lasik enhancement: clinical and surgical management · lasik enhancement/moshirfar et al ly,...

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R E V I E W

mong the wide assortment of techniques available for the correction of ametropia, LASIK has obtained considerable international reception over the past

two decades and is currently the procedure of choice within the field of refractive surgery. LASIK can result in residual refractive errors causing undercorrection, overcorrection, re-gression, and surgically induced astigmatism. If visually sig-nificant, correction requires that an enhancement procedure be performed. Notwithstanding customary efficacy associ-ated with postoperative refractive outcomes, postoperative enhancement rates vary between 5% and 28%.1,2 Details con-cerning postoperative enhancement requirements and related predisposing factors, clinical considerations, and treatment options will be discussed herein.

METHODSAn extensive literature search was performed using the

PubMed database and Google search engine. Search terms included “LASIK,” “laser-assisted in situ keratomileusis,” “refractive surgery,” “PRK,” “photorefractive keratectomy,” “surface ablation,” “RK,” “radial keratotomy,” “CK,” “con-ductive keratoplasty,” “intacs,” “intrastromal ring segments,” “ISAK,” “intrastromal astigmatic keratotomy,” “AK,” “arcu-ate keratotomy,” “MMC,” “mitomycin C,” “undercorrection,” “overcorrection,” “enhancement,” “mini-flap,” “regression,” “astigmatism,” “retreatment,” “ablation,” “re-lift,” “re-cut,” “side cut,” “microkeratome,” “femtosecond laser,” “excimer laser,” and “keratectasia.”

AABSTRACT

PURPOSE: To review refractive regression and current therapeutic options for patients who have residual re-fractive error after LASIK.

METHODS: An extensive literature search using PubMed was performed for terms such as “LASIK,” “PRK,” “en-hancement,” “overcorrection,” “undercorrection,” “re-lift,” “mini-flap,” and related terms.

RESULTS: The presence of residual refractive error fol-lowing LASIK is a challenging situation. After excluding anatomical causes (eg, ocular surface disease, cata-ract, and macular pathology) and intraoperative flap complications, evaluation of the residual stromal bed must be performed. Depending on the length of time since primary LASIK, procedures such as flap re-lift, flap re-cut, and surface ablation may be performed.

CONCLUSIONS: Residual refractive errors can be seen after LASIK surgery and may benefit from an enhance-ment procedure. Different options are available for en-hancement, each requiring proper evaluation and an analytical approach to make the procedure safe and effective.

[J Refract Surg. 2017;33(2):116-127.]

LASIK Enhancement: Clinical and Surgical ManagementMajid Moshirfar, MD, FACS; Naz Jehangir, MD; Carlton R. Fenzl, MD; Michael McCaughey, MD

From the Department of Ophthalmology and Visual Sciences, Moran Eye Center, University of Utah, HDR Research, Hoopes Vision, Salt Lake City, Utah (MMoshirfar); the Department of Ophthalmology, Francis I. Proctor Foundation, University of California–San Francisco, San Francisco, California (NJ); Eye Surgeons Associates Bettendorf, IA (CRF); and SUNY Downstate Medical Center, Brooklyn New York (MMcCaughey).

Submitted: October 8, 2015; Accepted: April 26, 2016

The authors have no financial or proprietary interest in the materials pre-sented herein.

Correspondence: Majid Moshirfar, MD, FACS, HDR Research Center, Hoopes Vision, 11820 South State Street, Suite #200, Draper, UT 84020. E-mail: [email protected]

doi:10.3928/1081597X-20161202-01

117Journal of Refractive Surgery • Vol. 33, No. 2, 2017

LASIK Enhancement/Moshirfar et al

ETIOLOGY OF RESIDUAL REFRACTIVE ERRORUndercorrection is a common complication of

LASIK. Diagnosis occurs rapidly, and vision remains suboptimal thereafter. Decreased incidences of under-correction and central island formation have been at-tributed to improvements in nomogram parameters, a switch from broad beam to scanning excimer laser technology, and improved awareness regarding intra-operative procedural considerations.3,4 Earlier studies noted a positive correlation between the rate of under-correction and the degree of preoperative refractive error. However, this correlation was not present in a recent prospective study of more than 30,000 eyes. Overcorrection has a lower reported estimated occur-rence. It has been positively correlated with the esca-lating quantity of preoperative spherical equivalent and spherical aberration treated.5

Residual or surgically induced astigmatism may eventually require an ensuing enhancement proce-dure. It has been demonstrated that patients receiving spherical ablations with low preoperative astigmatism (< 1.00 diopter [D]) had statistically significant in-creases after ablation. In addition, greater degrees of surgically induced astigmatism were correlated with higher preoperative astigmatic values.6 Other risk fac-tors associated with the development of surgically in-duced astigmatism include a preoperative keratomet-ric power of greater than 44.00 D and an ablation zone diameter of less than 6 mm.

The underlying etiology of postoperative regres-sion is not fully understood and is likely multifacto-rial (Table 1). It refers to a gradual tendency to return to the original preoperative ametropic state, whether myopic or hyperopic. Regression generally occurs 3 to 6 months following LASIK, although it can continu-ally progress up to 3 years postoperatively.7,8 Proposed factors contributing to its occurrence include stromal remodeling, epithelial hyperplasia, flap thickness, central corneal thickness,9 lenticular nuclear sclerosis, forward shifting of the cornea, increased intraocular pressure,10,11 and vitreous chamber elongation. Regres-sion is typically found after spherical corrections and is present to a greater degree after hyperopic compared to myopic LASIK.1 In both groups, patients seem to ex-perience an equally distributed increased propensity for postoperative regression with a higher magnitude of preoperative refractive error.3,7 Despite this predom-inant observation, a recently performed large-scale study of purely myopic patients did not find a posi-tive association between regression and high refrac-tive errors. The authors documented an enhancement rate of only 0.5%.4 They speculated that the sparsity of enhancement procedures within the highly myopic

patient population may have been attributable to for-feiture of patient candidacy as a result of inadequate postoperative residual stromal bed thickness.

Epithelial hyperplasia has been well substantiated as a cause of regression in both myopic and hyper-opic eyes.12,13 The incorporation of additional epithe-lial cells within the center or periphery of the cornea results in myopic and hyperopic shifts, respectively, according to the Law of Thicknesses established by Barraquer. The quantity of postoperative epithelial hy-perplasia has been demonstrated to increase linearly with progressively higher ablation depths. However, regressive effects resulting from epithelial prolifera-tion appear to affect lower myopes more severely than higher myopes due to a progressively decreasing rate of thickening per each additional diopter of myopia.12

Decreased flap thickness has been hypothesized to contribute to regression due to closer apposition of stromal and epithelial surfaces. When exposed to growth factors intended for the epithelium, stromal keratocytes may differentiate into fibroblasts. These newly formed cells have been postulated to play a role in regression after LASIK. However, a direct compari-son of eyes with corneal flap thickness of 100 µm or less and 110 µm or more showed no increased rate of regression within the group containing thinner flaps over a period of 1 year.

Postoperative stromal remodeling has been associat-ed with myopic and hyperopic regression, due to stro-mal thickening and thinning, respectively. Interesting-

TABLE 1Etiology of Residual Refractive Error

After Primary LASIK TreatmentCause Associated Factors

Regression Postoperative epithelial hyperplasia

Decreased flap thickness

Postoperative stromal remodeling

Lenticular nuclear sclerosis

Postoperative progression of presbyopia

Vitreous chamber elongation

Older age

Preexisting dry eye

Preexisting astigmatism

Smaller ablative zone

Overcorrection Preoperative high refractive error

Astigmatism Preoperative high astigmatic value

Keratometric power > 44.00 diopters

Ablation zone diameter < 6 mm



ly, Ivarsen et al. did not find a significant association between refractive regression and stromal thickness specifically, but rather central corneal thickness over a period of 3 years postoperatively.9 Development of so-called ‘index myopia’ resulting from nuclear sclerotic cataract formation can induce a myopic shift, repre-senting another potential precipitant of postoperative regression.14 Risk factors for lenticular nuclear sclero-sis include advanced age, smoking, ultraviolet radia-tion exposure, and diabetes mellitus type 2.15 Accel-erated progression of presbyopia after LASIK has also been documented, with relatively younger patients ex-periencing a more pronounced alteration in refractive power required for near vision.16

Vitreous chamber enlargement in adulthood is an-other possible etiology of regression. In a longitudinal study of individuals who had not undergone refractive surgery, vitreous chamber enlargement and a myo-pic shift were found in both myopes (48%) and em-metropes (39%) over a 2-year period. Of significance, the corneal curvatures and lens thicknesses did not significantly change. The cause of enlargement was unknown, but occupation-related accommodation was thought to be a possible etiology.

Multiple factors likely play a role in the incidence and progression of regression. The initial phase within the 3-month to 3-year period is likely due to a com-bination of epithelial hyperplasia and stromal remod-eling. These anatomical changes occur in direct re-sponse to surgical manipulation. Regression past this time period often occurs to a lesser degree and is more than likely due to natural phenomena that would have transpired if no surgery had taken place. Likely etiolo-gies include vitreous chamber enlargement and lens nucleus changes. Additional predisposing factors for the development of postoperative regression include older age, pre-existing astigmatism, dry eye, and small-er ablative zone.17,18 The annual rate of hyperopic re-gression has been reported as +0.03 to +0.07 D in low, +0.15 D in medium, and +0.37 D in high hyperopic subsets. The annual rate of myopic regression has been reported as -0.10 ± 0.18 D in moderate and -0.18 ± 0.22 D in high preoperative myopic error.3,19

RE-TREATMENT EVALUATIONThe initial approach to any patient experiencing

subjective change in visual acuity after refractive sur-gery includes evaluation of the manifest refraction, cycloplegic refraction, dilated funduscopic examina-tion, topographic/tomographic evaluations, under-lying systemic comorbidities (ie, hypertension and diabetes mellitus), current medications, postoperative dry eye, and other adnexal abnormalities.20 A detailed

appraisal for the presence of visually influential len-ticular nuclear sclerosis and alternative pathological mechanisms conceivably engendering postoperative regression, such as axial elongation and choroidal thinning, should be performed.21 Suitable candidates for enhancement should demonstrate refractive stabil-ity for an interval that comprises at least two separate clinic visits. However, specific parameters have not been thoroughly defined.22

Residual stRomal Bed thicknessAccurate determination of residual stromal bed thick-

ness is of paramount importance prior to performing an enhancement procedure. Miscalculations and/or mis-interpretations can result in overestimation or underes-timation of the residual stromal bed thickness, which increases the risk of ectasia or inappropriate rejection of candidacy for enhancement, respectively. There are currently several devices that can measure corneal thickness, including ultrasound, confocal microscopy through focusing, optical coherence tomography (OCT), slit-scanning tomography, and rotating Scheimpflug tomography.23 Direct preoperative measurement of re-sidual stromal bed thickness using OCT, confocal mi-croscopy, or very high-frequency digital ultrasound (VHFDU) appears to be the most optimal method for en-suring measurement accuracy. Intraoperative measure-ment of residual stromal bed thickness using handheld ultrasound may cause potential measurement errors, including inter-user variability, alteration of stromal bed hydration, applicative force, and variation of per-pendicular orientation to the cornea.24 Single site analy-ses during intraoperative ultrasonic computation may neglect potential areas of even greater attenuation than that which is measured (ie, irregular flap creation).25

Comparative analysis between these instruments demonstrated higher residual stromal bed thickness values reported by handheld ultrasound and OCT in contrast to VHFDU measurements.26 Each instrument’s tendency for measurement bias should be considered when obtaining evaluative parameters for corneal thickness and residual stromal bed thickness. If there is any doubt about residual stromal bed thickness, sur-face ablation is a safer alternative for enhancement. Santhiago et al.27 provided a new metric, called percent cornea tissue altered, for the detection of ectasia risk that can be used for LASIK enhancement. Compared to residual stromal bed, percent cornea tissue altered may provide a more sensitive and individualized mea-surement for eyes undergoing LASIK enhancement. It has been observed that percent cornea tissue altered exceeding 40% increases the risk of ectasia in enhance-ment patients.



epithelial thicknessEvaluation of epithelial thickness might be useful

for the better evaluation of patients interested in en-hancement. Epithelial hyperplasia has been postulated as a cause of regression, as mentioned above. Enhance-ments should be modified in patients who experience epithelial hyperplasia, especially when receiving sur-face ablation. Two patients receiving surface ablation enhancements who have the same amount of postop-erative regression, one with an epithelial thickness of 50 µm and the other with a thickness of 75 µm, need different degrees of ablation.

The determination of epithelial thickness after LASIK has been performed using different imaging modalities. VHFDU has been used to document in-creases in postoperative central and peripheral epi-thelial thickness. Individual measurements for epithe-lium, stroma, LASIK flap, and residual stromal bed can be obtained with good repeatability.28 OCT has also been used to follow changes in epithelial thickness.29 However, direct comparisons have not been performed to determine which modality is superior. After review-ing the literature carefully, we found no guidelines on epithelial changes for patients needing enhancement. Such information can be helpful in creating nomo-grams for patients undergoing enhancement, thereby making the results of enhancements more predictable.

keRatectasiaKeratectasia may masquerade as regression and

must be ruled out prior to enhancement. Development occurs days to years after the performance of primary LASIK with a mean time of 13 months.30 Structural changes can be identified in these corneas, includ-ing disorganized stromal lamellae and degeneration of collagen fibrils.31 Although multiple enhancements can also independently increase the potential for kera-tectasia, the presence of unidentified preoperative risk factors including forme fruste keratoconus, pellucid marginal degeneration, thin cornea, and steep cornea have been strongly correlated with the development of postoperative ectasia.30 Topography, OCT, and Scheimpflug imaging have been used in an attempt to identify specific parameters that will assist in the im-proved delineation of these overlapping patient popu-lations.32-34 After primary LASIK, there have been ob-servations of both significant35 and non-significant36-38 posterior corneal elevation. It has previously been in-terpreted that a posterior corneal elevation of greater than 40 µm should represent a basis for preclusion of enhancement procedure performance.7 However, dif-ferences in imaging modality results remain the princi-pal obstacle in confirming a standard threshold.

Studies in which significant changes have been not-ed have used slit-scanning tomography-based imaging, whereas investigations that have not observed signifi-cant changes have used Scheimpflug imaging. Due to the current absence of an absolute reference standard regarding corneal tomographic imaging, it is unclear whether slit-scanning tomography-based imaging may overestimate posterior corneal measurement or whether Scheimpflug imaging may underestimate the true value of this parameter. Attempting to correlate the degree of posterior corneal elevation with the risk of postop-erative ectasia in pre-enhancement candidates does not seem to be a beneficial strategy at this point. Also, pa-tients with ectasia can present with abnormal anterior and normal posterior corneal findings.39 Consequently, we recommend performing both anterior and posterior surface imaging to identify ectasia during the preop-erative evaluation preceding enhancement. Stability in both parameters is a sign that ectasia does not exist and regression is more likely present. Patients with changes in posterior curvature should be observed at regular in-tervals with the assumption that ectasia rather than re-gression is the most likely etiology.

ENHANCEMENT MODALITIESNumerous procedural techniques exist for the per-

formance of enhancement. These include flap re-lift, surface ablation, re-cut, side cut, mini-flap flap pos-terior surface ablation, conductive keratoplasty, and arcuate keratotomy. Appropriate selection is contin-gent on the accurate determination of several factors, including the postoperative time period, previous in-traoperative complications, and preoperative screen-ing parameters (Figures A-C, available in the online version of this article). Customarily, it is recommended that enhancement procedures be performed no earlier than 3 months following primary LASIK, due to the preponderant proportion of refractive alterations that occurs within that time period.

common suRgical techniques foR enhancementFlap Re-lift. LASIK flap re-lift is the most commonly

performed enhancement method. This technique has demonstrated a high degree of efficacy, predictability, and safety for the correction of residual refractive error. Re-lifting has been conducted using several approaches. In the ‘flaporhexis’ technique, a lens-manipulating hook is used to open the flap edge, which is then grasped by forceps and retracted.2 An alternative method of initial flap-edge delineation using a needle rather than a hook has been described. The flaporhexis method is rapid, is less likely to cause epithelial inoculation than other procedures because of its minimal use of instruments,



and causes smooth tearing of the epithelium overlying the flap edge.40 After lifting the flap, the ablation must be performed in the appropriate manner. Re-treatment ablation diameters should be 5 mm or more and prefer-ably the same size as the initial ablation to reduce the incidence of glare and decreased contrast sensitivity. Additionally, wavefront-guided ablations have been advocated for the correction of higher order aberrations and undesired sequelae following primary LASIK.41,42

Certain circumstances may preclude the perfor-mance of flap re-lifting, such as intraoperative compli-cations during primary LASIK (ie, incomplete/irregular

flaps, flap edge necrosis, or buttonholes) (Figures 1-2), previous incisional surgery (astigmatic/radial kera-totomy), inadequate flap-diameter (for cases of hyper-opic enhancement), stromal scar, and existing interface haze.7 Time is also a concern and re-lifting becomes more difficult the longer the flap remains in place. Microkeratome-created flaps have, on average, a longer phase of availability for flap re-lifting than femtosecond laser-assisted flaps. Successful reported effectuation of this procedure has been documented more than 10 years after primary LASIK.43 There have been reports in which even microkeratome-created flaps have been re-sistant to re-lifting following a period of approximately 2 to 3 years after primary LASIK.18

Difficulty in re-lifting the flap increases the probability of the inadvertent intraoperative creation of epithelial de-fects, which can increase susceptibility for development of epithelial ingrowth, diffuse lamellar keratitis, infec-tion, and flap melts.18 It is therefore recommended that enhancement procedures be performed within a period of 6 to 8 months following primary LASIK in patients who possess femtosecond laser-created flaps.43 It may also be prudent to attempt flap re-lifting enhancements no later than 3 years after primary LASIK for microkeratome-created flaps to decrease the likelihood of the aforemen-tioned complications.

Despite adequate patient selection, surgical tech-nique, and postoperative timing, epithelial ingrowth re-mains the most common associated complication of flap re-lifting, with a reported incidence as high as 31% (Fig-ure 3). It has been hypothesized that epithelial ingrowth results from stromal inoculation that can occur during the process of flap re-lifting or as a result of peripheral

Figure 1. LASIK flap edge necrosis may occur after primary LASIK or enhancement procedures. It occurs in conjunction with epithelial ingrowth and is a contraindication to a flap re-lift. Figure 2. LASIK flap buttonholes are areas in the central cornea where

the flap is not complete. A central island tissue with epithelium intact is often present when the flap is lifted. They occur more often in patients with high keratometric values and are a contraindication to flap re-lift procedures.

Figure 3. Epithelial ingrowth of the LASIK flap interface. Flap edge necro-sis is seen inferiorly. Ingrowth may occur after primary LASIK, but is more common following enhancements.



epithelial cell invasion through non-adherent portions of the flap periphery.44 Inadequate peripheral flap adhe-sion represents the most probable current hypothesis, in which the implantation of epithelial cells failed to in-stigate epithelial ingrowth. Documented risk factors for the development of epithelial ingrowth include induc-tion of intraoperative epithelial defects, epithelial base-ment membrane dystrophy, flap instability, hyperopic LASIK treatment, surgical technique, recurrent corneal erosions, and use of a bandage contact lens.44-46

Microkeratome-created flaps have been associated with a higher incidence of epithelial ingrowth following pri-mary LASIK and successive enhancement procedures.47 In accordance with the peripheral-invasion hypothesis, the gradual attenuation of flap edges produced by micro-keratomes is believed to allow easier access for the migra-tion of epithelial cells into the interface, compared to the more vertical configuration of a femtosecond laser-creat-ed flap edge.48 A recent study by Caster et al.49 analyzing rates of epithelial ingrowth after re-lifting between fem-tosecond laser-created flaps and microkeratome-created flaps found that the propensity for the development of epithelial ingrowth increased substantially after 3 years following the primary procedure.

Epithelial ingrowth can result in significant visual impairment as a result of visual axis intrusion or induc-tion of astigmatic error, in addition to flap melt and/or necrosis. Epithelial ingrowth can be effectively man-aged by flap reflection with debridement of epithelial cells from both the posterior-flap surface and stromal bed, without application of accompanying sutures dur-ing flap replacement.50,51 Recurrence of epithelial in-growth after primary debridement merits supplemen-tary operative measures during secondary treatment, such as YAG laser administration,52 flap sutures, or use of fibrin glue53 (Figure 4). Despite the problems associ-ated with re-lifting the flap, good uncorrected distance visual acuities (UDVA) have been obtained.

Surface Ablation. Surface ablation possesses a high degree of applicative value within the expanding as-semblage of enhancement techniques. Early dissua-sion of post-LASIK photorefractive keratectomy (PRK) for the correction of refractive error was initially ad-vanced due to several observed cases of severe postop-erative haze. Successive investigations supplementing intraoperative mitomycin C (MMC) during PRK (with-in previously non-ablated eyes) reported favorable re-sults with a lower incidence of post-procedural haze and improved visual outcomes. PRK has since been used for the enhancement of postoperative astigma-tism, myopia, and hyperopia, with generally favorable results. It is particularly useful in patients with thin residual stromal bed thicknesses.

Traditionally, epithelial debridement prior to laser application was performed using ethanol. The 7- to 9-mm alcohol well for the removal of epithelium has enabled us to have a faster resolution of epithelial de-fect. There has been a trend for the substitution of eth-anol-based epithelial debridement for phototherapeutic keratectomy (PTK) and the removal of 42 to 50 µm of epithelium in association with intraoperative MMC during enhancement procedures. Despite this, the use of ethanol in conjunction with intraoperative MMC has also demonstrated a high degree of safety and efficacy for post-LASIK enhancement.54 The intraoperative ad-ministration of MMC appears to constitute a safe treat-ment option for haze prophylaxis, without adversely affecting endothelial cell morphology or density.55-57 However, there are accounts of statistically significant reductions in endothelial cell counts after MMC appli-cation.58,59 Patients with low endothelial cell counts are not good refractive surgery candidates to begin with and should be expressly avoided if MMC use is necessary.

The elapsed time period following the primary LASIK procedure has been proposed as an important factor influencing the probability of postoperative haze and resultant visual decline. Recovery of corneal nerve function after primary LASIK has been suggested as a principal determinant for modulating the manifesta-tion of corneal scarring; in accordance, the authors of this study proposed a minimum recovery period of at least 2 years prior to the performance of PRK for en-hancement.60 Conversely, earlier performance of PRK has been advocated only for the management of acute flap complications (eg, doughnut flap) following the original LASIK surgery for avoiding corneal scarring.

The duration of intraoperative MMC has steadily declined from 2 minutes to a typical length of 30 sec-onds,54 which appears to represent an optimal time

Figure 4. Recurrence of epithelial ingrowth after primary debridement may need suturing of flap during secondary treatment.



interval for haze prophylaxis. An instance of slight de-viation from this time span (15 seconds) described by Liu and Manche61 resulted in recalcitrant postoperative haze that was unresponsive to corticosteroid therapy.

PRK in association with PTK has been employed for the successful treatment of flap complications result-ing from primary LASIK. Wavefront-guided PRK has also demonstrated excellent results for reduction of higher order aberrations in eyes previously treated with LASIK.62 Several critical adaptations must be enacted when performing PRK in synchrony with PTK and MMC; PTK pulses should be evenly distributed to re-duce the occurrence of unintended hyperopic outcomes. It has been approximated that 48 discharged pulses will result in 12 µm of stromal ablation in addition to 1.00 D of hyperopic change during PTK. In addition, effects resulting from MMC administration will attenuate the postoperative reparative process, conceivably resulting in further post-procedural hyperopic variation.

The advent of topography-guided photoablation has promising results. Successful outcomes have been reported after topography-guided enhancements after unsuccessful primary LASIK and PRK treatments. In addition, there is now promise for the treatment of ir-regular astigmatism with topographical guidance and even post-LASIK ectasia when combined with colla-gen cross-linking.63,64

Laser-assisted sub-epithelial keratectomy (LASEK) is another viable enhancement option for mild degrees of residual myopia (-1.50 to -2.00 D).65-67 Hyperopic en-hancements and myopic enhancements for ametropia of greater than 2.00 D have been associated with a high in-cidence of postoperative haze.65,66 As a result of this ob-servation, the use of intraoperative MMC was proposed to potentially prevent this complication for attempted corrections greater than -2.00 D.66 Variable results have been reported, including good visual outcomes for low power corrections and high degrees of postoperative re-fractive unpredictability after the application of MMC.68

Additional surface ablation techniques include PTK following manual epithelial debridement for enhance-ment in patients with post-LASIK epithelial basement membrane degeneration.69 Exclusive ablation of the epithelial layer has been evaluated through the use of intraepithelial PRK for the treatment of postop-erative epithelial hyperplasia. Statistically significant improvements in visual acuity and refraction were obtained compared to preoperative values. However, 47.6% of treated eyes suffered from residual myopia.

otheR alteRnative techniquesFlap Re-cut. Re-cut designates an additional proce-

dural option for enhancement (Figures 5-6). The new

flap is ideally created more posterior to the original, but may end up at a similar depth. However, if re-lift or surface ablation can be done, they should be the proce-dures of choice.

Indications for re-cut are essentially identical to many of the contraindications that exist for performing flap re-elevation with stromal bed ablation (ie, thin/in-complete flap, previous radial/astigmatic keratotomy, inadequate flap diameter, buttonhole, or vertical gas bubble). Currently, re-cutting a corneal flap can be ac-complished using either a microkeratome or femtosec-ond laser.70 Although it is a viable option, microkera-tome flap creation has been associated with a higher than average rate of complications. These include in-accurate incision depth, complete flap transection, in-complete flap creation, and formation of intrastromal tissue fragments as a result of unintentional encroach-ment of the original interface. The creation of intrastro-mal tissue fragments characterizes a particularly diffi-cult complication that may be irrevocable and result in severe visual dysfunction. In addition, a comparison between flap re-lifting and flap re-cut yielded superior long-term uncorrected visual acuities and refractive er-ror stability for those receiving re-lifts. The frequency of microkeratome use during enhancement procedures appears to have decreased over time due, in part, to the aforementioned complications and advent of the femtosecond laser in producing more precise and reli-able outcomes.

Vertical Side Cut. A variation of re-cut involves the creation of a vertical side cut within the estab-lished flap using a femtosecond laser (Figure 7). This technique has resulted in a lower incidence of postop-erative epithelial ingrowth when compared to flap re-lifting, and is believed to represent a safer alternative for performing flap re-cut.71,72 Accessibility of appro-priate preoperative imaging devices (ie, OCT or ultra-sound) is essential for identifying the peripheral inter-face boundary.73 Complications associated with this technique include a potential for side cut overlap of the previous flap edge and creation of displaced tissue fragments resulting from inadequate distance between the new and established side cuts. It has therefore been recommended that a side cut 1 mm smaller in diam-eter than the established flap and careful confirmation of side cut centration be performed to ensure that uni-formity between each edge margin is achieved.74 Ideal patients to benefit from this technique include those with old femtosecond laser flaps (> 1 year) where re-lifting may be problematic and those at risk for re-cut complications. A limitation, analogous to that of flap re-lifting, is the inability for expansion of an ablative zone beyond the original flap diameter.



Mini-Flap. The ‘mini-flap’ (Figure 8) represents an ad-ditional femtosecond laser-assisted technique. This sup-plemental flap is created within the anterior portion of

the original flap. It is smaller in thickness and diameter in comparison to the original flap and offers an alternative enhancement option for patients who would otherwise

Figure 5. Common indications for performing flap re-cut include thin/incomplete flap, buttonhole, vertical gas bubble, inadequate flap diameter, and history of radial/astigmatic keratotomy.

Figure 6. Flap re-cut. The new flap is ideally created posterior to the original one.



be ineligible for enhancement due to inadequate residual stromal bed thickness. In the pilot study describing this technique, no problems were encountered during flap creation or flap lifting.75 However, this technique is lim-ited by the thickness of the original flap. Microkeratome-created flaps are traditionally thicker and can be as thick as 180 µm. A mini-flap of 100 µm can then safely be per-formed. Femtosecond laser-created flaps are currently be-tween 90 and 120 µm. This precludes the establishment of a secondary flap within the original.

Arcuate Keratotomy. Arcuate keratotomy has been successfully used in the past for the correction of post-LASIK topographical abnormalities and postopera-tive astigmatic error.76 Arcuate keratotomy functions through incisional flattening and simultaneous recip-rocated steepening, resulting in a “coupling effect.” It can be performed manually or with the assistance of the femtosecond laser. This procedure provides a relative degree of safety, in that procedural failure will not typically adversely affect corrected distance visual acuity. Disadvantages of arcuate keratotomy include unpredictable outcomes with higher levels of astigma-tism (often requiring re-treatment) and complications such as epithelial ingrowth into the incision.77

Femtosecond Laser-Assisted Intrastromal Astig-matic Keratotomy. A relatively new procedure for the enhancement of mixed astigmatism has exploited sev-eral beneficial attributes afforded by the femtosecond laser to advance the technique of arcuate keratotomy; this procedural iteration has been termed femtosecond laser-assisted intrastromal astigmatic keratotomy. Fem-tosecond laser-assisted intrastromal astigmatic kera-

totomy enables the performance of paired intrastromal incisions, which remain unopened postoperatively.78 Greater precision of incisional placement, faster pro-cedural time, and lack of epithelial trauma represent advantages conferred by this technique.

Intracorneal Stromal Rings and Collagen Cross-linking. Although these procedures are not commonly used and are not the primary procedures of choice in enhancement, intracorneal stromal ring implantation has been shown to correct small myopic error and can facilitate refractive stability.79 The addition of corneal cross-linking to intracorneal stromal ring implantation is a second option for improvement of UDVA and cor-neal topography in patients with post-LASIK ectasia.80

miscellaneous techniquesAlthough the literature describes the following sur-

gical techniques as options for enhancement of vision after original LASIK surgery, many clinicians question the safety, predictability, and long-term efficacy of these procedures.

Flap Undersurface Ablation. This treatment is con-troversial, but it is another variation involving flap ma-nipulation and combines re-lifting with concomitant ablation of the posterior stromal surface of the flap.81 This procedure is particularly appealing to patients who have low residual stromal bed thicknesses and are at increased risk of developing ectasia following re-treatment. In addition to preserving the previously ab-lated region of stromal tissue, flap ablation results in a lesser degree of postoperative posterior corneal shifting when compared to stromal bed ablation.82

Figure 7. Vertical cut is a variation of re-cut and a vertical side cut is created within the established flap using a femtosecond laser. It has a lower incidence of epithelial ingrowth compared to flap re-cut.

Figure 8. Mini-flap is usually indi-cated in patients with inadequate residual stromal bed thickness and is created within the anterior portion of the original flap. The creation is limited by the thick-ness of the original flap, making it difficult to perform in femtosec-ond laser-created flaps that are thinner compared to microkera-tome-created flaps.



However, over-ablation of the flap stroma is difficult and unpredictable. If Bowman’s membrane becomes involved, significant flap wrinkling and loss of visual acuity may occur. To prevent this, a minimum flap thickness of greater than 150 µm has been recommend-ed. Appropriate positioning and fixation of the reflect-ed flap and centration difficulty represent procedural challenges. When performing this procedure, the cen-ter of the visual axis must first be marked. After reflec-tion of the flap, the ablative zone axis must be rotated 90° to mirror a traditional ablation to the residual stro-mal bed. The patient must be able to maintain the eye in a stable infraducted position (if a superior hinge is present) because no tracking capabilities exist with this variation. Replacement of the flap on the stromal bed allows for realignment of the tissue along the proper axis. Observed complications of this procedure include microstriae, interface debris, and epithelial ingrowth. Despite these difficulties, multiple head-to-head com-parisons of flab ablation versus stromal bed ablation resulted in similar refractive outcomes.81,82

Conductive Keratoplasty. Conductive keratoplasty as an ancillary enhancement technique can be used for myopic overcorrection83 and hyperopic undercor-rection.84 However, important issues related to initial overcorrection, delayed onset of refractive stability, re-gression, and lack of an established nomogram detract from consistent use of this technique.

CONCLUSIONThe presence of residual refractive error following

LASIK is a precarious situation that may occur as a re-sult of overcorrection, undercorrection, or regression. Clinical evaluation of the refractive state and the pa-tient’s subjective status is extremely important. Ocular and systemic diseases with the potential to affect visual acuity require appropriate treatment prior to the estab-lishment of a diagnosis of residual refractive error. If the need for enhancement has been identified, review of the primary LASIK procedure should be performed. Preoperative measurements and the ablation depth are important for establishing accurate residual stromal bed thicknesses. The operative report should be inves-tigated for evidence of complications such as button-holes or incomplete flaps. Additionally, the postopera-tive course must be reviewed in case of postoperative haze, epithelial ingrowth, or other significant issues. Supplemental imaging via VHFDU and OCT may also help confirm flap and stromal bed thicknesses. Once the data have been collected, the proper enhancement procedure can be selected. Patients with thick residual stromal beds and recent primary LASIK treatments are

good candidates for re-lifts. Patients with thin stromal beds and LASIK treatments several years prior may benefit to a greater degree from PRK surface ablation. Even patients who suffer from post-LASIK ectasia have treatment options to improve visual acuity.

AUTHOR CONTRIBUTIONSStudy concept and design (MMoshirfar, NJ, MMcCaughey, CRF);

data collection (MMcCaughey); analysis and interpretation of data

(MMcCaughey); writing the manuscript (MMoshirfar, MMcCaughey,

CRF); critical revision of the manuscript (MMoshirfar, NJ, CRF); su-

pervision (MMoshirfar)

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Figure A. Flowchart describing the flap re-lift modality for enhancement. D = diopters

Figure B. Flowchart describing the surface ablation modality for enhancement using microkeratome-assisted original flaps. FS = femtosecond laser; RSB = residual stromal bed; PRK = photorefractive keratectomy

Figure C. Flowchart describing the surface ablation modality for enhancement using femtosecond laser-assisted original flaps. PRK = photorefractive keratectomy; RSB = residual stromal bed

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