Corneal Coupling—Importance in Incisional and Ablative Procedures
Purpose
To redefine corneal coupling so that it can be applied to laser ablations as well as incisional surgery. This can then be used clinically to adjust for any effect the astigmatic treatment has on the spherical correction.
Methods
The previously used terms of coupling ratio and coupling constant are redefined for their application in both incisional and laser vision correction procedures. In addition, a new measure coupling adjustment is introduced to determine the amount of spherical adjustment required to compensate for coupling that occurs as a result of astigmatism treatment. Retrospective incisional and ablative data is analyzed to demonstrate the applicability of these terms.
Results
Compound myopic and hyperopic astigmatism excimer laser treatments (3818 eyes) showed a coupling ratio close to zero, a coupling constant close to 0.5, and a coupling adjustment close to zero. Incisional limbal relaxing incisions (74 eyes) demonstrated a coupling ratio close to one, and a coupling constant close to zero.
Conclusion
The revised definitions of coupling ratio and coupling constant can be used with both incisional and ablative surgery. Incorporating the coupling adjustment into the planning of spherocylindrical treatments enables the surgeon to factor in the effect of the astigmatic treatment on the spherical component for better outcomes.