<title>Laser thermokeratoplasty by means of a continuously emitting laser diode in the mid IR</title>

Abstract

Laser thermokeratoplasty (LTK) has been performed with a continuously emitting, tunable laser diode at 1.86 jim. A study on enucleated porcine eyes was conducted in order to demonstrate the feasibility of this laser source for LTK and to determine the useful irradiation modalities. Refractive changes achieved with different application systems and a standard coagulation pattern, consisting of 8 coagulation spots on a 6 mm ring, were measured. The most promising sets of parameters were carrried out in a first animal study with mini pigs. Initial refractive changes up to 6 D could be achieved in vitro and in vivo with laser powers between 120 mW and 200 mW and irradiation times of several seconds. In conclusion, the mid-JR laser diode operated at a wavelength of 1.86 tm seems to be the optimal source for a clinical LTK system. key words: coagulation, collagen shrinkage, cornea, denaturation, hyperopia, refractive change, temperature calculation 1 . INTRODUCTIONLaser thermokeratoplasty (LTK) is a minimally invasive treatment for hyperopia and astigmatism. The increase in refractive power is achieved by thermally induced shrinkage of stromal collagen in the corneal periphery'3. For a spherical correction, typically 8 coagulations are equidistantely applied around the corneal center with a diameter between 5 and 8 mm. The resulting zone of annular stress is believed to increase the central corneal curvature. In order to treat hyperopic astigmatisms asymmetric patterns concentrated on the lower hyperopic, flat meridian can be used4.A variety of different laser sources and application systems have been investigated for LTK. Early attempts have been made with the Er:Glass laser (X=1.54 jim)', but also CO2 lasers (?.=9.6-10.6 jim)2 have been used. Currently the pulsed holmium laser system (A=2. 1 jim) is favorized due to the fact, that its wavelength has a penetration depth in the order of the corneal thickness, which allows coagulations deep into the corneal stroma. Two different holmium based laser systems are under phase II FDA approval:The first system (Sunrise Techn. Inc.) simultaneously applies the desired number of coagulations on the cornea in a non contact mode via slit lamp using collinear beams of about 600 tm in diameter. Mild hyperopic corrections of about +1 D were reported in the long term follow up of two years5. The advantage of this system, coupled to a slit lamp, is its non contact and very precise application of all coagulation at one time.The second system (Summit Techn. Inc.) uses a handpiece with a lens at the distal fiber tip to focus the radiation into the corneal stroma3'4'6. With this device deeper coagulations and a more homogeneous temperature distribution can be achieved. Patients treated here show a refractive change of +2-3 D change in the long term of one to two years6. For both systems a strong regression within the first 3 months post-op and a slower gradual regression in the following 6-18 months was reported6. 66 /SPIE Vol. 2930 0-8194-2332-7/96/$6.0Q Downloaded From: http://proceeding...

A boundary element model of the human eye undergoing laser thermokeratoplasty

“…In the continuous wave laser treatment, laser diodes which exhibit a wider range of wavelength due to its tunability are used [8]. Typical values of the laser wavelength may lie between 1.85 and 1.87µm [9].…”

“…According to Brinkmann et al [8], when using continuous wave laser, the cornea must be heated to a minimum temperature of 85 o C in order to produce maximum amount of corneal shrinkage. Our model however, only predicted a maximum temperature of approximately 75 o C at (r, z) = (0, 0).…”

“…This model was subsequently used by Brinkmann and co-workers to investigate the thermal responses of the human cornea during L-TKP under various conditions [7], [8], [9], [10]. In [11] and [12], the cornea was modeled as a rectangular strip of finite thickness.…”

Mathematical modeling of ultrashort pulsed laser irradiation in the cornea

“…Brinkmann et al, [5] also developed a cylindrical model and solve it using the finite difference method. This model has been used by Brinkmann and his co-workers to understand the thermal responses of the human cornea during LTK treatment under various conditions [6][7][8][9]. The cornea was modeled as a rectangular strip of finite thickness by [10].…”