Purpose: The Zeiss IOLMaster optical biometry system provides superior prediction of refractive outcome of cataract surgery compared to applanation ultrasound. However, measurement is not always possible in the presence of dense cataract. The purpose of this study was to elucidate the rate of measurement acquisition failure due to cataract and how this varies with morphology and severity.
Methods: A total of 149 subjects were prospectively enrolled and visual acuity, Lens Opacities Classification System III (LOCS III) scores, ultrasonic A-scan and IOLMaster axial lengths were measured. Chi-squared analysis was used to test the null hypothesis that cataract severity has no effect on measurement failure rate.
Results: Measurements could not be obtained with the IOLMaster in nine cases for reasons other than cataract. Cataract caused measurement acquisition failure in 22 (15.9%) cases, including all mature cataracts (n=3) and posterior subcapsular cataracts (PSC) with LOCS III (p>3.5; n=18). The null hypothesis was accepted for cortical (chi(2)=2.94, d.f.=2, p>0.05) and nuclear (chi(2)=7.91, d.f.=4, p>0.05) ataract, but rejected for PSC (chi(2)=111, d.f.=3, p<0.001). All cases could be measured with ultrasound.
Conclusions: The IOLMaster fails to acquire axial length measurement in approximately 20% of UK public hospital cataract patients. Failure is principally due to PSC, whereby the LOCS III score of p=3.5 defines the limit of PSC severity that the IOLMaster can measure.