Depth-dependent transverse shear properties of the human corneal stroma.

PURPOSE: To measure the transverse shear modulus of the human corneal stroma and its profile through the depth by mechanical testing, and to assess the validity of the hypothesis that the shear modulus will be greater in the anterior third due to increased interweaving of lamellae. METHODS: Torsional rheometry was used to measure the transverse shear properties of 6 mm diameter buttons of matched human cadaver cornea pairs. One cornea from each pair was cut into thirds through the thickness with a femtosecond laser and each stromal third was tested individually. The remaining intact corneas were tested to measure full stroma shear modulus. The shear modulus from a 1% shear strain oscillatory test was measured at various levels of axial compression for all samples. RESULTS: After controlling for axial compression, the transverse shear moduli of isolated anterior layers were significantly higher than central and posterior layers. Mean modulus values at 0% axial strain were 7.71 ± 6.34 kPa in the anterior, 1.99 ± 0.45 kPa in the center, 1.31 ± 1.01 kPa in the posterior, and 9.48 ± 2.92 kPa for full thickness samples. A mean equilibrium compressive modulus of 38.7 ± 8.6 kPa at 0% axial strain was calculated from axial compression measured during the shear tests. CONCLUSIONS: Transverse shear moduli are two to three orders of magnitude lower than tensile moduli reported in the literature. The profile of shear moduli through the depth displayed a significant increase from posterior to anterior. This gradient supports the hypothesis and corresponds to the gradient of interwoven lamellae seen in imaging of stromal cross-sections.

Sectorial and annular quantitative area pachymetry with the Orbscan II.

PURPOSE: To present a new application of quantitative area pachymetry of the cornea. METHODS: Sectorial quantitative area pachymetry of small corneal zones located at the center and surrounding areas is possible by means of the statistical analysis device and tools menu of Orbscan II (Bausch and Lomb). RESULTS: Average pachymetry of 1 central zone and 12 small zones around the center may be shown in the pachymetry map by simply formatting the system. Average pachymetry, standard deviation, rate of thickness variation (slope), and volume of analyzed annular corneal zone can also be automatically shown in a data window. The examination results can be printed and kept on record. CONCLUSIONS: The method is simple and descriptive. This type of preoperative evaluation may guide the manual or laser preparation of channels for intracorneal segments and the corneal trephination for lamellar or penetrating keratoplasty.