Research Of Corneal Biomechanics And Femtosecond LASIK

Purpose: To investigate the corneal hysteresis(CH) and the come al resistance factor(CRF) in myopic eyes and its influencing factors. And to investigate the dynamic changes before and after femtosecond LASIK.Methods:(1) 355 eyes of 181 patients combined with myopia were included in the retrospective study. Collect datas of corneal hysteresis(CH) and comeal resistance factor(CRF) by Ocular response analyzer(ORA), and analyze the relationship with age, spherical equivalen(SE), corneal curvature(K), intraocular pressure(IOP), central corneal thickness(CCT) and axial length(AL).(2) 125 eyes of 63 patients combined with femtosecond LASIK were examined by Ocularresponse analyzer(ORA). The central cormeal thickness(CCT) was also measured by ultrasonic pachymetry. Parameters including corneal hysteresis and comeal resistance factor were compared between postoperation and preoperation.Results:1. For all myopia eyes in our research, the average CH value was 10.42±1.40 mmHg, the average CRF value was 10.35±1.67 mmHg. For minors, the average CH value was 11.12±1.20 mmHg, the average CRF value was 11.01±1.67 mmHg. For adults, the average CH value was 10.26±1.34 mmHg, the average CRF value was 10.20±1.64 mmHg.2. In multiple stepwise regression analysis, spherical equivalen, cormeal thickness, corneal curvature and axial length were independent affecting factors for both CH and CRF, intraocular pressure was an independent affecting factor for CRF, age was not an independent affecting factor for both CH and CRF. And he partial regression coefficent of cormeal thickness was the largest one.3. According to the CCT, the patients were divided into group A(CCT<500um), group B(500um≤CCT<540um), group C(540um≤CCT<580um), group D(CCT ≥580um). CH and CRF sequentially increase. Significant differences were between every two groups(P < 0. 05) for both CH and CRF, which reminding CH and CRF increase with CCT.4. According to the SE, the patients with CCT among 500 um to 540 um were divided into mild myopia group(SE≤-3. 00 D), moderate myopia group(-3. 00 D to-6. 00D), high myopia group(SE≥-6 D). CH and CRF sequentially increase. Significant differences were between mild myopia, moderate myopia groups and high group(P < 0. 05) for CH value and CRF value.5. After matching CCT, there were no significant differences between the minors group and the adults group(P < 0. 05) for both CH and CRF. Reminding that age does not effect corneal biological parameters itself.6. CH and CRF declined obviously after femtosecond LASIK, and decreased 23.5% and 38.8% separately. The significant differences were found between any time-points from 1m to 1years postoperatively and preoperation(all P < 0.05). No significant differences were among 1m to 6m(P >0. 05) for CH value, but CH value increased at 1 year(P < 0. 05). No significant differences were among 1m to 1 year(P < 0. 05) for CRF value.7. The CH and CRF at 1m and 1year postoperatively were positive related to CCT, corneal curvature, residual bed thickness(RBT) and preoperative SE, were negative related to ablation depth. Both CH and CRF had no correlation to corneal flap diameter.The CH and CRF changes between postoperation and preoperation( △ CH 、 △ CRF) were positive related to CCT changes, corneal curvature changes and preoperative SE, were negative related to ablation depth. Meanwhile, Both △CH and △CRF had no correlation to corneal flap diameter and RBT.8. According to the corneal flap diameter, the patients were divided into group A(8.3mm), group B(8.5mm). No significant differences were found between the two groups(P < 0. 05) for CH and CRF value before and after operation.9. According to the preoperative SE, the patients were divided into mild myopia group(SE≤-3. 00 D), moderate myopia group(-3. 00 D to-6. 00D) and high myopia group(SE≥-6. 00D). The changing trends of CH and CRF were the same in every group. No significant differences were among 1m to 6m(P >0. 05) for CH value in every group, but CH value increased at 1 year in mild and high myopia group(P < 0. 05). No significant differences were among 1m to 1 year(P > 0. 05) for CRF value in every group.10. According to the CCT, the patients were divided into group A(CCT < 400um), group B(400um ≤ CCT < 480um), group C(CCT ≥480um), at 1m, 6m and 1year separately. Significant differences were between every two groups(P < 0. 05) for both CH and CRF at any same time-points, but no significant differences were between any two time-points(P > 0. 05) in the same CCT ranges.Conclusions:1.For the total myopic population, the average CH value was 10.42±1.40 mmHg, the average CRF value was 10.35 ±1.67 mmHg. Central cormeal thickness is the main influence factors of corneal biological parameters. And CH and CRF increase with the incrassation of central cormeal thickness. Spherical equivalen, corneal curvature and intraocular pressure also affect them in a way, and intraocular pressure affects CRF more than CH. The CH and CRF for high myopia population are lower than them for mild myopia and moderate myopia population. And age does not affect corneal biological parameters itself, but may works by influencing central cormeal thickness.2. CH and CRF values decline obviously after femtosecond LASIK, and decrease 23.5% and 38.8% separately. CH is relatively steady in half year after femtosecond LASIK, and increases to a certain extent at 1 year postoperation. While CRF is steady in 1 year after operation, and has no tend to recover. The ablation of stroma is the primary cause of decline of corneal biomechanics, while reaidual bed thickness and central corneal thichness is the main factors influencing the postoperative corneal biomechanical performance levels. Keepping maximum reaidual bed thickness by thin flap design and controling ablation depth reasonably can reduce the influence of surgery on corneal biomechanical performance.