Back-calculation to model strategies for pretreatment adjustment of the ablation sphere in myopic wavefront laser in situ keratomileusis.

PURPOSE: To examine strategies for adjusting the ablation sphere in myopic wavefront laser in situ keratomileusis (LASIK) with reference to preoperative manifest refraction.

SETTING: Refractive Surgery Service, Moorfields Eye Hospital, London, United Kingdom.

METHODS: The variance in the 3-month postoperative manifest refraction spherical equivalent (MRSE) results in 295 consecutive cases of myopic wavefront LASIK treated with a nonsystematic, intuitive approach to pretreatment adjustment of the ablation sphere was compared with the variance in modeled results obtained by back-calculation using 4 systematic strategies: (1) no adjustment (No Adj), (2) addition of the difference between the preoperative MRSE and the preoperative 4.0 mm pupil wavefront refraction spherical equivalent (WRSE) (4.0 mm), (3) addition of the difference between the preoperative MRSE and the preoperative maximum pupil WRSE (Max), and (4) addition of the difference between the preoperative MRSE and the preoperative effective blur (EB). The EB is a theoretical value for the dioptric SE of all aberrations at the pupil size calculated for the Hartmann-Shack images acquired (wavefront diameter). A stratified analysis of results for different wavefront diameters was also performed.

RESULTS: Variance in the postoperative MRSE was least (0.116) when nonsystematic, intuitive adjustments to the ablation sphere were used. In ascending order, the back-calculated variance was as follows: EB (0.142; P = .09, Bartlett test), 4.0 mm pupil (0.163; P = .004), No Adj (0.171; P = .001), and Max (0.225; P<.0001). There was a strong trend toward reduced variance in results in patients with a larger wavefront diameter.

CONCLUSION: Back-calculation to model results with different pretreatment ablation adjustment strategies may be useful to eliminate unpromising new approaches before clinical trials.