The human brain is made up of functional regions governing movement, sensation, language, and cognition. Unintentional injury during neurosurgery can result in significant neurological deficits and morbidity. The current standard for localizing function to brain tissue during surgery, intraoperative electrical stimulation or recording, significantly increases the risk, time, and cost of the procedure. There is a need for a fast, cost-effective, and high-resolution intraoperative technique that can avoid damage to functional brain regions. We propose that optical coherence tomography (OCT) can fill this niche by imaging differences in the cellular composition and organization of functional brain areas. We hypothesized this would manifest as differences in the attenuation coefficient measured using OCT. Five functional regions (prefrontal, somatosensory, auditory, visual, and cerebellum) were imaged in ex vivo porcine brains (n=3), a model chosen due to a similar white/gray matter ratio as human brains. The attenuation coefficient was calculated using a depth-resolved model and quantitatively validated with Intralipid phantoms across a physiological range of attenuation coefficients (absolute difference < 0.1cm-1). Image analysis was performed on the attenuation coefficient images to derive quantitative endpoints. We observed a statistically significant difference among the median attenuation coefficients of these five regions (one-way ANOVA, p<0.05). Nissl-stained histology will be used to validate our results and correlate OCT-measured attenuation coefficients to neuronal density. Additional development and validation of OCT algorithms to discriminate brain regions are planned to improve the safety and efficacy of neurosurgical procedures such as biopsy, electrode placement, and tissue resection.
Previous research showed that mid-infrared free-electron lasers could reproducibly ablate soft tissue with little collateral damage. The potential for surgical applications motivated searches for alternative tabletop lasers providing thermally confined pulses in the 6- to-7-μm wavelength range with sufficient pulse energy, stability, and reliability. Here, we evaluate a prototype Raman-shifted alexandrite laser. We measure ablation thresholds, etch rates, and collateral damage in gelatin and cornea as a function of laser wavelength (6.09, 6.27, or 6.43 μm), pulse energy (up to 3 mJ/pulse), and spot diameter (100 to 600 μm). We find modest wavelength dependence for ablation thresholds and collateral damage, with the lowest thresholds and least damage for 6.09 μm. We find a strong spot-size dependence for all metrics. When the beam is tightly focused (∼100-μm diameter), ablation requires more energy, is highly variable and less efficient, and can yield large zones of mechanical damage (for pulse energies >1 mJ). When the beam is softly focused (∼300-μm diameter), ablation proceeded at surgically relevant etch rates, with reasonable reproducibility (5% to 12% within a single sample), and little collateral damage. With improvements in pulse-energy stability, this prototype laser may have significant potential for soft-tissue surgical applications.
Optical coherence tomography imaging is widely used in ophthalmology and optometry clinics for diagnosing retinal disorders. External microscope-mounted OCT operating room systems have imaged retinal changes immediately following surgical manipulations. However, the goal is to image critical surgical maneuvers in real time. External microscope-mounted OCT systems have some limitations with problems tracking constantly moving intraocular surgical instruments, and formation of absolute shadows by the metallic surgical instruments upon the underlying tissues of interest. An intraocular OCT-imaging probe was developed to resolve these problems. A disposable 25-gauge probe tip extended beyond the handpiece, with a 36-gauge needle welded to a disposable tip with its end extending an additional 3.5 mm. A sealed 0.35 mm diameter GRIN lens protected the fiber scanner and focused the scanning beam at a 3 to 4 mm distance. The OCT engine was a very high-resolution spectral-domain optical coherence tomography (SDOCT) system (870 nm, Bioptigen, Inc. Durham, NC) which produced 2000 A-scan lines per B-scan image at a frequency of 5 Hz with the fiber optic oscillations matched to this frequency. Real-time imaging of the needle tip as it touched infrared paper was performed. The B-scan OCT-needle was capable of real-time performance and imaging of the phantom material. In the future, the B-scan OCT-guided needle will be used to perform sub-retinal injections.
Optical coherence tomography (OCT) has had a tremendous global health impact upon the current ability to diagnose,
treat, and monitor multiple eye diseases. We propose that a miniature forward-imaging OCT probe can be developed for
real-time ocular imaging. A miniature 25-gauge forward-imaging probe was designed and developed to use with an
850 nm spectral-domain optical coherence tomography (SDOCT) system (Bioptigen, Inc. Durham, NC). Imaging
parameters were determined. Ocular tissues were examined with the miniature OCT probe. A miniature SDOCT probe
was developed with the scanning driver within the hand piece. The SDOCT fiber-scanning probe maximally transmitted
power of 800 μW. The scanning range was 3 mm when the probe tip was held 3 to 5 mm from the tissue surface. The
axial resolution was 6 μm and the lateral resolution was 30-35 μm. The 25-gauge forward-imaging probe was used to
image cellophane tape, eyelid skin, cornea, conjunctiva, sclera, iris, anterior lens, anterior chamber angle, retina, retinal
tear, retinal detachment, optic nerve head, and optic nerve sheath. Images obtained from the miniature probe appeared
similar to images from a 3 mm scanning range of a commercial large handheld OCT probe (Bioptigen, Inc. Durham,
NC).
Purpose: It is hypothesized that 6.1 μm produced by a portable laser would be useful for incising tissue layers such as performing a retinectomy in detached retina with extensive anterior proliferative vitreoretinopathy.
Methods: An alexandrite laser system, which provides a high-intensity Q-switched pulse (780 nm, 50-100 ns duration, 10 Hz), is
wavelength-shifted by a two-stage stimulated Raman conversion process into the 6-7 μm range (Light Age, Inc.). Fresh cadaver porcine retinas were lased with 6.1 μm using a 200 μm diameter spot at 0.6 mJ after removal of the vitreous. Specimens were examined grossly and prepared for histological examination.
Results: The Raman-shifted alexandrite laser produced a smooth Gaussian profile. A narrow spectrum was produced at 6.1 μm. A full-thickness retinal incision with minimal thermal damage was obtained at a low energy level of 0.6 mJ in the retinas. However, the depth of the incision did vary from an incomplete incision to a full-thickness incision involving the underlying choroidal layer in
attached retinas. Conclusions: The 6.1 μm mid-infrared energy produced by a portable laser is capable of incising
detached retinas with minimal thermal damage.
Purpose: Orbital tumors and pseudotumor cerebri are sometimes treated with surgical
approaches. Our previous studies suggest that potentially endoscopy may be useful for
minimally invasive orbital surgery. This study proposed to improve the approach
technique for accessing the posterior orbital space via endoscopy, as well as assess
visibility improvements with CO2 insufflation to posterior orbital tissues.
Methods: An inferior transconjunctival approach accessed the posterior orbital space in
non-survival pigs. Various guidance tubes were compared to assess ability to guide the
endoscope to the posterior orbit with the greatest ease and visibility. FEL energy
application (6.1 &mgr;m, 2.7 ± 0.5 mJ, 30 Hz, delivered via glass-hollow waveguide) was
attempted via endoscopy. The effect of CO2 gas insufflation was assessed by analyzing
visibility of the stuctures before and after CO2 application.
Results: The posterior orbit was accessed via endoscopy in all except the first attempted
eye. A beveled transparent butyrate tube provided the best guidance for the endoscope
and an opaque metal tube provided the worst guidance. The optic nerve was encountered
and FEL energy was applied with the butyrate tube in 8 orbits. Visibility was adequate
without CO2 insufflation, and did not improve with CO2.
Conclusions: The posterior orbit was successfully accessed using endoscopy. The optic
nerve was exposed and treated with FEL energy. CO2 insufflation did not further
enhance visibility in this study. Application of endoscopy for posterior orbital
procedures is feasible, but extreme surgical care is required and further study with human
cadaveric eyes is needed.
Our previous studies using rabbits and monkeys showed that the Amide II wavelength (6.45 micrometers ) produced by the FEL could efficiently produce an optic nerve sheath fenestration with minimal damage. In order to determine if the technology safely could be applied to human surgery, we used 2 blind human eyes during enucleation to compare the results of producing fenestrations with the FEL or a scissors. FDA and Vanderbilt IRB approvals, and individual patient consents were obtained. The FEL energy was transmitted to a human operating room. After disinsertion of the medial rectus muscle, an optic nerve sheath fenestration (2 mm diameter) was made with either the FEL (6.45 micrometers , 325 micrometers spot size, 30 Hz, 3 mJ) through a hollow waveguide surgical probe or with a scissors. The enucleation was then completed. The optic nerve was dissected from the globe and fixed. Specimens were examined histologically. Dural incisions were effective with both methods. FEL energy at 6.45 micrometers can be transmitted to an operating room and delivered to human ocular tissue through a hollow waveguide surgical probe. This FEL wavelength can produce an optic nerve sheath fenestration without acute direct damage to the nerve in this case report.
We have investigated the experimental consequences of two picosecond infrared lasers, both tuned to 6.45micrometers and focused on ocular tissue. The exposure conditions were comparable, other than pulse repetition rate, where an optical parametric oscillator/amplifier laser (OPA) system operates at a kilohertz and the Mark-III FEL at 3 gigahertz. In both cases, the peak intensity was near 2x1014 W/m2 and the total delivered energy was approximately 125 mJ. The Mark-III consistently ablates tissue, while the OPA fails to ablate or to damage corneal tissue. In particular, there is no experimental evidence for protein denaturation due to OPA irradiation. WE account for these observations in terms of a theoretical model based on thermal diffusion and threshold conditions for superheating and chemical kinetics. We comment on the relevance of tissue geometry.
One glaucoma challenge is the treatment of leaking trabeculectomy blebs. Simple methods such as patching, autologous blood injection, compression sutures or cyanoacrylate glue application often fail. Because the conjunctiva is thin and ischemic, it often can't be sutured together so major surgery is required to excise the thin tissue and advance healthy conjunctiva. We report the preliminary results of Tisseel and Tisseel treated with two wavelengths from Vanderbilt's free electron laser placed on leaking trabeculectomy bleb holes in Dutch belted rabbits. The holes were healed at one week in the sutured group and in the 7.7 micrometer FEL-treated Tisseel group. One hole was healed in the cyanoacrylate glue-treated group. Holes remained in the other treatment groups. Tisseel irradiated with 7.7 micrometer energy from the free electron laser may promote healing of trabeculectomy bleb holes.
Purpose: To determine if the free electron laser (FEL) energy can be delivered to a small space to perform optic nerve sheath fenestration with minimal acute nerve damage.
Methods: A 530 mm hollow waveguide probe was designed. Optic nerve sheath fenestration (1.0 mm diameter) was performed in 8 rabbits using either the FEL (4 eyes, 6.45mm, 10 Hz, 2 mJ) or a knife (4 eyes). Within 2 hours following surgery, the animals were perfused with aldehyde fixative. The integrity of the optic nerve and glial response at the site of fenestration were evaluated on tissue selections with H&E, and antibodies to S100β or GFAP.
Results: Surgery using the FEL probe was found to be technically superior to the knife. The glial reaction was limited to a zone adjacent to the fenestration and was similar in both the FEL and knife incisions.
Conclusions: The FEL appears capable of efficiently performing an optic nerve sheath fenestration in a small space with minimal acute damage. Both the FEL and knife incisions result in a rapid glial response at the site of fenestration even when optic nerve integrity is not compromised.
The Free Electron Loss (FEL) at Vanderbilt University is tunable form 2 μm to 9 μm in the mid-IR spectrum, which is capable of controlling predicted laser-tissue interaction by selecting a specific wavelength. However, delivery of this laser into the internal portion of the eye is difficult because of strong water absorption at this spectrum range and the high peak power of the FEL. We used a metallic coated hollow waveguide with a 530 μm inner diameter and 1 meter in length, and delivered the FEL beam to an autoclaved surgical probe. The prove tip was an 18 gauge canula with a mini CaF2 window fixed in front of it to protect the waveguide from contacting water. Human and animal cadaver eyes were used to perform an open sky retinal cutting procedure. The system was able to deliver 60 percent of FEL energy to the intraocular tissues. Up to 6 X 105w peak power was reached without damage to the waveguide or the surgical probe at the spectrum range of 2.94 μm to 7.7 μm. In conclusion, the hollow waveguide is suitable for delivering the IR FEL for intraocular microsurgical procedures.
Purpose: To determine if the Heidelberg Retinal Laser Tomograph (HRT) and Flowmeter (HRF) can be modified to obtain images in supine patients. Methods: A mount was customized to securely attach the Heidelberg scanning head to an operating microscope stand. This mount was designed to allow rotation for viewing of the macula or optic nerve head region in either eye. The HRT was used to acquire 3 consecutive images of the optic nerve head in supine subjects to obtain a mean topographic image. The HRF was then used to obtain capillary flow measurements in supine subjects. Results: The optic nerve area in either the right or left eye can be safely and easily visualized with the modified Heidelberg system in supine patients for evaluation of optic nerve head topography or capillary flow. However, the configuration of the Heidelberg scanning head requires the images to be taken 180 degrees from the normal orientation of the scanning head to the patient's body. Therefore, the images are rotated 180 degrees from those taken in upright subjects. This must be considered when analyzing the data. Conclusion: The Heidelberg Retinal Tomograph and Flowmeter are capable of acquiring images in supine subjects. However, these images are rotated 180 degrees.
Purpose: To study the healing process in cultured human corneas after Er:YAG laser ablation. Methods: Human cadaver corneas within 24 hours post mortem were ablated with a Q- switched Er:YAG laser at 2.94 micrometer wavelength. The radiant exposure was 500 mJ/cm2. The cornea was cultured on a tissue supporting frame immediately after the ablation. Culture media consisted of 92% minimum essential media, 8% fetal bovine serum, 0.125% HEPES buffer solution, 0.125% gentamicin, and 0.05% fungizone. The entire tissue frame and media container were kept in an incubator at 37 degrees Celsius and 5% CO2. Serial macroscopic photographs of the cultured corneas were taken during the healing process. Histology was performed after 30 days of culture. Results: A clear ablated crater into the stroma was observed immediately after the ablation. The thickness of thermal damage ranges between 1 and 25 micrometer. Haze development within the crater varies from the third day to the fourteenth day according to the depth and the roughness of the crater. Histologic sections of the cultured cornea showed complete re- epithelization of the lased area. Loose fibrous tissue is observed filling the ablated space beneath the epithelium. The endothelium appeared unaffected. Conclusions: The intensity and time of haze development appears dependent upon the depth of the ablation. Cultured human corneas may provide useful information regarding the healing process following laser ablation.
Trabeculectomy with early postoperative slit lamp laser suture lysis is a controlled means of maximizing bleb filtration and reducing intraocular pressure. However, this procedure is not possible in children and even some adults. Thus, an effective alternative method for postoperative laser suture lysis was investigated. Dissection of 15 scleral flaps was performed on two human cadaver eyes. Each flap was closed with two 10-0 nylon sutures and the conjunctiva repositioned to cover the sutures. Laser suture lysis was performed using an optical fiber probe for the Argon/Dye laser and a Hoskins laser lens. Five different wavelengths were studied: red (630 nm), orange (595 nm), yellow (577 nm), blue-green (488 - 514 nm), and green (514 nm). Each individual wavelength was studied using six scleral flap sutures, and a single laser application was applied to each suture. Suture lysis was attainable with each wavelength, however the argon green lysed 100 percent of the sutures. Histologic analysis demonstrated no conjunctival injury with any of the above wavelengths. These findings suggest that multiple wavelengths are effective in laser suture lysis without a slit lamp biomicroscope.
The purpose of this study is to investigate the healing process in cultured human cornea after infrared Free Electron Laser ablation. Fresh human cadaver cornea was ablated using the Free Electron Laser at the amide II band peak (6.45 micrometers ). The cornea was then cultured in an incubator for 18 days. Haze development within the ablated area was monitored during culture. Histologic sections of the cornea showed complete re-epithelialization of the lased area, and ablation of the underlying Bowman's layer and stroma. The endothelium appeared unaffected. Cultured human corneas may provide useful information regarding the healing process following laser ablation.
To determine if a method can be used with the Heidelberg Retinal Tomograph (HRT) to improve measurement of retinal vessel diameters. The HRT was used to acquire 3 consecutive images of the optic nerve head in 7 normal subjects to obtain a mean topographic image. The files were transferred to `CorelPHOTO-PAINT The diameters of vessels with well- defined edges were measured using the `line tool'. Each vessel diameter was measured 5 times, and an average and standard deviation were calculated. Fifty-five arterioles and 49 venules were measured from the 7 normal subjects using `CorelPHOTO-PAINT `interactivemeas' submenu. The standard deviations of the vessels ranged from 0 to 10 microns with the majority less than 5 microns in subjects using `CorelPHOTO-PAINT of the measurable vessels ranged from 2 to 17 microns with the majority greater than 5 microns. The HRT `interactivemeas' submenu permits distance measurements, but it does not enhance the image enough to clearly define the vessel edges. Enlargement and improvement of contrast reduces user error in measuring retinal vessel diameters of HRT topographic images.
The smoothness and accuracy of PMMA ablations with a prototype scanning photorefractive keratectomy (SPRK) system were evaluated by optical profilometry. A prototype frequency- quintupled Nd:YAG laser (Laser Harmonic, LaserSight, Orlando, FL) was used (wavelength: 213 nm, pulse duration: 15 ns, repetition rate: 10 Hz). The laser energy was delivered through two computer-controlled galvanometer scanners that were controlled with our own hardware and software. The system was programmed to create on a block of PMMA the ablations corresponding to the correction of 6 diopters of myopia with 60%, 70%, and 80% spot overlap. The energy was 1.25 mJ. After ablation, the topography of the samples was measured with an optical profilometer (UBM Messtechnik, Ettlingen, Germany). The ablation depth was 10 to 15 micrometer larger than expected. The surfaces created with 50% to 70% overlap exhibited large saw-tooth like variations, with a maximum peak to peak variation of approximately 20 micrometer. With 80% overlap, the rms roughness was 1.3 micrometer and the central flattening was 7 diopters. This study shows that scanning PRK can produce smooth and accurate ablations.
Purpose: To develop a simple suture lysis technique for post-trabeculectomy examinations under anesthesia since slit lamp laser suture lysis in the clinic cannot be performed on infants and young children. Methods: An argon endolaser probe lysed 10-0 nylon suture through conjunctiva harvested from human cadaver eyes. Since suture lysis failed with the thick Hoskins lens, clear plastic from the suture package compressed the conjunctiva. The conjunctiva was examined histologically. Results: Argon laser suture lysis (250 mW, 0.1 sec, 488 - 514 nm) was achieved without conjunctival damage. Conclusion: The argon endolaser probe is effective for suture lysis when the slit lamp cannot be used.
A free electron laser (FEL) may be tuned to novel wavelengths to explore laser-tissue interactions for development or improvement of laser surgical procedures. This study investigated the effect of selected infrared wavelengths upon human cornea and optic nerve tissues. Human cadaver eyes were placed in 10% dextran solution to normalize corneal thickness, and solution was injected intraocularly to achieve a physiologic intraocular pressure. The corneas and optic nerves were lased with the 6.0 micrometer amide I band, 6.1 micrometer water absorbency peak, 6.45 micrometer amide II band, and 7.7 micrometer. The Vanderbilt FEL produces 5 microsecond long macropulses at 10 Hz with each macropulse consisting of 1 ps micropulses at 3 GHz. Histologic examination of the corneal tissue showed the least amount of collateral damage (10 - 20 micrometers) with the 6.0 micrometer amide I band, while marked shrinkage occurred with the 7.7 micrometer wavelength. For optic nerve tissue, the least amount of collateral damage (0 micrometer visible) occurred at 6.1 micrometer water absorbency peak and 6.45 micrometer amide II band, while the most damage (30 - 50 micrometers) was observed with the 7.7 micrometer wavelength. We conclude that different tissues may have different optimal wavelengths for surgical laser procedures.
A method for calculating pulse distribution maps for scanning laser corneal surgery is presented. The accuracy, the smoothness of the corneal shape, and the duration of surgery were evaluated for corrections of myopia by using computer simulations. The accuracy and the number of pulses were computed as a function of the beam diameter, the diameter of the treatment zone, and the amount of attempted flattening. The ablation is smooth when the spot overlap is 80% or more. The accuracy does not depend on the beam diameter or on the diameter of the ablation zone when the ablation zone is larger than 5 mm. With an overlap of 80% and an ablation zone larger than 5 mm, the error is 5% of the attempted flattening, and 610 pulses are needed per Diopter of correction with a beam diameter of 1 mm. Pulse maps for the correction of astigmatism were computed and evaluated. The simulations show that with 60% overlap, a beam diameter of 1 mm, and a 5 mm treatment zone, 6 D of astigmatism can be corrected with an accuracy better than 1.8 D. This study shows that smooth and accurate ablations can be produced with a scanning spot.
An endoscope allows visualization of the anterior chamber angle in porcine eyes despite the presence of cloudy corneas. The pectinate ligaments in the anterior chamber angle are a surgical model for primary infantile glaucoma. This study investigated the histologic results, one month after treating the anterior chamber angle with a goniotomy needle, the holmium:YAG laser, or the erbium:YAG laser coupled to a small endoscope. The anterior chambers were deepened with a viscoelastic material in one-month-old anesthetized pigs. An Olympus 0.8 mm diameter flexible endoscope was externally coupled to a 23 gauge needle or a 300 micron diameter fiber. The angle was treated for 120 degrees by one of the three methods, and the probe was removed. During the acute study, all three methods cut the pectinate ligaments. The histologic findings one month after healing demonstrated minimal surrounding tissue damage following goniotomy with a needle and the most surrounding tissue damage following treatment with the holmium:YAG laser.
A scanning beam of nano-second pulses at 213 nm flattens the cornea as predicted. However, there is a considerable variability in the flattening and the ablation is not safe. Ablation for 16 D flattening with an active spot overlap of 50% induced 8.9 +/- 5.3 D (n equals 7) as measured by the TMS topography system (ring 7 average) and 5.8 +/- 4.1 D (n equals 5) as measured with the SK-1 (2 mm zone) system. Ablation for 6 D flattening with an active spot overlap of 70% induced approximately 2 D flattening to 2 D steepening (n equals 3) as measured with the TMS (ring 7) and 6.6 +/- 4.33 D (n equals 7) flattening as measured by SK-1 (2 mm zone) keratometry. There was no change in IOP at 12 weeks after as compared to before ablation. There was a net increase of central and peripheral corneal thickness at 12 weeks after the ablation as compared to preoperatively. Epithelial defects remained up to 4 weeks after ablation. After four weeks, vessels had invaded the cornea in 30% of the cases and remained throughout the three months observed. It is concluded that 213 nm nano-second pulses can be used for flattening the cornea but the system should not be used for clinical trials in humans until the adverse effects can be avoided.
Pilot studies for laser scleral buckling made it clear that quantification of scleral shrinkage was required for precision and reproducibility of the treatment. For the quantification either the Ho:YAG (2.10 micrometers ) or the Tm:YAG (2.01 micrometers ) lasers were applied to the equatorial sclera of human cadaver eyes. Two slightly overlapping spots (2.8 mm (phi) ) were applied. Shrinkage rate was expressed as: [(Scleral length before treatment--Schleral length after treatment)/Schleral length before treatment] X 100(%). Shrinkage rate was measured changing several parameters. Total fluence, energy/pulse, scleral thickness, tissue temperature, age, and intraocular pressure. Shrinkage rate was found to be mainly function of total fluence attaining a maximum of 26 - 30% in adult and 46% in infant eyes at a 3 - 4 mm Hg intraocular pressure. Rising tissue temperature from room temperature to physiologic levels reduced the laser energy requirements but not the maximum shrinkage level. From the same shrinkage effect in the practical range of total fluence, less energy (56 - 60%) was required with the Tm:YAG laser. The data acquired in this study will help us construct an algorithm to predict the outcome of laser scleral buckling in patients.
The cloudy cornea in primary infantile glaucoma may contraindicate a goniotomy if a view of the anterior chamber angle is not obtainable through the operating microscope. The purpose of this study was to determine if a gradient-index (GRIN) endoscope was capable of improving the imaging of the anterior chamber angle structures in vitro. The anterior chambers of cadaver porcine eyes were deepened with a viscoelastic material. The GRIN lens endoscope entered the anterior chamber through a corneal incision near the limbus. The anterior chamber structures including the pectinate ligaments in the angle were clearly visualized on a video screen as the endoscope was advanced into the anterior chamber angle. The amount of illumination was automatically controlled. A low OH silica fiber having a core diameter of 100 microns and delivering 10 mJ of Holmium:YAG energy was inserted into the endoscope. Due to the low amount of energy, the pectinate ligaments were inconsistently cut. The specimens were examined with a dissecting microscope, fixed in formalin, and processed for light microscopy. A thin endoscope allows intraoperative examination of anterior segment structures when a cloudy cornea is present. Therefore, it may become a useful instrument for performing a goniotomy in primary infantile glaucoma when direct visualization of the anterior chamber angle is not possible with a goniotomy lens.
A scanning pulsed beam at 213 nm creates predictable correction of myopia but the surgical procedure with the presently used laser system is not yet sufficiently safe to proceed to human clinical trials. It was found that a 70% overlap of approximate active spot size provides a refractive change as predicted 1 week after ablation. There was a minimal transient increase of IOP shortly after the ablation. There was an immediate swelling of the cornea after the ablation that gradually decreased during the following 12 weeks. The corneal epithelium did not heal until around 4 - 7 weeks after the ablation. The epithelium stained with fluorescein late after the ablation. When considerable haze was seen it decreased towards the end of the 12 weeks of observation. Approximately 30% of ablated corneas showed vessel ingrowth at 12 weeks after ablation. The 213 nm beam was obtained by frequency quintupling of the output of a Nd-Yag laser. The beam was moved with an X-Y scanner.
It was found that ablation at radiant exposures below 1 kJ/m2 both for polymethylmetacrylate (PMMA) and for the human cornea is minimal. Above 1 kJ/m2 the ablation rate increased approximately linearly with radiant exposure, the increase being twice as quick for cornea [0.26 (micrometers /pulse)/(kJ/m2)] as for PMMA [0.13 (micrometers /pulse)/(kJ/m2)]. Laminae of PMMA and human cornea of known thickness were ablated with nano second pulses at 213 nm. It is anticipated that the current data will serve as a basis for design of ablation procedures in the cornea.
The optimal values of the parameters of a scanning system for laser corneal surgery are determined by using a computer model. The correction of myopia is simulated. The accuracy of the correction, the smoothness of the corneal surface after ablation, and the time needed for surgery are computed. With a beam diameter of 0.5 mm, an overlap of 60% to 80%, and a 5 mm diameter treatment zone, the simulated error is less than 0.1 Diopter for a correction of 6 Diopters or less; the roughness is less than 7% of the central ablation depth; the number of pulses per Diopter of correction is 2500 if the beam intensity distribution is Gaussian and 580 if the beam intensity distribution is uniform.
Goniotomy is an effective treatment for primary infantile glaucoma. Because a cloudy cornea may prevent a clear view of the anterior chamber angle through the operating microscope, we investigated whether an endoscope can be combined with a cutting laser to perform laser goniotomy in a surgical model of primary infantile glaucoma. The anterior chambers of cadaver procine eyes were deepened with a viscoelastic material. A 300-micron-diameter silica fiber coupled to an Olympus 0.8-mm-diameter flexible fiber optic endoscope entered the anterior chambers through 4-mm corneal incisions. The anterior chamber angles were clearly observed on a videoscreen as the endoscopic fiber optic laser scalpel approached the pectinate ligaments. With the guidance of a He-Ne aiming beam, the anterior chamber angle pectinate ligaments were cut over a 160 degree arc with a pulsed Ho:YAG laser (2.1 micrometers wavelength, 50 mJ, 5 Hz repetition). The specimens were fixed in glutaraldehyde and processed for scanning electron microscopy, or fixed in formalin and processed for light microscopy. The treated area demonstrated incision of the pectinate ligaments with opening of the underlying trabecular meshwork. The edoscopic fiber optic laser scalpel is capable of cutting the pectinate ligaments in a surgical model of primary infantile glaucoma. Therefore, it may be a useful instrument for performing goniotomy when a cloudy cornea in primary infantile glaucoma prevents visualization of the anterior chamber angle with a goniotomy lens.
We investigate the laser refractive scleroplasty (LRS) as a potential minimal-invasive method for correcting post-operative astigmatism. The scleral shrinkage near limbus was induced on 6 cadaver eyes using a 200 micrometers fiber optic probe coupled to a pulsed Ho:YAG laser. The diameter of the treatment spot was 0.8 mm. The output energy measured at tip was 60.2+/- 0.6 mJ. The treatments consisted of multiple sector patterns placed along the major axis of astigmatism parallel to the limbus, and round patterns placed along the limbus. Three treatment spots were applied on each side of the sector. The separation among sectors and limbus is 1 mm. Keratometry and topography of the cornea were measured after each sector or round pattern treatment. Effect of 5 and 10 pulses at each treatment spot were compared. Histology was performed to evaluate laser tissue damage. The major axis of astigmatism was shifted 90 degrees after the sector pattern treatment and amount of dioptric change increased when adding a new treatment or using more treatment pulses. However, the spherical equivalent of the eyes was essentially unchanged. The keratometry of the corneas remained the same after the round pattern treatment. Laser refractive scleroplasty may be applied for the correction of post-operative astigmatism.
Scleral indentation was induced in cadaver eyes by shrinkage of scleral collagen fibers using a pulsed solid state Ho:YAG (2.1 micrometers ) laser with fiber optic delivery. Applying Ho:YAG laser radiation permits control of the amount of laser induced buckling effect by selecting laser treatment parameters such as beam spot, radiant exposure, and number of pulses. With treatment using 11.3 +/- 1.2 J/cm2 laser radiant exposure and 5 pulses, laser induced scleral shrinkage affected only the external two-thirds of scleral tissue. No thermal damage or disruption was observed in subjacent retinal pigment epithelium, chorioid, or retina. Coupling of two appropriately selected lasers may allow laser induced scleral buckling and transscleral retinal photocoagulation using the same laser probe for retinal reattachment surgery.
Replacing the gas ArF (193 nm) excimer laser with a solid state laser source in the far-UV spectrum region would eliminate the hazards of a gas laser and would reduce its size which is desirable for photo-refractive keratectomy (PRK). In this study, we investigated corneal reshaping using a frequency-quintupled (213 nm) pulsed (10 ns) Nd:YAG laser coupled to a computer-controlled optical scanning delivery system. Corneal topographic measurements showed myopic corrections ranging from 2.3 to 6.1 diopters. Post-operative examination with the slit-lamp and operating microscope demonstrated a smoothly ablated surface without corneal haze. Histological results showed a smoothly sloping surface without recognizable steps. The surface quality and cellular effects were similar to that of previously described excimer PRK. Our study demonstrated that a UV solid state laser coupled to an optical scanning delivery system is capable of reshaping the corneal surface with the advantage of producing customized, aspheric corrections without corneal haze which may improve the quality of vision following PRK.
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