Abstract
Purpose: :
Residual corneal astigmatism is a frequent outcome after penetrating keratoplasty (PK). Surgical correction of residual astigmatism is possible with astigmatic keratotomy (AK) techniques in which arcuate incisions are made in the steepest meridian of the cornea. The resulting reduction in astigmatism after both PK and AK is often variable and at times unpredictable. There are few nomograms outlining the ideal parameters to use for AK in order to achieve a desirable outcome especially after PK.
Methods: :
We conducted a preliminary retrospective study investigating data from 14 patients who underwent AK after PK surgeries and achieved a significant reduction in corneal astigmatism. Topographical analysis was used to determine the extent and axis of corneal astigmatism prior to and after AK. Jaffe and Clayman’s formula for surgically induced astigmatism (SIA) was used to quantify the results taking into account changes in cylinder axis. A multiple linear regression analysis was conducted with SIA as the dependent variable. Predictor variables included in the analysis were: 1. pre-AK corneal cylinder; 2. pachymetry at site of arcuate incisions; 3. depth of the incisions; 4. length of arcuate incisions.
Results: :
The mean age of participants was 69.2 years (SD=17.5 years). The mean pachymetry measure was 588nm (SD=73nm) and the average length of each arcuate incision was 59 deg (SD=24 deg). The mean pre-operative corneal cylinder and SIA were, respectively, 9.3D (SD=2.8D) and 9.5D (SD=4.3D). The resultant linear regression model was able to account for variability in observed SIA to a significant extent (R2=0.65, p<.05).
Conclusions: :
We present a nomogram based on our preliminary data which provides arcuate lengths to use for AK surgeries after-PK (assuming that incisions are paired and made at 90% of the corneal thickness using a 6mm optic zone). The nomogram provides incision lengths for corneal thicknesses ranging from 400nm to 700nm and for pre-operative astigmatism ranging from 1.00D to 25.00D. We are currently conducting a prospective study to investigate the validity of this nomogram and will present upon these results.
Keywords: cornea: clinical science • astigmatism • refractive surgery