Iglesias et al. BMC Ophthalmology (2022) 22:79 https://doi.org/10.1186/s12886-022-02309-x RESEARCH Open Access Intraocular pressure after myopic laser refractive surgery measured with a new Goldmann convex prism: correlations with GAT and ORA María Iglesias1*, Bachar Kudsieh2, Andrea Laiseca1, Cristina Santos3, Jeroni Nadal1, Rafael Barraquer1,4 and Ricardo P. Casaroli‑Marano5 Abstract Background: The purpose of this study is to describe measurements using a newly developed modified Goldmann convex tonometer (CT) 1 year after myopic laser refractive surgery. Intraocular pressure (IOP) measurements were compared with IOP values obtained by Goldmann applanation tonometer (GAT), and Ocular Response Analyzer (ORA). Methods: Prospective double-masked study performed on thirty eyes of thirty patients that underwent laser in situ keratomileusis (LASIK; n = 19) or photorefractive keratectomy (PRK; n = 11). IOP was measured before and 3 and 12 months after surgery. Intraclass correlation coefficient (ICC) and Bland-Altman plot were calculated to assess the agreement between GAT, CT, IOPg (Goldmann-correlated IOP) and IOPcc (corneal-compensated IOP) from ORA. Results: Twelve months after LASIK, IOP measured with CT showed the best correlation with IOP measured with GAT before surgery (GATpre) (ICC = 0.886, 95% CI: 0.703–0.956) (15.60 ± 3.27 vs 15.80 ± 3.22; p < 0.000). However, a moder‑ ate correlation was found for IOP measured with IOPcc and CT 12 months after LASIK (ICC = 0.568, 95% CI: − 0.185 – 0.843) (15.80 ± 3.22 vs 12.87 ± 2.77; p < 0.004). Twelve months after PRK, CT showed a weak correlation (ICC = − 0.266, 95% CI: − 3.896 – 0.663), compared to GATpre (17.30 ± 3.47 vs 16.01 ± 1.45; p < 0.642), as well as poor correlation (ICC = 0.256, 95% CI: − 0.332 – 0.719) with IOPcc (17.30 ± 3.47 vs 13.38 ± 1.65; p < 0.182). Conclusions: Twelve months after LASIK, IOP measured with CT strongly correlated with GAT before surgery and could therefore provide an alternative method for measuring IOP after this surgery. More studies regarding this new convex prism are needed to assess its accuracy. Keywords: Tonometry, Corneal biomechanics, Intraocular pressure, Glaucoma, Myopia, LASIK, PRK Background Laser in situ keratomileusis (LASIK) is the most popular corneal refractive surgery performed in the last decade [1], with an estimated one million myopic patients under- going LASIK every year [2]. In view of this circumstance, *Correspondence: mariaiglesiasalvarez@gmail.com it is expected that clinical practice will involve an increas- 1 Instituto Universitario Barraquer, Barraquer Ophthalmology Centre, ing number of patients that have undergone laser refrac- Laforja 88, 08012 Barcelona, Spain tive surgery (LRS) in the past. Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/. The Creative Commons Public Domain Dedication waiver (http://​creat​iveco​ mmons.​org/​publi​cdoma​in/​zero/1.​0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Iglesias et al. BMC Ophthalmology (2022) 22:79 Page 2 of 9 It is known that corneal biomechanics (CB) are altered of the ablated zone of an OC, a different phenomenon can after LRS [3, 4]. The long-term results of postoperative be expected: initial contact pressure rises from the centre visual acuity and the safety of this frequent procedure of the OC, resulting in a balance of forces comparable to have been widely reported [5, 6]. Regardless of the sur- that before surgery. By applying this force with the CT, we gery performed, CB can vary with age, which may affect get to estimate the patient’s IOP from before their corneal corneal topography, visual outcomes and variations procedure. In addition, this device functions exactly like in tonometry measurements [7–10]. Corneal stiffness GAT prisms and its measurements are reproducible by dif- may increase with time due to a growth in glycogen- ferent observers [4], which implies that it is universal and induced cross-links, and lead to different wound healing simple to use, since it can be inserted in any slit-lamp or responses for PRK and LASIK [11, 12]. Moreover, biases Perkins tonometer (Fig. 1B). in IOP measurements may lead to glaucoma misdiagno- Recording pre-operative CCT and IOP measurements ses, especially in myopic eyes that are also known as a is very convenient for patient management after LRS risk factor for developing open-angle glaucoma [13, 14]. [15] in addition to follow-up IOPm, taking into account The Ocular Response Analyzer (ORA) is a non-contact that pre-operative measurement values do not remain tonometer that is less influenced by CB modifications after stable throughout a patient’s life [4]. Given that IOP is LRS [3] than the Goldmann applanation tonometer (GAT), the only risk factor in glaucoma progression that can be which is still the gold standard for measuring intraocular modified [21, 22], knowledge of the IOP baseline seems pressure (IOP) in normal corneas [15–17]. However, IOP mandatory to create a prognosis profile [23, 24], even underestimations in GAT measurements after LRS have more so if LRS is not recent. Following our previous been widely described [18, 19], due to changes in corneal research in which we found a strong correlation between central thickness (CCT) and CB [4, 20]. To overcome this IOP using CT 3 months after myopic LASIK and GAT underestimation of IOP in myopic eyes post LRS, we have before surgery [4], we describe the clinical outcomes of developed a new method for measuring IOP in this sub- this new Goldmann modified device in the long term, its group of patients: the modified Goldmann convex tonom- correlations with GAT and with ORA by evaluating IOP eter (CT) (Fig. 1A) [4]. Using finite element analysis, we measurements (IOPm) before and 3 and 12 months after demonstrated that the applicability of the Imbert-Fick law myopic LRS. is compromised after myopic LRS. When GAT reaches the 3.06 mm area of applanation, the flattened centre of Methods an operated cornea (OC) is consistent with the idea of​​ Study design Imbert-Fick behaviour, but not the corneal edges [4]. This A prospective, double-masked, single-centre, compara- indicates that there is lower resistance from the centre of tive study was carried out on a sample of thirty myopic an OC when we make physical contact with GAT, and as subjects who were going in for femtosecond-assisted a result the IOP registered will be lower in these patients. LASIK or photorefractive keratectomy (PRK) at the Despite this, if a convex force is applied towards the centre Barraquer Ophthalmology Centre in Barcelona. These Fig. 1 Modified CT applanation tonometer. Observe the convex reflection in the tip (A) and in the side image (B, *) CT tonometer inserted in a handling tonometer device (B) Iglesias et al. BMC Ophthalmology (2022) 22:79 Page 3 of 9 subjects first underwent a general medical history to the small sample size, and a significance level of 5% review and a detailed eye examination. The study proto- was considered in all analyses. We conducted descrip- col adhered to the tenets of the Declaration of Helsinki tive analyses for all variables before surgery and 3 and and the centre’s Institutional Review Board approved 12 months after surgery. Descriptive values are presented this study. Written consent was obtained from all as mean ± standard deviation (MD ± SD) unless stated participants. otherwise. Comparisons between preoperative and post- operative values were performed using the Wilcoxon test, Exploration protocol and comparisons between LASIK and PRK patients were All right eyes were randomly selected, adding up to a performed using the Mann-Witney test. total of 30 eyes. The inclusion criteria for the study were: GATpre was considered the current reference, and its myopic Caucasian patients over 18 years of age with a correlation with IOPm using different tonometers was stable myopic refractive error of less than − 9 spherical evaluated by: i) calculating the mean differences (md) dioptres (dpts), and myopic astigmatism of less than − 4 between measurements with GATpre and all devices dpts. Subjects with previously diagnosed ocular pathol- after surgery and testing for the absence of differences ogy, previous ocular surgery or receiving treatment with using the Wilcoxon test; ii) calculating the intraclass cor- medications that may affect IOP levels were excluded. relation coefficient (ICC) based on absolute agreement; All patients underwent a standardised examination at values lower than 0.5, between 0.5 and 0.8, and greater baseline, 24 h, 1 week, 1, 3, and 12 months after surgery. than 0.8 were indicative of poor or weak, good, and excel- To simplify comparisons in the long term, only 3 IOPm lent reliability, respectively [29]; and iii) constructing the controls were considered for the study: at baseline and Bland-Altman plot. 3 and 12 months after surgery. These included measure- ment of visual acuity (spherical equivalent refraction, Results SE), slit-lamp anterior biomicroscopy, posterior segment Thirty eyes of thirty patients were enrolled in the study ophthalmoscopy and IOPm. Age, gender, and refrac- and met the inclusion criteria in pre- and post-controls. tive error were recorded. Before recording IOPm, cor- Nineteen (63.3%) patients underwent LASIK and eleven neal topography was carried out by Pentacam, (Oculus, (36.6%) PRK. The mean age was 30.7 ± 6.5 years. Table 1 Wetzlar, Germany), to obtain anterior simulated kerato- shows descriptive statistics of the normally distributed metry (simK) and CCT. Moreover, maximum ablation variables in the pre- and post-surgical evaluations. depth (Max.Abl) and percentage of ablated tissue (PTA) Before surgery, ORA and GAT IOPm where simi- were analysed. Corneal characteristics including Corneal lar between groups. In contrast, CT overestimated Hysteresis (CH), Corneal Resistance Factor (CRF), Gold- IOP in relation to the other tonometers. After surgery, mann-correlated IOP (IOPg) and corneal-compensated all tonometers significantly registered lower IOP val- IOP (IOPcc) were also obtained by ORA (Reichert Oph- ues, except for IOPcc in the PRK subgroup, with IOPg thalmic Instruments, New York) after 3 different meas- recording the lowest compared to GATpre (Table 1). In urements with a waveform score (WS) higher than 3.5 as the LASIK subgroup after 3 months (Fig. 2A), GAT and recommended [25], and the best score was selected. IOPcc underestimated IOP, whereas CT at 3 months pro- In each patient, IOPm were taken between 10:00 and vided similar values to those obtained with GATpre and 13:00 h (10 am and 1 pm) [18] and ORA was performed ORA. After 12 months, CT values were like GAT and at least 10 min before GAT [26] and CT measurements. ORA before surgery, whereas the rest of the tonometers Each applanation tonometer measurement was carried kept IOPm below its pre-surgery estimations (Fig. 2A). out with a double-mask protocol on the same slit lamp In the PRK subgroup (Fig. 2B), an IOP reduction was Goldmann tonometer, which is regularly calibrated by a also found for all devices at 3 and 12 months after surgery. certified assistant. The main observer (AL) measured IOP However, in this case, CT overestimated pre-surgery with different devices – GAT and CT – randomly given GAT, IOPg and IOPcc values; and IOPcc significantly by a second observer (MI), and with 2-min rests between registered the most stable IOPm compared to pre-sur- measurements to avoid tonographic effects [27, 28]. gery values. The ICC, calculated for all patients and considering Statistical analysis was performed using SPSS® (Statisti- Statistical analysis LASIK and PRK separately, presented poor or moderate correlation between GAT measurements before and after cal Package for the Social Sciences, version 22.0; SPSS surgery (Table 2). The best correlations were observed Inc., Chicago, IL, USA), with the exception of the Bland- in the LASIK subgroup between GATpre and CT at Altman analysis, for which jamovi 1.2 (jamovi project, 3 months (15.60 ± 3.27 vs 14.05 ± 2.67; ICC = 0.808, 2020) was used. A nonparametric test was used due 95% CI: 0.429–0.927; p < 0.000); and at 12 months Iglesias et al. BMC Ophthalmology (2022) 22:79 Page 4 of 9 Table 1 Descriptive statistics of variables in the evaluations Table 1 (continued) before and 3 and 12 months after surgery Maximum corneal ablation, CH Corneal hysteresis, CRF Corneal resistance factor, M Months PRE-SURGERY POST 3 M POST 12 M MD ± SD MD ± SD MD ± SD GAT (mmHg)a (15.60 ± 3.27 vs 15.80 ± 3.22; ICC = 0.886, 95% CI: 0.703– • ALL 15.80 ± 2.72 12.00 ± 2.32 13.04 ± 2.55 0.956; p < 0.000). Moderate correlation was found for CT • LASIK 15.60 ± 3.27 11.01 ± 2.31 12.90 ± 2.64 and IOPcc at 12 months (15.80 ± 3.22 vs 12.87 ± 2.77; • PRK 16.01 ± 1.45 13.05 ± 1.13 14.20 ± 2.27 ICC = 0.568, 95% CI: − 0.185 – 0.843; p < 0.004) CT (mmHg)a (Table 2). The PRK subgroup – compared to GATpre – • ALL 21.07 ± 4.35 15.80 ± 3.07 16.30 ± 3.34 showed poor correlation for GAT and CT at 3 months • LASIK 21.05 ± 4.59 14.05 ± 2.67 15.80 ± 3.22 (16.01 ± 1.45 vs 17.09 ± 2.55; ICC = − 0.196, 95% CI: • PRK 22.00 ± 4.01 17.09 ± 2.55 17.30 ± 3.47 − 1.817 – 0.619; p < 0.638) and also 12 months after sur- IOPcc (mmHg)a gery (16.01 ± 1.45 vs 17.30 ± 3.47; ICC = − 0.266, 95% CI: • ALL 15.89 ± 3.07 13.07 ± 2.52 13.06 ± 2.40 − 3.896 – 0.663; p < 0.642). A weak correlation was also • LASIK 16.40 ± 2.93 12.48 ± 2.20 12.87 ± 2.77 found between CT and IOPcc at 3 and 12 months in the • PRK 15.02 ± 3.26 14.08 ± 2.83 13.38 ± 1.65 PRK subgroup (17.09 ± 2.55 vs 14.08 ± 2.83; ICC = 0.272, IOPg (mmHg)a 95% CI: − 0.328 – 0.729; p < 0.116), (17.30 ± 3.47 vs • ALL 15.49 ± 2.98 8.95 ± 2.79 9.62 ± 2.73 13.38 ± 1.65; ICC = 0.256, 95% CI: − 0.332 – 0.719; • LASIK 15.81 ± 2.77 8.24 ± 2.57 9.62 ± 3.00 p < 0.182) respectively (Table 2). • PRK 14.93 ± 3.37 10.17 ± 2.87 9.61 ± 2.32 Bland-Altman plots showed significant differences for SER (dpts) GATpre and post measurements, and for CT and IOPcc • ALL − 4.98 ± 2.28 0.00 ± 0.30 0.02 ± 0.22 after surgery. The mean differences for the entire cohort • LASIK −5.83 ± 2.21 0.05 ± 0.33 0.03 ± 0.25 and between subgroups for IOPm before and after sur- • PRK −3.52 ± 1.33 0.05 ± 0.23 0.03 ± 0.15 gery can be found in Table 3. Considering LASIK sepa- simK rately, CT showed a small difference with GATpre at • ALL 43.70 ± 1.13 39.80 ± 1.96 39.90 ± 2.00 3 months (md: − 1.11 mmHg, p = 0.440; LoA: − 3.32 • LASIK 43.70 ± 1.16 39.30 ± 2.06 39.40 ± 2.04 – 5.53), the difference for CT at 12 months being the • PRK 43.60 ± 1.11 40.60 ± 1.53 40.08 ± 1.62 smallest (md: 0.158 mmHg, p = 0.438; LoA: − 5.58 – CCT (μ) 5.26). On the other hand, CT showed small overestima- • ALL 543 ± 27.08 457 ± 35.90 469 ± 41.30 tions at 3 months in the PRK subgroup (md: 1.82 mmHg, • LASIK 554 ± 22.30 460 ± 40.70 466 ± 42.40 p = 0.106; LoA: − 3.09 – 7.06) and at 12 months (md: • PRK 524 ± 26.40 452 ± 26.60 473 ± 41.10 1.18 mmHg, p = 0.512; LoA: − 6.60 – 7.86). Additionally, CH CTpost significantly overestimated IOPccpost in LASIK • ALL 10.53 ± 1.28 7.88 ± 1.16 8.07 ± 1.03 and PRK respectively, at 3 months (md: 2.04 mmHg, • LASIK 10.44 ± 1.12 7.85 ± 1.10 8.04 ± 1.05 p = 0.008; LoA: − 3.5 – 7.58) (md: 3.83 mmHg, p = 0.008; • PRK 10.67 ± 1.56 7.92 ± 1.32 8.11 ± 1.05 LoA: − 3.05 – 7.74), and 12 months (md: 2.91 mmHg, CRF p < 0.000; LoA: − 2.53 – 8.35) (md: 3.89 mmHg, p = 0.03; • ALL 9.88 ± 1.12 6.29 ± 1.40 6.59 ± 1.25 LoA: − 2.95 – 8.04) respectively (graphics not shown). • LASIK 9.97 ± 1.09 6.05 ± 1.37 6.54 ± 1.20 • PRK 9.72 ± 1.21 6.70 ± 1.43 6.68 ± 1.39 PTA Discussion • ALL 12.90 ± 4.73 Tonometry nowadays has become a complex concern • LASIK 14.40 ± 4.94 since a single tonometer cannot be referred universally • PRK 10.30 ± 2.99 for every CCT, nor can CB and prior surgical procedures Max.Abl such as LRS or transplants be dismissed when measuring • ALL 70.50 ± 26.80 IOP [30, 31]. • LASIK 79.90 ± 27.50 A reduced CCT is not only an independent factor for • PRK 54.10 ± 16.40 developing glaucoma in the future [32], but also chal- Significant differences between LASIK and PRK IOPm are marked in bold lenge for estimating IOP if such reduction is due to a Comparisons between pre- and post-surgery: a, Wilcoxon test, p < 0.001. All laser-assisted procedure, regardless of the device used to values are in mean ± standard deviation (MD ± SD). GAT​Goldmann applanation obtain IOPm. It is known that after laser ablation, thicker tonometer, CT Convex tonometer, IOPcc Corneal-compensated IOP, IOPg Goldmann-correlated IOP, SER Spherical equivalent refraction, simK simulated corneas preserve more biomechanical properties than keratometry, CCT​Central corneal thickness, PTA Percent tissue altered, Max.Abl thinner corneas [33, 34], and high myopia has a greater decrease in CH and CRF properties after LASIK due to Iglesias et al. BMC Ophthalmology (2022) 22:79 Page 5 of 9 Fig. 2 Mean IOP changes over time obtained using different tonometer devices in pre- and post-refractive surgery evaluations after 3 and 12 months. CX: L, LASIK (A); CX:P, PRK (B); GAT, Goldmann applanation tonometer; CT, convex tonometer; IOPg, Goldmann-correlated intraocular pressure and IOPcc, corneal-compensated IOP. M, months greater anterior flap stroma lamellae reduction [35]. Tou- boul et al. described that the lower the CH, the lower the IOP underestimation with GAT, defining CH as a risk factor for underestimating IOP [33]. Taking into account Table 2 Intraclass correlation coefficient (ICC) results that CH is lower in glaucoma [21, 36, 37] and myopic LRS Tonometer pair N ICC (95% CI) P patients [20, 38], GAT should be avoided in this subgroup All patients GAT3M - GATpre 30 0.400 (− 0.227–0.741) < 0.001 of patients since they will have low CH values regardless GAT12M - GATpre 30 0.478 (− 0.113–0.758) < 0.006 of glaucoma. Therefore, seems crucial to focus on using CT 3 M - GATpre 30 0.669 (0.296–0.843) < 0.002 only specific tonometers with these patients. CT 12 M - GATpre 30 0.704 (0.383–0.859) < 0.001 This convex prism has already been tested 3 months after LRS in comparison with GAT, registering higher IOPcc3M- 30 0.450 (− 0.136–0.740) < 0.010 CT3M IOPm in PRK than in LASIK subjects (1.62 ± 2.65 mmHg IOPcc12M - 30 0.465 (− 0.196–0.764) < 0.003 vs − 0.19 ± 2.60 mmHg, respectively) [4], concurring CT12M with the third month outcomes of the present study LASIK GAT3M - GATpre 19 0.418 (−0.221–0.781) < 0.003 (1.82 ± 2.55 mmHg vs − 1.11 ± 2.67 mmHg). Still, this GAT12M - GATpre 19 0.516 (−0.171–0.811) < 0.015 new modified prism has never before been compared CT 3 M - GATpre 19 0.808 (0.492–0.927) < 0.000 in the long term. In a global overview of our meas- CT 12 M - GATpre 19 0.886 (0.703–0.956) < 0.000 urements, IOPm at 12 months were slightly higher IOPcc3M- 19 0.405 (−0.252–0.748) < 0.076 than IOPm at 3 months. This could be related to a par- CT3M tial recovery of CB after this period. Nonetheless, this IOPcc12M- 19 0.568 (−0.185–0.843) < 0.004 assumption must be interpreted with caution, since the CT12M main limitation of the present study is that the number PRK GAT3M - GATpre 11 0.134 (−0.234–0.590) < 0.261 of patients undergoing measurements is not appropri- GAT12M - GATpre 11 0.262 (0.581–0.752) < 0.251 ate for estimating significant IOP correlations with CB CT 3 M - GATpre 11 −0.196 (−1.817–0.619) < 0.638 or biological factors such as age or degree of myopia. CT 12 M - GATpre 11 −0.266 (−3.896–0.663) < 0.642 Nevertheless, our CTpost LASIK results concur with IOPcc3M- 11 0.272 (− 0.328–0.729) < 0.116 CT3M the outcomes referred to above, where a significant cor- IOPcc12M- 11 0.256 (− 0.332–0.719) < 0.182 relation with CRF and CH reduction was found between CT12M CTpost and GATpre in 73 patients 3 months after LASIK ICC is based on absolute agreement. Significant differences are marked in bold. (0.15 ± 3.22 vs − 0.19 ± 2.60 mmHg, respectively) [4]. CI, confidence Interval; p, value for correlation Likewise, despite our small sample in the PRK subgroup, GAT​Goldmann applanation tonometer, CT Convex tonometer, IOPg Goldmann- our outcomes in the current study are comparable with correlated IOP, IOPcc Corneal-compensated IOP, M Months Iglesias et al. BMC Ophthalmology (2022) 22:79 Page 6 of 9 Table 3 Mean differences for IOP measurements in pre-surgery with GAT and at 3 and 12 months after surgery with GAT, CT and IOPcc IOP difference GAT3M - GATpre GAT12M - CT3M - GATpre CT12M - GATpre CT3M -IOPcc3M CT12M -IOPcc12M GATpre All md (bias) −3.80 −2.43 −0.0333 0.533 2.70 3.27 Wilcoxon test Z = -4.747;p < 0.000 Z = -3904;p < 0.000 Z = -0.361;p = 0.718 Z = -0.888;p = 0.375 Z = -3.754;p < 0.000 Z = -4.618;p < 0.000 LASIK md (bias) −4.52 −2.74 −1.11 0.158 2.04 2.91 GV Wilcoxon test Z = -3.836;p < 0.000 Z = -3.209;p < 0.001 Z = -2.017; Z = -0.776;p = 0.438 Z = -2.636; Z = -3.582;p < 0.000 p = 0.440 p = 0.008 PRK md (bias) −2.54 −1.91 1.82 1.18 3.83 3.89 Wilcoxon test Z = -2.185;p < 0.005 Z = -2.203;p = 0.028 Z = -1.616;p = 0.106 Z = -6.55;p = 0.512 Z = -2.667;p = 0.008 Z = -2.936;p = 0.03 Mean differences between GATpre, GAT, IOPcc and CT after surgery (at 3 and 12 months) were calculated and tested for the absence of differences using the Wilcoxon test. Significant differences are marked in bold. md, mean difference; Z, Wilcoxon test; p, probability value GAT​Goldmann applanation tonometer, CT Convex tonometer, IOPg Goldmann-correlated IOP, IOPcc Corneal-compensated IOP, M Months those of our previous publication: we measured 29 PRK and ORA subgroups after femtosecond laser-assisted patients after 3 months and the IOP deviation was higher LASIK compared to TransPRK. In addition, Schallhorn than in the LASIK subgroup [4], confirming that we have et al., in a larger cohort of LASIK and PRK patients enough evidence to assure that the modified Goldmann measured with NCT [34], found a higher reduction of CT applicability in PRK patients and in standard corneas IOP after 3 months of LASIK (− 4.57 ± 2.42 mmHg), is not reliable due to preservation of CB. Further studies compared to PRK (− 3.16 ± 2.53 mmHg). In our previ- in larger cohorts will focus on determining if this convex ous research, despite a strong correlation found between prism performs accurately in LASIK patients. CT and LASIK compared to PRK, GAT IOPm were sig- Biases in estimating IOP may come from diverse nificantly lower in the first group (− 3.94 ± 2.17 mmHg sources, like introducing different devices in our daily vs − 2.62 ± 2.16 mmHg) [4]. clinical routine or determining inaccurate IOPm for a This is the first time that this new convex prism is same patient [15, 39]. To minimize the biases of GAT being compared to ORA IOPm. ORA has been proven post LRS, we can use non-contact tonometers such as the to improve GAT measurements by introducing new ORA, the non-contact tonometer (NCT), and the Cor- parameters that can evaluate CB modifications and IOP vis ST; or contact tonometers like the Pascal Dynamic after LRS [3, 42]. IOPcc is known for being less influ- Tonometer (PDCT) [40]. Among these, bIOP (biome- enced by CCT variations, compared to the IOP under- chanical corrected IOP) from Corvis ST has shown to estimation of IOPg and GAT post LRS [3, 43, 44]. Mean be the most stable and accurate parameter after surface differences between GAT and IOPcc readings in stand- ablation or lamellar procedure [39, 41]. Nonetheless, ard corneas are ±1.5 mmHg [45]. However, despite GAT ophthalmologists do not always have access to devices and IOPcc show higher correlations than IOPg [26, 46], that evaluate post-operative IOP accurately. In applana- they should not be used interchangeably because there tion tonometry after myopic LRS, IOP estimations must can be differences of more than 2 mmHg between them be considered carefully since they do not seem to follow which could have clinical implications [45]. In our sam- CCT reduction linearly due to CB modifications. We ple, although a significantly good correlation was found believe it could be related in part to that contact pres- between IOPcc and CT in the LASIK subgroup after 1 sure profiles between applanation tonometry and the year – compared to a very poor correlation in the PRK anterior corneal surface are very different in PRK com- subgroup –, mean differences were significantly higher pared to LASIK patients [4]. Since less corneal tissue is than > 2 mmHg in both groups. Moreover, even though removed from the anterior stroma in low myopia, CB are IOPcc has shown better agreement with GAT than expected to be less altered in PRK, and therefore these with IOPg before LRS, and is more stable after sur- corneas may behave like non-OC. In this study, we have gery [26, 45, 46], we believe that an agreement between compared a non-contact tonometer with two contact the convex prism and IOPcc cannot be expected after tonometers. Considering all of them together, we have LRS. To begin with, CTpost has already demonstrated found a significant IOP decrease in the LASIK subgroup bad agreement with GATpost [4]; besides, in our study, compared to PRK. These findings coincide with recent CTpost overestimated IOPcc and IOPg values at 3 and results for other devices. Chen et al. measured IOP after 12 months, having similar correlations with GAT and 3 months with GAT, ORA, PDCT and Corvis ST [41], IOPcc after 1 year, but those correlations were weak and also found a significant IOP reduction in the GAT compared to correlations with GAT before surgery. This Iglesias et al. BMC Ophthalmology (2022) 22:79 Page 7 of 9 could be associated to the fact that matching static with diminish the risk of glaucoma progression in this sub- dynamic tonometry seems to be incongruous since it type of patients. has been widely proven that comparing these devices is not accurate [26, 46, 47]. An exact comparison must Abbreviations not be extrapolated considering that the central areas CB: Corneal biomechanics; CCT​: Corneal central thickness; CH: Corneal hyster‑ from which the IOP is measured are different between esis; CI: Confidence interval; CPP: Contact pressure profile; CRF: Corneal resist‑ them: in applanation tonometry after LRS, both GAT ance factor; CT: Convexly-shaped tonometer; dpts: Dioptres; GAT​: Goldmann applanation tonometer; ICC: Intra-class correlation coefficient; IOP: Intraocular and CT contact pressure profiles are not consistent with pressure; IOPcc: Corneal-compensated IOP; IOPg: Goldmann-correlated 3.06 mm and could encompass a larger amount of tissue IOP; IOPm: IOP measurements; LASIK: Laser-assisted in situ keratomileusis; especially in LASIK [4] compared to the exact central LoA: Limits of agreement; LRS: Laser refractive surgery; M: Months; Max.Abl: Maximum ablation depth; md: Mean differences; MD ± SD : Mean ± standard corneal 3.0 mm diameter from which ORA estimates deviation; NCT: Non-contact tonometer; OC: Operated cornea; ORA: Ocular IOP, regardless of the surgical procedure performed Response Analyser; p: Probability value; PDCT: Pascal Dynamic Tonometer; [48, 49]. These results suggest that the three tonometers PRK: Photorefractive keratectomy; PTA: Percentage of ablated tissue; SER: Spherical equivalent refraction; simK: Simulated keratometry; Z: Wilcoxon test. should not be used interchangeably after LRS due to clinically significant IOP variations between them. Acknowledgements Another important limitation affecting our work, is The authors would like to thank Emilio Iglesias Touriño MD, PhD, an ophthal‑ mologist whose extraordinary knowledge has been indispensable for the that modified Goldmann CT measurements do not apply development of the new Goldmann CT device from the beginning of our for every OC and may not represent real IOP, in the same project. way that GAT is not suitable for every non-OC, except Authors’ contributions for standard corneas with CCT around 520 μm (microns) MI contributed to ensure the double-masking. She also collected, analysed, and accurate measurement technique [50]. Since true and interpreted the sample data and was a major contributor in writing IOP can only be obtained from invasive intracameral the original manuscript. BK analysed and interpreted the clinical results. AL collected IOP measurements before, and 3 and 12 months after surgery. CS readings [43, 51], and IOPm may have not been taken at interpreted the statistical analysis. RPCM, JN and RB reviewed the manuscript. the same time of day after several months entailing IOP All authors read and approved the final manuscript. fluctuations between measurements [52], IOP stabil- Authors’ information ity after LRS was taken as a reference for the quality of Not applicable. tonometer readings [43]. In our study, IOPcc in the PRK subgroup was the most stable IOPm. It seems obvious to Funding The author(s) received no financial support for the research, authorship, and/ affirm that GAT and CT measurements before and after or publication of this article. surgery are not reliable compared to themselves. How- ever, as previously discussed, CT appears to be ineffective Availability of data and materials The research data generated during the current study cannot be shared due in non-OC, but its use is to register IOP after surgery in to exclusive intellectual property rights of the new Goldmann “CT” convex correlation to GAT before surgery. Nevertheless, CTpost tonometer secured by a Spanish patent filed with number P201631280 mean differences between IOPm at 3 and 12 months were (granted patent), and a European patent filed with number 3520682 (granted patent). However, the data are available from the corresponding author upon not significantly different, and they registered high corre- reasonable request. lations with GATpre values in the LASIK subgroup, con- cluding that convex prism IOPm in the sample presented Declarations may be considered acceptably stable after this procedure. A future study in larger LASIK populations will be car- Ethics approval and consent to participate An informed consent was obtained from all participants and/or their legal ried out comparing the Goldmann CT and other tonom- guardian/s before their participation in the study, and the Institutional Review eters to shed some light on its accuracy. Board of the Barraquer Ophthalmology Centre approved this study. Consent for publication Conclusions The written consent of all participants approved by the Institutional Review Board of the centre included a statement for consent to publish any personal In summary, though GAT is still the current reference data regarding their refractive surgery, as well as any additional related technique in standard corneas, a certain IOPm vari- information. ability is expected with applanation tonometry due to Competing interests influencing corneal factors, especially after LRS [53, Each of the co-authors has seen and agrees with the financial and non-finan‑ 54]. The new modified Goldmann CT is the first appla- cial competing interests statement presented by MI (as corresponding author) nation tonometer to offer precise IOP estimations in on behalf of all the authors of the paper: MI has personal conflict of interest being the inventor of the new Goldmann LASIK patients after 3 [4] and 12 months in relation “CT” applanation tonometer. She had full access to all the data in this study to GAT before surgery. It could be an affordable and and takes complete responsibility for the integrity of the data and the accu‑ effortless method for monitoring IOP by providing an racy of the data analysis. MI has exclusive personal rights to the intellectual additional reference to IOP assessment and therefore Iglesias et al. BMC Ophthalmology (2022) 22:79 Page 8 of 9 property of this invention secured by the patents referred to above, and is the 15. Bagnasco L, Bagnis A, Bonzano C. EGS_guidelines_5_English.Pdf. Euro‑ owner of every figure presented in this study. pean Glaucoma society terminology and Guidelines for Glaucoma. 5th The rest of the co-authors declare no financial competing interests. ed; 2020. p. 49. MI and the rest of the co-authors declare they do not have non-financial 16. Nakao Y, Kiuchi Y, Okumichi H. Evaluation of biomechanically corrected competing interests. intraocular pressure using Corvis ST and comparison of the Corvis ST, noncontact tonometer, and Goldmann applanation tonometer in Author details patients with glaucoma. PLoS One. 2020;15(9 September):1–9. Disponible 1 Instituto Universitario Barraquer, Barraquer Ophthalmology Centre, Laforja en. https://​doi.​org/​10.​1371/​journ​al.​pone.​02383​95. 88, 08012 Barcelona, Spain. 2 Department of Ophthalmology, Hospital 17. Fu D, Li M, Knorz MC, Wei S, Shang J, Zhou X. Intraocular pressure Universitario Puerta De Hierro, 28222 Madrid, Spain. 3 Unitat Antropologia changes and corneal biomechanics after hyperopic small-incision lentic‑ Biològica, Department Biologia Animal, Biologia Vegetal i Ecologia, Univer‑ ule extraction. BMC Ophthalmol. 2020;20(1):2–7. sitat Autònoma de Barcelona (UAB), 08193 Barcelona, Spain. 4 International 18. Fernández J, Rodríguez-Vallejo M, Martínez J, Tauste A, Piñero DP. 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The effect of diurnal fluctuation in intraocular pressure on the evaluation of risk factors of progression in normal tension glaucoma. PLoS One. 2016;11(10):1–14. 53. Ramm L, Herber R, Spoerl E, Raiskup F, Pillunat LE, Terai N. Intraocular Ready to submit your research ? Choose BMC and benefit from: Pressure Measurement Using Ocular Response Analyzer, Dynamic Contour Tonometer, and Scheimpflug Analyzer Corvis ST. J Ophthalmol. • fast, convenient online submission 2019;2019:3879651. https://​doi.​org/​10.​1155/​2019/​38796​51. 54. Shousha SMA, Steit MAHA, Hosny MHM, Ewais WA, Shalaby AMM. • thorough peer review by experienced researchers in your field Comparison of different intraocular pressure measurement techniques • rapid publication on acceptance in normal eyes, post surface and post lamellar refractive surgery. 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