BERLIN – Refractive surgery performed with the ArF excimer laser is gaining widespread acceptance and is among the well established procedures to correct nearsightness. In contrast to angloamerican citizens most German patients are still treated with photorefractive keratectomy (PRK). Laser in situ keratomileusis is less often performed due to additional risks of the procedure.

The patients undergo various diagnostic examinations as refraction and topometry before laser treatment. The settings of the laser parameter are a result of these examinations and of the personal experience of the clinician. So called individual nomograms are deduced of these parameters. Until now nomograms have been inevitable as there were no means to control the laser treatment online and to get an actually measured value of the amount of removed tissue. A miscorrection cannot be avoided if the relevant parameters of the corneal tissue differ from the expected normal values. The incidence of this deviation is in the order of several percent. These unhappy patients have the choice between continuing to wear glasses and receiving another treatment which is commonly referred to as an enhancement.

For the first time there is a method to measure the topology of the cornea during the laser procedure. This allows to obey the ongoing ablation process at any time of the treatment. The results of the measurement is communicated to the laser control unit. This unit continuously adapts the process to the current state of the cornea. Miscorrections due to wrong assumptions of the corneal tissue are thus eliminated. Individual tissue properties guide the procedure rather than relying on doubtfully standardised parameters.

The measuring principle is adopted from the non tactile method of fringe projection. Fringe projection is a very popular topometry method in industrial applications. One or several patterns of parallel lines are projected onto the surface of the object. Detection of the pattern with a digital camera under a tilted angle results in a bending of the lines. The amount of the bending is directly related to the topology. The result of the measurement is obtained within a few seconds.

Only contributions from the most outer layer of the cornea without tear film are analysed. This results in an exact topometry especially during laser surgery when the tear film is suppressed. The accuracy of the method is in the range of a few microns. It works with PRK and with LASIK procedures.

A German patent is pending. An international patent is currently being prepared. Next steps include further developments of the prototype together with medical laser companies.