Laser in situ keratomileusis for myopia and compound myopic astigmatism using the Technolas 217 scanning-spot laser

The Impact of Image Registration for Ablation Orientation on Clinical Outcomes After Wavefront-Optimized Refractive Surgery in Eyes with Myopia and Astigmatism

Purpose

To compare the clinical outcomes from laser refractive surgery performed with the same laser with and without incorporating iris registration technology to compensate for ocular cyclotorsion.

Design

Single-site, two-arm, retrospective chart review.

Methods

Clinical outcomes at a single site after wavefront-optimized LASIK using the Wavelight excimer laser with and without the Vario imaging system for iris registration (IR) were evaluated. Eligible subjects were those that received on-label wavefront-optimized treatment of myopia with astigmatism >1.5 D. Measures of interest were the amount of residual refractive cylinder after surgery, the refractive error, and the best-corrected (BCVA) and uncorrected (UCVA) visual acuities, with a target follow-up of around 90 days.

Results

A total of 112 eligible eyes that were treated with IR and 126 similar eyes treated without IR (NO IR) were included. The refractive sphere and spherical equivalent refractions were statistically significantly different between groups (p < 0.05), but the mean differences were <0.1 D in both cases. Refractive cylinder averaged around 0.12 D and was not statistically significantly different between groups. The number of eyes with residual cylinder >0.50 D was higher in the NO IR group vs the IR group (6% vs 1%, respectively, p = 0.04). The mean logMAR UCVA and BCVA were statistically significantly better in the IR group, with a difference of 1.5 letters for UCVA and 1.0 letters for BCVA (p < 0.001 for both). Significantly more eyes in the IR group had a UCVA (p = 0.01) and a BCVA of 20/15 or better (p = 0.003). Overall, 96% of eyes in the IR group and 91% of eyes in the NO IR group had uncorrected visual acuity of 20/20 or better.

Conclusion

Iris registration with the VARIO imaging device demonstrably reduced the overall variability in clinical outcomes.

Twelve-Year Follow-Up of Laser In Situ Keratomileusis for Moderate to High Myopia

Purpose

To assess the long-term clinical outcomes of conventional laser in situ keratomileusis (LASIK) for moderate to high myopia.

Methods

We retrospectively examined sixty-eight eyes of 37 consecutive patients who underwent conventional LASIK for the correction of myopia (−3.00 to −12.75 diopters (D)). At 3 months and 1, 4, 8, and 12 years postoperatively, we assessed the safety, efficacy, predictability, stability, mean keratometry, central corneal thickness, and adverse events.

Results

The safety and efficacy indices were 0.82 ± 0.29 and 0.67 ± 0.37, respectively, 12 years postoperatively. At 12 years, 53% and 75% of the eyes were within 0.5 and 1.0 D, respectively, of the targeted correction. Manifest refraction changes of −0.74 ± 0.99 D occurred from 3 months to 12 years after LASIK (p < 0.001). We found a significant correlation of refractive regression with the changes in keratometric readings from 3 months to 12 years postoperatively (Pearson correlation coefficient, r = −0.28, p = 0.02), but not with the changes in central corneal thickness (r = −0.08, p = 0.63). No vision-threatening complications occurred in any case.

Conclusions

Conventional LASIK offered good safety outcomes during the 12-year observation period. However, the efficacy and the predictability gradually decreased with time owing to myopic regression in relation to corneal steepening.

Ocular residual astigmatism's effect on high myopic astigmatism LASIK surgery

PURPOSE

To analyze the effect of ocular residual astigmatism (ORA) on the effectiveness of LASIK for treating high myopic astigmatism.

METHODS

This is an observational, cross-sectional study. We studied 116 consecutive myopic eyes with -3 diopters (D) or more of astigmatism that underwent LASIK surgery. The magnitude of uncorrected residual refractive astigmatism 3 months postoperatively was measured.

RESULTS

The mean preoperative cylinder was -4.0±0.83 D (range, -7.5 to -3 D) and the mean preoperative ORA was 0.82±0.5 D. The mean residual refractive cylinder 3 months postoperatively was -0.78±0.83 D (range, -3 to 0 D). No correlation was found between ORA and the refractive cylinder 3 months postoperatively (P=0.6).

CONCLUSION

In eyes with high myopic astigmatism undergoing LASIK, ORA was not correlated with the residual postoperative cylinder.

Clinical results of using a high-repetition-rate excimer laser with an optimized ablation profile for myopic correction in 10 235 eyes

PURPOSE

To evaluate the visual outcomes, safety, stability, efficacy, and predictability of laser in situ keratomileusis (LASIK) to correct myopia using a high-repetition-rate excimer laser with an optimized ablation profile.

SETTING

Private clinic, Tokyo, Japan.

DESIGN

Retrospective noncomparative study.

METHODS

In this study, patients had LASIK using the Schwind Amaris excimer laser for myopic correction.

RESULTS

The study comprised 10  235 eyes of 5191 patients. The patients' mean age was 33.9 years ± 7.84 (SD) (range 18 to 56 years). The mean preoperative manifest refraction spherical equivalent (MRSE) was -5.02 ± 2.17 diopters (D) (range -2.75 to -11.50 D). Three months postoperatively, 82.0% of patients achieved an uncorrected distance visual acuity of -0.18 logMAR or better and 96.9% achieved 0.00 logMAR or better. The MRSE was within ±0.50 D of the intended refractive target in 88.4% of eyes and within ±1.00 D in 98.8%. Despite using the profile designed to minimize postoperative aberrations, the postoperative corneal and ocular higher-order aberrations increased.

CONCLUSION

Laser in situ keratomileusis using a high-repetition-rate excimer laser was a safe and effective procedure, yielding predictable results for a wide range of myopic patients.

FINANCIAL DISCLOSURE

Mr. Magnago is an employee of Schwind Eye-Tech Solutions GmbH & Co. KG. No other author has a financial or proprietary interest in any material or method mentioned.

[AcrySof® Toric (SN60T) intraocular lens implantation: refractive predictibility and aberrometric impact of decentration]

PURPOSE

To evaluate refractive outcomes of toric intraocular lens (IOL) implantation with a detailed analysis of decentration and its effect on aberrometry.

METHODS

This retrospective study enrolled 19 eyes implanted with SN60T AcrySof(®) Toric (Alcon - USA) IOL's. Spherocylindric correction was studied in depth by the Alpins method, and retro-illumination images were used to analyze the alignment of the IOL with its intended axis. IOL decentration as well as its aberrometric impact were evaluated with a strict, novel protocol.

RESULTS

Three months postoperatively, uncorrected distance visual acuity was greater or equal to 20/40 in 94.74% of cases, postoperative subjective cylinder was less or equal to 0.5 D in 68.42% of eyes, with a mean index of success of 0.24. Mean error of toric IOL alignment was 5.68° (0 to 14). Mean IOL decentration was 0.78 mm (0 to 1.78) with a mean coma and trefoil of 0.18 μ (0.06 to 0.33) and 0.19 μ (0.05 to 0.51), respectively. The larger the IOL decentration, the higher the optical aberrations were.

CONCLUSION

Toric intraocular lens implantation is an effective, safe and predictable method of spherocylindrical correction during cataract surgery, with a refractive accuracy similar to that of LASIK in the treatment of astigmatism in young patients. IOL decentration produces optical aberrations including coma and trefoil, which interfere with visual performance.

Time course of refractive and corneal astigmatism after laser in situ keratomileusis for moderate to high astigmatism

PURPOSE

To assess the time course of refractive and corneal astigmatism after laser in situ keratomileusis (LASIK) in eyes with moderate to high astigmatism.

SETTING

Department of Ophthalmology, Kitasato University, Kanagawa, Japan.

DESIGN

Retrospective case series.

METHODS

Keratometric readings and corneal astigmatism were determined with an autokeratometer in consecutive patients who had LASIK for moderate to high astigmatism (≥2.00 diopters [D]).

RESULTS

The study enrolled 48 eyes of 35 patients with a mean age of 34.0 years ± 8.1 (SD), a mean spherical error of -5.10 ± 2.11 D, and a mean cylindrical error of -2.74 ± 0.99 D. Postoperatively, the mean spherical refraction changed significantly from 0.38 ± 0.80 D at 1 week to -0.13 ± 0.90 D at 1 year (P<.001, Wilcoxon signed-rank test). The mean cylindrical refraction showed no significant change (-0.67 ± 0.54 D at 1 week to -0.63 ± 0.63 D at 1 year) (P=.54). There were significant increases in the flattest and steepest keratometry readings at 1 week and at 1 year. However, no significant change in corneal astigmatism was found at either time point (P=.10).

CONCLUSIONS

After LASIK, there was significant refractive regression in the spherical component but not in the cylindrical component. This suggests that refractive regression occurs by corneal steepening in the spherical component and that astigmatic regression does not occur, even in moderately to highly astigmatic eyes.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Visual acuity after laser in situ keratomileusis to correct high astigmatism in adults with meridional amblyopia

PURPOSE

To analyze the effect of laser in situ keratomileusis to correct high myopic astigmatism in patients with suboptimal best spectacle-corrected visual acuity (BSCVA).

DESIGN

Retrospective, interventional series of consecutive cases.

METHODS

SETTING

Vissum Madrid, Madrid, Spain.

PATIENTS

Two hundred five eyes of consecutive patients with suboptimal BSCVA and high myopic astigmatism of -3 diopters or more.

INTERVENTION

Laser in situ keratomileusis surgery.

MAIN OUTCOME MEASURES

BSCVA.

RESULTS

The BSCVA (decimal Snellen fraction notation) improved significantly (P = .0001) from a mean preoperative 0.77 ± 0.18 (range, 0.05 to 0.90) to 0.81 ± 0.19 (range, 0.05 to 1.25) 3 months after surgery. The mean change in lines of BSCVA (decimal Snellen fraction notation) was 0.04 ± 0.11 (range, -0.25 to 0.4). A significant inverse relationship (P = .001) was found between the preoperative BSCVA and the improvement in BSCVA.

CONCLUSIONS

In eyes with high myopic astigmatism and suboptimal preoperative BSCVA, laser in situ keratomileusis may result in a significant improvement in BSCVA.

Aspheric profile versus wavefront-guided ablation photorefractive keratectomy for the correction of myopia using the Allegretto Eye Q

PURPOSE

To compare the results of photorefractive keratectomy (PRK) with Q-factor customized aspheric ablation and wavefront-guided customized ablation for correction of myopia compound with astigmatism.

METHODS

Fifty-six patients were enrolled in this prospective controlled study. In each patient, one eye was randomly assigned for treatment with the Q-factor customized PRK (custom-Q study group) and the other eye treated with wavefront-guided customized PRK. Preoperative and 3-month postoperative manifest refraction, contrast sensitivity, visual acuity, aberrometry, and asphericity of the cornea were compared between the two groups. All eyes were treated with the Wavelight Eye-Q 400 Hz excimer laser in a single refractive surgery center by a single surgeon.

RESULTS

Uncorrected visual acuity (UCVA) improved to 20/20 or more and 20/25 or more in 34 eyes (60.7%) and 56 eyes (100%), respectively, in the wavefront-guided ablation group and in 36 eyes (64.2%) and 54 eyes (96.4%) in the custom-Q ablation group. All eyes had UCVA of 20/40 or better. A total of 54 eyes (96.4%) in the wavefront-guided ablation group and 56 (100%) in the custom-Q ablation group had spherical equivalent (SE) within -/+0.5 D. One eye in each group (2%) lost >or=2 lines of best-corrected visual acuity (BCVA). High order root mean square (RMS) in the wavefront-guided group was 0.3630-/+0.13 mum preoperatively and 0.427-/+0.17 at 3 months (p=0.2). In the custom-Q ablation group it was 0.329-/+0.092 preoperatively and 0.4730-/+0.181 at 3 months after PRK (p=0.08).

CONCLUSIONS

Regarding safety and refractive efficacy, custom-Q ablation profiles were clinically equivalent to wavefront-guided profiles in corrections of myopia up to -6.00 D and astigmatism up to 2.50 D.

Corneal limbal marking in the treatment of myopic astigmatism with the excimer laser

PURPOSE

To determine whether preoperative marking of the limbal cornea improves treatment of myopic astigmatism with the excimer laser.

METHODS

Retrospective study on 108 eyes with myopic astigmatism that underwent LASIK or laser epithelial keratomileusis (LASEK) with the Technolas 217 (Bausch & Lomb) excimer laser. Preoperative limbal marking was performed in 47 eyes (marked group). The 12-month results were used for refractive and visual analysis.

RESULTS

The achieved cylinder reduction, spherical reduction, and refractive predictability were similar for the marked and unmarked groups in the overall study collective, in the LASIK and LASEK subgroup analysis, and in a higher astigmatism (> 1.25 diopters) subgroup analysis. Limbal marking showed no influence on the refractive results, and vector analysis showed no significant difference in angle of error among groups.

CONCLUSIONS

Corneal limbal marking failed to improve the refractive outcome in LASIK and LASEK for myopic astigmatism.

Wavefront-guided laser in situ keratomileusis in the treatment of high myopia by using the CustomVue wavefront platform

PURPOSE

To assess the efficacy, predictability, and safety of wavefront-guided laser in situ keratomileusis (LASIK) in the treatment of high myopia by using the Visx S4 CustomVue wavefront platform.

METHODS

A retrospective analysis of consecutive cases of eyes with high myopia (manifest refraction spherical equivalent >or= -6.00 D) that underwent non-physician-adjusted wavefront-guided LASIK by using the Visx S4 CustomVue wavefront platform. Forty-three eyes of 29 patients were included. Preoperative best spectacle-corrected visual acuity (BSCVA), manifest refraction, WaveScan refraction, postoperative uncorrected visual acuity (UCVA) and BSCVA, and manifest refraction were determined. The clinical outcomes were evaluated on the basis of standard formats and criteria. Data at 3 months postoperatively are presented.

RESULTS

Preoperatively, we found mean sphere was -6.89 +/- 1.08 D, mean cylinder was -0.97 +/- 0.75 D, and mean spherical equivalent (SE) was -7.38 +/- 1.20 D. Postoperatively, mean sphere was 0.02 +/- 0.40 D, mean cylinder was -0.40 +/- 0.40 D, and mean SE was -0.18 +/- 0.43 D. UCVA was 20/15 or better in 27.9% and cumulatively 20/20 or better in 58% of eyes. All eyes treated had at least 20/50 UCVA. Efficacy index was 0.94. Eighty-two percent of eyes were within 0.50 D and 97.6% were within 1.00 D of emmetropia at the 3-month follow-up visit. Ninety-one percent of eyes either maintained or gained 1 line of BSCVA. No eye lost >1 line of BSCVA. The safety index was 1.1.

CONCLUSIONS

The 3-month follow-up results of our study indicate that wavefront-guided LASIK by using the Visx S4 CustomVue wavefront platform is an effective, predictable, and safe treatment of high myopia.

Conventional vs Wavefront-guided LASIK Using the LADARVision4000 Excimer Laser

PURPOSE

To compare outcomes after LASIK surgery using the conventional LADARVision4000 laser and the wavefront-guided LADARWave CustomCornea wavefront system.

METHODS

A prospective study was performed involving 140 myopic eyes receiving conventional or CustomCornea LASIK between May and October 2003. The preoperative manifest spherical equivalent refraction was limited to myopia < or = -7.00 diopters (D). The preoperative manifest cylinder was limited to < or = -2.50 D of astigmatism. Patients were evaluated for 3 months following surgery. Results evaluated were uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity, manifest refraction, dilated wavefront measurements, contrast sensitivity, and patient responses to subjective questionnaires.

RESULTS

For the CustomCornea eyes at 3 months, 80% (70/87) had UCVA > or = 20/20 and 95% (83/87) had UCVA > or = 20/25. For the conventional eyes at 3 months, 45% (9/20) had UCVA > or = 20/20 and 80% (16/20) had UCVA > or = 20/25. At the 3-month postoperative visit, 85% (74/87) of the CustomCornea eyes and 55% (11/20) of the conventional eyes were within +/- 0.50 D of their intended correction. At 1 and 3 months, the CustomCornea treated eyes had a statistically significant lower mean increase in higher order aberrations than conventionally treated eyes (P < .05).

CONCLUSIONS

CustomCornea wavefront-guided LASIK surgery appears safe and effective and provides clinical benefits that appear to exceed those of conventional LADARVision surgery.

Comparison of LASIK Outcomes With the Alcon LADARVision4000 and the VISX STAR S2 Excimer Lasers Using Optimized Nomograms

PURPOSE

To evaluate the difference in clinical outcomes between LASIK patients treated with the Alcon LADARVision4000 laser and the VISX STAR S2 laser using optimized nomograms.

METHODS

Data from 572 LASIK surgeries of two groups that were sequentially performed by one surgeon were retrospectively reviewed. The first group of 286 eyes were the last cases performed with the VISX STAR S2 laser. Following these surgeries, a second group of 286 eyes were the first cases performed with the Alcon LADARVision4000 laser, after an initial 50-eye adaptation period. Treatments were guided by the Refractive Surgery Consultant software.

RESULTS

For myopic eyes 3 months postoperatively, the percentage of eyes with uncorrected visual acuity (UCVA) > or = 20/20, accuracy of treatment within +/- 0.5 D, and loss of > or = 2 lines of best spectacle-corrected visual acuity (BSCVA) for the LADARVision4000 and STAR S2, respectively was: 89% vs 63%, 84% vs 80%, and 0.8% vs 3.8%. For hyperopic eyes 3 months postoperatively, the percentage of eyes with UCVA > or = 20/20, accuracy of treatment within +/- 0.5 D, and loss of > or = 2 lines of BSCVA for the LADARVision4000 and STAR S2, respectively was: 74% vs 33%, 69% vs 74%, and 2% vs 18.5%.

CONCLUSIONS

Although good results are evident for both platforms, superior results were observed with the Alcon LADARVision4000 laser.

LASIK for Myopia With the Zeiss Meditec MEL 80

PURPOSE

To prospectively evaluate a new high-speed, small spot-scanner laser for the correction of myopia and myopic astigmatism.

METHODS

Seventy-six consecutive eyes with myopia and myopic astigmatism between -1.00 and -8.25 diopters (D) and up to -2.75 D astigmatism underwent LASIK treatment using the MEL 80 laser (Carl Zeiss Meditec, Jena, Germany). Parameters evaluated were uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), residual refractive error, regression of correction, and aberrometry.

RESULTS

Mean preoperative BSCVA was 20/20, which improved to 20/18 postoperatively. Postoperative UCVA was 20/20 at 1 month and 20/18 at 1 year. Uncorrected visual acuity > or = 20/20 was achieved in 58 (83%) of 70 eyes at 1 month and in 60 (88%) of 68 eyes at 1 year. The average refractive error before LASIK was -4.41 +/- 1.98 D. The mean residual refractive error was 0.14 +/- 0.31 D at 1 month and 0.13 +/- 0.30 D at 1 year. At 1-month and 1-year follow-up, respectively, 66 (94%) of 70 eyes and 65 (96%) of 68 eyes were within +/- 0.50 D of intended refractive correction. No eye lost two lines. At 1 month 17% of eyes and at 1 year 13% of eyes gained two lines or more. Between 1-month and 1-year follow-up, 100% of eyes were stable. Mean root-mean-square high order aberration changed from 0.20 microm preoperatively to 0.28 microm postoperatively.

CONCLUSIONS

The MEL 80 is effective and safe in the treatment of myopia and myopic astigmatism.

Refractive outcomes of laser in situ keratomileusis after flap complications

PURPOSE

To study the refractive and visual outcomes, including surgically induced astigmatism, of laser in situ keratomileusis (LASIK) after flap buttonholes and incomplete flaps.

SETTING

Rajendra Prasad Center for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India.

METHODS

This was a retrospective case series of 15 patients (15 eyes) who had LASIK after flap complications (6 eyes with buttonholes and 9 with incomplete flaps). The second surgery was performed after the refraction was stable for 2 months. The refractive and visual outcomes were studied.

RESULTS

After the flap complication, the spheroequivalent did not change significantly (P = .78) but the mean pachymetry changed from 523.6 microm +/- 23.14 (SD) to 530.4 +/- 23.96 microm (P = .04) and the mean absolute cylinder changed from 0.63 +/- 1.01 diopter (D) to 0.88 +/- 0.87 D (P = .026). The second surgery was performed at a mean interval of 6.2 months (range 3 to 17 months). Two eyes had flap complications (irregular flaps) again after the second surgery. At the 6-month follow-up, the mean spheroequivalent was -0.34 +/- 0.65 D and the mean cylinder was 0.15 +/- 0.19 D x 83.6. Thirteen eyes (86.66%) were within 0.5 D of the attempted correction. Post-LASIK complications included diffuse lamellar keratitis (1 eye), epithelial ingrowth (1 eye), and flap striae (2 eyes). After the flap complication, no eye lost 2 or more lines of best spectacle-corrected visual acuity (BSCVA); but after the second surgery at 6 months, 1 eye had lost 2 lines of BSCVA. Six months postoperatively, all eyes had a BSCVA of 20/40 or better.

CONCLUSION

Laser in situ keratomileusis after flap complications is associated with good refractive and visual outcomes; however, there is a higher risk for intraoperative and postoperative complications after the second surgery.

Nomogram Adjustment of Laser in situ Keratomileusis for Myopia and Myopic Astigmatism With the Alcon LADARVision System

PURPOSE

To evaluate the visual and refractive results of conventional (non-wavefront) laser in situ keratomileusis (LASIK) for treatment of myopia and myopic astigmatism using the Alcon LADARVision 4000 excimer laser system and nomogram adjustment techniques.

METHODS

A retrospective analysis of 499 eyes that had LASIK for myopia and myopic astigmatism was performed. Preoperative manifest spherical equivalent refraction ranged from -0.43 to -6.00 D and preoperative astigmatism ranged from 0 to -4.75 D. Patients were evaluated during 3 months following surgery.

RESULTS

One month after surgery, 72% of eyes examined (298/415 eyes) had uncorrected visual acuity (UCVA) of 20/20 or better. Three months after surgery, 83% of eyes examined (216/261 eyes) had UCVA of 20/20 or better. One and three months after surgery, 82% and 83% of eyes, respectively, were within +/-0.50 D of attempted correction; 97% of eyes were within +/-1.00 D at both 1 and 3 months. No eye lost more than 1 line of best spectacle-corrected visual acuity (BSCVA) at 3 months after surgery. At the 3-month examination, 83% of eyes had UCVA better than or equal to preoperative BSCVA.

CONCLUSIONS

Conventional LASIK to correct myopia and myopic astigmatism was safe and effective using the Alcon LADARVision 4000 excimer laser system. Outcomes were substantially improved throughout development of an accurate nomogram, derived from continually updated regression analysis of previous refractive results.

Outcomes of Laser in situ Keratomileusis in a Refractive Surgery Fellowship Program

PURPOSE

The purpose of this study was to document the learning curve and evaluate the success of laser in situ keratomileusis (LASIK) performed by fellows in a Cornea/External Disease and Refractive fellowship program.

METHODS

Two surgeons performed LASIK within a fellowship training program between July 2000 and August 2001. A retrospective review was conducted to determine the visual outcomes and operative complications from this non-comparative case series of 755 consecutive eyes. All LASIK procedures were performed using a Bausch & Lomb Technolas 217A laser and the Bausch & Lomb Hansatome or the Automated Corneal Shaper microkeratome. All eyes were evaluated for operative complications. Visual acuity was reported with a minimum of 1 month follow-up.

RESULTS

Data analysis revealed that uncorrected visual acuity of 20/40 or better was attained in 99.4% of treated eyes, and 20/20 or better in 77.2%. With both eyes open, 100% of patients saw 20/30 or better, 94% saw 20/20 or better, 70% saw 20/15 or better, and 5.5% saw 20/10. Flap related complications occurred in 7 of 755 eyes (0.9%) and all were successfully treated. Forty-two eyes (5.6%) were lost to follow-up prior to the 1-month examination and were excluded from analysis of visual acuity.

CONCLUSIONS

Favorable operative and visual acuity 1-month results in 755 eyes after LASIK with the Bausch & Lomb Technolas 217A laser were in part due to a well organized system of education, patient examination, execution of surgery, and postoperative care. A team approach was essential.

Comparison of designs of laser systems utilized for refractive surgery

Several different laser types are available on the market for the practitioner to use in refractive surgery. Each laser type has certain parameters that the surgeon must understand to obtain the best refractive outcomes. Studies published in peer-reviewed literature between February 2002 and March 2003 show that refractive results across all types of lasers have improved compared with those of several years ago. A difference in refractive outcomes between lasers is impossible to directly compare, as there are no randomized studies that hold surgeon or patient characteristics constant.