PURPOSE. Success in fitting toric soft lenses rests, in part, with the acuity that can be achieved and maintained. Thus, sensitivity to astigmatism axis rotation and stability of the contact lens may influence successful fitting. We evaluated patients with various amounts of astigmatism to determine blur sensitivity to axis rotation. METHOD. Fifty-nine consecutive contact lens patients (104 eyes; patient age 12-58; mean=29.9, SD=10.5) with astigmatism of 0.75 to 3.00D in at least one eye and vision correctable to 6/6- (20/20-) or better in each eye were evaluated in two optometric practices. With the patient monocularly viewing the 20/30 line on a standard acuity chart through the phoropter, after subjective refraction was complete, the examiner slowly rotated the astigmatism axis by hand (about 2deg/sec) until blur was first reported, for the right and left eye in turn. Mean and standard deviation were determined for clock-wise (CW) and counter clock-wise (CCW) rotation.
RESULTS. There was no statistical difference in rotation sensitivity between with- and against-the-rule astigmatism. There was increased sensitivity to rotation as astigmatism power increased.
Astigmatism N Clock-wise Counter clock-wise
(eyes) Rotation Rotation
Mean SD Mean SD
0.75 to 1.00 46 23.8 12.2 25.1 10.6
1.25 to 1.50 27 19.4 7.0 21.0 8.6
1.75 to 2.00 17 18.6 10.2 15.5 7.5
2.25 to 2.50 08 12.0 5.3 14.3 6.5
2.75 to 3.00 06 7.3 2.2 7.8 3.6
CONCLUSIONS. Our results suggest that for most astigmatic patients sensitivity to axis rotation is large enough that proper alignment should be easily achievable using current compensation-for-rotation techniques (e.g., LARS). Of more importance will be lens rotational stability. If variation in lens orientation is more than one standard deviation of blur sensitivity it is likely that the patient will report unstable vision with toric lenses. For clinical purposes the stability desired can be estimated by the formula:
13deg minus 3deg per diopter of astigmatism (e.g., with 1.5DC; 13-3(1.5)=8.5).