Developing entry criteria for studies of severe postkeratoplasty astigmatism

Investigators who use keratometry or topography systems to plan operations for the correction of severe postkeratoplasty astigmatism assume the study group is homogeneous. To test this hypothesis, we established four entry criteria we believed were necessary to ensure a homogeneous group. The criteria were as follows: (1) best-corrected or pinhole visual acuity greater than or equal to 20/40; (2) graft well centered relative to the corneal light reflex; (3) corneal light reflex within 0.5 mm of the center of the entrance pupil; and (4) a minimum of ten central keratoscope rings of the Corneal Modeling System accurately digitized. A retrospective analysis of the keratoscope images of 20 consecutive patients (20 eyes) referred for correction of postkeratoplasty astigmatism was performed to determine the percentage of patients who met the proposed criteria. Only seven (35%) simultaneously met all four entry criteria. Investigators interested in comparing techniques to correct this problem are faced with a dilemma. Strict entry criteria will ensure a homogeneous study group but will exclude most patients. More lenient entry criteria will increase the number of eligible subjects but will introduce heterogeneity, which may confound results.