Application of the modulation transfer function to evaluate the early visual quality after full femtosecond laser refractive surgery

Abstract

Background Full femtosecond laser refractive surgery has been generally applied,but the improvement of visual quality after surgery is a vital problem.It is well-known that modulation transfer function (MTF) is a major evaluation indicator.Objective To investigate the change of the MTF of operated eyes after the full femtosecond laser refractive surgery.Methods A retrospective series of cases study was adopted.Forty eyes of 23 patients with the spherical equivalent of-2.25 to-5.75 D who received full femtosecond laser refractive surgery were enrolled in Nanjing General Hospital of Nanjing Military Command from July to October,2012.The change of visual acuities was analyzed and compared among before operation and 1 day,1 week and 1 month after operation.MTF value in total aberration,low and high order aberrations under the 3.0 mm and 6.0 mm pupil size conditions were detected with i-Trace Visual Function Analyzer at above time points.The MTF values among various time points,between different pupil diameters and different spatial frequencies (5,10,15,20,25 c/d) were compared respectively.Results Compared with the preoperation,the visual acuity improved in postoperative 1 day,1 week and 1 month,showing a significant improvement among various time points (F =10.341,P =0.000).Corrected diopter was 3.795 D at average.MTF values under the 3 mm pupil diameter gradually elevated with the lapse of time after operation,without significant differences among different time points whatever total aberration,low and high order aberrations(all at P＞0.05).The MTF values under 6 mm pupil diameter of total aberration and low order aberration were significantly higher in postoperation than those in preoperation(all at P＜0.05),but no significant difference was seen in MTF of high order aberration among various time points (F =0.260,P =0.854).Compared with the MTF under the 3 mm pupil diameter,those of low and high order aberrations under the 6 mm pupil diameter considerably declined in preoperation and postoperative 1 month (preoperation:P =0.050,0.001 ; postoperation:P =0.012,0.001).MTF values under the 3 mm pupil diameter were not different at various spatial frequencies (all at P＞ 0.05).However,MTF values under the 6 mm pupil diameter were gradually increased from preoperation through postoperative 1 month at 10,15,20 and 25 c/d (P =0.044,0.043,0.024,0.014).Conclusions Full femtosecond laser refractive surgery can eliminate low aberration,increase the MTF and acuity,and therefore improve the visual quality.MTF is one of the key indicators assessing visual quality after full femtosecond laser refractive surgery. Key words: Modulation transfer function; Full femtosecond laser; Visual quality; Refractive error