ESCRS 2024: Measuring the Biomechanics of the Cornea
Speaker: Alejandra Consejo
Biomechanics of the cornea refers to the study of the mechanical properties and behavior of corneal tissue under various forces. The cornea, a biomechanical tissue, has several essential characteristics: it is elastic, meaning it can return to its original shape after being deformed; it is stiff, indicating the amount of force required to cause deformation; and it exhibits viscoelasticity, which refers to its resistance to permanent deformation and its time-dependent recovery behavior. To study corneal biomechanics, mechanical stimulation must be applied to the cornea, as biomechanics involves the study of tissue movement under force. This can be done ex vivo (outside the living body) or in vivo (within the living body). In ex vivo studies, mechanical forces like stretching are applied to corneal samples, providing valuable research insights but with no clinical application. In vivo methods allow for non-invasive assessment of corneal biomechanics, crucial for clinical practice.
Corneal biomechanics is of particular interest in conditions like keratoconus, glaucoma, and refractive surgery. Pathological corneal tissue often exhibits altered biomechanical properties compared to healthy tissue. For example, in keratoconus, corneal stiffness decreases, leading to tissue weakening. In glaucoma, there is controversy surrounding whether chronic intraocular pressure (IOP) elevation affects corneal stiffness, but a consensus exists that the tissue loses viscosity, which may contribute to optic nerve damage. In refractive surgery, corneal biomechanics is studied to understand the tissue's response to surgical intervention, while in orthokeratology (OrthoK), research has suggested that biomechanical factors may predict treatment success. Corneal biomechanics is crucial for evaluating the effectiveness of OrthoK treatments and refractive surgery outcomes. It is established that a stiffer cornea improves the likelihood of successful OrthoK treatment. Although OrthoK modifies corneal biomechanics during treatment, the cornea typically returns to its original state after the treatment concludes. In the context of refractive surgery, corneal biomechanics is vital for preoperative screening, especially for early detection of keratoconus.
Postoperatively, it is recognized that refractive surgery alters corneal biomechanics, but there is ongoing debate about which surgical techniques are more or less invasive and how long it takes for the cornea to return to its baseline biomechanics, if at all. An important limitation in studying corneal biomechanics is the interaction between corneal tissue properties and IOP. For example, experiments with porcine eyes have demonstrated that varying IOP significantly affects the corneal response to stimuli, even when tissue properties remain unchanged. Additionally, scleral stiffness, or the "boundary condition," influences how the cornea reacts to stimuli, with stiffer or softer sclera altering the corneal response.
Clinically, corneal biomechanics is most commonly measured using air-puff tonometry, which involves an air puff that induces corneal movement while a camera records the response. Two primary clinical devices used to assess corneal biomechanics are the Ocular Response Analyzer (ORA) and the Corvis ST. The ORA measures a single point on the cornea and provides key parameters such as corneal hysteresis (CH) and corneal resistance factor (CRF), which reflect differences in corneal pressures and thickness. However, it does not offer imaging, limiting its analysis to one corneal point. In contrast, the Corvis ST employs a high-speed Scheimpflug camera to capture a sequence of 140 images of the cornea during deformation, allowing for the measurement of multiple parameters such as applanation length, deformation amplitude, peak-to-peak distance, and IOP, alongside pachymetry. This device provides a more comprehensive assessment of corneal biomechanics compared to the ORA.
Researchers have long recognized the significance of corneal biomechanics, and various alternatives to current techniques have been explored, though many are still not clinically available. For instance, placido disc indenters, which require physical contact with the cornea, present a significant limitation due to their invasive nature. In contrast, non-contact methods such as air puff tonometry are more widely used, though high-resolution corneal ultrasound is less practical due to its lengthy measurement process. Another advanced technique under investigation is Optical Coherence Elastography (OCE).
Unlike traditional methods that use strong air puffs, OCE employs gentle vibrations, akin to a mild massage, to assess corneal biomechanics, potentially making it less invasive and more patient-friendly. Despite its promise, OCE remains in the research phase and is not yet commercially available. Additionally, there is significant research into computational models of corneal biomechanics, which aim to predict individual responses to surgical interventions based on patient-specific topography. While this approach shows potential, it remains time-consuming and is not yet used in clinical settings. Artificial intelligence may expedite these computations in the future, offering hope for quicker and more efficient biomechanical assessments.
Another promising area is densitometry, which evaluates corneal tissue density without the need for mechanical stimulation. This method has shown potential for early keratoconus detection and assessment. Analyzing images from devices like the Pentacam, researchers have found that densitometry can provide valuable biomarkers even before visible changes in corneal shape occur. Despite its current limitations in clinical applications, densitometry offers a non-invasive alternative that could complement existing methods.
In summary, while corneal biomechanics holds significant promise for diagnosing and monitoring various conditions, including keratoconus and glaucoma, ongoing research and development are essential for integrating these advanced techniques into clinical practice. The future may see these innovative methods, particularly those leveraging AI and non-invasive imaging, becoming more widely available and enhancing patient care.
42nd Congress of the European Society of Cataract and Refractive Surgeons, 6 – 10 September 2024, Fira de Barcelona, Spain.
