Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
ARTICLE Influence of stromal surface humidity in intraoperative ultrasound pachymetry during LASIK Rafael Bilbao-Calabuig, MD1; Valentín Jiménez Mateo-Sidrón, MD1; Fernando Gómez-Sanz, OD1,2 PURPOSE: To investigate the influence of surface stromal humidity in intraoperative ultrasound pachymetry during LASIK. SETTING: Clínica Baviera, Instituto Oftalmologico Europeo, Madrid, Spain. DESIGN: Evaluation of diagnostic test or technology. METHODS: In this prospective study, 267 consecutive eyes were operated by 2 experienced surgeons: after the corneal flap was cut with the Moria LSK-One microkeratome, it was lifted and the degree of stromal surface humidity was evaluated on a subjective scale; we then measured stromal bed thickness with ultrasonic pachymetry before and after drying the stromal surface with a Weck-cel sponge. Flap thickness was calculated taking into account preoperative pachymetry. Statistical analysis was performed using the paired Student t-test to evaluate the difference between wet and dry stromal bed pachymetry, and a multivariate regression analysis of this difference with several clinical parameters was made. RESULTS: Mean flap thickness before drying was 93 ± 15 μm, and after drying 102 ± 15 μm. The mean difference between wet and dried stromal bed pachymetry was 8.8 ± 5.5 μm (P<0.001). This difference was not associated with the surgeon, preoperative pachymetry or keratometry, flap thickness, suction ring or right or left eye. The only variable showing a significant influence to explain the difference found between wet and dry measurements was stromal bed humidity (P<0.05). CONCLUSIONS: When performing intraoperative LASIK ultrasonic pachymetry, the actual stromal thickness should be considered to be around 10 microns less than the thickness measured without drying. In case of thin preoperative pachymetry, large ablations or a very wet stromal surface, drying the surface before pachymetry could improve the safety of the procedure. J Emmetropia 2013; 4: 9-12 Submitted: 12/02/2012 Revised: 12/22/2012 Accepted: 01/08/2013 Clínica Baviera, Instituto Oftalmológico Europeo, Madrid, Spain 1 Hospital del Henares, Coslada, Madrid, Spain 2 Finantial disclosure: The authors have no commercial or proprietary interest in the products mentioned herein. Corresponding Author: Rafael Bilbao-Calabuig, MD Clínica Baviera, Paseo Castellana, 20, Madrid 28046, Spain Mail: [email protected] © 2013 SECOIR Sociedad Española de Cirugía Ocular Implanto-Refractiva LASIK is the most popular refractive surgery procedure performed worldwide, but this technique permanently distorts corneal biomechanics. In extreme circumstances this can lead to progressive corneal ectasia. An insufficient residual stromal bed thickness left after this surgery is considered an important factor for this complication1,2,3. Currently, LASIK surgeons calculate the residual stromal bed by subtracting the expected flap thickness and the planned ablation depth from the preoperative central corneal thickness measured by ultrasound pachymetry. However, significant differences in flap thickness have been described between intended and ISSN: 2171-4703 9 10 INFLUENCE OF STROMAL SURFACE HUMIDITY IN INTRAOPERATIVE US PACHYMETRY real values with several microkeratome models, and great variability may occur with a specific microkeratome head, with standard deviations of around 30 microns having been published4. Thus, intraoperative assessment of the corneal flap is important for adjusting the treatment and ensuring an acceptable postoperative stromal remnant5. For this reason, since 1999 our group has been performing intraoperative ultrasonic pachymetry on every patient as part of our surgical protocol. Some surgeons prefer drying the stromal surface with a Weckcell® sponge (Beaver-Visitec International Inc. Waltham, MA, USA) before placing the probe on the corneal bed, whereas others do not. In this study we investigated the influence of surface stromal humidity on intraoperative ultrasonic pachymetry during LASIK, with the aim of improving the accuracy of our measurements, thus minimising the risk of an unexpected or insufficient residual stromal bed thickness. PATIENTS AND METHODS In this prospective study, 267 eyes of 135 patients (81 women and 54 men) underwent LASIK surgery at the same centre over a three-month period (JanuaryMarch 2007). The study followed the tenets of the Declaration of Helsinki. Informed consent was obtained from each patient after they received verbal and written explanation. The protocol received institutional review board approval. Preoperative clinical examination involved a complete medical history including the review of ocular symptoms and refraction stability, manifest and cycloplegic refraction, external ocular motility examination, slit-lamp microscopy, keratometry (KR30, Topcon Corp., Tokyo, Japan), corneal topography (Orbscan II, Bausch&Lomb, Rochester, NY, USA), ultrasound pachymetry and dilated fundus examination. All surgeries were performed by two experienced refractive surgeons (R. B-C and V. J. M-S) using the same two-step protocol: first, corneal flaps were cut for both eyes under a surgical microscope in the operating theatre, and once completed, laser ablation was then performed on both eyes in the adjacent laser room, approximately five minutes later. Preoperative ultrasonic corneal pachymetry was repeated on the day of surgery, just before the flap cut, using a DGH pachymeter (DGH 5100E, 50-60 Hz probe, DGH Technology Inc., Exton, PA, USA). After administration of one drop of topical anaesthesia, the patient was prepared in a sterile fashion. A lid speculum was used to expose the globe, and one more drop of proparacaine hydrochloride was instilled. Corneal flaps were created with the MQ Moria LSK-One microkeratome (Moria S.A., Antony, France) using in every case the same 100 micron head, with either the −1 or H suction ring (8.5 or 9 mm in flap diameter) according to our surgical protocol. Corneal rinsing with saline solution was performed during flap creation to minimise epithelial damage. Flaps were then lifted, 20 to 30 seconds later, using a 30G dry cannula, trying to avoid interface wetting; immediately, the degree of superficial humidity on the stromal bed was graded on a subjective scale (0 was an apparently dry surface, 2 was a completely moist surface and 1 was intermediate moistness). We then measured stromal bed thickness with the same ultrasound pachymeter, before and after drying the stromal surface with a Weck-cel® sponge, and thus determined the difference between wet and dry stromal bed pachymetry. Finally taking into account preoperative pachymetry, we calculated flap thickness before and after drying. Three pachymetric readings were taken at each time and then averaged. Once finished, the corneal flap was repositioned and immediately the same procedure was performed for the other eye. Finally, approximately five minutes after the flap creation and flap thickness measurements, laser ablation was performed for both eyes using the Technolas 217-Z (Bausch&Lomb, Rochester, NY, USA). The aim was to maintain a minimum residual stromal thickness of 300 microns for all eyes. Any eyes with any even minor complications during flap creation (epithelial defect, surface irregularities, corneal bleeding, etc.) were excluded from the study. Statistical analysis was then performed using the SPSS for Windows (SPSS Inc., Chicago, IL, USA). The paired Student t-test was used to evaluate the difference between wet and dry stromal bed thickness measurement, and a multivariate regression analysis of this difference was then performed with several clinical parameters (surgeon, right or left eye, suction ring used, preoperative pachymetry, stromal residual pachymetry and keratometry, and degree of stromal surface humidity). A p-value of <0.05 was considered statistically significant. RESULTS For the 267 eyes of 135 patients (81 women and 54 men) in the study, patient age ranged from 21 to 62 (mean 36.4). O these,135 were right and 132 left eyes; 211 eyes received a myopic treatment, 52 a hyperopic treatment and 4 received treatment for mixed astigmatism. A total of 82 eyes were operated using the H suction ring (9 mm in flap diameter) and 185 with the −1 ring (8.5 mm). Mean preoperative keratometry was 43.74 ± 1.46 D and mean preoperative pachymetry ranged from 495 and 630 microns (mean 549.5 ± 30.9). JOURNAL OF EMMETROPIA - VOL 4, JANUARY-MARCH 11 INFLUENCE OF STROMAL SURFACE HUMIDITY IN INTRAOPERATIVE US PACHYMETRY During intraoperative pachymetry, all eyes were graded according to the degree of superficial humidity on the stromal bed, 81 being grade 0 (dry surface), 139 grade 1 (intermediate) and 47 grade 2 (moist surface). For the 267 eyes, mean flap thickness before drying was 93 ± 15 microns, and after drying 102 ± 15 microns. The mean difference between wet and dried stromal bed pachymetry was 8.8 ± 5.5 microns, ranging between −2 and 33 microns; this difference was statistically significant (p < 0.001) The multivariate regression analysis revealed that the only variable showing a significant influence to explain the difference found between wet and dry corneal bed thickness measurements was stromal bed humidity (p < 0.05): a greater degree of surface humidity found when lifting the created flap would be related with a greater difference between wet and dry stromal bed pachymetry. The mean difference between the wet and dry measurements in grade 0 eyes was 5.77 ± 3.2 microns, in grade 1 eyes 9.47 ± 4.7 microns and in grade 2 eyes 10.55 ± 4.3 microns (Figure 1). The surgeon, right or left eye, suction ring used, preoperative pachymetry, stromal residual pachymetry and keratometry showed no correlation with the difference wet-dry pachymetry. DISCUSSION Several factors may influence the precision of LASIK intraoperative ultrasonic pachymetry: during the flap cut the microkeratome can compress stromal tissue just before the measurements are obtained; there may be imprecision associated with the manual placement of the probe in the three measurements; corneal irrigation to minimize epithelial injury during the microkeratome pass can increase surface humidity and excimer laser ablation can dehydrate corneal stroma if the measurements are taken at the end of the ablation. The results shown in Table 1 revealed no statistically significant differences between both surgeons, neither in their mean flap thickness, nor in the mean stromal pachymetric reduction after drying the stromal bed. Besides, in a series of 42 eyes using the same Moria LSKOne microkeratome and the 100-micron head, Duffey reported a mean flap thickness after drying the stromal bed during intraoperative ultrasound (US) pachymetry of 107 ± 14 microns, comparable to that obtained in our study (102 ± 18 microns)6. Figure 1. Difference in intraoperative pachymetry between wet and dry stromal bed surface. Corneal hydration and surface humidity have been extensively described in the literature as factors influencing US corneal pachymetry. Moreover, Duffey6 reported a mean reduction of 9 microns in preoperative US pachymetry after drying the corneal epithelium. Similarly, in our study we found a mean decrease of 8.8 μm after drying the stromal bed. However, to our knowledge this is the first report studying the influence and quantifying the effect of surface stromal humidity in LASIK intraoperative ultrasonic pachymetry. Multivariate regression analysis revealed that the only variable that significantly influenced the difference between wet and dry stromal bed pachymetry was the degree of superficial stromal bed humidity. Thus, when a greater surface humidity is observed after lifting the flap, the greater the risk of underestimating the real flap thickness calculated by subtraction, if the surface is not dried before the pachymetric measurement. This would overestimate the residual bed assessment and, in certain eyes, could lead to an insufficient residual stromal bed thickness after laser ablation. Other pachymetric imaging methods are used to determine flap thickness and determine stromal residual bed7,8, among which are confocal microscopy, very high-frequency ultrasound or corneal and anterior segment OCT; they all show excellent resolution and reproducibility, but they are more technologydependent and more difficult to perform than US Table 1: Intraoperative pachymetric results with different surgeons. Surgeon Preoperative Pachymetry Flap thickness with wet surface Flap thickness with dried surface Difference wet/ dried flap thickness Jimenez (n=59) 545.7 μm ± 30 94.4 μm ± 15 103.4 μm ± 15 9.0 μm (9.5%) Bilbao-Calabuig (n=208) 550.2 μm ± 31 93.3 μm ± 15 102.1 μm ± 15 8.8 μm (9.4%) JOURNAL OF EMMETROPIA - VOL 4, JANUARY-MARCH 12 INFLUENCE OF STROMAL SURFACE HUMIDITY IN INTRAOPERATIVE US PACHYMETRY corneal pachymetry, especially if intraoperative measurements are needed. In our group, we have been using intraoperative US pachymetry routinely in our surgical protocol for more than 10 years now without finding any evidence of increased risk of infectious or inflammatory complications over this period. In a retrospective study recently published by our group with more than 200,000 operated eyes, the incidence of post-LASIK infectious keratitis was 0.035%9,10; this was similar to other published reports11-14 in which this intraoperative measurement was not used. Intraoperative drying of the stromal surface significantly improves the accuracy of flap thickness measurement. Although the mean difference is statistically significant (9 microns or 9% of flap thickness), this overestimation of residual stromal thickness has little clinical relevance. However, in selected cases with a very wet surface, the pachymetric difference found after drying was as large as 33 microns. In conclusion, we would recommend the use of ultrasonic intraoperative pachymetry. When it is performed, the real residual stromal bed thickness should be considered to be around 10 microns less than measured without drying. In cases with thin preoperative pachymetry, large ablations or a very wet stromal surface, drying the surface before the intraoperative pachymetric measurement, could improve the safety of the surgery. REFERENCES 5. Cheng HC, Chen YT, Yeh SI, Yau CW. Errors of residual stromal thickness estimation in LASIK. Ophthalmic Surg Lasers Imaging. 2008; 39: 107-113. 6. Duffey RJ. Thin flap laser in situ keratomileusis: flap dimensions with the Moria LSK-One manual microkeratome using the 100-microm head. J Cataract Refract Surg. 2005; 31: 1159-1162 7. Reinstein DZ, Archer TJ, Gobbe M, Silverman RH, Coleman DJ. Repeatability of layered corneal pachymetry with the artemis very high-frequency digital ultrasound arc-scanner. J Refract Surg. 2010; 26: 646-659 8. Neuhann IM, Lege BA, Bauer M, Hassel JM, Hilger A, Neuhann TF. Online optical coherence pachymetry as a safety measure for laser in situ keratomileusis treatment in 1859 cases. J Cataract Refract Surg. 2008; 34: 1273-1279. 9. Llovet F, de Rojas V, Interlandi E, et al. Infectious keratitis in 204 586 LASIK procedures. Ophthalmology. 2010; 117: 232-238. 10. Llovet-Osuna F, de Rojas Silva V, Martínez del Pozo M, Ortega-Usobiaga J. Infectious keratitis in 266,191 laser in situ keratomileusis. Arch Soc Esp 2010; 85: 355-359. 11. Mozayan A, Madu A, Channa P. Laser in-situ keratomileusis infection: review and update of current practices. Curr Opin Ophthalmol. 2011; 22: 233-237. 12. Linke SJ, Richard G, Katz T. Infectious keratitis after LASIKupdate and survey of the literature. Klin Monbl Augenheilkd. 2011;228: 531-536. 13. Donnenfeld ED, Kim T, Holland EJ, et al. American Society of Cataract and Refractive Surgery Cornea Clinical Committee. ASCRS White Paper: Management of infectious keratitis following laser in situ keratomileusis. J Cataract Refract Surg. 2005; 31: 2008-2011. 14. Solomon R, Donnenfeld ED, Azar DT, et al. Infectious keratitis after laser in situ keratomileusis: results of an ASCRS survey. J Cataract Refract Surg. 2003; 29: 2001-2006. 1. Binder PS, Trattler WB. Evaluation of a risk factor scoring system for corneal ectasia after LASIK in eyes with normal topography. J Refract Surg. 2010; 26: 241-250. 2. Binder PS. Risk factors for ectasia after LASIK. J Cataract Refract Surg. 2008; 34: 2010-2011. 3. Dawson DG, Randleman JB, Grossniklaus HE, et al. Corneal ectasia after excimer laser keratorefractive surgery: histopathology, ultrastructure, and pathophysiology. Ophthalmology. 2008; 115: 2181-2191. 4. Viestenz A, Langenbucher A, Hofmann-Rummelt C, Modis L, Viestenz A, Seitz B. Evaluation of corneal flap dimensions and cut quality using the SKBM automated microkeratome. J Cataract Refract Surg. 2003; 29: 825-831. JOURNAL OF EMMETROPIA - VOL 4, JANUARY-MARCH First author: Rafael Bilbao-Calabuig, MD Clínica Baviera, Instituto Oftalmológico Europeo, Madrid, Spain