Normal young healthy subjects were recruited. Exclusion criteria included subjects with rigid contact lens wear, corneal astigmatism more than 2D, and history of any ocular diseases. Soft contact lens wearers were required to stop contact lens wear for at least 24hours. This study was approved by the ethics committee of the University and was carried out with due regard to the tenets of the Declaration of Helsinki. Details of the study were given to subjects before informed consent was obtained.

Two brands of silicone hydrogel contact lenses with different modulus were used. They were Ciba Vision Air Optix® Focus Night & Day® Aqua (N&D) and Acuvue® Advance® (AA). All the lenses used had back vertex power of −3.00D. These lenses shared similar lens overall diameter (13.8mm versus 14.0mm), center thickness (0.08mm versus 0.07mm) and base cure (8.4mm versus 8.3mm). Their lens modulus values were different (1.5MPa versus 0.43MPa, respectively).

After informed consent, each subject had CH and CRF measured by ORA (Reichert Ophthalmic Instruments, Buffalo, NY), corneal topography and central corneal thickness (CCT) measured by Pentacam Classic (Oculus Inc, Germany) on both eyes. All these procedures were non-contact. In ORA, three measurements were obtained each with a waveform score of at least 3.50.16 In corneal topography, three images were captured using the 50-scan mode and all scans were registered as “OK” for quality assurance.21 The corneal power, in terms of SimK, was converted to its vector form (M, J0 and J45) for analysis.22

After the above baseline measurements, one eye was randomly selected to be fitted with N&D and the fellow eye with AA. After 10min of lens stabilization, CH and CRF were measured by ORA followed by DCT (Ziemer Ophthalmic Systems, Port, Switzerland). Although the latter is a contact procedure, no topical anesthetic was used with the contact lens in situ.7 After the ORA and DCT measurements, lenses were removed and rinsed. The subject had a 5-min rest with no contact lens wear. The same lenses were then re-inserted again for the other eye, followed by ORA and DCT measurements in the same manner. After lens removal and another 5min of rest, one drop of 0.5% proparacaine was instilled in each eye. IOP and OPA were measured again “without lens”. In DCT, three readings of quality score between Q1 and Q3 were obtained, with readings at Q4 or Q5 being discarded.23

The standard deviation of IOP difference without lens and with low minus power silicone hydrogel lens was 0.92mmHg (personal communication with Dr. Daniela Nosch). A sample size of 40 has a 80% power to detect a difference of 0.41mmHg without and with contact lens wear.9 Kolmogorov–Smirnov test was used to check for normality. Paired t-tests were used to compare the baseline measurement results between the two eyes. Pearson correlations were studied between CCT and CH, CRF as well as DCT results for each eye. CH and CRF were compared “without lens” and with different contact lenses in situ. DCT measurements (IOP and OPA) were also compared. Repeated measures analysis of variance (ANOVA) was used. Whenever significant difference was found, paired t-tests were performed to find out any pair difference. Alpha was maintained at 0.05 without any Bonferroni adjustment.24 Agreements in DCT without and with lens were compared using the Bland and Altman plots.25

We had 47 subjects (21 males) with a mean age of 22.3 years, standard deviation 1.2 years (19–26 years). There were no significant differences between the two eyes at baseline, except the corneal J45 component and CCT (Table 1). The mean difference was 0.23 D for J45 (standard deviation, SD, of 0.42 D) and 4.7μm for CCT (SD of 8.1μm). There were no significant correlations between CCT and DCT parameters in both the right and left eyes. Significant positive correlations were found between CCT and CRF (right eye: r2=0.59, p<0.01; left eye: r2=0.29, p<0.01), and between CCT and CH (right eye: r2=0.37, p<0.01; left eye: r2=0.14, p<0.01).

Twenty-six subjects had their right eyes fitted with N&D first followed by AA. The remaining subjects had a reverse wearing sequence. All lens fitting had good centration with adequate lens movement. Table 2 shows the ORA and DCT results with lenses and “without lens”. No significant difference was found in CH and CRF with and without lens (repeated measures ANOVA: p>0.05 in all cases). Comparing with the “without lens” condition, significant difference was found in IOP (repeated measures ANOVA: F=5.185, p=0.007) when it was measured with N&D followed by AA, but not in the reverse sequence of lens wear. Paired t-tests did not identify any significant pair difference comparing with the “without lens” condition. The difference between N&D and AA was significant (Paired t-test: t=3.054, p=0.004) (Table 3).

Comparing the IOP obtained with and without lens, a greater difference was found with the N&D lenses in situ (Table 3) (Figs. 1 and 2). Regression analyses could not demonstrate any significant trends. The 95% limits of agreement (1.96×standard deviation of the difference) were greater over N&D lenses.

Figure 1.

Bland and Altman plots of agreement between intraocular pressure (IOP) from contact lens wear and no lens. Night & Day® was inserted first followed by Acuvue® Advance®. The solid line represents the mean difference, whereas the dotted lines represent the upper and lower limits of agreement. The correlation coefficients (r) are reported. (a) Night & Day® versus no lens (r=−0.074, p=0.62). (b) Acuvue® Advance® versus no lens (r=−0.147, p=0.32). (c) Night & Day® versus Acuvue® Advance® (r=−0.037, p=0.80).

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Figure 2.

Bland and Altman plots of agreement between intraocular pressure (IOP) from contact lens wear and no lens. Acuvue® Advance® was inserted first followed by Night & Day®. The solid line represents the mean difference, whereas the dotted lines represent the upper and lower limits of agreement. The correlation coefficients (r) are reported. (a) Acuvue® Advance® versus no lens (r=0.062, p=0.68). (b) Night & Day® versus no lens (r=−0.094, p=0.53). (c) Acuvue® Advance® versus Night & Day® (r=−0.155, p=0.30).

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OPA demonstrated a significant difference when it was measured with AA followed by N&D, but not in the reverse sequence of lens wear (Table 2). Paired t-tests found that both lens types demonstrated a lower OPA than without lens (Table 4). Figs. 3 and 4 show the Bland and Altman plots.

Figure 3.

Bland and Altman plots of agreement between ocular pulse amplitude (OPA) from contact lens wear and no lens. Night & Day® was inserted first followed by Acuvue® Advance®. The solid line represents the mean difference, whereas the dotted lines represent the upper and lower limits of agreement. The correlation coefficients (r) are reported. (a) Night & Day® versus no lens (r=−0.272, p=0.06). (b) Acuvue® Advance® versus no lens (r=−0.120, p=0.42). (c) Night & Day® versus Acuvue® Advance® (r=0.213, p=0.15).

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Figure 4.

Bland and Altman plots of agreement between ocular pulse amplitude (OPA) from contact lens wear and no lens. Acuvue® Advance® was inserted first followed by Night & Day®. The solid line represents the mean difference, whereas the dotted lines represent the upper and lower limits of agreement. The correlation coefficients (r) are reported. (a) Acuvue® Advance® versus no lens (r=−0.265, p=0.07). (b) Night & Day® versus no lens (r=−0.178, p=0.23). (c) Acuvue® Advance® versus Night & Day® (r=0.113, p=0.45).

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Lenses used in this study have different modulus. The Focus Night & Day® lens was the first few brands of silicone hydrogel lenses with high modulus, or is stiffer. The two lenses are different in the water content, 24% versus 47%, respectively. Most patients, especially daily disposal soft hydrogel lens wearers, complained about the lens awareness immediately after lens insertion. The Acuvue® Advance® lens is relatively newer and softer and comes with increased lens comfort.13

Dynamic contour tonometry can be a substitute for GAT on eyes after refractive surgeries.26,27 There has been no significant change in IOP before and after laser in situ keratomileusis (LASIK) while GAT26,27 and NCT27 can be significantly reduced. We shared the same finding as previous studies that DCT parameters were not influenced by CCT.

Nosch et al.9 found similar IOP measurement with and without soft hydrogel contact lens. Gogniat et al.10 included both hydrogel and silicone hydrogel materials. They found stable DCT measurements over silicone hydrogel materials. Similar findings could be observed in rebound tonometry, that is less effect from silicone hydrogel than hydrogel materials on IOP measurements.28 This may be in concordance with the speculation that silicone hydrogel contact lens can provide a more stable contact surface than hydrogel contact lens.10 Although a significant IOP difference was found in the current study over one lens wearing sequence, this was not substantiated in the reverse wearing sequence. Agreement analyses revealed a slightly greater IOP difference when DCT was performed over Focus Night & Day® (Figs. 1 and 2). The difference may be attributed to the higher lens modulus of the material. Gogniat and co-workers10 concluded that DCT over silicone hydrogel contact lens was not influenced by lens power. The lens modulus of their silicone hydrogel lens was 0.66MPa, similar to Acuvue® Advance® in our study. Clinically, IOP variation within 3mmHg is considered acceptable.29,30 From the Bland and Altman plots (Figs. 1 and 2), only one or two subjects had IOP difference beyond this limit in each scenario. From the agreement analyses (Table 3), DCT over silicone hydrogel contact lens of low lens modulus has reasonable clinical acceptance. Optometrists not eligible to use topical anesthetics may put a low modulus silicone hydrogel lens in situ for DCT.

Nosch et al.9 found lower OPA (2.08mmHg versus 2.20mmHg) when it was measured over soft contact lenses. We demonstrated similar findings over silicone hydrogel lens (Table 2). Post hoc tests revealed significantly lower OPA with both lens types (Table 4). Agreement was within 1mmHg. OPA is a relatively new ocular parameter. Higher OPA means greater IOP fluctuation that could be associated with glaucomatous visual field loss.31 The OPA of our subjects was similar to Gogniat and co-workers10 but smaller than other studies.32,33 It may be due to longer axial length in our population,32,34 but unfortunately we did not measure their refractive status.

Ocular Response Analyzer provides corneal biomechanical properties. We found no significant change in CH and CRF when measurements were taken over silicone hydrogel lenses. We originally thought that corneal biomechanical properties could be measured differently in our experimental protocol, such as a higher corneal resistance factor over contact lens of higher modulus. From the current results, CH and CRF are measured clinically the same. There is no speculation that ORA is not measuring corneal biomechanical properties because contact lens of higher modulus over the cornea is different from a stiffer cornea. On the other hand, whether ORA is really measuring corneal biomechanics is still a question.35 For example, ORA could not find significant changes in corneal biomechanics before and after CXL in keratoconus.36–38

There were some limitations of this study. We provided a 5-min rest before inserting the other lens type because the wearing period was 10min only for each lens type. We did not anticipate much effect on IOP and corneal biomechanical properties from such a short wearing schedule. On the other hand, such short duration of lens wear could not represent the real clinical situation of long hours silicone hydrogel lens wear. Thirdly, the DCT results for comparison were actually obtained after wearing all the lenses. Since DCT measurement over the cornea requires topical anesthesia, it was therefore the last procedure to be conducted. Our study design involved repeated corneal contact, even though some of them were through contact lenses. Whether repeated DCT would affect the accuracy of IOP measurement is not clear. Perhaps massaging effect from DCT, if any, could be less than the Goldmann applanation tonometry because of less corneal deformation arising from the former.39 Both ORA and DCT were performed by the same investigator. Acceptance of measurement results was based on waveform score from ORA16,40 and quality score from DCT23 to eliminate bias from investigator. Our study included only young subjects with normal IOP. The study should be repeated on subjects who are more prone to have glaucoma, such as those over the age of 40 years.

Asian population has high prevalence of myopia. Soft contact lens is a very popular corrective means. It could be inconvenient for patients to remove their contact lenses for ophthalmic procedures. From the current results, removal of silicone hydrogel contact lenses may not be necessary for DCT measurements. For countries where optometrists are not allowed to use topical anesthetics, they may put a low modulus silicone hydrogel contact lens in situ for DCT.

The authors have no proprietary or commercial interest in any materials discussed in this paper.