Patient-specific biomechanical properties of the human cornea are rarely used with finite elements analysis. In order for that to be possible, a proper formulation for biomechanical properties that is based on patient-specific measurable values must be used. In this study, we propose a formula that simulates hyperelastic stress-strain curves based on non-invasive clinical measurements that can be acquired in vivo. These consist of, but are not limited to, center corneal thickness and center corneal curvature as well as corneal resistance factor and applanation diameter that are measured during non-contact tonometry. The presented formulation was demonstrated and validated through several computer simulations. First, mean values that were reported in literature were inputted into the formula to simulate a curve that represents a healthy case. This case was compared to two independent in vitro studies. Then, a sensitivity analysis was carried to identify inputs that have the most dominant effect. Finally, a finite element analysis simulating elevations in intraocular pressure was conducted; the corneal model comprised of patient-specific corneal geometry that was measured in vivo in our clinic as well as the current formulation for patient-specific corneal biomechanics. "Strong" and "weak" corneal tissue cases were simulated and deformations as well as instantaneous curvature optical maps were derived. Results for the simulated healthy curve showed good agreement with the in vitro studies. The sensitivity analysis found the corneal resistance factor and applanation diameter to have the most dominant influence. The finite element analysis of strong and weak biomechanical properties resulted in corneal deformations and instantaneous curvature optical maps that are common for healthy and pathological conditions respectively. In conclusion, the presented modeling technique can be used to assess corneal biomechanics in vivo and therefor may enhance follow-up on the effectiveness of clinical treatments, rehabilitation of vision and perhaps improve the diagnosis of pathologies that are related to corneal biomechanics.
Meniscal tears often lead to degenerative arthritis, attributed primarily to the changes in the magnitude and pattern of stress distribution in the knee. While meniscal replacement traditionally involves implantation of allografts, problems related to availability, size matching, and cost limit their use. In this regard, there are significant potential advantages to a bio-stable synthetic meniscus implant that combines long-term durability with a dependable biomechanical performance resembling that of the natural meniscus. The Nusurface ® medial Meniscus Implant is a poly-(carbonate-urethane) implant that recreates the functionality of the meniscus, as evidenced by compression tests to determine its pressure distribution capability [1] and finite element analyses [2] in its virgin state. However, the overall success of such an implant is dependent on its long-term functional properties, and biomechanical testing of the NUsurface ® device has not yet examined the effect of varying and dynamic testing conditions. In particular, the pre-test soaking in physiologic solution and the loading rate can have non-trivial influences on the properties of the device. Therefore the aims of this study were to characterize the strain-rate response, as well as the viscoelastic properties of the implant as measured by creep, stress relaxation, and hysteresis after simulated use.
Background: The etiology of keratoconus most likely involves substantial biomechanical interactions. The goal of this study was to characterize corneal biomechanics using computer modeling techniques in order to elucidate the pathogenesis of keratoconus in biomechanical terms.
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