The IOP measurement was overestimated after bandage contact lens (BCL) wearing, either by NCT (0.9 ± 2.9mmHg) or by Corvis ST (1.3 ± 2.4mmHg) in this study. This is consistent with previous studies15–18, while the average difference of IOP before and after contact lens wearing varies among these studies. Previous studies suggest that IOP measurement overestimation with soft contact lenses may be associated with lens power, central thickness, as well as the rigidity of the lens material16, 19, 20. BCL as a special type of soft contact lens, may affect the IOP measurement by those factors mentioned above. BCL wearing not only increases of the thickness of the eye wall that is flattened by the air puff of tonometer, but also changes the biomechanics of the cornea-BCL complexus.
Previous studies have confirmed that IOP measurement increases when central corneal thickness (CCT) increases. According to Zhang et al.21, the IOP measured by Goldmann applanation tonometer and NCT increased 0.39 mmHg and 0.64 mmHg respectively for each 10 µm increase of CCT. Doughty et al.22 considered that the IOP difference was 3.4 mmHg for every 10% difference in CCT. However, previous studies had different views on the effect of central thickness of soft contact lens on IOP measurement. Ogbuehi et al.20 considered that the effect of central thickness of soft contact lens with high water content was similar to the effect of central corneal thickness. They also deemed that the influence of high water content lenses with central thickness less than 0.3mm could be ignored. Nevertheless, McMonnies et al.15 believed that the influence was significant and nonnegligible when the central thickness of the contact lens was greater than 0.15 mm.
The wearing of BCL increased the apparent CCT measuring values by Pentacam or Corvis ST, which may partially explain the change of IOP measurement values. After BCL wearing, Pentacam indicated that the apparent CCT increased by 12 ± 13 µm; according to Zhang et al.21, the corresponding increased IOP was about 0.768 mmHg; according to Doughty et al.22, the corresponding increased IOP was about 0.748 mmHg; in our study, the measured IOP values by NCT increased 0.9 ± 2.9 mmHg. After BCL wearing, Corvis ST indicated that apparent CCT increased by 24 ± 15 µm; according to Zhang et al.21, the corresponding increased IOP was about 1.536 mmHg; according to Doughty et al.22, the corresponding increased IOP was 1.496 mmHg; in our study, the IOP measured values by Corvis ST increased 1.3 ± 2.4mmHg.
Since BCL wearing increased the apparent CCT values, we may refer to CCT correction method of IOP to minimize the influence of BCL wearing on IOP measurement. Pentacam's five IOP correction formulas were tried and evaluated respectively. We compared the corrected IOP after BCL wearing with the uncorrected IOP before BCL wearing, and found that the difference of Kohlhaas formula was the most significant. Kohlhaas formula was initially developed to amend the IOP measurement after Laser Assisted In-situ Keratomi (LASIK) in which a corneal flap is created, and a modified constant of 0.75mmHg was added 12. So it might not be suitable to use Kohlhaas formula for the correction of IOP after BCL wearing, which would cause overestimation of IOP measurements. Among the other formulas, the difference of Ehlers formula was the smallest, followed by Orssengo/Pye formula. Therefore, when using a NCT to measure the IOP over BCL wearing, it is recommended to use the Pentacam IOP correction system with Ehlers formula and Orssengo/Pye formula for IOP correction.
Previous studies had shown that the interaction among central thickness, lens power and elastic modulus of corneal contact lenses could better explain the effect of corneal contact lenses wearing on IOP measurements17, 23. Patel et al.16 studied siloxane hydrogel contact lenses with low water content (24%) and high modulus of elasticity (1.2MPa) and hydrogel contact lenses with high water content (69%) and low modulus of elasticity (0.91MPa). They found out that the increase of IOP of low elastic modulus contact lenses was related to the central thickness of corneal contact lenses, and the increase of IOP of high elastic modulus contact lenses was more related to the material. The BCL used in our study was lotrafilcon A silicon hydrogel lens with 24% water content, which is a high elastic modulus lens. Therefore, in addition to the increase of the thickness of corneal contact lens, the elastic modulus of the lens material may also be a factor affecting the measured value of IOP in our study.
Corvis ST is supposed to biomechanically correct the elastic modulus7. An IOP correction formula was developed by Joda et al.7 through the analysis of clinical data, which is based on the study of finite element parameters. It can reduce the influence of corneal stiffness and age on IOP measurements, and biomechanically correct the IOP values measured by Corvis ST. In fact, study has found out that the biomechanical correction of Corvis ST measurements is closely related to the CCT, and the BIOP can well eliminate the effect of corneal thickness14. In our study, the biomechanically corrected IOP (BIOP) after BCL wearing was significantly lower than the uncorrected IOP before BCL wearing. We speculated BIOP may overcorrected the IOP of eye with BCL. The BCL wearing may affect the accurate measurement of CCT by Corvis ST, therefore an overcorrection of IOP might happen due to the underlying correction logic of BIOP. In addition, this formula was developed to eliminate the effect of age on the elastic modulus of cornea, but the change of elastic modulus caused by BCL is unrelated to age. Therefore, the BIOP offered by Corvis ST may not be suitable for IOP correction in BCL wearers.
In summary, BCL wearing can overmeasure the IOP, either by NCT or Corvis ST. It was related to the increase of the apparent corneal thickness measurement, as well as the change in the elastic property of cornea-BCL complexus. Certain IOP correction systems have been developed to minimize the effect of BCL wearing on IOP measurement. Pentacam IOP correction system with the Ehlers formula and Orssengo/Pye formula are recommended to correct the IOP values measured by NCT over BCL. Ehlers formula is also recommended when Corvis ST is used to measure the IOP over BCL.