Manual intrastromal corneal keratotomy: An alternate encouraging approach for refractive error correction.

Results of femtosecond based intrastromal astigmatic keratotomy have been reported to be encouraging for correction of Astigmatism. We report a new surgical technique-manual intrastromal corneal keratotomy (MICK) for correction of simple refractive astigmatism (-1.5 DC against the rule). The technique involves the creation of a 100 μm thickness corneal flap creation using Moria M2 evolution LSK MicroKeratome and 300 μm depth, 4 mm long manual transverse astigmatic keratotomy on both sides of the steep axis 3 mm from the pupillary center along with four incision peripheral radial keratotomy outside the optic zone (based on the mesopic pupil). The flap was repositioned and routine post-operative regimen was followed. Patient achieved 20/20 vision in both eyes post-operatively on the first day, which was maintained even at the 1 month follow-up. This simple technique could be useful as an alternative method for correction of refractive errors in patients not suitable for excimer and/or femtosecond laser treatment.

Introduction

Currently, femtosecond lasers have been employed to place intrastromal cuts to reduce astigmatism (intrastromal astigmatic keratotomy [ISAK]) with encouraging results and are being evaluated in a clinical trial.[12] These lasers have been used to overcome potential complications of manual transepithelial incisional surgical procedures such as astigmatic keratotomy and radial keratotomy, which were popular for a major part of the last century after being initially described in 1885.[3] Such complications include irregular corneal shape remodeling, corneal perforation, rupture, diurnal fluctuation of refractive error with corneal instability, hyperopic shift, overcorrection/undercorrection and of course, keratectasia. However, we report a surgical technique employed by us that can be used in a setting without employing either the expensive excimer laser or the femtosecond laser in the correction of astigmatism and provide visual rehabilitation in a safer and cost-effective manner. This method may also be potentially useful in correcting refractive errors in cases where stromal ablation is not possible or desirable for any reason whatsoever after corneal flap creation has been achieved. Such situations are likely to be encountered in situations such as mechanical failure of excimer laser, poor fixation, uncontrollable excessive oozing from peripheral corneal new vessels, thick flap, borderline corneal thickness cases, thin flaps, mildly decentered flaps, etc., As the results have been demonstrated successfully in only two eyes of a single case in our report, further studies involving a larger number of cases can also be potentially considered in different ethnic populations and geographic zones and explored further by surgeons as an evolving technique toward achieving correction of refractive error in a setting where excimer laser and/or femtosecond lasers are not available or recommended.

Background

A 28-year-old female patient with stable −1.50 diopters of refractive astigmatism.

Right eye (RE): −1.50 DC at 90° and left eye (LE): −1.50 DC at 60° was evaluated for correction of astigmatism. She underwent routine evaluation to exclude factors that were thought to induce potential complications of the procedure. She had uncorrected visual acuity of 20/80 oculus dexter (OD) and 20/80 oculus sinister (OS) and a BCVA of 20/20 OU. Her central pachymetry (Sonomed, USA) was recorded at 480 μm in both eyes. Except for high K readings around 50 diopters in both eyes indicative of corneal curvature myopia, her topography was otherwise normal and almost spherical with minimal corneal astigmatism of around < 0.50 diopters against the rule [Figures 1 and 2]. Her axial length, as measured with ultrasound biometry and was recorded at 21.70 OD and 21.75 OS confirming the axial hyperopia, which was compensating for the curvature myopia. The refractive astigmatism in her case was presumed to be primarily lenticular in origin. Pachymetry measured along the proposed incision sites were recorded between 495 and 525 μm along the radial incisions and around 495-500 μm along the transverse incision sites in both eyes.

Figure 1

Pre-operative topography of the right eye

Figure 2

Pre-operative topography of the left eye

Surgical Plan

In view of her borderline corneal thickness, high corneal K readings, short axial length, and possible lenticular cause of her astigmatism, she was not considered as an ideal candidate for ablative procedures to be used primarily. Minimally incisional procedures altering the topography of the cornea to accommodate the refractive astigmatism would be acceptable especially if the option of future treatments either incisional or ablative could be preserved as lenticular changes in the future could affect the long-term refractive stability requiring additional interventions in the future. Direct transepithelial radial keratotomy or astigmatic keratotomy would affect future excimer laser ablative treatments if needed and so this new technique-manual intrastromal corneal keratotomy (MICK) was devised.

The surgical plan was devised to raise a superficial stromal flap of about 90-100 μm and perform a transverse keratotomy on the steeper refractive axis along with four incision radial keratotomy outside the optic zone on the stromal bed to alter the topography of the stromal bed.[45] Since conceptually, the current surgical plan is similar in principle to transepithelial intrastromal keratotomy where precision cutting of the corneal lamellae induced a flattening of the steeper meridian and since it has always been thought that the superficial layers of the cornea contribute little in terms of structural support to the integrity of the cornea, it was decided to follow the operating surgeons personal nomogram, which was used previously for transepithelial intrastromal corneal keratotomy (TICK) with one modification. The nomogram used in this case is an adaptation of the transepithelial keratotomy well-described and referenced by Chu et al.[6] and the modification that was applied was that the depth of the incision was planned for around 300 μm leaving behind a residual intact depth of around 70-80 μm. This was based on the previous studies using femtosecond laser for ISAK where a residual depth of 100 μm was left behind and still produced good results.[7] It was decided that these incisions could be deepened later postoperatively if the desired flattening effect was not achieved due to under correction while overcorrections may be more difficult to handle. Repositioning the flap over a topographically modified stromal bed would allow the cornea to assume a shape that compensates for the refractive error and lead to improvement in vision.

Surgical Technique

After the informed consent and a course of pre-operative topical antibiotics for 3 days with ofloxacin 0.3% 4 times a day, under topical anesthesia with 0.5% proparacaine hydrochloride, the steep axis was marked in both eyes using gentian violet under the slit lamp (RE-180° and LE-150°) along with markers on the inferotemporal and inferonasal position for flap realignment. Then, patient was taken to the operating room where topical anesthesia with 0.5% proparacaine hydrochloride was again instilled. Care was taken to position the eye perpendicular to the microscope by ensuring that the upper and lower limbal vessels were getting focused on the same plane. Under sterile conditions, the speculum was inserted into the LE with minimal discomfort and another drop of the topical anesthetic was instilled prior to the placement of the suction ring of the Moria M2 Evolution Microkeratome. Suction was applied and the hand piece with the 90 μm blade was attached to the ring and the to and fro movement initiated. After completion of the reverse movement, the suction was released and the cut inspected for any abnormalities. The flap was raised such that the hinge was placed superiorly at about 70° instead of the usual 90° position. This was performed to avoid interference with the stromal incisions as per the surgical plan. The flap was inspected for any irregularities and was folded onto itself and the bed later inspected for any irregularities. Ultrasonic pachymetry was performed on the stromal bed again. The residual bed thickness (central) was recorded at 380 μm. After verifying the centration as well as the perpendicularity of the eyeball, the central optic zone was marked with indentation using a 6 mm optic zone marker after patient was asked to fixate on the operating microscope light source, which was turned to a low level. The marker blades are coated with the ink from the marker pens and imprinted on the stromal bed as per the surgical plan. A 300 μm transverse cut was placed in the stromal bed using a guarded knife (Sharepoint, USA) as per the surgical plan shown in Figure 4 outside the 6 mm optic zone well outside and correlating with the mesopic pupil size of 4.96 mm for approximately 4 mm length perpendicularly on either side of the steep axis at 180° bisecting it for 2 mm on either side. Four radial cuts were placed again on the stromal bed as shown outside the optic zone such that the superior cuts were aligned on either side of the flap hinge. The Transverse cuts were meant to relax the steeper axis while the radial cuts were meant to reduce the myopic shift due to the coupling effect. Care was taken to ensure that none of the cuts transverse or radial intersected with each other to eliminate stromal healing irregularities. After the cuts were placed in position, the flap was placed back into position and interface irrigation with balanced salt solution was carried out. The flap was allowed to float back into position along the markers placed previously and a 3 min drying period was allowed to ensure flap adhesion. Topical antibiotic drops ofloxacin 0.3% and lubricants were applied and the speculum removed. The same procedure was repeated in the other eye too except that the flap hinge was placed superiorly at the usual 90° position. All other steps were the same and the incision was placed as per the surgical plan for the RE [Figure 3]. Patient was examined on a slit lamp immediately after the procedure and advised topical antibiotic ofloxacin 0.3% to be instilled every 3 h during the waking hours on that day. She was seen on day 1, day 7 and day 30 post-operatively. She was advised antibiotic-steroid combination eye drops (ofloxacin 0.3%-dexamethasone 0.1%) for 1 week and topical lubricants (hydroxypropyl methyl cellulose 0.3%) for 3 months. The topical lubricants were tapered slowly and were completely withdrawn after 3 months.

Figure 4

Surgical plan for manual intrastromal corneal keratotomy in the left eye

Figure 3

Surgical plan for manual intrastromal corneal keratotomy in the right eye

Result

Patient reported excellent visual outcome of 20/20 OD and 20/15 OS on day 1, which was maintained on day 30. Post-operative topography on day 7 revealed a toric corneal shape remodeling to compensate for the refractive cylindrical error [Figures 5 and 6]. Central pachymetry performed on day 7 recorded 485 μm, which was expected to settle down to the pre-operative level by 3 months. A thinning of 60 μm was seen on the transverse incisions while a thinning of 40-60 μm was seen on the peripheral radial incisions with more thinning toward the center as compared with the periphery, from the pre-operative levels as measured with ultrasound pachymetry (Sonomed, USA). Patient satisfaction level was very high with the overall outcome of the procedure.

Figure 5

Post-operative topography after 1 week in the right eye

Figure 6

Post-operative topography after 1 week in the left eye

Discussion

MICK appears to provide an encouraging alternative to already established methods of refractive error correction as demonstrated in this case. Theoretically, there are many benefits of MICK over transepithelial keratotomy as also Femtosecond intrastromal keratotomy. The incisions when placed in the stromal bed need not be near full thickness in the first stage to alter the topography of the stromal bed and hence complications like perforations can be avoided. Additional deepening of such incisions may be attempted as in TICK to titrate the end result in the post-operative period after reflecting the same flap again, which can be carried out in a similar fashion as applicable in Lasik Enhancements. Nomograms that are used for transepithelial incisions have been used in this case with good effect with slight modifications. While traditionally, it was thought that the flap created by the mechanical keratome was too thin to contribute to the structural integrity of the cornea after ablative refractive surgery and that it was the bed, which is responsible for the corneal structural integrity, Recent reports of the femtosecond laser being used for creating stromal tracks, which can later be partially opened manually in the office to further enhance the flattening effect and titrate the effect on the correction of astigmatism may suggest that the superficial layers too may contribute partially to the structural integrity of the cornea especially after corneal stromal keratotomy, which is why partially opening and relaxing them transepithelial in the office enables a titratable, progressive correction of Astigmatism beyond the effect achieved with just plain intrastromal cuts. This effect cannot be seen in similar flaps after ablative procedures on the corneal. This may be because ablative procedures act across a large area of the cornea and act by removing corneal tissue to achieve curvature change while stromal incisions are localized and act by cutting across corneal lamellae without any removal to achieve flattening of a meridian and so are inherently different in their mechanisms in causing topographical change. Hence, in our case, the superficial flap postoperatively may also add to improving the corneal integrity by providing a bridging and strengthening effect across the localized gaping trans-incisional stromal wounds once the stromal interface healing occurs. Further studies may be required to evaluate if further incisions on the corneal epithelium in these cases over the stromal incisions result in enhancement of the flattening effect, which may conclusively prove the strengthening effect of the flap over these incisions. Since the flap is repositioned after the keratotomy is done, This bridging and strengthening effect would be secondary to the desired incision effect on the wound gaping and would not limit the desired topographical alteration as may occur with femtosecond ISAK where the epithelium and superficial stroma may be stretched, but is intact throughout, which might limit the desired topographical alteration and may not correct the refractive error fully in some cases. These benefits however, need to be demonstrated on a sustained long-term basis in a larger number of cases to be considered significant. One of the significant benefits is that the refractive error correction has been achieved predictably in two eyes without using either the excimer laser or the femtosecond laser, which are expensive and may not be available in an average practice. While safety and predictability of any refractive procedure is very important, some patients would like to avoid excimer laser ablation due to corneal melting disorders, medications etc., which may aggravate stromal melting post-excimer laser treatment due to the thermal burning effect of the excimer laser on the corneal tissue. MICK may provide a safer alternative to other established procedures as refractive correction of mild to moderate astigmatism may be obtained with safer partial thickness simple incisions and without using thermal energy, which may aggravate corneal melting disorders. In the absence of femtosecond laser, MICK provides an alternative encouraging approach to correction of mild to moderate astigmatism as demonstrated in this case. Further studies may be required to demonstrate the effectiveness in other refractive errors to determine the reproducibility and accuracy of the results and to refine the nomograms to establish optimal outcomes in MICK. A thinning of 40-60 μm (10-15% approximately) was seen on the incision sites in this case from their pre-operative level, which in our opinion was an added safety advantage in terms of induced iatrogenic corneal ectasia over other procedures like TICK where a greater degree of thinning is expected as the incision is more than 90% of the entire corneal thickness. In addition, Corneal Stromal incision healing and regeneration is expected to be much smoother as compared with transepithelial incisions, which may contribute toward stronger corneal biomechanical strength as well as preventing abrupt curvature changes in the post-operative corneal topography due to the blending effect of the superficial flap over the actual topography of the stromal bed where the incisions are placed. The effect of such flaps on the stromal bed incisions in MICK may also reduce the induced higher-order aberrations as compared with TICK, which however needs to be evaluated separately. In addition, Long-term follow-up of these cases are also required to establish their safety with respect to iatrogenic corneal ectasia, which is seen as a major impediment to performing ablative procedures in borderline cases. We do strongly recommend further studies involving a larger cohort size as well as longer follow-ups in a variety of refractive errors in various ethnic populations as well as geographic zones, which we expect will provide further clarity on the various issues outlined in this paper.

What was known?

TICK is commonly used to modify the corneal topography to compensate for refractive errors.

The procedure declined in popularity after the introduction of excimer laser stromal ablative treatments as the complication rates of TICK were considered high as compared with LASIK or PRK.

However, with the introduction of Expensive femtosecond lasers, intrastromal keratotomy has encouraging results and it is considered to be safer than either TICK or Lasik or PRK.

What this paper adds

MICK can be safely and easily performed for most mild to moderate refractive errors in a cost-effective and safe manner to alter the corneal topography to compensate for refractive errors.

Since many patients may not be suitable candidates for either femtosecond or excimer lasers due to various reasons outlined above, MICK may be considered as an acceptable and alternative method in the approach to correction of refractive errors.