There were a total of 117, 129, and 112 American Caucasian, American Chinese, and mainland Chinese subjects, respectively, who had UBM images available for this study. Two subjects were excluded for use of glaucoma medications, and no subject was excluded for corneal or conjunctival abnormities. UBM images were of right eyes except in cases in which the right eye did not meet the inclusion criteria. Left eye UBM data were used in six (5%), three (2.3%), and five (4.5%) participants from each cohort, respectively. All three groups had a similar average age. The American Caucasian cohort had a mean age of 59.7 ± 11.6 years, and the American Chinese and mainland Chinese cohorts had mean ages of 59.4 ± 12.0 and 59.63 ± 11.7 years, respectively (
P = 0.98,
Table 2). Proportions of males and females were also similar among the three cohorts (
P = 0.43,
Table 2). Refractive error and ocular measurements by A-scan ultrasound were also compared across the three groups using ANOVA; there were significant differences in the SE (
P < 0.0001), AL (
P < 0.0001), LT (
P = 0.002), and ACD (
P < 0.0001) (
Table 2) among the 3 groups.
Proportions of different types of iris insertion were compared between Caucasians and ethnic Chinese cohorts using Pearson χ
2 tests. Because results did not differ significantly between American Chinese and mainland Chinese cohorts (all
P values > 0.05,
Table 3), we combined the two ethnic Chinese groups for the purpose of further analysis in this study. In the superior quadrant, basal insertion was the most common type in both Caucasians and Chinese (
P = 0.60,
Table 3). In the nasal quadrant, Caucasians had more than 52% basal insertions, whereas the most frequently observed type of insertion in the Chinese cohort was the middle type (
P < 0.0001,
Table 3). In the inferior quadrant, Caucasians had approximately 41% of middle and 50% of basal insertions. This was also significantly different from that of the Chinese, in whom nearly 55% were the middle type (
P = 0.04,
Table 3). Similar results were found in the temporal quadrant, where Caucasians had almost 49% of the basal type, whereas Chinese had approximately 63% middle and 21% basal insertions (
P < 0.0001,
Table 3). Since the apical insertion type was found to be rare in all four quadrants, especially in the Caucasian cohort, the apical and middle insertion types were combined and redefined as the nonbasal type for further analysis. After this combination, Chinese were found to have a significantly higher percentage of nonbasal insertion in all quadrants but the superior one (superior,
P = 0.55; nasal,
P < 0.0001; inferior,
P = 0.01; temporal,
P < 0.0001).
When anterior segment biometry measured by ASOCT (horizontal scan) was compared, significantly shallower ACD and smaller ACW, ACA, and ACV were found in eyes with nonbasal insertion in the nasal, inferior and temporal quadrants independently (
Table 4). Iris characteristics were also compared based on iris insertion (basal versus nonbasal) in the nasal and temporal quadrants separately. IT750, Iarea, and Icurv were all found to be significantly different between the two cohorts in both quadrants. The nonbasal cohort showed thicker peripheral irides (
Table 4, IT750,
P = 0.02 for nasal,
P = 0.02 for temporal), larger iris area (
Table 4,
P = 0.009 for nasal,
P = 0.004 for temporal), and larger iris curvature (
Table 4,
P = 0.0003 for nasal,
P < 0.0001 for temporal). Sex composition did not differ between the basal and nonbasal cohorts (
Table 4, superior,
P = 0.15; nasal,
P = 0.84; inferior,
P = 0.48; temporal
P = 0.89).
The difference in iris insertion type between Caucasian and ethnic Chinese was further verified with a multivariate logistic regression model adjusting for potential confounding factors, including age, sex, ethnicity, AL, SE, ACD, ACW, ACV, IT750, Icurv, and Iarea (
Table 5). Because ASOCT measured iris parameters only in the horizontal meridian, the regression analysis was carried out for the nasal and temporal quadrants only. Our results showed that in both the nasal and temporal quadrants, Chinese ethnicity (nasal, odds ratio [OR]: 3.1,
P = 0.002; temporal, OR: 4.8,
P < 0.0001), shallower ACD (nasal,
P = 0.04; temporal,
P = 0.006), smaller ACW (nasal,
P = 0.0002; temporal,
P = 0.006), smaller ACV (nasal,
P = 0.0004; temporal,
P = 0.0003), larger Icurv (nasal,
P < 0.0001; temporal,
P < 0.0001), and larger Iarea (nasal,
P = 0.02; temporal,
P = 0.03) are all significantly associated with nonbasal iris insertion. Older age was found to be associated with nonbasal iris insertion in the temporal quadrant only (nasal,
P = 0.26; temporal,
P = 0.01).
To further elucidate the relationship between age and iris insertion, we divided the study population into four age groups with 10-year increments. An apparently increasing number of nonbasal insertion was observed in all four quadrants with increasing age based on Pearson χ2 tests, although statistical significance was detected only in the superior and temporal quadrants (superior, 13.82% [40–49 years old], 41.11% [50–59 years old], 48.9% [60–69 years old], 57.8% [70–80 years old], P = 0.003; nasal, 53.4%, 62.2%, 66.7%, 70.0%, P = 0.12; inferior, 48.9%, 61.0%, 61.1%, 66.67%, P = 0.10; temporal, 55.7%, 67.8%, 74.4%, 82.2%, P = 0.001). We further analyzed this observation by comparing the proportion of nonbasal insertion in the top age group with the bottom age group using Pearson χ2 tests for each ethnicity separately. In Caucasians, higher percentages of nonbasal insertion were identified in all quadrants in the top age group, and statistical significance was found in the superior (57.7% vs. 27.6%; P = 0.02) and temporal (69.3% vs. 37.9%, P = 0.02) quadrants. Similarly, in ethnic Chinese, a significantly higher proportion of the nonbasal insertion was detected in all four quadrants in the older group (superior, 54.7% vs. 33.9%, P = 0.01; nasal, 76.8% vs. 59.3%, P = 0.04; inferior, 70.0% vs. 50.9%, P = 0.03; temporal, 90.0% vs. 64.4%, P = 0.0009).