The intrastromal correction of presbyopia using a femtosecond laser was first described in 2009.1,2 Early near visual acuity outcomes were good, promising a new potential method for the correction of presbyopia. Longer single and multicenter follow-up studies of 12 and 18 months revealed a stable gain of uncorrected near visual acuity (UNVA) and integrity of the cornea.3,4 However, the significant postoperative gain of UNVA can lead to a one- to two-line loss of corrected distance visual acuity (CDVA) in some patients.
The standard INTRACOR pattern (Technolas 520F Femtosecond Workstation; Technolas Perfect Vision GmbH, Munich, Germany) consisting of five intrastromal ring cuts received CE-mark approval in 2009 for the treatment of slightly hyperopic and presbyopic patients. Plano emmetropic or myopic patients are not recommended candidates for standard INTRACOR due to a mild postoperative myopic shift.2
The aim of our study was to evaluate a modified INTRACOR pattern, which consists of eight intrastromal midperipheral radial cuts in addition to the five standard central ring cuts. The radial cuts are presumed to prevent the known myopic shift and should make INTRACOR treatment suitable for plano emmetropes.
Patients and Methods
Twenty eyes from 20 patients were included in the study. The study was approved by the ethics committee of the medical faculty of the University of Heidelberg and written informed consent was obtained from all patients.
Inclusion criteria were presbyopia ⩾2.00 diopters (D) at 40 cm, emmetropic refraction (spherical equivalent) ranging from −0.375 to +0.125 D, cylinder ⩽0.50 D, CDVA of 20/25 or better, and a corneal thickness of a least 500 μm at the thinnest point. Exclusion criteria were history of ocular trauma, prior ocular surgery, or ocular pathology.
Examinations were performed within 45 days preoperatively, as well as 1, 3, 6, and 12 months postoperatively. At 1 and 3 months, 19 patients were examined. At all other time points all 20 patients were available for examination. For the true net power analysis, 1 patient could not be analyzed due to technical problems with the topography unit.
Preoperative examinations included subjective refraction, UNVA (Sloan ETDRS charts at 40 cm), distance corrected near visual acuity (DCNVA, Snellen charts at 40 cm), and CDVA (Snellen charts at 5 m). For evaluation of corneal integrity, endothelial cell density (Tomey EM-3000 specular microscope; Tomey Corp, Nagoya, Japan), corneal topography, corneal asphericity, and pachymetry (Pentacam HR; Oculus Optikgeräte GmbH, Wetzlar, Germany) were examined. To analyze wavefront aberrations, the Ocular Wavefront Analyzer (Schwind eye-tech-solutions, Kleinostheim, Germany) was used. Stray light measurements were performed using C-Quant (Oculus Optikgeräte GmbH). At 12 months postoperatively, patients filled out a subjective questionnaire. Slit-lamp, intraocular pressure, and funduscopic examinations were performed. Central corneal steepening was evaluated using Pentacam difference maps of total corneal power (Pentacam true net power analysis). Pre- and postoperative values were compared and the highest point of steepening in the treated area was evaluated. Midperipheral corneal flattening was also analyzed using true net power maps. Flattening occurs at the 4- to 4.5-mm radius. The 4-mm radius was the closest area that could be analyzed. The mean of four measurements (0°, 90°, 180°, and 270°) at the 4-mm radius was calculated and compared to preoperative values.
INTRACOR treatment was performed on the non-dominant eye with the Technolas 520F femtosecond laser (Technolas Perfect Vision GmbH). Dominance of the eye was determined using the hole-in-the-card test and pointing test.5–7 The nondominant eye was chosen following outcome data of the standard INTRACOR treatment and due to IRB requirements. Details of the standard five ring INTRACOR treatment have been described previously.1–4 The modified pattern differs from the standard pattern by inducing eight radial intrastromal cuts in the midperipheral cornea in addition to the five standard central intrastromal rings (Fig 1). Standard INTRACOR five-ring pattern with inner diameter of 1.8 mm and outer diameter of 3.4 mm was performed first.4 Typical settings of the radial cuts were: inner diameter 4.0 mm, outer diameter 9.5 mm, distance to endothelium 200 μm, and distance to epithelium 100 μm. The cuts should not overlap the rings; the clear zone between the outer ring and radial cuts was 600 μm. The cuts were applied in a centripetal fashion intrastromally and not “opened” as is done with classical radial keratotomies.
Figure 1. Modified INTRACOR pattern A) 1 hour and B) 1 year postoperatively.
All data sets were tested for normal distribution using the Kolmogorov-Smirnov test (MedCalc, version 126.96.36.199; MedCalc Software bvba, Mariakerke, Belgium). Not all data sets showed normal distribution, which resulted in use of median, minimum, and maximum values. For statistical analysis between two paired data sets, the Wilcoxon test was performed. For statistical analysis between more than two paired data sets, the Friedman test was used. The level of significance was set at P=.05.
Median age of the 20 patients was 52.5 years (range: 46 to 65 years) with a median preoperative presbyopia requiring 2.25 D near addition (range: 2.00 to 3.00 D).
Visual Acuity and Refraction
Median UNVA at 40 cm improved significantly from 0.60 logMAR (20/80) preoperatively to 0.10, 0.20, 0.20, and 0.10 logMAR (20/25) at 1, 3, 6, and 12 months postoperatively, respectively (P<.0001 at all time points) (Fig 2A). The postoperative values did not change significantly (P=.062). Overall, patients gained between one and eight lines of UNVA, with a median gain of four to five lines at follow-up (Fig 2B). Distance corrected near visual acuity showed similar results with a median improvement from 0.49 logMAR (∼20/63) preoperatively to 0.10, 0.20, 0.20, and 0.10 logMAR (20/25) 1, 3, 6, and 12 months postoperatively (P<.0001 at all time points), respectively, with no significant change noted in the postoperative values (P=.265).
Figure 2. A) Uncorrected near visual acuity (UNVA) at 40 cm and C) corrected distance visual acuity (CDVA) pre- (n=20) and postoperatively at 1 month (n=19), 3 months (n=19), 6 months (n=20), and 12 months (n=20). Lines lost and gained for B) UNVA and D) CDVA.
Analysis of CDVA revealed a median decrease from −0.10 logMAR (20/16) (range: −0.20 to 0.00 logMAR) preoperatively to 0.00 logMAR (20/20) (range: −0.10 to 0.20 logMAR) at all postoperative examinations (P⩽.001) (Fig 2C). At 12 months, postoperative CDVA ranged between −0.10 and 0.10 logMAR (20/16 and 20/25). Overall, postoperative CDVA showed no significant change during follow-up (P=.463). Over the course of the study, patients ranged between a 1-line gain and 3-line loss with a median loss of 1 line (Fig 2D).
Results of the subjective refraction are shown in Table 1. Median spherical equivalent (SE) was 0.00 D preoperatively and 0.00 D at all postoperative follow-up examinations with no statistically significant change (P=.054, Friedman test, n=18). When comparing the preoperative SE values (median 0.00 D; range: −0.25 to 0.25 D) to the 12-month postoperative values (median 0.00 D; range: −0.375 to 0.625 D), the maximum values of an induced refractive shift were a myopic shift of −0.50 D in one patient and a hyperopic shift of +0.625 D in three patients. Statistically, these changes were not significant (P=.1937, n=20).
Table 1: Subjective Refraction Over 12 months Following Modified INTRACOR Treatment
After treatment with the modified INTRACOR pattern, central corneal steepening and midperipheral flattening occurred, which can be seen in the Pentacam difference maps (Fig 3). Postoperative median central steepening was 1.40, 1.20, 1.00, and 1.40 D at 1, 3, 6, and 12 months postoperatively, respectively (Fig 4A), which was statistically significant compared to the preoperative measurement (P⩽.0001 for all values). Postoperatively, no significant change was noted in the steepening values (P=.192).
Figure 3. Pentacam true net power maps. Left panel) 6 months postoperatively, middle panel) preoperatively, and right panel) difference map showing the postoperative central corneal steepening (red center) and midperipheral flattening (green ring).
Figure 4. Corneal changes over the 12-month follow-up. Boxplots represent median, minimum, maximum, 25th percentile, 75th percentile, and outliers (o,▪). Postoperative central corneal A) steepening and B) midperipheral flattening compared to preoperative values. C) Pachymetry at the thinnest point and D) endothelial cell density pre- and postoperatively.
A significant midperipheral flattening of −0.40, −0.40, −0.50, and −0.50 D at 1, 3, 6, and 12 months postoperatively, respectively, was measured at the 4-mm radius (P<.0001), which remained statistically unchanged throughout the postoperative period (P=.131) (Fig 4B).
Pachymetry at the thinnest point remained statistically unchanged during the study when comparing the median preoperative value (552 μm) to postoperative values (554, 556, 552, and 545 μm, at 1, 3, 6, and 12 months postoperatively, respectively [P=.917]) (Fig 4C).
Median endothelial cell density also showed no significant change after treatment (Fig 4D): preoperatively, 2555 cells/mm2; 3 months, 2547 cells/mm2; 6 months, 2616 cells/mm2; and 12 months, 2593 cells/mm2 (P=.335).
Median anterior corneal asphericity (Q-value at 6 mm, Pentacam HR) was −0.12 (range: −0.27 to 0.06) preoperatively and −0.08 (range: −0.36 to 0.10) 12 months postoperatively (P=.3465). Median posterior corneal asphericity (Q-value at 6 mm) changed significantly from −0.27 (range: −0.62 to 0.01) preoperatively to −0.39 (range: −0.60 to 0.02) 12 months postoperatively (P=.0328), ie, the prolate asphericity increased.
For the analysis of wavefront aberrations, only patients who reached 6-mm pupil size were evaluated (n=18). Only spherical aberration () showed a statistically significant change and decreased from a median of 0.18 μm (range: −0.04 to 0.35 μm) preoperatively to 0.12 μm (range: −0.09 to 0.29 μm) 12 months postoperatively (P=.0002). All other analyzed aberrations (horizontal coma, vertical coma, higher order root-mean-square) did not change significantly (P>.05) (Table 2).
Table 2: Wavefront Aberrations Pre- and 12 Months Postoperatively
Subjective Questionnaire and Stray Light Measurements
Patients were asked about the perception of glare, halos, and problems when driving at night (n=18). On a scale from 0 (“no”) to 10 (“very much”), patients scored glare at a median of 1.83 (range: 0.00 to 7.96), halos 2.51 (range: 0.00 to 7.91), and problems when driving at night 2.36 (range: 0.00 to 8.48). Of 18 patients, 15 (83%) would undergo the treatment again.
Monocular evaluation of stray light (C-Quant) of the treated eye revealed a significant increase when comparing pre- to postoperative values (P<.001). Stray light increased from 1.05 log(s) (range: 0.80 to 1.37, n=20) preoperatively to 1.30 log(s) (range: 1.04 to 1.55, n=19), 1.20 log(s) (range: 1.02 to 1.64, n=20), and 1.24 log(s) (range: 1.00 to 1.54, n=20) at 3, 6, and 12 months postoperatively, respectively. At 12 months postoperatively, 8 (40%) of 20 patients were outside of the normal C-Quant range; however, 4 of these patients were already outside of the range preoperatively, leaving a total of 4 (20%) patients with values higher than typically seen in patients of the same age.
Treating presbyopia in the setting of spectacle-independence for near and distance vision is a challenging task. Different types of corneal treatments to correct presbyopia using excimer lasers or other techniques have been developed in recent years.8–16 Since 2009, when the standard INTRACOR pattern was introduced, corneal treatment using a femtosecond laser has become available. However, the standard INTRA-COR procedure is limited to the treatment of slightly hyperopic patients because of a known postoperative myopic shift. Therefore, a modified pattern was developed to prevent myopic shift.
In our patient group, the modified INTRACOR pattern improved UNVA by a median of four lines after 12 months. The highest gain was seven lines (two patients) and the lowest gain was one line (one patient). These results are similar to the standard treatment pattern, which was evaluated in a multicenter study3 and where a median gain of four lines (range: one to nine lines) was reported. It remains unclear as to why certain patients only show a slight gain in near vision. Although near vision is improved, such an outcome can be disturbing. Patient informed consent should be performed carefully with specific emphasis on the variability of outcomes and treatment only done monocularly.
Corrected distance visual acuity decreased in our patient group by a median of one line (range: zero to two line loss) after 12 months. Median CDVA of 0.00 logMAR (20/20), however, was still excellent and the lowest CDVA, which was 0.10 logMAR (20/25), was acceptable for a presbyopic patient cohort. After 12 months, 15% of patients had a loss of two lines. One patient had a loss of three lines after 6 months, but only a loss of one line at 12 months postoperatively. Using the standard pattern, Holzer et al3 reported 7.1% of patients losing two lines. This decrease in CDVA can be disturbing and disappointing for patients.
The improvement in UNVA can be explained by a central corneal steepening of 1.40 D after 12 months as a result of the modified treatment pattern. With the standard treatment, steepening of 1.00 D after 12 months was reported.4 When comparing the median postoperative steepening values (at 1, 3, 6, and 12 months) of this study with the median postoperative steepening values of the standard treatment pattern described by Menassa et al,4 no statistically significant differences were found (unpaired Mann-Whitney U test, P values between 0.0462 and 0.4379). The central corneal steepening in our patients and those reported by Menassa et al remained statistically unchanged during the postoperative course with no evidence of further steepening or corneal ectasia induced by the procedure. Corneal integrity was preserved in our patient group with no significant changes in pachymetry (thinnest point) or endothelial cell density, which has also been described in the 12-month results of the standard pattern.4
The standard pattern is only recommended for slightly hyperopic patients because of a mild postoperative myopic shift of approximately −0.50 D.2,4 Using the modified pattern with its eight additional radial intrastromal corneal cuts, this postoperative myopic shift can be avoided (median values) and thus enlarges the treatment range to plano emmetropic or slightly myopic patients. The induction of a myopic shift is most likely directly counteracted by the midperipheral flattening of the cornea, which shows a median value of −0.50 D after 12 months. Although only a limited number of patients are eligible for the modified treatment pattern, it nevertheless enlarges the INTRACOR treatment range and provides useful data on the topic of intrastromal refractive treatments.
Whether the radial intrastromal cuts avoid the pitfalls of traditional radial keratotomy (RK) in the future, including fluctuating refractions and a tendency for progressive hyperopic shift over time, cannot be answered at the moment. However, it is important to take into account that in contrast to traditional RK, the femtosecond intrastromal cuts do not dissect Bowman layer or the epithelium. Therefore, the biomechanical effect should be much smaller than what has been observed with RK cuts. The fluctuations seen in the steepening are thus far not statistically significant and might possibly be due to the limited accuracy of Pentacam measurements in the central cornea or the manual analysis method.
In our patient cohort, median stray light (C-Quant)17 was increased after treatment. This is consistent with changes in optical quality after the standard INTRACOR pattern, in which a slight decrease in mesopic contrast sensitivity and increase in glare sensitivity have been described.18
One important topic that needs to be discussed is whether the INTRACOR treatment affects other ophthalmological refractive measurements or procedures. For the standard five ring treatment pattern, it was shown that postoperative subjective refraction and measurements with autorefractometers had a high correlation.19 In cataract surgery after INTRACOR, intraocular lens power calculation was reliable and no adaption of formulas was needed as reported in a theoretic approach and one case report.20,21 Most likely this can also be assumed for the modified INTRACOR pattern; however, none of the patients treated with the modified INTRA-COR treatment have undergone cataract surgery to date.
The modified INTRACOR treatment combining five intrastromal rings and eight intrastromal radial cuts significantly improves near visual acuity in most patients up to 12 months after surgery. More data are needed to determine whether the effect remains stable over a longer period of time. A possible loss of CDVA lines and possible increase in stray light must be taken into account and needs to be discussed carefully with potential patients preoperatively. Patient selection has to be done carefully and it is important to only treat the nondominant eye. The known myopic shift of the standard treatment pattern can be avoided and thus enlarges the treatment range to include plano emmetropes.
- Ruiz LA, Cepeda LM, Fuentes VC. Intrastromal correction of presbyopia using a femtosecond laser system. J Refract Surg. 2009;25(10):847–854. doi:10.3928/1081597X-20090917-05 [CrossRef]
- Holzer MP, Mannsfeld A, Ehmer A, Auffarth GU. Early outcomes of INTRACOR femtosecond laser treatment for presbyopia. J Refract Surg. 2009;25(10):855–861. doi:10.3928/1081597X-20090917-06 [CrossRef]
- Holzer MP, Knorz MC, Tomalla M, Neuhann TM, Auffarth GU. Intrastromal femtosecond laser presbyopia correction: 1-year results of a multicenter study. J Refract Surg. 2012;28(3):182–188. doi:10.3928/1081597X-20120203-01 [CrossRef]
- Menassa N, Fitting A, Auffarth GU, Holzer MP. Visual outcomes and corneal changes after intrastromal femtosecond laser correction of presbyopia. J Cataract Refract Surg. 2012;38(5):765–773. doi:10.1016/j.jcrs.2011.11.051 [CrossRef]
- Cheng CY, Yen MY, Lin HY, Hsia WW, Hsu WM. Association of ocular dominance and anisometropic myopia. Invest Ophthalmol Vis Sci. 2004;45(8):2856–2860. doi:10.1167/iovs.03-0878 [CrossRef]
- Khan AZ, Crawford JD. Ocular dominance reverses as a function of horizontal gaze angle. Vision Res. 2001;41(14):1743–1748. doi:10.1016/S0042-6989(01)00079-7 [CrossRef]
- Portal JM, Romano PE. Major review: ocular sighting dominance: a review and a study of athletic proficiency and eye-hand dominance in a collegiate baseball team. Binocul Vis Strabismus Q. 1998;13(2):125–132.
- Reinstein DZ, Couch DG, Archer TJ. LASIK for hyperopic astigmatism and presbyopia using micro-monovision with the Carl Zeiss Meditec MEL80 platform. J Refract Surg. 2009;25(1):37–58.
- Reinstein DZ, Archer TJ, Gobbe M. LASIK for myopic astigmatism and presbyopia using non-linear aspheric micro-monovision with the Carl Zeiss Meditec MEL 80 platform. J Refract Surg. 2011;27(1):23–37. doi:10.3928/1081597X-20100212-04 [CrossRef]
- Pinelli R, Ortiz D, Simonetto A, Bacchi C, Sala E, Alio JL. Correction of presbyopia in hyperopia with a center-distance, paracentral-near technique using the Technolas 217z platform. J Refract Surg. 2008;24(5):494–500.
- Farid M, Steinert RF. Patient selection for monovision laser refractive surgery. Curr Opin Ophthalmol. 2009;20(4):251–254. doi:10.1097/ICU.0b013e32832a0cdb [CrossRef]
- Epstein RL, Gurgos MA. Presbyopia treatment by monocular peripheral presbyLASIK. J Refract Surg. 2009;25(6):516–523.
- El Danasoury AM, Gamaly TO, Hantera M. Multizone LASIK with peripheral near zone for correction of presbyopia in myopic and hyperopic eyes: 1-year results. J Refract Surg. 2009;25(3):296–305.
- Stahl JE. Conductive keratoplasty for presbyopia: 3-year results. J Refract Surg. 2007;23(9):905–910.
- Becker KA, Jaksche A, Holz FG. PresbyLASIK: treatment approaches with the excimer laser [German]. Ophthalmologe. 2006;103(8):667–672. doi:10.1007/s00347-006-1391-y [CrossRef]
- Alio JL, Amparo F, Ortiz D, Moreno L. Corneal multifocality with excimer laser for presbyopia correction. Curr Opin Ophthalmol. 2009;20(4):264–271. doi:10.1097/ICU.0b013e32832a7ded [CrossRef]
- van den Berg TJ, Franssen L, Coppens JE. Straylight in the human eye: testing objectivity and optical character of the psychophysical measurement. Ophthalmic Physiol Opt. 2009;29(3):345–350. doi:10.1111/j.1475-1313.2009.00638.x [CrossRef]
- Fitting A, Menassa N, Auffarth GU, Holzer MP. Effect of intrastromal correction of presbyopia with femtosecond laser (INTRACOR) on mesopic contrast sensitivity. Ophthalmologe. 2012;109(10):1001–1007. doi:10.1007/s00347-012-2624-x [CrossRef]
- Fitting A, Ehmer A, Rabsilber TM, Auffarth GU, Holzer MP. Agreement of subjective and objective refraction measurements following INTRACOR femtosecond laser treatment [German]. Ophthalmologe. 2011;108(9):852–858. doi:10.1007/s00347-011-2398-6 [CrossRef]
- Rabsilber TM, Haigis W, Auffarth GU, Mannsfeld A, Ehmer A, Holzer MP. Intraocular lens power calculation after intrastromal femtosecond laser treatment for presbyopia: theoretic approach. J Cataract Refract Surg. 2011;37(3):532–537. doi:10.1016/j.jcrs.2010.10.042 [CrossRef]
- Fitting A, Rabsilber TM, Auffarth GU, Holzer MP. Cataract surgery after previous femtosecond laser intrastromal presbyopia treatment. J Cataract Refract Surg. 2012;38(7):1293–1297. doi:10.1016/j.jcrs.2012.04.023 [CrossRef]
Subjective Refraction Over 12 months Following Modified INTRACOR Treatment
||Median (Range) (D)
||1 Month (n=19)
||3 Months (n=19)
||6 Months (n=20)
||12 Months (n=20)
||0.00 (−0.25 to 0.25)
||0.00 (−0.50 to 0.50)
||0.25 (−0.25 to 0.50)
||0.00 (−0.25 to 0.50)
||0.00 (−0.25 to 0.75)
||0.00 (−0.50 to 0.00)
||0.00 (−0.50 to 0.00)
||0.00 (−0.75 to 0.00)
||0.00 (−0.50 to 0.00)
||0.00 (−0.50 to 0.00)
||0.00 (−0.25 to 0.25)
||0.00 (−0.50 to 0.375)
||0.00 (−0.375 to 0.50)
||0.00 (−0.25 to 0.50)
||0.00 (−0.375 to 0.625)
Wavefront Aberrations Pre- and 12 Months Postoperatively
||Median (Range) (μm)
||12 Months Postoperatively
||0.18 (−0.04 to 0.35)
||0.12* (−0.09 to 0.29)
||−0.09 (−0.46 to 0.13)
||−0.10 (−0.37 to 0.16)
||0.02 (−0.16 to 0.28)
||0.05 (−0.25 to 0.39)
||0.36 (0.19 to 0.54)
||0.40 (0.21 to 0.59)