Corneal swelling, shape alterations, refractive stability or induced optical aberrations are crucial factors for normal vision1 that must also be specially controlled in severe corneal pathologies such as keratoconus5, ectasia28 or other ocular degenerative diseases with corneal manifestations such as glaucoma, for instance.
In this sense, there has been an increasing effort to understand how corneal biomechanics is affected by soft contact lenses wearing. It is well-known that both hydrogel and silicone hydrogel soft contact lenses induce structural corneal alterations: thickness14, warpage29 or morphology16.
Lau and Pye30 reported biomechanical overestimation measured with applanation tonometry due to the temporal hydration or induced corneal edema as a consequence of wearing hydrogel soft contact lenses. This overestimation is due to an increasing in corneal stiffness induced by corneal hyper-hydration.
In addition, Lu and coworkers studied biomechanical changes after inducing corneal edema by eye closure while wearing Hydroxyethyl-methacrylate soft CLs21 during 3 hours. Immediately after lens removal they found significant correlation between IOP and CRF with corneal thickness but no association of CH with corneal edema. Both CRF and CCT recovered the baseline values after 100 minutes. These two studies revealed short-term time-dependent changes in CRF and CCT because of the induced corneal edema.
In this work we have investigated the corneal biomechanics, IOP and CCT alterations after wearing SiH-CLs during two temporal groups: 10 and 20 days. Results herein showed that after 10 days the IOP is significantly reduced, whereas CH slightly increases and CRF and CCT remain without important variations.
The longer period of follow-up (20 days) revealed a stronger decrease in IOP and a statistical increase in CH. No induced edema or increased corneal stiffness (CRF) were observed. This inverse relationship can be explained by the La Place´s law: as the IOP increases the rigidity is greater and therefore the viscous damping reduces, that is, IOP and CH are negatively correlated31,32.
In this sense, RadaieMoghadam and collaborators23 extended the following time of corneal biomechanics after using soft toric contact lenses up to three months. They found a significant reduction in both CH and CRF after 1 month of CLs wearing, but not a significant variation in CCT or corneal swelling. This last finding is consistent with our results, however the main discrepancies in our study and the study by Radaie-Monghadam, et al23 can be due to an absence of IOP analysis that could explain an increase in CH instead of the observed reduction in our study. A second discrepant factor may lie in the relationship between corneal biomechanics stiffness and the refractive error22 and the fact that all the subjects were chosen to present regular astigmatism. Other factors such as contact lens properties or patient´s age difference could explain the discrepancies.
Although it is clear the direct influence of soft CLs wear in corneal biomechanics, a deep knowledge on how biomechanical and physical corneal parameters are related is necessary. In our work we have analyzed the whole data set of the study (including control and the two temporal groups) in order to find the possible relationships between pure physical (IOP and CCT) and biomechanical corneal parameters (CH and CRF).
Our results revealed that IOP is positively correlated to CRF, that is, the higher the elastic property the higher the measured IOP. CRF was also linearly associated with CCT, these finding are consistent with the results reported by Bhan, et al33.
From a physiopathological point of view, an increase in CCT has been associated to ocular hypertension34–36 what could explain the observed increase in CRF while measuring the IOP according to the results previously reported by Sha et al37. They observed highest values of CRF (and therefore corneal rigidity) in ocular hypertension eyes. More recently, the positive correlation between corneal thickness and stiffness was demonstrated using optical coherence elastography38.
In 2009, Mangouritsas, et al39reported a strong association between CH and CCT in healthy eyes that becomes weaker as IOP increases in glaucomatous eyes. Our results are in good agreement with this study as revealed a positive correlation between CH and CCT (Fig. 3d).
Finally, the exponential relationship found between CH and CRF allowed to obtain an analytical biophysical expression that unifies IOP, CH, CRF and CCF parameters. This equation allows explaining the follow-up biomechanics after SiH-CLs wearing represented in Fig. 1 as follow:
If the IOP decreases while CCT remains without significant fluctuations, in order to preserve the corneal resistance the viscoelastic response increase for mechanical compensation. These findings suggest that while the elastic property is weakening, the viscosity increases to compensate the corneal biomechanics and preserve ocular stability.
To conclude, unveiling the biomechanical properties of the human cornea is crucial for understanding the development of corneal diseases and new treatments. The main findings of our work lie in fact that the use of SiH-CL during a short-term period of wearing time reduces the IOP as the CH increases. Considering that CH has been proved a biomarker of visual field loss in glaucomatous eyes40, future work of this study could help in the development of new biomechanical treatment against the progression of glaucoma.