Keratorefractive surgery, success, and the public health

Evaluation of a clinical aberrometer for lower-order accuracy and repeatability, higher-order repeatability, and instrument myopia

BACKGROUND

Refractive surgery has stimulated the development of aberrometers, which are instruments that measure higher-order aberrations. The purpose of this study was to test one clinical aberrometer, the Complete Ophthalmic Analysis System (COAS), for its accuracy, repeatability, and instrument myopia for measuring sphere and astigmatism and its repeatability for measuring higher-order aberrations.

METHODS

Aberrations of 56 normal eyes (28 subjects) were measured with and without cycloplegia using a COAS, a conventional autorefractor and by subjective refraction. We evaluated lower-order accuracy (sphere and astigmatism) of the COAS and autorefractor by comparing that data with that of subjective refraction. We also tested COAS lower- and higher-order repeatability for 5 measurements taken in less than 1 minute. We evaluated instrument myopia by comparing cycloplegic and noncycloplegic measurements of the same eye. Data were analyzed for a 5.0-mm-diameter pupil.

RESULTS

Mean COAS spherical error was between -0.1 and +0.4 diopters (D), depending on cycloplegia and the kind of sphere power computation selected. Cylinder power errors were less than 0.1 D. COAS repeatability coefficients were better than 0.25 D, and instrument myopia was less than 0.4 D. These were comparable with those of autorefraction. Higher-order repeatability was sufficient to allow reliable measurement of normal third-order aberrations and spherical aberration.

CONCLUSIONS

Accuracy, repeatability, and instrument myopia of the COAS are similar to those of a conventional autorefractor. Accuracy and repeatability are also similar to those of subjective refraction. Like an autorefractor, the COAS provides instantaneous, objective measurements of sphere and astigmatism, but it also measures higher-order aberrations. We found that it is capable of reliably measuring problematic higher-order aberrations and is therefore a valuable asset for modern clinical eye care.

Responsiveness of the National Eye Institute Refractive Error Quality of Life instrument to surgical correction of refractive error

OBJECTIVE

Refractive error and the means by which it is corrected may impact substantially on quality of vision and health-related quality of life in ways not captured adequately by standard measures of visual acuity. The goal of this analysis was to evaluate the responsiveness of the National Eye Institute Refractive Error Quality of Life (NEI-RQL) instrument to surgical correction of refractive error.

DESIGN

Prospective, multicenter cohort study.

PARTICIPANTS

The NEI-RQL, a 42-item measure with 13 scales, was self-administered by 185 patients before and after undergoing surgical correction of myopic or hyperopic refractive error. Preoperative and postoperative clinical information was collected, including refractive error and corrected visual acuity.

METHODS

Differences between preoperative and postoperative NEI-RQL scores were examined. Responsiveness was assessed using the standardized response mean and the responsiveness statistic. We also compared scales using relative efficiency estimates.

MAIN OUTCOME MEASURES

Changes in NEI-RQL scales (clarity of vision, expectations, near vision, far vision, diurnal fluctuations, activity limitations, glare, symptoms, dependence on correction, worry, suboptimal correction, appearance, and satisfaction with correction).

RESULTS

For myopes and hyperopes combined, refractive surgical correction was associated with statistically significant (P<0.05) improvements in scores for 11 of 13 scales. The largest improvements, ranging from 26 to 58 points on the 0 to 100 possible score range, were seen in expectations, activity limitations, dependence on correction, appearance, and satisfaction with correction. Separate analysis of myopes and hyperopes revealed similar effects in the 2 groups. Baseline scores were found to be predictive of change after surgery.

CONCLUSIONS

The NEI-RQL is responsive to changes in vision-targeted health-related quality of life resulting from keratorefractive surgery. This instrument may prove useful for evaluating the beneficial and adverse impacts of surgical and nonsurgical methods of refractive error correction.

Night vision disturbances after corneal refractive surgery

A certain percentage of patients complain of "glare" at night after undergoing a refractive surgical procedure. When patients speak of glare they are, technically, describing a decrease in quality of vision secondary to glare disability, decreased contrast sensitivity, and image degradations, or more succinctly, "night vision disturbances." The definitions, differences, and methods of measurement of such vision disturbances after refractive surgery are described in our article. In most cases of corneal refractive surgery, there is a significant increase in vision disturbances immediately following the procedure. The majority of patients improve between 6 months to 1 year post-surgery. The relation between pupil size and the optical clear zone are most important in minimizing these disturbances in RK. In PRK and LASIK, pupil size and the ablation diameter size and location are the major factors involved. Treatment options for disabling glare are also discussed. With the exponential increase of patients having refractive surgery, the increase of patients complaining of scotopic or mesopic vision disturbances may become a major public health issue in the near future. Currently, however, there are no gold-standard clinical tests available to measure glare disability, contrast sensitivity, or image degradations. Standardization is essential for objective measurement and follow-up to further our understanding of the effects of these surgeries on the optical system and thus, hopefully, allow for modification of our techniques to decrease or eliminate post-refractive vision disturbances.

Functional outcome and satisfaction after photorefractive keratectomy. Part 1: development and validation of a survey questionnaire

OBJECTIVE

The aim of this study was to develop a valid, reliable, and easy-to-administer instrument to assess patient satisfaction and perceived outcome after bilateral excimer laser photorefractive keratectomy.

DESIGN

Development and validation of a psychometric questionnaire.

PARTICIPANTS

Consecutive patients who underwent bilateral excimer laser photorefractive keratectomy from May 1994 through May 1997 by 12 surgeons from four collaborating centers. To be eligible, a minimum of 4 months since the last surgery and a maximum of 30 months since the first surgery was required.

METHODS

The new instrument was derived in part from the Prospective Evaluation of Radial Keratotomy (PERK) study 10-year psychometric questionnaire and the Visual Functional Index (VF-14), an index of functional impairment in patients with cataract. Questions were grouped in seven scales, each covering a specific aspect of quality of vision. These included global satisfaction, quality of uncorrected vision, quality of corrected vision, quality of night vision, glare, daytime driving, and night driving.

MAIN OUTCOME MEASURES

Acceptability, reliability, validity, and interpretability of the instrument, as well as its ease of administration.

RESULTS

The instrument scale structure was examined and scale scores were created. Item-discriminant validity ensured that questions belonged to their hypothesized scale, based on multitrait correlation analysis. The instrument was shown to be reliable by a high level of internal consistency, and all Cronbach's alpha coefficients were superior or equal to 0.83. Construct-related validity and interpretability were assessed based on correlations between scale scores and clinically recognized success criteria such as visual acuity and refraction. Respondent burden was shown to be minimal. Acceptability of the instrument was shown to be very good, with a participation rate of 74.3% (690 of 929 patients). The instrument is available in English and in French and the translation was shown to be reliable.

CONCLUSIONS

The acceptability, reliability, and interpretability of the instrument, as well as its ease of administration, were shown to be adequate. This questionnaire appears clinically useful to document patient satisfaction after excimer laser photorefractive keratectomy.

Contrast sensitivity measured by 2 methods after photorefractive keratectomy

PURPOSE

To study contrast sensitivity in eyes that had flying-spot excimer laser photorefractive keratectomy (PRK) and to compare a subjective method (Vision Contrast Test System [VCTS] 6500) and an objective method (visual evoked potential [VEP]) of measuring contrast sensitivity.

SETTING

Istituto Clinica Oculistica, Università degli Studi, Bari, Italy.

METHODS

Contrast sensitivity changes over time were evaluated in 26 eyes. The baseline values were compared with measurements 3, 6, and 12 months after PRK using the VCTS 6500 and VEP. Contrast threshold and VEP amplitude were classified by myopic correction.

RESULTS

Contrast threshold values changed significantly over time (P <.001). Significant differences were found between mean contrast threshold preoperatively and 12 months postoperatively (P <.001) at all spatial frequencies. A significant relationship was established between baseline and 12 month measurements at 18 cycles per degree. Mean VEP amplitude measurements also changed significantly over time (P <.001) and showed a significant relationship between baseline and 12 month measurements (P <.001). A significant relationship was also established between baseline and 12 month VEP amplitude values at 100% of grating contrast. Patients with high myopia complained significantly more at a lower contrast threshold and at lower VEP amplitude values than patients with low myopia.

CONCLUSIONS

Three months after PRK, contrast threshold and VEP amplitude values were reduced. Partial recovery was established at 6 months, although patients reported permanent impairment under low-contrast conditions, especially if the myopia correction was more than 6.0 diopters.

Laser in situ Keratomileusis Enhancement After Radial Keratotomy

PURPOSE

To present results of laser in situ keratomileusis (LASIK) enhancement after radial keratotomy (RK).

METHODS

Sixteen eyes of 10 patients were treated with LASIK for residual myopia and hyperopia after RK. Mean preoperative spherical equivalent refraction was -3.14+/-3.04 D (range, -6.675 to +6.00 D). Best spectacle-corrected visual acuity was 20/20 in 9 eyes, 20/25 in 6 eyes, and 20/30 in 1 eye. Uncorrected visual acuity was better than 20/40 in only 2 eyes. Patients were followed at 1 day, 1 week, 1, 3, and 6 months, and 1 year. Mean follow-up was 8.3 months (range, 1 to 17 mo).

RESULTS

All eyes received one LASIK enhancement. Mean final spherical equivalent refraction was +0.16+/-0.68 D (range, -1.00 to +1.75 D). No eyes experienced any visual loss. Five eyes gained 1 line of best spectacle-corrected visual acuity. Uncorrected visual acuity was 20/20 in 9 eyes, 20/25 in 6 eyes, and 20/30 in 1 eye. Two eyes of one patient had the previous RK incisions open.

CONCLUSION

LASIK was an effective treatment for correction of residual myopia and hyperopia after RK.

Detection and classification of mild irregular astigmatism in patients with good visual acuity

Videokeratography has been available for a decade, and this test is essential for determining the presence and type of irregular corneal astigmatism. Three eyes diagnosed with myopic astigmatism and considered good candidates for refractive surgery with conventional examination were studied. Color-coded maps with videokeratography showed regular astigmatism in one eye and the existence of irregular astigmatism in two eyes. Videokeratography showed that one of these eyes had a keratoconus suspect pattern and the second showed a pattern consistent with pellucid marginal degeneration. Videokeratography can detect and classify irregular astigmatism in cases where routine examination shows no abnormal findings.

Corneal Aberrations and Visual Performance After Radial Keratotomy

BACKGROUND

Refractive surgery and videokeratography have allowed us to study the effects on visual performance of relatively large changes in corneal aberration structure induced by surgical changes in corneal shape.

METHODS

We quantified in one eye of nine normal and 23 radial keratotomy patients, the area under the log contrast sensitivity function (AULCSF) and corneal first surface wavefront variance for two artificial pupil sizes (3 and 7 mm). Contrast sensitivity was measured with sine-wave gratings at six spacial frequencies. Wavefront variance was derived from videokeratographs using Zernike polynomials.

RESULTS

For normals eyes there were no significant changes over time. For eyes that had radial keratotomy, there were significant pupil size-dependent changes. For the 3 mm pupil, there were significant surgery-induced changes in the corneal wavefront variance which became large (approximately 30 times preoperative values) at 7 mm. Significant correlated changes in AULCSF for the 7 mm pupil but not for the 3 mm pupil occurred immediately following surgery and remained.

CONCLUSIONS

Radial keratotomy, like photorefractive keratectomy, shifts the distribution of aberrations from third order dominance (coma-like aberrations) to fourth order dominance (spherical-like aberrations). Radial keratotomy-induced aberrations and loss in contrast sensitivity are reduced with increasing clear zone diameter. Radial keratotomy induces an increase in the optical aberrations of the eye and the increase for large pupils (7 mm) but not small (3 mm) is correlated to a decrease in contrast sensitivity.

Excimer laser photorefractive keratectomy for high myopia: four-year experience with a multiple zone technique

PURPOSE

The purpose of the study is to evaluate the results of the authors' 4-year experience with excimer laser photorefractive keratectomy (PRK) and multiple optical zone corneal ablation in highly myopic eyes.

METHODS

The authors retrospectively evaluated 56 eyes of 44 patients (mean refraction, -11.3 diopters [D]; range, -5.75 to -24.5 D) who underwent PRK with a Visx Model 20/20 laser (Visx, Santa Clara, CA). Preoperative visual acuity of 20/40 or better was present in 46 eyes. Corneal ablation was divided into concentric optical zones (4, 5, and 6 mm), allowing corrections of up to 18 D, with a refractive goal of within -1 D from emmetropia in 49 eyes. A hand-held fixation system was always used, and a nitrogen-blowing system (NBS) was used in the first 21 eyes only.

RESULTS

Before retreatment, the range of final cycloplegic refraction from emmetropia in eyes treated with NBS versus not was within +/-1 D in 6 (28.6%) and 15 eyes (44.1%), between -1.25 and -3 D in 5 (23.8%) and 14 eyes (41.1%), and more than -3 D in 10 (47.6%) and 5 eyes (14.7%), respectively. No lines of visual acuity were lost in 37 eyes (80.4%) with 20/40 or better visual acuity before surgery. Three eyes showed vision loss due to worsening of myopic maculopathy and one due to corneal haze. Correction stabilized within 9 months, and at a mean time of 25.6 months, the correction attained was of -8.5 +/- 3.6 D, achieving 90.3% of attempted correction. Eyes with preoperative myopia less than -10 D (n = 27) showed regression less than -1 D in 8 eyes (29.6%), between -1.25 and -3.00 D in 5 eyes (18.5%), and greater than -3.00 D in 1 eye (3.7%); eyes with more than -10 D (n = 29) regressed in 3 (10.3%), 6 (20.7%), and 1 eye (3.4%), respectively. Severe haze was observed in 11 eyes (19.6%) 3 months after surgery. Two eyes showed decentration greater than 1.5 mm. At last examination, night driving problems were reported by 12 (41.4%) of 29 patients evaluated who drive.

CONCLUSIONS

After the NBS was eliminated, the multiple-zone technique achieved a long-term, stable 83.1% reduction of preoperative myopia. Patients with severe myopia appreciated reduction of most of the refractive defect, although perception of halos was noted by 16 patients.

Clear lens extraction to correct hyperopia

PURPOSE

To evaluate the effectiveness and safety of clear lens extraction with intraocular lens (IOL) implantation to correct hyperopia and to determine the most accurate and predictable method of IOL power selection.

SETTING

The Eye Institute of Utah, Salt Lake City, Utah, USA.

METHODS

The outcome of 20 phacoemulsification and IOL implantation procedures was assessed at a mean follow-up of 23.2 months (range 3 to 60 months). Intraocular lens power and predicted refraction were retrospectively evaluated in each eye with the SRK II and Holladay formulas with different attempted refractions based on the stabilized postoperative refraction.

RESULTS

Uncorrected visual acuity (UCVA) improved from 20/200 preoperatively to 20/30 postoperatively. At the final examination, 89% of eyes achieved 20/40 or better UCVA. All eyes had 20/25 or better best corrected visual acuity (BCVA). No eye lost two or more Snellen lines of BCVA. There were no surgical or postoperative complications. The Holladay formula was more accurate than the SRK II formula. With the Holladay formula aiming for -1.00 diopter (D), the predicted mean postoperative spherical equivalent would be -0.21 D +/- 0.89 (SD); with the SRK II aiming for -1.50 D, it would be +0.43 +/- 1.10 D. The Holladay formula reduced the chance of postoperative residual hyperopia.

CONCLUSION

Clear lens extraction with IOL implantation was an effective, safe procedure for the correction of hyperopia. However, this method was less accurate and less predictable for hyperopia below +3.00 D. With the Holladay formula aiming for -1.00 D, good visual and refractive results can be expected. Further study with a larger sample of patients and longer follow-up is needed to assess long-term safety and effectiveness.

Mesopic vision in myopia corrected by photorefractive keratectomy, soft contact lenses, and spectacles

PURPOSE

To evaluate contrast vision and glare sensitivity under mesopic conditions in eyes having uncomplicated excimer laser photorefractive keratectomy (PRK) for myopia and in eyes corrected by disposable soft contact lenses, soft contact lenses, and spectacles.

SETTING

Division of Experimental Ophthalmic Surgery, University of Tübingen, Germany.

METHODS

The Mesoptometer II test was used to evaluate mesopic vision (glare sensitivity and contrast vision) in 28 eyes of 14 patients wearing disposable soft contact lenses, 20 eyes of 10 patients wearing soft contact lenses, 39 eyes of 20 patients wearing spectacles, 30 eyes of 15 emmetropic patients, and 33 eyes of 22 patients after PRK with 5.0 mm optical zone. Follow-up was between 15 and 60 months after PRK (mean 34.5 months).

RESULTS

The guidelines of the German Ophthalmologic Society state that patients must recognize Mesoptometer II contrast levels of 1:5 or better with and without glare to meet the minimum legal night-driving standards for private cars. All eyes with disposable soft contact lenses and soft contact lenses, all emmetropic eyes, and 38 eyes corrected by spectacles recognized contrast levels of 1:5 or better without glare. In contrast, 18 eyes in the PRK group were unable to recognize contrast level 1:5 without glare. With glare, 1 eye in the disposable soft contact lens group, 1 in the soft contact lens group, and 7 with spectacles were unable to recognize the 1:5 contrast level. All emmetropic eyes recognized contrast levels of 1:5 or better; 22 PRK eyes were unable to recognize contrast level 1:5 with glare.

CONCLUSION

Myopic PRK may lead to long-term impairment of mesopic vision, while soft contact lens use does not seem to markedly influence mesopic vision in eyes with low to moderate myopia.

Characteristics influencing outcomes of excimer laser photorefractive keratectomy. Summit Photorefractive Keratectomy Phase III Study Group

PURPOSE

To identify preoperative and intraoperative characteristics associated with outcomes of photorefractive keratectomy (PRK).

METHODS

In the phase III multicenter clinical trials of the Summit Technology excimer laser for corrections of 1.5 to 6.0 diopters (D) of myopia, three principal outcomes of PRK on 612 patients were examined: (1) uncorrected visual acuity of 20/40 or better, (2) predictability of refractive outcome within 1.0 D of attempted correction, and (3) stability of refractive result between 12 and 24 months. Multiple logistic regression was used to test for independent associations of multiple preoperative and intraoperative characteristics with each of these outcomes.

RESULTS

Older age was independently associated with lesser likelihood of achieving 20/40 or better uncorrected visual acuity (odds ratio = 1.08 per incremental year of age, 95% confidence interval [CI] = 1.04-1.12) and with decreased predictability, specifically with overcorrection (odds ratio = 1.09, 95% CI = 1.06-1.12), but age was not associated with stability of refraction. Greater attempted correction was associated independently with a decreased likelihood of 20/40 or better uncorrected visual acuity (odds ratio = 2.78 for corrections of 3.5-5.5 D, 95% CI = 1.18-6.75; odds ratio = 4.19 for corrections of > or = 5.5 D, 95% CI = 1.66-10.58), with decreased predictability (odds ratio = 1.72 for corrections of 3.5-5.5 D, 95% CI = 1.05-2.85; odds ratio = 2.95 for corrections of > or = 5.5 D, 95% CI = 1.65-5.26), and with a reduced likelihood of stability of refraction (odds ratio = 3.46 for corrections of > or = 5.0 D, 95% CI = 1.32-9.11). No intraoperative characteristics were associated with any of the outcomes assessed.

CONCLUSIONS

Using this specific excimer laser system with an optical zone of 4.5 or 5.0 mm, patient age and attempted correction are important preoperative characteristics associated with postoperative uncorrected visual acuity and predictability of PRK. Stability of refraction is strongly associated with attempted correction. Such information may help guide patient selection, determine timing of fellow eye treatment, and suggest changes in the laser treatment algorithm for individual patients. Although these findings may be representative of PRK in general, similar analyses should be performed before modifying patient treatments using either a 6.0-mm treatment zone or other laser systems.

Corneal Ablation Profiles for Correction of Hyperopia with the Excimer Laser

PURPOSE

To calculate smooth corneal ablation profiles for the correction of hyperopia by means of excimer laser photorefractive keratectomy and to quantify the typical topographical changes, especially in the transition zone, in terms of axial distance and instantaneous radius of curvature.

METHODS

Conditions were determined to yield a smooth transition surrounding a hyperopic photorefractive keratectomy. Functions for the ablation depth in the transition zone were calculated. Theoretical optical zones of 4-mm diameter with 1 or 2-mm transition zones were required. The variation in axial and instantaneous power along one semimeridian was subsequently calculated. The power profile of a +10 diopters (D) correction on a spherical surface (47 D) was presented as an example for both ablation profiles.

RESULTS

In photorefractive keratectomy for hyperopia, the transition zone is necessarily flatter than the original cornea. The more abrupt the transition, the more pronounced the flattening. A central steepening of +10 D entailed a mid-peripheral flattening of -7.00 D or -15.00 D in transition zones of 2 and 1 mm width, respectively. In the transition zone, the mean rate of change of axial power was 15 and 45 D/mm, respectively.

CONCLUSION

Making the central cornea steeper by means of photorefractive keratectomy implies that the periphery should be flattened (axial power). The amount of flattening is directly proportional to the degree of steepening, and is relatively larger if the transition is more abrupt.

Disability Glare After Excimer Laser Photorefractive Keratectomy for Myopia

BACKGROUND

A change of corneal topography and haze after excimer laser photorefractive keratectomy (PRK) can reduce contrast sensitivity and cause glare. Both glare and contrast sensitivity can be examined in a reproducible manner with one instrument.

METHODS

We have used the Berkeley Glare Test to examine 46 eyes of 32 patients before and 1, 3, 6, 9, and 12 months after excimer laser PRK for moderate to high myopia. Multiple regression analysis was used for statistical analysis.

RESULTS

High contrast visual acuity showed a statistically significant deterioration during the first 6 months after PRK (p = 0.01); 1 year after treatment visual acuity returned to almost pretreatment levels (p = 0.2). High- and low contrast visual acuity under glare deteriorated significantly 3 months after PRK and had only risen slightly 1 year later (p < or = 0.0065). A similar development could be observed for the low contrast visual acuity without glare.

CONCLUSION

Although high contrast visual acuity recovers by 1 year after PRK, low contrast visual acuity and glare deteriorate significantly and do not recover, even after 1 year.

Preliminary results of photorefractive keratectomy in active-duty United States Navy personnel

PURPOSE

To evaluate the safety, efficacy, and quality of vision after photorefractive keratectomy (PRK) in active-duty military personnel.

METHODS

Photorefractive keratectomy (6.0-mm ablation zone) was performed on 30 navy/marine personnel(-2.00 to -5.50 diopters [D]; mean, -3.35 D). Glare disability was assessed with a patient questionnaire and measurements of intraocular light scatter and near contrast acuity with glare.

RESULTS

At 1 year, all 30 patients had 20/20 or better uncorrected visual acuity with no loss of best-corrected vision. By cycloplegic refraction, 53% (16/30) of patients were within +/- 0.50 D of emmetropia and 87% (26/30) were within +/- 1.00 D. The refraction (mean +/- standard deviation) was +0.45 +/- 0.56 D (range, -1.00 to 1.63 D). Four patients (13%) had an overcorrection of more than 1 D. Glare testing in the early (1 month) postoperative period demonstrated increased intraocular light scatter (P<0.01) and reduced contrast acuity (with and without glare, (P<0.01). These glare measurements statistically returned to preoperative levels by 3 months (undilated) and 12 months (dilated) postoperatively. Two patients reported moderate to severe visual symptoms (glare, halo, night vision) worsened by PRK. One patient had a decrease in the quality of night vision severe enough to decline treatment in the fellow eye. Intraocular light scatter was increased significantly (>2S D) in this patient after the procedure.

CONCLUSIONS

Photorefractive keratectomy reduced myopia and improved the uncorrected vision acuity of all patients in this study. Refinement of the ablation algorithm is needed to decrease the incidence of hyperopia. Glare disability appears to be a transient event after PRK. However, a prolonged reduction in the quality of vision at night was observed in one patient and requires further study.

Surface Roughness in PMMA is Linearly Related to the Amount of Excimer Laser Ablation

OBJECTIVE

To determine if surface roughness after excimer laser ablation is a function of the amount of ablation and to identify a standard unit for ablation roughness.

METHODS

We used a VISX 20/20 excimer laser to perform a series of single zone 6-mm diameter ablations (photorefractive keratectomy [PRK]) in polymethylmethacrylate (PMMA). Corrections ranged from -1.00 diopter (D) to -15.00 D. A scanning white light interferometry microscope (Zygo Corp, Middlefield, Conn) was used to quantify the surface roughness at the center of each ablation.

RESULTS

We found a linear increase in surface roughness as the refractive correction increased. Each diopter increment resulted in an approximately 300 nm increased peak-to-valley measurement. This represented an increase of 25 nm roughness per micron of ablation in PMMA.

CONCLUSIONS

Surface irregularities in PMMA increase with ablation depth. We propose a unit of measure of roughness, the "ablation," expressed as the peak-to-valley distance in nm/divided by m of ablation.