens more slowly for higher amounts of hyperopia. In the case of hyperopia and astigmatism the computer directs the opening slit to pause momentarily over the flatter astigmatic meridian of the hyperopic astigmatic cornea. This corrects, simultaneously, both the hyperopia and astigmatism. This module is activated by the computer only if the surgeon programs in hyperopic correction. The third module actually contains the off-set lens that rotates 360 degrees around the center of the optical path of the oncoming laser beam. This directs the slit shaped laser beam, shaped by the second module, on to the mid-periphery of the cornea.

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