Induction of Astigmatism by Straight Transverse Corneal Incisions, 45° Long, at Different Clear Zones in Human Cadaver Eyes
Authors: Sabates Mario A , Buzard Kurt A , Friedlander Miles H , Cortinas Maria B 
Published: April 2021

ABSTRACT
BACKGROUND: Two of the major factors affecting the amount of astigmatism correction are the length of the transverse incision and its distance from the center of the cornea. Many nomograms used in clinical practice have been created by varying the length or clear zone diameter of the incisions. A simplification of this situation has been suggested by Thornton, who has theorized that straight transverse incisions, subtending 45° of arc, have equal astigmatic corrective effect at different clear zones. Our study tested Thornton's theory in human donor eyes.
METHODS: Ten eyes were tested at four clear zones: 5.0, 6.0, 7.0, and 8.0 mm. Paired straight transverse incisions, subtending an arc of 45° (2.1 to 3.3 mm long), were centered on the 90-degree meridian. Preoperative keratometric readings at the 180and 90-degree meridians were compared to the postoperative readings; the difference was the total astigmatism induced.
We also calculated the coupling ratio.
RESULTS: Student's t-tests comparing clear zones 6.0 and 7.0 mm revealed a statistical difference (p = . 0085) in total astigmatic induction, greater for the 6.0-millimeter zone. The coupling ratio decreased as the clear zone diameter increased, presumably as a result of diminished flattening effect along the incised meridian. One-way analysis of variance indicated that the groups were different (p = .0001), and that the theory noted above was incorrect.
CONCLUSIONS: The effect of transverse incisions subtending the same angular length, drops off dramatically with clear zones larger than 6.0 mm, contrary to the theory of Thornton. This effect may be due to reduction in coupling as the clear zone diameter increases, suggesting that the greatest efficacy is achieved for transverse incisions placed between 5.0- and 6.0-millimeter zones. [J Refract Corneal Surg. 1994;10:327-332.]
RESUME
INTRODUCTION: La longueur de l'incision transverse et sa distance du centre corneen ont un effet sur le degré de correction d'astigmatisme dans la chirurgie refractive. Bien des nomogrammes utilisés aujourd'hui furent crées en variant la longueur ou le diamètre de la zone optique. Thornton a émis l'hypothèse que les incisions transverses droites sous-tendant 45° d'arc ont le même effet correcteur de l'astigmatisme au niveau de différentes zones claires. Nous avons experimenté cette hypothèse dans les yeux de cadavres humains.
METHODES: Dix yeux furent essayé aux 4 zones clairs chacun: 5,0, 6,0, 7,0, et 8,0 mm. Des incisions droites et transverses en paires, sous-tendant 45° (2,1-3,3 mm) furent centrées sur le mérithen 90°. Les mesures kératométriques de 180° et 90° en préopératoire furent comparées avec celles en postopératoire afin de déterminer l'astigmatisme total induit. Nous avons calculé aussi la raison de couplage.
RESULTATS: L'essai "t" selon Student montra signifícativement plus d'astigmatisme induit par la zone claire de 6,0 nun que pour la zone de 7,0 nun (p = 0,0085). Le rapport de couplage diminuait quand le diamètre du zone clair augmentait, probablement le résultat de moins d'effet aplanissement le long du mérithen incisé. L'analyse de variance unidirectionnelle démontra un a différence significative entre les deux groupes (p = 0,0001), ce qui réfute l'hypothèse ci-dessus.