COMPARISON OF KERATOMETRY AND VIDEOKERATOGRAPHY AFTER PENETRATING KERATOPLASTY

BACKGROUND: As new methods for corneal curvature measurement have evol ved, users of videokeratscopes need to know the practical limitations of these instruments. We assessed agreement between keratometry and vi deokeratography in measuring highly astigmatic corneas. METHODS: Two i ndependent examiners made three keratometric and videokeratographic me asurements on each of 33 corneas after penetrating keratoplasty. The n on-orthogonal keratometric readings obtained with a Zeiss 10 SL/O kera tometer (Carl Zeiss Ltd.) were compared to the non-orthogonal simK, re adings (maxK, minK) calculated by the algorithms of a TMS-1 videokerat oscope (Tomey). Measurement agreement was evaluated for steep and flat meridian power and location, and astigmatism magnitude (D). RESULTS: A systematic bias of the TMS-1 in measuring steeper than keratometry f or the steep meridian was demonstrated (95% confidence interval: -0.34 to -1.20 D). The limits of agreement (d-2SD to d+2SD) between the two instruments were found to be unacceptable for clinical purposes in me asuring steep meridian power (-3.17 to +1.63 D), flat meridian power ( -4.92 to +4.48 D) and astigmatism magnitude (-5.84 to +4.87 D). Clinic ally acceptable differences were observed in identification of steep a nd flat meridian location. CONCLUSIONS: The Zeiss 10 SL/O keratometer and the TMS-1 videokeratoscope showed poor measurement agreement for i rregular corneal surfaces, despite the good correlation previously sho wn between keratometry and videokeratography in calibrated spheres and regular corneas. The TMS-1 showed a systematic bias, measuring a grea ter power in the steeper meridian than the Zeiss 10 SL/O keratometer. It is suggested that the two instruments cannot be used interchangeabl y in comparing the curvature of corneas after penetrating keratoplasty .