Changes of axial length and keratometry after keratoplasty for keratoconus using the guided trephine system.

PURPOSE: The purpose of this study was to examine the cause of increasing myopia after penetrating keratoplasty for keratoconus with the guided trephine system.

DESIGN

Prospective interventional study cohort.

METHODS

Thirty eyes (28 patients) after keratoplasty for keratoconus were examined. Preoperatively and 1, 3, 6, 12, and 24 months postoperatively subjective refraction was evaluated. Keratometry was calculated with a computerized videokeratoscope (TMS-1). Axial length was measured using applanation ultrasonography before surgery and 2 years after surgery. Anterior chamber depth, lens thickness, and vitreous length were taken into consideration.

RESULTS

The mean spherical equivalent was + 2.22 +/- 3.47 diopters 1 month postoperatively and had a continuous myopic shift to -1.02 +/- 2.65 diopters 2 years postoperatively. This was associated with a significant increase in mean keratometric levels from + 41.72 +/- 2.96 diopters 1 month postoperatively to + 43.77 +/- 2.29 diopters 2 years postoperatively (r(s) = -0.36, P =.05). Overall, no significant changes in axial length were observed. However, vitreous length showed a small but statistically significant increase. As expected, mean anterior chamber depth decreased significantly postoperatively (P </=.05). In eyes with a decrease in keratometric values (n = 8) a myopic shift was also observed. In this group a significant elongation of the axial length and the vitreous length was found.

CONCLUSIONS

A study of eyes after keratoplasty for keratoconus demonstrated continuous myopic shift over time. Increasing myopia was associated with increasing keratometric values. Thus, the major cause of increasing myopia after keratoplasty in keratoconus eyes seems to be continuing keratectasia. However, ongoing bulbus growth may contribute to continuing myopia in some eyes.