Conversion of keratometer readings to polar values

Abstract

Corneal astigmatism is a complex entity that has direction and magnitude. This study reports a new method to describe corneal astigmatism within the with- and against-the-rule concept. Each net astigmatism of the maximal power M in the meridian alpha may be divided into two dioptric components: a with-the-rule astigmatism projected on the 90-degree meridian and an against-the-rule component projected on the 180-degree meridian. The former figure has the dioptric value M X sin2 alpha, the latter M X cos2 alpha. The polar value is defined as the difference between these magnitudes: M X (sin2 alpha - cos2 alpha). The polar value calculates the balance between the with- and against-the-rule components for any given net astigmatism. The entire model allows an exact description of surgically induced with- or against-the-rule astigmatism following cataract extraction. The advantage of the model is that a corneal astigmatism may be expressed by a single figure. The system enables each surgeon to evaluate the contribution of the preoperative astigmatism, incision type, suture technique, and postoperative treatment on the final astigmatism. This in turn allows the surgeon to estimate a number of different surgical techniques empirically. By disposing of and choosing between several known techniques the surgeon may be able to minimize final astigmatism even in cases of significant preoperative astigmatism.

Mudanças na refração após cirurgia de correção de esotropia

“…A utilização do método do valor polar (11)(12)(13) nos permitiu fazer comparação direta: demonstramos um incremento significante no astigmatismo afavor-da-regra, de aproximadamente 0,6 dioptrias, tanto na amostra de 21 pacientes pré e pós-operatório 1 mês), como na amostra completa (pré, pós-operatório 1 mês e pós-operatório 6 meses). Não obtivemos dados significantes no componente a 180°, no entanto, notamos decréscimo sistemático nos valores de pós-operatório, sugerindo um aplanamento neste meridiano.…”

“…Os dados de astigmatismo ceratométrico e refracional foram transcritos a valores polares, conforme descrito por Naeser (11)(12)(13) para permitir a manipulação matemática e estatís-tica. Assim, cada astigmatismo, tendo seu poder máximo M no meridiano α foi dividido em dois componentes dióptricos: um astigmatismo a-favor-da-regra projetado no meridiano 90°( D90) e um componente contra-regra projetado no meridano 180° (D180).…”

Possible factors modifying the surgically induced astigmatism in cataract surgery

“…We used a calibrated manual Javal-Schiotz keratometer at all examinations except at the 6-year visit where an automatic keratometer (Alcon Renaissance) was used. The keratometric astigmatism is expressed as Naeser's polar values converted from the keratometric readings (Naeser 1990). Astigmatism is a complex entity having both magnitude and direction.…”

“…Surgically induced astigmatism is defined by means of preoperative and postoperative keratometry, and can be calculated by at least six different methods (Dam-Johansen et al 1994). In this paper we have chosen Naeser's method due to its simplicity (Naeser 1990, Ninn-Pedersen 1996. The surgically induced astigmatism is modified by many factors: the length of the incision (Neuman et al 1989;Steinert et al 1991), the location of the incision (Rij & Waring 1984;Cravy 1989), the suturing technique (Jaffe & Clayman 1975;Cravy 1989), and suture cutting (Steiner et al 1982;Kronish & Forster 1987).…”

Comparison between postoperative astigmatism after classic extracapsular lens extraction and after phacoemulsification with implantation of a Pearce tripod or Pearce vaulted Y-loop intraocular lens

“…A statistical analysis of these data posed a very great problem and for that reason, was not performed. (2) Naeser's polar value method [1] for determining with-the-rule or against-the-rule change. In short, Naeser describes corneal astigmatism by a single parameter, the so-called polar value (p).…”