EFFECT OF VITREOUS LENGTH AND TREPHINE-SIZE DISPARITY ON

REFRACTIVE STATUS AFTER DEEP ANTERIOR LAMELLAR KERATOPLASTY

Mohammad Ali Javadi, MD.

Sepehr Feizi, MD.

Shahid Beheshti University of Medical Sciences, Tehran, Iran.World Cornea Congress VI, Boston, April 2010

The authors have no financial interest in the subjective matter of this presentation.

INTRODUCTION Deep anterior lamellar keratoplasty (DALK) is considered

as an acceptable alternative procedure to penetrating keratoplasty (PK) for corneal pathologies not affecting the endothelium and Descemet’s membrane (DM).

Nowadays, achieving a clear graft is not the only concern after DALK. As the majority of the keratoconic patients who undergo corneal transplantation are young, establishing an acceptable visual acuity with minimal refractive error which lasts for a lifetime has become the major aim.

Like PK, astigmatism and myopia remaining after complete suture removal is the most common morbidity after DALK preventing patients from achieving acceptable visual acuity.

INTRODUCTION (CONTINUED)

A spherical equivalent refractive error ranging from -13.0 D to +7.0 D has been reported by several studies recruiting DALK for different corneal stromal pathologies.

Refractive status after PK depends on various factors

including vitreous length, donor-recipient disparity, and suture tensions. (Javadi et al. Cornea 1993)

However, there is no report to address this issue in DALK. In this study, we investigate the effect of vitreous length and donor-recipient disparity on the postoperative refractive outcomes in keratoconic eyes undergoing DALK with the big-bubble technique.

MATERIALS & METHODS

In this retrospective study, patients with moderate (mean keratometry < 55 D) or advanced keratoconus (mean keratometry ≥ 55 D or immeasurable keratometry) who received successful DALK using the big-bubble technique between January 2003 and December 2007 were enrolled.

Preoperatively, complete ocular examinations including Snellen uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BSCVA) with a rigid gas-permeable contact lens (if possible) using the Snellen acuity chart, slit-lamp examination, tonometry, dilated funduscopy, manifest refraction (when possible), keratometry, corneal topography, and vitreous length measurement using an ultrasonic contact probe were performed.

MATERIALS & METHODS (CONTINUED)

Vitreous length defined as the distance between the posterior lens surface and the vitreoretinal interface was considered to choose donor-recipient disparity because in contrast to anterior chamber depth and axial length, it remains unchanged after keratoplasty.

The method of surgery was DALK using the big-bubble technique

The size of recipient trephine was chosen according to the vertical corneal diameter (VCD) as follows: a 7.5-mm trephine for VCD < 10.0 mm, a 7.75-mm trephine for 10.0 mm ≤ VCD < 10.5 mm, and an 8-mm trephine for VCD ≥ 10.5 mm.

Approximately 80% of corneal thickness was cut. A #27 gauge needle was advanced to the center of the cornea and air was injected gently into the mid-stroma until a big-bubble was formed. It was followed by stromal removal leaving a bared DM.

MATERIALS & METHODS (CONTINUED)

For eyes with a vitreous length of ≥ 16.0 mm, a 0.25-mm-oversize graft was chosen and for those with a vitreous length of < 16.0mm, a 0.50-mm disparity was considered.

Donor DM and endothelium was gently stripped off followed by suturing the donor tissue to the recipient bed.

A combined suturing technique consisting of a 16-bite single running and 8-bite interrupted 10-0 nylon sutures was recruited in all cases. Astigmatism was adjusted with a hand-held keratoscope intraoperatively.

MATERIALS & METHODS (CONTINUED)

Selective suture removal was started at least 2 months after the operation, based on the keratometry readings, and continued over the follow-up period until an acceptable keratometric astigmatism was achieved.

Afterwards, the remaining sutures were removed when they broke or became loosened and hence appearing ineffective. Therefore, the participants had all sutures removed at the final examination.

The refractive outcomes evaluated in this study consisted of postoperative keratometry readings and manifest refraction and BSCVA obtained at least 3 months after complete suture removal.

RESULTS (CONTINUED)

Eighty-five (47 right) eyes of 83 patients (55 men) with keratoconus, aged 26.5±7.7 (range 15 to 46) years were enrolled.

Being measurable in 62 eyes, mean preoperative keratometery was 55.4±5.5 D (range, 44.25 to 71.9 D).

Donor-recipient disparity was 0.25 mm in 75 eyes (group 1) and 0.50 mm in 10 eyes (group 2). Patients were followed up for 22±10 (range, 13 to 51) months after the operation and for 9±9 (range, 3 to 40) months after complete suture removal.

RESULTS

A significant correlation is observed between vitreous length (VL) and postoperative SE (R2=0.48, P<0.001) in group 1, expressed by the equation: SE = -1.9 VL + 27.4.

A significant correlation is demonstrated between postoperative mean keratometry (K) and SE (R2=0.19, P=0.001) in group 1, expressed by the equation: SE = -0.7 K + 28.1.

The vitreous length was not significantly associated with postoperative keratometric astigmatism in group 1(R2=0.004, P=0.65; Figure-3).

RESULTS Multiple linear regression showed that 17.66% of the

variation in the postoperative SE was explained by the vitreous length (P<0.001), whereas a less variation (7.03%) was explained by postoperative mean keratometry (P<0.001).

However, Linear regression analysis failed to show any significant correlation between recipient trephination size and SE in group 1 (R2 = 0.013, P = 0.48) or in group 2 (R2 = 0.002, P = 0.81).

Comparing the two groups of disparity, the patients with 0.5-mm disparity had significantly steeper corneas postoperatively. However, postoperative SE, keratometric astigmatism, and BSCVA in group 1 did not differ from those in group 2.

DISCUSSION Similar to PK, postoperative refractive error ranging between -13.0 D

and +7.0 D is the main reason for patient’s dissatisfaction after DALK. The axial length of keratoconic eyes has been found to be greater than

emmetropic controls which explains why high post-keratoplasty myopia is frequently encountered in these eyes.

Several factors including vitreous length, suture tension, and donor-recipient disparity influence the post-keratoplasty refractive error. Among these factors, graft size disparity has received more attention and there are several studies on DALK which use different disparities and report a wide range of postoperative refractive error.

In an attempt to reduce post-DALK myopia, some surgeons use a same-size graft. Although they report a mean final spherical equivalent refractive error closer to emmetropia than those applying a 0.25- or 0.50-mm-oversize graft, high myopic refractive error can still be found in their results. On the other hand, more patients ended up with hyperopic shift up to +7.0 D.

DISCUSSION (CONTINUED)

A same-size or even undersize donor can lead to several problems. It makes wound closure difficult and needs a tight suture resulting in a flat graft resulting in inadequate tear film distribution and hence delayed epithelialization, post-keratoplasty hyperopia, and difficulty in contact lens fitting, as well as more severe interface wrinkling may develop if an undersize graft is used.

The results of our study show that the main contributor to the final post-DALK refractive error is the posterior segment length and the donor-recipient disparity plays a less important role in this regard.

To sum up, in order to make a compromise between acceptable post-DALK refractive outcomes in keratoconic eyes and reduction in postoperative complications encountered with a flat graft, we suggest donor trephines 0.25 mm and 0.50 mm larger than the recipient trephines for a vitreous length of ≥ 16.mm and < 16.0 mm, respectively.