3. Refractive stability - Usually 1-3 months after operation - The lower the correction, the sooner refraction will be stable myopia < -3 D : 1 month myopia 3-6 D : 2 months myopia > -6 D : 3 months

4. Why Retreatment ? - Residual refractive error. - Complications of primary operation.

5. Residual refractive error - Under or Over correction(occures in the immediate postoperative period). - Regression(occures during the first 3 to 6 months after surgery may continue up to 1 year).

6. Undercorrection If an eye was treated by LASIK for -8 D and on day 1, through to 1 week, 3 months, 6 months, and 1 year, the eye was found to be stable at -1 D, this would be defined as a primary undercorrection. Causes:Corneal and Non-Corneal.

7. Non-Corneal causes Inaccurate preoperative refraction. Inadequate laser energy delivery (eg, excess bed hydration, room humidity, inappropriate laser energy calibration, laser head energy instability).

8. Corneal causes Cases where the biomechanics of the cornea change due to the keratectomy and a stable but unpredicted curvature change is obtained. For example, this can happen if the residual stromal thickness was much less than 250 microns but not thin enough to cause long- term destabilization (ectasia).

9. Regression regression is defined narrowly to the observation of a shift in refraction postoperatively that tends to reverse the intended effect. excluding the situation where the refraction does not stabilize and continues to change due to plastic deformity of the cornea-a process known as keratectasia

10. Regression myopic eye, undergoing LASIK for -4 D, that was found to be plano on postoperative day 1,-0.25 at 1 month, -0.75 at 3 months, 6 months, and-1.25 at 1 year and became stable.

11. Regression Is it because the epithelium thickened in the center(CEH)? Is it because the flap was too thick, and there was bowing of the central cornea forward? It is because stromal synthesis(wound healing)? Axial length change? Nuclear sclerosis?

12. Regression in under corrected eye An eye with 5 diopter myopia treated by PRK had been -0.75 D at postoperative day 1, -1.50 D at 1 month, -1.75 D at 3 months, but then stable through to 1 year, then the eye would have been said to have had a primary undercorrection of -0.75 D followed by regression from -0.75 to -1.75 D

13. Pre-op evaluation - Visual acuity - CSF and Glare testing - Refraction - Schirmer test - Corneal imaging ( Topo, Orbscan, Pentacam, Artemis ) - Abberometry - Residual stromal thickness - Specular microscopy

14. Visual acuity - Does UCVA matches refraction ?( 1 line drop for each 0.25 D ) - Is there significant difference between UCVA and BCVA ? ( At least 2 line difference )

15. Corneal imaging - Using Topo, Orbscan, Pentacam, Artemis, etc, KCN should be seriously ruled out. - Decentered ablation can be seen in Topo and Orbscan Posterior corneal elevation

23. Schirmer test - In cases with severe dry eye, there is higher risk of regression after enhancement

24. Refraction - Stability of refraction ( < 0.25 D change in two months ) - Retinoscopic and Autorefraction

25. Residual stromal thickness'(RST) Indirect : Pachymetry, Orbscan(not reliable) RST = central thickness – estimated flap thickness. Direct : OCT, Artemis, Confocal Intra-operative pachymetry.

27. Ideal RST - At least 250 mic or ideally 300 mic - More than 50% of total pachymetry - Total post-op pachymetry > 400µ

28. Reoperation Rate between 5% -30%. Lasik ASA

29. Re-operation ( LASIK) - Flap creation - Stromal ablation

30. Flap creation - Lifting the original flap - Recut the original flap

31. Flap lifting - Usually possible 6-12 months post-op - More difficult in cases with : a) aggressive wound healing b) Prolonged time between primary operation and flap lifting c) Vicinity of flap edge to limbus d) Fibrosis in interface

32. Flap recut - Conventional microkeratome - Femtosec. Laser

33. Advanced Surface Ablation Smoothing Non-Smoothing Transepithelial On lasik flap

34. Smoothing PRK while retreatment of PRK improved the refractive results, there remained two major problems: the scatter of refractive results, and the loss of BSCVA, which was often associated with haze. Both of these parameters were improved in the smoothing vs. the nonsmoothing groups.

35. Smoothing PRK. The “smoothing” technique, developed by Dr. Julian Stevens, consisted of soaking the cornea with normal saline solution, waiting 30 seconds for stromal hydration, after which a 5 um PTK was applied in short bursts. For the smoothing group, 81% of the intended refraction by retreatment was achieved, compared to 48% in the non- smoothing group, a difference that was significantly significant. Also, no patients in the smoothing group lost 2 or more lines of BSCVA, compared to 8% of patients in the non-smoothing group. UCVA was better, and haze less, in the smoothing vs. the nonsmoothing group.

36. Transepithelial customized PRK The technique is a “no touch” modification of conventional PRK in which the epithelium is removed by laser rather than manually. When performing a second treatment, epithelial removal is often a problem and manual approaches might be inaccurate and traumatic.

37. Contraindication of Enhancement Refractive instability. Suboptimal pachymetry(total pachymetry <400 and RST<300) Suboptimal posterior corneal elevation(>40µm reveal ectasia even if anterior elevation and power appear normal).

38. Retreatment Prior to re-treatment the surgeon must be sure that regression has stabilized and is not due to progressive ectasia. Flap lifting with ablation or surface ablation may be considered to correct postoperative refractive error after either PRK or LASIK. Topical steroids and mitomycin C (MMC) may be useful to mitigate haze formation. Intrastromal pachymetry is very helpful for subsequent decisions regarding re-treatment to reduce the risk of postoperative ectasia. Patients having custom LASIK may achieve better vision that those having conventional (noncustom) LASIK. However, there is mixed evidence on the safety of wavefront guided re-treatments.

39. Effects of Antiglaucoma Drugs in Eyes with Myopic Regression The preliminary data show that antiglaucoma drugs are effective for the reduction of the refractive regression, especially of the spherical errors, after LASIK. It is suggested that backward movement of the cornea may occur, possibly flattening the corneal curvature by lowering the IOP. Reduction of the IOP may contribute to improving regression after keratorefractive surgery.

40. Thanks for your attention