Abstract
PurposeTotal 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.MethodsParticipants 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°; opposite-axis, a difference of 90 ± 10°; and the rest defined as oblique-axis.ResultsThe 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%; χ2 = 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%).ConclusionsORA 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.
Highlights
Astigmatism, which is defined as the difference in the refractive error between two principal meridians of the eyeball, is the most common type of refractive error [1,2,3,4]
Opposite-axis was the major type of axis difference (90.96%) of clinical significance (i.e., ! 1.00 D) between corneal astigmatism (CA) and ocular residual astigmatism (ORA), which prevailed in all levels of Total astigmatism (TA)
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
Summary
Astigmatism, which is defined as the difference in the refractive error between two principal meridians of the eyeball, is the most common type of refractive error [1,2,3,4]. CA is theoretically composed of astigmatisms from both the anterior and posterior surfaces of the cornea, generally CA only refers to an astigmatism of the anterior corneal surface [5]. This is partly because of difficulties in measuring the posterior corneal surface and, the posterior corneal curvature has a relatively small impact on the total corneal astigmatism because of relatively small changes in the refractive index between the cornea and the aqueous humor [6]. ORA is defined as an astigmatism of the posterior corneal surface, plus that of both surfaces of the crystalline lens and any retinal astigmatism. ORA is defined as an astigmatism of the posterior corneal surface, plus that of both surfaces of the crystalline lens and any retinal astigmatism. [7,8]
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