• Written By: Jason E. Stahl, MD
    Refractive Mgmt/Intervention

    The authors explore the biomechanical effects of laser flap creation on human donor corneas, demonstrating and quantifying three important principles: (1) corneal strain is predominantly due to the vertical side cut and not the horizontal lamellar cut, (2) thin flaps produce less strain than thick flaps, and (3) angulating side cuts to produce a stromal flap diameter greater than the epithelial diameter can decrease the strain.

    They divided 42 organ-cultured human corneas into a control group and three investigative groups, each undergoing different incision types at both 90- and 160-μm depth using a femtosecond laser. In the first group, typical LASIK flaps were created; in the second group, only the bed was cut (delamination); and in the third group, side cuts alone were affected. Corneal strain was measured using radial shearing speckle pattern interferometry before and after treatment following an increase in hydrostatic pressure from 15.0 to 15.5 mmHg and again after one week of incubation in culture medium.

    The flap group demonstrated a weakening of strength related to the depth of cut, with strain increasing by 9 percent and 32 percent at 90 and 160 μm, respectively. Similar changes, 9 percent and 33 percent, were observed following execution of side cuts to the same depths. By contrast, strain increase following delamination showed no relationship with depth, increasing by 5 percent in both instances. When the side-cut angle was made more acute, strain decreased.

    The authors conclude that with this finding thatthin-flap LASIK causes only insignificant mechanical changes and with other investigations demonstrating that patient recovery is equivalent after thin-flap LASIK to that following conventional LASIK and much more rapid than after PRK, these results support the clinical trend toward creation of thinner flaps.