Dimitri Chernyak

Computational modeling of mechanical anisotropy in the cornea and sclera

Purpose: To determine the biomechanical deformation of the cornea resulting from tissue cutting and removal by use of a new computational model and to investigate the effect of mechanical anisotrophy resulting from the fibrillar architecture. Setting: Department of Mechanical Engineering, Stanford University, Stanford, California, USA. Methods: A mathematical model for a typical lamella that explicitly accounts for the strain energy of the collagen fibrils, extrafibrillar matrix, and proteoglycan cross‐linking was developed. A stromal model was then obtained by generalized averaging of the lamella properties through the stromal thickness, taking into account the preferred orientations of the collagen fibrils, which were obtained from x‐ray scattering data. Results: The model was used to predict astigmatism induced by a tunnel incision in the sclera, such as is used for cataract extraction and intraocular lens implantation. The amount of induced cylinder was in good agreement with published clinical data. Results show it is important for the model to incorporate preexisting corneal physiological stress caused by intraocular pressure. Conclusions: The mathematical model described appears to provide a framework for further development, capturing the essential features of mechanical anisotropy of the cornea. The tunnel incision simulation indicated the importance of the anisotropy in this case.

Cyclotorsional eye motion occurring between wavefront measurement and refractive surgery.

Purpose: To quantify the cyclorotation occurring between wavefront measurement and laser refractive surgery. Setting: LaserVue Eye Center Ophthalmic Clinic, Santa Rosa, California, USA. Methods: The pupil camera of the Visx WaveScan® wavefront device was used to obtain images of 51 eyes (26 patients) from 5 to 20 minutes before refractive laser surgery. Additionally, an infrared camera was mounted on the Visx Star S3 ActiveTrak® excimer laser system to obtain another image immediately before the laser was fired. After surgery, the 2 sets of images were compared to determine the amount of cyclotorsion between the measurement and surgery. Results: Cyclorotation of individual eyes was as high as 9.5 degrees. The mean was approximately 2.0 degrees for each eye. Binocular excyclotorsion was the predominant trend, affecting 19 of 24 patients. Conclusions: A low to moderate amount of cyclotorsion was observed in the transition from seated to supine position. Comparison of eye position at the time of measurement to eye position at the time of surgery can be used to adjust the laser ablation algorithm to compensate for this rotational displacement.

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.

Comparison of wavefront aberrations in rabbit and human eyes

The rabbit is one of the most common animal models used for preclinical safety evaluation of new cataract surgery and laser vision‐correction technologies in ophthalmic research; however, the distributions of wavefront aberrations in rabbit eyes are unknown. The purpose of this study was to investigate the similarities and differences of wavefront aberrations between rabbit and human eyes.

Depth Dependent In-Plane Shear Properties of the Corneal Stroma

The popularity of refractive surgery to correct the vision of individuals with hyperopia or myopia is increasing. These procedures alter the tissue of the human cornea to cause a change in curvature (refractive power) of the cornea. Radial keratotomy, photorefractive keratectomy, LASIK, and LASEK are all types of refractive surgery. The outcomes of refractive surgical procedures must depend significantly on the biomechanical response of the tissue and therefore on the biomechanical properties of the cornea, or more specifically the corneal stroma which makes up 90% of the tissue. The missing link between computer models of these procedures and predicting patient outcomes is the biomechanical properties of the tissue, including shear modulus. This study aims to characterize the in-plane shear modulus of the corneal stroma through the depth by mechanical testing. Scant data, if any, exists about the shear stiffness and no data includes depth dependence. The stroma consists of sheets of collagenous lamellae in which fibrils are maintained at uniform spacing by glycoaminoglycan molecules. Studies have shown increased interweaving of the lamellae in the anterior third of the stroma compared to the central and posterior thirds [1]. Figure 1 shows the distinct interweaving in the anterior third [2]. It is hypothesized that more interweaving lamellae increases the in-plane shear stiffness. The shear modulus of the full cornea, as well as individual thirds, is examined in this study.© 2010 ASME