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  • Refractive Mgmt/Intervention

    Researchers have developed an effective nomogram for femtosecond laser astigmatic keratotomy (AK) in patients with post-keratoplasty astigmatism. The formula takes into consideration a variety of incision-related factors and the degree of preexisting astigmatism.

    Several nomograms are available for planning manual AK incisions in native corneas, but this is the first to take into account the fact that grafted corneas respond differently after incisional surgery.

    What the researchers found highlights the importance of considering a variety of incision-related factors and the degree of preexisting astigmatism when managing post-keratoplasty astigmatism.

    Using data from 3 eye centers, the authors first assessed the safety and efficacy of 140 femtosecond AK procedures performed after penetrating keratoplasty (PKP) or deep anterior lamellar keratoplasty (DALK). At 1 month postop, there was a significant increase in mean UDVA and CDVA, and a significant reduction in mean keratometric astigmatism. Investigators also noted a trend towards under-correction, resulting in a low 6.7% incidence of overcorrection.

    The surgical procedure was also relatively safe. The corneal transplant incision remained sound and no patient developed infectious keratitis.

    Additionally, the researchers recorded the arc length, depth, and diameter of the corneal incisions and related them to surgically induced astigmatic correction using regression analysis. From this analysis, the authors generated a nomogram for planning femtosecond laser AK after PKP and DALK.

    The coefficient of determination of the nomogram was 0.67, therefore 67% of the variation in the degree of surgically induced astigmatic correction can be explained by the studied variables: preop astigmatism, arc length, depth and incision diameter. However, 33% of variation is attributed to an as-yet undetermined etiology.

    The authors believe this nomogram will likely increase the utility of this already established procedure.

    Nomogram for femtosecond laser AK after keratoplasty:

    Preop DKIncision MagnitudeCorrection (%)Incision Depth (% Corneal Thickness)Arc Length (Degrees)Optical Zone Diameter (mm)
    2 1.311 0.87 85 60 7.0
    3 1.784 0.82 85 75 6.8
    4 2.176 0.78 85 85 6.7
    5 2.508 0.72 85 90 6.6
    6 2.791 0.98 90 90 6.6
    7 3.036 0.91 90 90 6.5
    8 3.249 0.87 90 90 6.4
    9 3.438 0.83 90 90 6.3
    10 3.605 0.81 90 90 6.2

    DK = delta K (difference between the steepest and flattest keratometry values)