Radiofrequency Heating of the Cornea: An Engineering Review of Electrodes and Applicators

This paper reviews the different applicators and electrodes employed to create localized heating in the cornea by means of the application of radiofrequency (RF) currents. Thermokeratoplasty (TKP) is probably the best known of these techniques and is based on the principle that heating corneal tissue (particularly the central part of the corneal tissue, i.e. the central stroma) causes collagen to shrink, and hence changes the corneal curvature. Firstly, we point out that TKP techniques are a complex challenge from the engineering point of view, due to the fact that it is necessary to create very localized heating in a precise location (central stroma), within a narrow temperature range (from 58 to 76ºC). Secondly, we describe the different applicator designs (i.e. RF electrodes) proposed and tested to date. This review is planned from a technical point of view, i.e. the technical developments are classified and described taking into consideration technical criteria, such as energy delivery mode (monopolar versus bipolar), thermal conditions (dry versus cooled electrodes), lesion pattern (focal versus circular lesions), and application placement (surface versus intrastromal)

Microwave treatment of the cornea leads to localised disruption of the extracellular matrix

Microwave keratoplasty is a thermo-refractive surgical procedure that can correct myopia (short-sightedness) and pathologic corneal steepening by using microwave energy to cause localised shrinkage around an annulus of the cornea leading to its flattening and vision correction. The effects on the corneal extracellular matrix, however, have not yet been evaluated, thus the current study to assess post-procedure ultrastructural changes in an in-vivo rabbit model. To achieve this a series of small-angle x-ray scattering (SAXS) experiments were carried out across whole transects of treated and untreated rabbit corneas at 0.25 mm intervals, which indicated no significant change in collagen intra-fibrillar parameters (i.e. collagen fibril diameter or axial D-period), whereas inter-fibrillar measures (i.e. fibril spacing and the degree of spatial order) were markedly altered in microwave-treated regions of the cornea. These structural matrix alterations in microwave-treated corneas have predicted implications for corneal biomechanical strength and tissue transparency, and, we contend, potentially render microwave-treated corneas resistant to surgical stabilization using corneal cross-linking procedures currently employed to combat refractive error caused by corneal steepening

Ring electrode for radio-frequency heating of the cornea: modelling and in vitro experiments

[EN] Radio-frequency thermokeratoplasty (RF-TKP) is a technique used to reshape the cornea curvature by means of thermal lesions using radio-frequency currents. This curvature change allows refractive disorders such as hyperopia to be corrected. A new electrode with ring geometry is proposed for RF-TKP. It was designed to create a single thermal lesion with a full-circle shape. Finite element models were developed, and the temperature distributions in the cornea were analysed for different ring electrode characteristics. The computer results indicated that the maximum temperature in the cornea was located in the vicinity of the ring electrode outer perimeter, and that the lesions had a semi-torus shape. The results also indicated that the electrode thickness, electrode radius and electrode thermal conductivity had a significant influence on the temperature distributions. In addition, in vitro experiments were performed on rabbit eyes. At 5 IN power the lesions were fully circular. Some lesions showed non-uniform characteristics along their circular path. Lesion depth depended on heating duration (60% of corneal thickness for 20s, and 30% for 10s). The results suggest that the critical shrinkage temperature (55-63degreesC) was reached at the central stroma and along the entire circular path in all the cases.Berjano, E.; Saiz Rodríguez, FJ.; Alió, J.; Ferrero, JM. (2003). Ring electrode for radio-frequency heating of the cornea: modelling and in vitro experiments. Medical & Biological Engineering & Computing. 41(6):630-639. https://doi.org/10.1007/BF02349970S630639416Alió, J. L., Ismail, M. M., Artola, A., andPérez-Santonja, J. J. (1997a): ‘Correction of hyperopia induced by photorefractive keratectomy using non-contact Ho: YAG laser thermal keratoplasty’,J. Refract. Surg.,13, pp. 13–16Alió, J. L., Ismail, M. M., andSanchez, J. L. (1997b): ‘Correction of hyperopia with non-contact Ho: YAG laser thermal keratoplasty’,J. Refract. Surg.,13, pp. 17–22Alió, J. 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Understanding the structural basis of corneal refractive function and its modification via novel therapeutic approaches

The studies comprising this thesis were conducted to further understand how structural changes to the corneal extracellular matrix can affect the cornea’s unique properties, with the ultimate goal of improving novel treatments and their outcomes. Following refractive surgery, changes in matrix structure can cause loss of structural integrity and transparency, which may adversely affect the surgical outcome. The first objective was to define what governs corneal shape and ultrastructural organisation by analysing the abnormal post-hatch corneal collagen arrangement in an avian model (beg). Structural information was also obtained post-in vivo microwave keratoplasty to assess the treatment as a suitable alternative to more invasive correction procedures. In addition, modifying the resident cell type as a means of improving post-treatment wound healing following LASIK was also investigated using corneal stromal stem cells. Finally, the efficiency of transepithelial riboflavin/UVA collagen cross-linking in terms of riboflavin uptake and post-treatment corneal stiffness, was evaluated. X-ray scattering studies revealed that corneal flattening in beg chickens is related to biomechanical changes brought about by an alteration in collagen arrangement at the corneal periphery. This highlights the importance of the limbal fibril annulus in corneal shape preservation. X-ray studies also revealed that microwave keratoplasty may impact on peripheral vision by introducing spatial disruption of stromal collagen, resulting in localised corneal opacity in the treatment area. Loss of fibrillar structure and order could also have further implications for corneal biomechanics and shape. The application of human corneal stromal stem cells under LASIK-like flaps was revealed to be a promising approach for increasing flap adherence strength whilst maintaining corneal clarity. Introducing these cells in the early stages of flap-healing appears to improve the repair process, conceivably through an embryonic-like mechanism. Lastly, stress-strain and riboflavin uptake results for transepithelial riboflavin delivery during corneal cross-linking were encouraging, however refinements to the testing procedure are required to fully assess the treatment efficacy. Collectively these findings accentuate the importance of the precise stromal collagen fibril arrangement and composition for maintaining corneal transparency, shape and general functionality. All these factors must be taken into consideration when implementing novel correction procedures or modifying existing treatments for corneal defects