Emmanuel Lacombe

Synthetic cornea: biocompatibility and optics

Purpose. Experimentally find a method to provide a safe surgical technique and an inexpensive and long lasting mesoplant for the restoration of vision in patients with bilateral corneal blindness due to ocular surface and stromal diseases. Methods. Identify the least invasive and the safest surgical technique for synthetic cornea implantation. Identify the most compatible biomaterials and the optimal shape a synthetic cornea must have to last a long time when implanted in vivo. Results. Penetrating procedures were deemed too invasive, time consuming, difficult and prone to long term complications. Therefore a non-penetrating delamination technique with central trephination was developed to preserve the integrity of Descemets membrane and the anterior segment. Even though this approach limits the number of indications, it is acceptable since the majority of patients only have opacities in the stroma. The prosthesis was designed to fit in the removed tissue plane with its skirt fitted under the delaminated stroma. To improve retention, the trephination wall was made conical with the smallest opening on the anterior surface and a hat-shaped mesoplant was made to fit. The skirt was perforated in its perimeter to allow passage of nutrients and tissues ingrowths. To simplify the fabrication procedure, the haptic and optic were made of the same polymer. The intrastromal biocompatibility of several hydrogels was found superior to current clinically used PMMA and PTFE materials. Monobloc mesoplants made of 4 different materials were implanted in rabbits and followed weekly until extrusion occurred. Some remained optically clear allowing for fundus photography. Conclusions. Hydrogel synthetic corneas can be made to survive for periods longer than 1 year. ArF excimer laser photoablation studies are needed to determine the refractive correction potential of these mesoplants. A pilot FDA clinical trial is needed to assess the mesoplant efficacy and very long-term stability.

Assessment of changes in corneal shape as a function of intraocular pressure: a pilot study

Purpose: To evaluate the effect of increased intraocular pressure (IOP) on the corneal shape of human cadaver eyes. Methods: Eight cadaver eyes, unsuitable for transplantation donated by the Florida Lions Eye Bank, were assessed for overall integrity. The epithelium was debrided, the eye placed into an artificial orbit, and a 30-gauge needle (connected to the IOP monitor) was inserted into the vitreous cavity. The IOP was altered as necessary by adjusting the height of the lactated ringers IV bottle. The surface contour was assessed at pressure levels: hypotony (2-4 mmHG), normal physiologic IOP (12-20 mmHg), hypertony (135-142 mmHg) and again at normal and low IOP. At each pressure level, two corneal topographic measurements (PAR CTSTM Vision Systems Corp.) were captured and averaged. Keratometric analysis was completed to examine the dioptric effects of varying IOP. An elevation analysis was performed to determine the corneal locations which conformed to the pressure adjustments. Results: The keratometric and elevation (both 0 and 90 degree meridians) data revealed decreasing radii with increasing IOP however, variability precluded statistical significance. Both keratometric and elevation data displayed probable plastic deformation, as the radii deviated from the original measurement upon the return to 2-4 mmHg. The elevation analysis did suggest an astigmatic conformation to pressure fluctuations, as 90 degree meridian radii were greater than 0 degree meridian radii. Corneal deformation is minimal in the 2 to 140 mmHg range and the PAR system not sensitive enough to accurately determine changes in curvature. Conclusion: The cornea does not uniformly conform to IOP variation. Further investigations with IOP levels of up to 500 mmHg will provide more information with respect to changes in curvature as a function of IOP.

Surface-modified porous PTFE for keratoprostheses: Biocolonization in the rabbit cornea

The purpose is to assess intracorneal biocolonization of expanded polytetrafluoroethylene materials covalently coated with collagen type-I.

Design improvement in keratoprosthesis

One of the major problems of keratoprosthesis (KPro) is postoperative expulsion resulting from inadequate haptic fixation. To overcome this obstacle, a new design was conceived using a retrocorneal fixation. An earlier version of our KPro was made of two pieces: an optical element threaded in a surrounding haptic. Both pieces were made of medical grade PMMA. Although encouraging results were obtained, some anatomic failures occurred (20% at 5 years). To improve fixation and sealing, a biocolonizable skirt was recently added to the keratoprosthesis. This skirt should prevent migration of biocontaminants inside the eye. Made of porous PTFE, the skirt was ultrasonically held inside a one-piece PMMA keratoprosthesis by ultrasonic microwelding. Thus far, implantations carried out in 8 patients show a good tolerance.

ArF photorefractive correction of keratoprostheses

Keratoprostheses (KPro) are optical elements replacing a cornea that is opacified and which cannot be replaced by transplantation. This occurs, for example, after severe burn or in trachoma. The KPro that consists of an optical cylinder mounted on a supporting plate (haptics) is implanted approximately in the middle of the cornea. It is intended to restore vision, i.e., should give a sufficient visual acuity and a comfortable field of view. Failures are due to rejection (expulsion) and optical decentration of the prosthesis. Postoperative corrections of the refractive power are possible with spectacles. However, in view of the rapid expansion of corneal reshaping with the ArF excimer laser emitting at 193 nm, a direct correction of the anterior surface of the KPro can be envisaged. For that purpose, various designs are analyzed in terms of image magnification and field of view. The possibility of a postoperative adaptation of the keratoprostheses by photoablation with the ArF is then presented.