Ocular residual and corneal astigmatism in a clinical population of high school students

Purpose Total refractive astigmatism is usually the first consideration that guides the selection of contact lens type (e.g., spherical or toric), while the ocular source of the astigmatism is a second, but more important consideration, for the final clinical decision. This study was conducted to provide detailed data on this topic by evaluating astigmatic components in Chinese adolescents. Methods Participants were recruited from healthy high school students undergoing an annual ocular examination at a local hospital. Total astigmatism (TA), corneal astigmatism (CA), and ocular residual astigmatism (ORA) were determined by a Hartmann-Shack wavefront analyzer system (KR-1W, Topcon) with the natural pupil. The axis relationship between CA and ORA was placed into three categories: on-axis, defined as an axis with a difference of 0 +/- 10 degrees; opposite-axis, a difference of 90 +/- 10 degrees; and the rest defined as oblique -axis. Results The study consisted of 1,466 students (57.84% girls, age: 16.49 +/- 1.05 years). ORA was present in 83.97%, 66.64%, and 45.23% of participants, according to the various criteria for astigmatism (>= 0.50 D, >= 0.75 D, and >= 1.00 D, respectively). While with-the-rule was the most common axis orientation for both TA (76.28%) and CA (89.94%), against-the-rule predominated in ORA (93.82%; chi(2) = 1688.544, p < 0.001). Opposite-axis was the major type of axis difference (90.96%) of clinical significance (i.e., >= 1.00 D) between CA and ORA, which also prevailed in all levels of TA (range: 56.25-82.26%). Conclusions ORA is common in high school students and usually demonstrates a compensation relationship with CA, which should be taken into consideration when determining the design of contact lenses to correct refractive error.