角膜后表面散光

Posterior corneal astigmatism

眼球的散光主要包括角膜散光和眼内散光（即晶状体的散光）,角膜散光由角膜前后表面散光组成.角膜后表面散光大小约0.3D,主要为逆规散光（占85％～96.1％）.角膜后表面非球面性并非完全由前表面的非球面性决定,它与年龄呈负相关.随年龄增长前表面散光由顺规向逆规漂移明显;后表面散光的趋势则相反,但总体变化很小.同时角膜前后表面半径比与年龄也呈负相关.后表面散光对前表面的补偿作用随年龄增加而减小.因此,对于青年人,后表面散光减小总角膜散光;对于老年人,后表面散光则增加总角膜散光.忽略后表面散光可能会导致总角膜散光的错误估计,测量角膜后表面散光以进一步准确计算角膜总散光是白内障屈光手术临床应用的迫切需求.

The eye astigmatism mainly includes corneal astigmatism and intraocular astigmatism （i.e.,the lens astigmatism）.Both the anterior and posterior corneal surfaces contribute to the total corneal astigmatism.In most eyes,the posterior corneal steep meridian is aligned vertically and approximately has 0.3D astigmatism.The asphericity of the posterior corneal surface decreas significantly with age and is not purely governed by that of the anterior surface.With aging,the anterior surface astigmatism shows a significant trend toward against-the-rule while the posterior corneal astigmatism shows toward with the rule,but the change of posterior astigmatism is smaller.The ratio of the anterior-to-posterior corneal radius also has a negative correlation with age.The compensating effects of the posterior corneal surface on anterior corneal astigmatism decreased with age.Therefore the posterior corneal astigmatism often leads to partially decrease of total corneal astigmatism in young adults while often adds to total corneal astigmatism in older individuals.Omission of the posterior corneal astigmatism may lead to significant inaccuracies in estimating the total corneal astigmatism.There is a clear clinical need to measure the posterior corneal surface for calculating the total corneal astigmatismin in cataract refractive surgery.