| Part I:Accuracy of intraocular pressure measurement and biomechanical properties of cornea (changed by corneal collagen cross-linking)Objective To study the relationship between biomechanical properties of the cornea and the accuracy of intraocular pressure measurements.Methods We included14New Zealand white rabbits in the study. The intervention eye was chosen randomly. One eye underwent corneal collagen cross-linking (CXL) performed with0.1%riboflavin solution and30minutes of3. Omw/cm2UVA irradiation after de-epithelialization. The remaining eye was used as blank-control. The changes in corneal biomechanical properties, CCT, corneal curvature and intraocular pressure were investigated one month after the surgery. Corneal biomechanics were examined by one dimension tensile test. The hydrostatically controlled reference pressure in anterior chamber was adjusted from10to30mmHg in5-mm Hg steps. Intraocular pressure (IOP) was measured by tono-pen XL. The corneas were then examined histologically by light microscopy and electron microscopy. Differences between tono-pen readings and reference pressure are recorded as IOP measurement errors.Results Compared to the control group, the CXL group showed a slight overestimation of mean IOP. When the hydrostatically controlled reference pressure is at the range of15-30mmHg,①there is a positive correlation between IOP measurement errors and corneal biomechanics. Regression equation:Y(IOP measurement errors, mmHg)=15.616+11.651×E(corneal biomechanical property, MPa), which is strongly statistically significant (F=31.17, p<0.01)②after CXL, there is a positive correlation between IOP measurement differences and corneal biomechanics differences. Regression equation:△Y (IOP measurement between two groups, mmHg)=0.065+12.553×△E(corneal biomechanical property between two groups, MPa), which is strongly statistically significant (F=21.18, p<0.01). By transmission-electron microscopy, rough endoplasmic reticulum was expanded in the cross-linking area. Light microscopy presents that in CXL group anterior corneal lamellar showed mergence.Conclusions Altered corneal biomechanical properties can cause inaccuracies in IOP readings. There is a positive linear correlation between IOP measurements and corneal biomechanical properties when reference pressure is between15-30mmHg. Every10%increase of corneal biomechanical properties, will tend to overestimate1.165mmHg in IOP readings. Part II:The effect of variance in corneal curvature on the measurement of intraocular pressure after LASEKObjective The aim of this study was to investigate the biomechanical properties of the cornea and their association with intraocular pressure (IOP), central corneal thickness(CCT) and the corneal curvature.Methods74eyes of37healthy individuals, who underwent LASEK surgery for myopia was studied. Central corneal thickness, corneal curvature, corneal biomechanics, and IOP were measured at each visit (before,3months after surgery). IOPcc, IOPg, corneal hysteresis (CH) and corneal resistance factor (CRF) were measured by the Ocular Response Analyzer (ORA). The corneal curvature and CCT were measured using the corneal topographer Pentacam. Other IOP parameters such as NCT, GAT were also studied. We measured changes in these parameters at different time points preoperatively and postoperatively, evaluated the relationship between biomechanical properties, thickness, curvature of the cornea and IOP.Results CCT, corneal curvature, biomechanical properties of cornea and IOP showed a statistically significant decrease3months after LASEK. All tonometry methods measured a significantly lower IOP after LASEK correction. The IOP reduction was largest after3months for IOPg(4.60±3.77mmHg) followed by NCT (3.19±2.98mmHg), GAT (1.62±3.17mmHg), and IOPcc (1.14±3.29mmHg). The results for IOP reduction after3months of NCT and IOPg showed a significant correlation with ocular parameters such as CCT, corneal curvature, ablation depth, CRF, ablation depth/CCT. IOPcc showed a significant correlation with CRF, GAT correlation with none. Linear regression equation:△NCT (mmHg)=-0.809+0.028×△CCT (μm)-0.214×△K (D)+0.765×△CRF(mmHg),△IOPcc=-1.118+0.016×△CCT-0.359×△K+0.712×△CRF,△IOPg=-0.882+0.012×△CCT-0.275×△K+1.596×△CRF.Conclusions CCT, corneal curvature, CH and CRF reduced after LASEK. All tonometry methods measured a significantly lower IOP after LASEK. NCT and IOPg can be affected by much more ocular parameters than GAT and IOPcc, which means GAT and IOPcc are more accurate than the formers. Every1D decrease of corneal curvature, will tend to underestimate0.21～0.36mmHg in IOP readings. |
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