Retreatment có thể được thực hiện đối với một trong những điều sau đây: I. Không hài lòng bệnh nhân do một lỗi còn lại hoặc một lỗi gây ra. Tuy nhiên, một số bệnh nhân (24%) có thể được thỏa mãn bởi một kết quả khúc xạ | 298 El-Shiaty and Boxer Wachler performed no sooner than 3 months postoperatively to allow for adequate healing and stability. Retreatments are usually indicated for errors of more than D away from the target refraction 4 6 13 and UCVA of 20 40 or worse 2 . Retreatment may be done for one of the following I. Dissatisfied patient due to a residual error or an induced error. However some patients up to 24 may be satisfied by a refractive outcome less than ideal and do not ask for retreatment or may not even come for follow-up 2 6 13 . The retreatment rate and number of retreatments may be influenced by such subjective factors as patient expectations and demands 10 . Presbyopic patients may well tolerate a small residual myopia. A. The residual error may either be initial undercorrection or regression of the achieved effect over time. 1. Undercorrection may be defined as a residual spherical equivalent error of more than D away from the target at the first postoperative week 5 . Most factories used to configure excimer lasers to provide correction for PRK on the surface of the cornea and a standard 10 reduction in programmed value was used for LASIK treatment. This standard reduction resulted in undercorrection especially for high errors and when simultaneous astigmatic corrections are undertaken 16 . LASIK nomograms with linear variability according to the attempted correction Fig. 1 have decreased the incidence of undercorrection 13 . Undercorrection may be induced intentionally in high myopia describing the golfing analogy to the patients. The first procedure will get them onto the green and the second procedure will putt them into the hole 8 . 2. Regression is a less common problem in LASIK than in PRK 17 . Although current laser algorithms aim at emmetropia they do not allow for possible external and internal influences that may modulate the corneal healing process. So following the excimer laser treat- Figure Percentage of reduction with the .
