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Key words: refractive lasers, corneal ablation, bioeffects, tech support, research
Refractive laser surgery continues to attract interest from the public, clinicians and the medical device community. The allure of reduced dependence on glasses is the primary motivation for patients although demand for the procedure may not be as strong as initially expected. Nevertheless, the large base of potential patients (approximately 25% of the US population is myopic) seems to be adequate incentive for practitioners and developers. While only two companies have PMA-approved devices, several firms are
in the latter stages of clinical trials, and PMA submissions from these companies are expected in the next year. The situation is further complicated by companies and physicians that either develop one-of-a-kind devices or re-import, refurbish, and use machines originally intended for the non-US market.
Regardless of the source of the device, the ablation process, especially as it affects the individual patients treatment, is still not fully understood. The lack of a thorough and quantitative view of ablation is particularly acute for the small scale developers. In many cases, these investigators have single site trials with small numbers of patients. Hence, the efficacy of unique treatment algorithms would be better assessed if the ablation process was more fully characterized. Also, next-generation devices are expected to incorporate topographic and/or real-time guidance of the ablation. Review of preliminary data from these devices will benefit from a greater understanding of the mechanisms of ablation and the factors that influence individual variations.
One critical aspect of the ablation process is the etch depth (depth of
material removed with each laser pulse). This quantity is predominantly a
function of the laser fluence (optical energy density per unit area) and the
optical absorption coefficient of the cornea tissue. In FY 97, OST scientists
completed a study to re-measure the absorption coefficient of corneal tissue
using an experimental technique called rotating-analyzer ellipsometry. Results
from these experiment agreed well with OSTs previous reflectometry measurements
and further suggest that the absorption coefficient is very large
(approximately 3.3 - 4.0 per micron).
Figure 8 - ETCH Rates Predicted by a Blow-Off Model
Figure 8 indicates the etch rates predicted by a blow-off model based upon absorption coefficients of Puliafito et al. (short dash), OSTs earlier reflectometry results (long dash), and the present ellipsometry measurements (solid line), compared with measured corneal etch rates from the literature (open circles).
Increasing understanding of the fundamental mechanisms of ablation is very beneficial and provides the basis of consultations with ODE on refractive lasers. This knowledge is particularly valuable for the evaluation of devices that incorporate unique and unusual treatment modes. In many cases, the performance of these devices do not simply build upon the base of information amassed by the relatively large clinical trials conducted by the PMA-approved devices. Hence, detailed and complete information about the physical mechanisms of ablation are important in assessing the effectiveness and safety of these devices.