Yousuf Khalifa

The location- and depth-dependent mechanical response of the human cornea under shear loading.

PURPOSE To characterize the depth-dependent shear modulus of the central and peripheral human cornea along the superior-inferior and nasal-temporal directions with a high spatial resolution. METHODS Cylindrical explants from the central and peripheral corneas of 10 human donors were subjected to a 5% shear strain along the superior-inferior and nasal-temporal directions using a microscope-mounted mechanical testing device. Depth-dependent shear strain and shear modulus were computed through force measurements and displacement tracking. RESULTS The shear modulus G of the human cornea varied continuously with depth, with a maximum occurring roughly 25% of the way from the anterior surface to the posterior surface. G also varied with direction in the superior region and (at some depths) was significantly higher for superior-inferior shear loading. In the anterior half of the cornea, the shear modulus along the nasal-temporal direction (GNT) did not vary with location; however, the superior region had significantly higher GNT in posterior cornea. In contrast, the shear modulus along the superior-inferior direction (GSI) was independent of location at all depths. CONCLUSIONS This study demonstrates that the peak shear modulus of the human cornea occurs at a substantial distance within the corneal stroma. Depth-dependent differences between central and peripheral cornea possibly reflect the location-dependent mechanical environment of the cornea. Moreover, the cornea is not a transverse isotropic material, and must be characterized by more than a single shear modulus due to its dependence on loading direction. The material properties measured in this study are critical for developing accurate mechanical models to predict the vision-threatening morphological changes that can occur in the cornea.

A computer vision-based approach to grade simulated cataract surgeries

To increase the timeliness, objectivity, and efficiency in evaluating ophthalmology residents’ learning of cataract surgery, an automatic analysis system for cataract surgery videos is developed to assess performance, particularly in the capsulorhexis step on the Kitaro simulator. We utilize computer vision technologies to measure performance of this critical step including duration, size, centrality, circularity, as well as motion stability during the capsulorhexis procedure. Consequently, a grading mechanism is established based on either linear regression or non-linear classification via Support Vector Machine of those computed measures. Comparisons of expert graders to the computer vision-based approach have demonstrated the accuracy and consistency of the computerized technique.

Assessment of the accuracy and cut-failure rates of eye bank-cut corneas for use in endothelial keratoplasty: A comparison of outcomes between 2010 and 2013

Purpose: To evaluate the accuracy of eye bank-prepared precut donor corneas over time by comparing cut-failure rates and corneal thickness measurements in 2010 and 2013. Methods: A total of 2511 human corneas cut by a technician-operated mechanical microkeratome intended for endothelial keratoplasty were evaluated prospectively at one large eye bank facility in 2010 and in 2013. The endothelium was evaluated by slit lamp, and specular microscopy both before and after cutting was performed. Graft thickness as measured by pachymetry and/or optical coherence tomography was collected to assess the accuracy of the cut tissue. Cut-failure rates were compared between normal donor tissue and tissue with significant preexisting scarring. Results: The combined cut-failure rate in 2010 and 2013 was 2.3% (23/1000) and 1.6% (24/1511), respectively (P = 0.23). The cut-failure rate among normal tissue in 2010 and 2013 was 2.0% (19/927) and 1.4% (19/1400), respectively (P = 0.24). The cut-failure rate among previously scarred tissue in 2010 and 2013 was 5.5% (4/73) and 4.5% (5/111), respectively (P = 0.74). The mean surgeon-requested graft thickness was 144.7 &mgr;m (range 100–150, SD 13.6) and 127.2 &mgr;m (range 75–150, SD 25.2) in 2010 and 2013, respectively (P < 0.0001). The mean deviation from target graft thickness was 21.3 &mgr;m (SD 16.3) and 13.6 &mgr;m (SD 12.5) in 2010 and 2013, respectively (P < 0.0001). Conclusions: From 2010 to 2013, the combined cut-failure rates trended toward improvement, while the accuracy of graft thickness improved. This study suggests that the accuracy and success rates of tissue preparation for endothelial keratoplasty improve with experience and volume.

Depth-Dependent Out-of-Plane Young’s Modulus of the Human Cornea

ABSTRACT Purpose/Aim: Despite their importance in accurate mechanical modeling of the cornea, the depth-dependent material properties of the cornea have only been partially elucidated. In this work, we characterized the depth-dependent out-of-plane Young’s modulus of the central and peripheral human cornea with high spatial resolution. Materials and Methods: Central and peripheral corneal buttons from human donors were subjected to unconfined axial compression followed by stress relaxation for 30 min. Sequences of fluorescent micrographs of full-thickness corneal buttons were acquired throughout the experiment to enable tracking of fluorescently labeled stromal keratocyte nuclei and measurements of depth-dependent infinitesimal strains. The nominal (gross) out-of-plane Young’s modulus and drained Poisson’s ratio for each whole specimen was computed from the equilibrium stress and overall tissue deformation. The depth-dependent (local) out-of-plane Young’s modulus was computed from the equilibrium stress and local tissue strain based on an anisotropic model (transverse isotropy). Results: The out-of-plane Young’s modulus of the cornea exhibited a strong dependence on in-plane location (peripheral versus central cornea), but not depth. The depth-dependent out-of-plane Young’s modulus of central and peripheral specimens ranged between 72.4–102.4 kPa and 38.3–58.9 kPa. The nominal out-of-plane Young’s modulus was 87 ± 41.51 kPa and 39.9 ± 15.28 kPa in the central and peripheral cornea, while the drained Poisson’s ratio was 0.05 ± 0.02 and 0.07 ± 0.04. Conclusions: The out-of-plane Young’s modulus of the cornea is mostly independent of depth, but not in-plane location (i.e. central vs. peripheral). These results may help inform more accurate finite element computer models of the cornea.

Vulnerability of corneal endothelial cells to mechanical trauma from indentation forces assessed using contact mechanics and fluorescence microscopy

Abstract Corneal endothelial cell (CEC) loss occurs from tissue manipulation during anterior segment surgery and corneal transplantation as well as from contact with synthetic materials like intraocular lenses and tube shunts. While several studies have quantified CEC loss for specific surgical steps, the vulnerability of CECs to isolated, controllable and measurable mechanical forces has not been assessed previously. The purpose of this study was to develop an experimental testing platform where the susceptibility of CECs to controlled mechanical trauma could be measured. The corneal endothelial surfaces of freshly dissected porcine corneas were subjected to a range of indentation forces via a spherical stainless steel bead. A cell viability assay in combination with high‐resolution fluorescence microscopy was used to visualize and quantify injured/dead CEC densities before and after mechanical loading. In specimens subjected to an indentation force of 9 mN, the mean ± SD peak contact pressure Symbol was 18.64 ± 3.59 kPa (139.81 ± 26.93 mmHg) in the center of indentation and decreased radially outward. Injured/dead CEC densities were significantly greater (p ≤ 0.001) after mechanical indentation of 9 mN (167 ± 97 cells/mm2) compared to before indentation (39 ± 52 cells/mm2) and compared to the sham group (34 ± 31 cells/mm2). In specimens subjected to “contact only” – defined as an applied indentation force of 0.65 mN – the peak contact pressure Symbol was 7.31 ± 1.5 kPa (54.83 ± 11.25 mmHg). In regions where the contact pressures was below 78% of Symbol (<5.7 kPa or 42.75 mmHg), injured/dead CEC densities were within the range of CEC loss observed in the sham group, suggesting negligible cell death. These findings indicate that CECs are highly susceptible to mechanical trauma via indentation, supporting the established “no‐touch” policy for ophthalmological procedures. While CECs can potentially remain viable below contact pressures of 5.7 kPa (42.75 mmHg), this low threshold suggests that prevention of indentation‐associated CEC loss may be challenging. Symbol. No caption available. HighlightsDeveloped platform to assess corneal endothelial cell vulnerability to indentation.Corneal endothelial cells (CECs) are highly susceptible to mechanical trauma.A low threshold contact pressure (<5.7 kPa) exists where CECs remain viable.Platform can be used to screen treatments protecting CEC against trauma.

Clinicopathologic correlations in eyes enucleated after uveal melanoma resection with positive surgical margins

We identified three eyes that had undergone enucleation after transscleral resection of uveal melanoma. Two enucleated eyes with microscopically positive margins of resection exhibited no evidence of residual melanoma and these patients were alive without metastasis with at least four years′ follow-up. One eye with a transected melanoma contained residual melanoma and that patient died with metastatic melanoma to the liver three years after enucleation. There appear to be at least two general types of positive surgical margins of resection of uveal melanoma: microscopically positive margins and macroscopically positive (transected) margins of resection.