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Biomedical subjects

J O Hjortdal

Publications and source records attributed to J O Hjortdal.

At least 19 recordsLinked to original sources
PubMed2000

[Surgery for nearsightedness].

Myopia can today be reduced or eliminated by refractive surgery. Excimer laser surgery of the cornea by surface sculpturing (photorefractive keratectomy) or intrastromal tissue removal (LASIK) are the most widely used techniques, although implantation of intra corneal ring segments for low myopia also appears promising. Treatment of high myopia (> 10 diopters) is still difficult although epikeratoplasty or phakic IOL implantation are present possibilities. The perfect surgery for myopia remains to be developed, but the existing techniques will without doubt be further optimised. In 10 years time, supra normal visual acuity may even be obtained when surgical, optical, and biological variables can be described and controlled in each individual undergoing refractive surgery.

Animals↗
PubMed2000

Comparison of endothelial cell density estimated by contact and non-contact specular microscopy.

PURPOSE: To compare a contact and a non-contact specular microscope in the determination of endothelial cell density. SUBJECTS AND METHODS: One hundred and twenty-one eyes from 70 patients who had undergone various degrees of photorefractive keratectomy for myopia were included. The endothelium was imaged by contact (Konan Clinical Specular Microscope) and non-contact (Topcon SP-1000) specular microscopy and the endothelial cell density estimated. RESULTS: The average endothelial cell density achieved by the contact specular microscope was 3011+/-298 cells/mm2 (mean+/-SD, n=121) and by the non-contact specular microscope 3015+/-265 cells/mm2 (n= 121). The difference in endothelial cell density between the contact and the non-contact specular microscope (contact minus non-contact) was -4+/-175 cells/mm2 (t=0.26, 2p>0.05 in a paired t-test). The sampling error on the estimated endothelial cell density was 76 cells/mm2 for the contact specular microscope and 74 cells/mm2 for the non-contact specular microscope. CONCLUSION: The average endothelial cell density and the precision of the measuring technique were similar for the contact and the non-contact specular microscope. Furthermore, the endothelial cell densities estimated by the two instruments at various values of anterior central corneal refractive power and central corneal thickness were similar. The two instruments can be used interchangeably.

Adult↗
PubMed2000

Refractive results of radial keratotomy: a ten-year retrospective study.

PURPOSE: To investigate the long-term effects and stability of refraction after radial keratotomy procedure. METHODS: Radial keratotomy was performed on 123 persons to reduce myopia (range: -1 to -13 diopters) in 1986 to 1989. A mean of 11.5 years later (range 10 to 13), 61 of these patients with 102 eyes underwent a standardised refractive examination where subjective spherical equivalent refraction was measured and compared to the preoperative and the one month postoperative refractive measurement collected from the patients records. RESULTS: There was a reduction in spherical equivalent from an average of -5.46 diopters (SD 2.38) preoperatively to -2.32 diopters (SD 1.96) 11.5 years postoperatively. The mean change in direction of myopia between 1 month and 11.5 years postoperatively was 0.17 diopters (SD 1.18). This change was not statistically significant. From 1 month to 11.5 years, 10 of the eyes had developed more than 1 diopter hyperopia, and 20% more than 1 diopter myopia. When asked directly, all patients were satisfied with the result of their operation in general; 2 patients still complained of glare. CONCLUSION: No significant changes in refraction were found between 1 month and 11.5 years after radial keratotomy. Previously reported long-term studies on this field have found a trend toward progressive hyperopia. No evidence of such change can be supported by this study.

Adolescent↗
PubMed2000

Stability of graft refractive power after penetrating keratoplasty.

PURPOSE: Patients needing penetrating keratoplasty (PK) and cataract extraction with intraocular lens (IOL) implantation may be handled with a single triple procedure or a two-stage procedure with initial keratoplasty and cataract surgery in a later session. The latter approach is considered more safe by some surgeons and allows adjustment of the IOL power to the power of the actual corneal graft. The purpose of this study was to estimate the optimal timing of cataract surgery with IOL implantation by studying the refractive stability of 8 mm penetrating keratoplasty grafts. METHODS: Penetrating keratoplasty (8.0 mm graft and recipient bed) was performed in 28 eyes of 28 patients. Corneal topography (TMS-1) was studied at 1, 2, 3, 6, and 12 months after surgery and after suture removal (30 months). The central spherical equivalent graft power was computed from the topographical data (rings 2 through 4). RESULTS: On average, the spherical equivalent graft power was stable from one month after surgery up to suture removal (range: 41.9 to 42.7 diopters). After suture removal the graft steepened slightly (0.7 diopters). Corneal refractive power of single grafts fluctuated considerably over time. The standard deviation on time-dependent changes in graft power was from 3 months efter PK smaller than the standard deviation on the graft powers at 12 months. CONCLUSION: The average central spherical equivalent power of an 8.0 mm donor graft in an 8.0 mm recipient bed was stable from one month after penetrating keratoplasty until suture removal. A two-stage procedure with cataract surgery performed 3 months after PK can, compared to the triple procedure, reduce postoperative ametropia at 12 months if graft topography is taken into consideration at the time of cataract surgery. We recommend that cataract surgery with IOL implantation takes place from 3 months after penetrating keratoplasty.

Adult↗
PubMed1999

Corneal transplantation with donor tissue kept in organ culture for 7 weeks.

PURPOSE: To study the fate of corneal grafts after extended organ culture (7 weeks). METHODS: Six patients with symmetrical eye diseases were grafted bilaterally, in one eye with a cornea prepared by routine organ culture (mean 16 days), in the other eye with a donor cornea kept for 7 weeks (mean 49 days) in organ culture. The outcome was evaluated by biomicroscopy, graft thickness, endothelial cell density and visual performance after an observation time of at least 1 year. RESULTS: Penetrating 7-8 mm grafting was uncomplicated in all cases. The endothelial densities were in both groups in the range 1000-2000 cells/mm2, and visual acuity 0.2-0.9 in cases with no other ocular pathology. Postoperative graft thickness and deswelling did not differ between 2- and 7-week cultured corneas. At final examination the thicknesses were 0.50 mm and 0.49 mm for 2- and 7-weeks cultured corneas. CONCLUSION: Seven-week cultured corneas give clinical results comparable to those obtained using shorter culture periods. An extended culture period may be used to improve other qualities of the graft (compatibility, cell number, cell metabolism) and microbiological control.

Adult↗
PubMed1999

Magnification changes in specular microscopy after corneal refractive surgery.

PURPOSE: To describe the effect of corneal refractive surgery on the magnification of a contact and a non-contact specular microscope. METHOD: The magnification of a contact specular microscope (Konan Clinical Specular Microscope) and a non-contact specular microscope (Topcon SP-1000) was experimentally and theoretically studied as a function of anterior corneal refractive power and central corneal thickness. RESULTS: The magnification of the contact and non-contact specular microscope was found to decrease slightly with decreasing central corneal thickness. In addition, the magnification of the non-contact specular microscope decreased slightly with decreasing anterior corneal refractive power. CONCLUSION: As the preoperative and postoperative measuring conditions are different in patients undergoing corneal refractive surgery a correction for magnification changes is necessary when small changes in endothelial cell density are looked for.

Cell Count↗
PubMed1999

Bivariate analysis of surgically induced regular astigmatism. Mathematical analysis and graphical display.

OBJECTIVE: The purpose of the study was to develop methods for simultaneous description of astigmatic direction and magnitude on aggregate data, with special reference to refractive surgery. DESIGN: Mathematical analysis of astigmatisms employing bivariate statistical methods. RESULTS: The mean of several astigmatisms is a new astigmatism of specific direction and magnitude, while the confidence region is an area, which may be determined exactly. CONCLUSIONS: Astigmatisms may conveniently be symbolized as an astigmatic direction and magnitude, but are actually composed of refractive powers in the form of polar values. We are operating with two different entities, a net astigmatism and a power vector in the form of polar values. There is an unequivocal point-to-point correlation between these entities. Mathematical conversions can only be performed with polar values, but never by using net astigmatisms. All net astigmatisms must be converted to their appropriate refractive powers and the relevant calculations performed with these entities. The final result, such as an average of several astigmatisms, variances or confidence areas, may be point-to-point reconverted to and symbolized by a net astigmatism. These principles allow for exact description and comparison of surgical methods, but may be employed to describe and analyze any other population of astigmatisms, such as subjective cylinders and spectacle corrections.

Astigmatism↗
PubMed1998

Paired arcuate keratotomy for congenital and post-keratoplasty astigmatism.

PURPOSE: To study the effect of arcuate keratotomy on corneal astigmatism in previously grafted eyes compared to eyes with naturally occurring astigmatism. SUBJECTS AND METHODS: Twenty-three eyes with naturally occurring astigmatism and 21 eyes with post-keratoplasty astigmatism were treated by arcuate keratotomies in the steepest corneal meridian. Visual acuity, spherically equivalent refraction, and refractive cylinder were measured before surgery and 1 to 10 years after the operation. RESULTS: The preoperative refractive cylinder was reduced from 5.0 dioptres (median) to 1.25 dioptres in eyes with natural astigmatism and from 7.0 dioptres to 3.25 dioptres in post-keratoplasty eyes. Spherical equivalent refraction changed from -0.6 dioptres to -1.5 dioptres in eyes with natural astigmatism and from -3.5 dioptres to -4.5 dioptres in previously grafted eyes. The induced change in astigmatism, as calculated by Fourier analysis, correlated strongly with the existing preoperative astigmatism. The effect of the procedure did not correlate with the type of astigmatism (congenital vs. post-keratoplasty), time after surgery, or with patient age or sex. CONCLUSION: Arcuate keratotomy is a simple procedure to reduce naturally occurring astigmatism as well as induced astigmatism after keratoplasty. Parallel to the astigmatic change, negligible changes in the spherical equivalent are induced.

Adolescent↗
PubMed1998

Endothelial cell loss after photorefractive keratectomy for myopia.

PURPOSE: To study the long-term effect of 193 nm excimer laser photorefractive keratectomy (PRK) on the human corneal endothelial cell density. SUBJECTS AND METHODS: One hundred and twenty-four eyes from 71 patients underwent photorefractive keratectomy for myopia or myopic-astigmatism. Endothelial cell density was examined a short time before the operation and on an average of 50 months after the operation using a contact specular microscope. A subgroup of 32 eyes from 20 patients treated only once was examined preoperatively and 7 and 52 months postoperatively. The endothelial cell densities were corrected for the changing magnification of the contact specular microscope with changing central corneal thickness and for the expected physiological cell loss with time. RESULTS: The average endothelial cell density was preoperatively 3098+/-283 cells/mm2 (mean+/-SD) and postoperatively 3048+/-294 cells/mm2 corresponding to a change of -50+/-157 cells/mm2. This was statistically significantly different from zero in a paired t-test (n=124, t=3.58, 2p<0.001). The average changes in endothelial cell density for the subgroup were -34+/-159 cells/mm2 for the postoperative time interval 0-7 months and -20+/-188 cells/mm2 for the postoperative time interval 7-52 months. These results were not statistically significantly different from zero in a paired t-test (n=32, t=1.21 and t=0.60, 2p>0.05). A statistically significant negative correlation between preoperative cell density and the change in cell density was found (n=124, r=-0.21, 2p<0.05). CONCLUSION: This study suggests a potentially harmful effect of PRK on the human corneal endothelium. It appears that most cells are lost during ablation or within the first period of time after PRK.

Adult↗
PubMed1997

[Biomechanical study of corneal stability after photorefractive keratectomy].

BACKGROUND: Photorefraktive keratectomy (PRK) is the most frequent refractive surgical procedure worldwide. The central corneal thickness is reduced due to removal of the anterior stroma, including Bowman's layer, with a laser beam. This procedure results in considerable alterations of the corneal structure. What does this mean for the mechanical properties of the cornea? METHODS: Intraocular pressure was increased via a 180 degrees tilt. Before and during this procedure, corneal topography was measured by photokeratoscopy. We examined 26 patients after PRK and 25 controls who had not undergone any surgical procedure. RESULTS: The corneal center flattened by 0.038 +/- 0.05 dpt (P > 0.05) in the PRK patients and by 0.187 +/- 0.045 dpt (P < 0.05) in the control group. PRK patients within 1 year after operation showed a minimal central corneal steepening, whereas PRK patients after more than 1 year showed a reaction similar to that in normal corneas (P < 0.05). CONCLUSION: Corneal stability is altered after PRK. After 1 year corneal stability seems to normalize due to stromal remodelling.

Adult↗
PubMed1997

Corneal versus scleral tunnel incision in cataract surgery: a randomized study.

PURPOSE: To compare the induced regular and irregular astigmatism after scleral and corneal tunnel incision. SETTING: University hospital outpatient cataract clinic. METHODS: One hundred phacoemulsification patients with less than 1.0 diopter (D) of preoperative astigmatism were randomly assigned to have a clear corneal incision (50 patients) or a scleral tunnel incision (50 patients). All incisions were 3.5 to 4.0 mm wide and were made in the steepest axis of the corneal astigmatism. The surgically induced astigmatism was analyzed by vector analysis from keratometric data, as well as by Fourier harmonic series analysis of the topographic data. RESULTS: One day after surgery, the surgically induced astigmatism (vector analysis, keratometry) was 1.41 D +/- 0.66 (SD) and 0.55 +/- 0.31 D in the corneal incision group and the scleral incision group, respectively (P < .01). Six months after surgery, the induced astigmatism was 0.72 +/- 0.35 D and 0.36 +/- 0.21 D in the two groups, respectively (P < .01) The corneal topography data confirmed the regular astigmatism changes found by conventional keratometry. However, in addition, Fourier harmonic series analysis of the topography data showed significantly more irregular induced astigmatism with the corneal approach than with the scleral approach. CONCLUSION: The clear corneal incision induces significantly more regular as well as irregular astigmatism than the scleral tunnel incision.

Adult↗
PubMed1997

Value of intraoperative keratometry in predicting outcome of radial keratotomy.

PURPOSE: To investigate whether the immediate change in corneal power during radial keratotomy correlates with the long-term postoperative change in subjective refraction, and thereby being predictive for refractive outcome. METHODS: Manual keratometry was performed on 45 consecutively operated eyes of 45 young persons with myopia of 5 dioptres and less and immediately after radial keratotomy. Automated keratometry and subjective spherical equivalent refraction were investigated during a follow-up period of 6 months and correlated to the intraoperative keratometric measurements. RESULTS: On average, the majority of the change in corneal curvature after radial keratotomy took place within 1 min. There was no correlation between the intraoperative curvature change and the changes in curvature measured up to 6 months after surgery. There was a weak significant positive correlation between intraoperative curvature change and the change in subjective refraction at 6 months after surgery (R = 0.48, p < 0.01). The prediction error in estimating subjective refractive changes from intraoperative keratometry changes was, however, similar in patients who had bilateral radial keratotomy. Inclusion of such fellow-eye information together with the age of the patient in a multiple linear regression analysis increased the correlation coefficient from 0.48 to 0.75. CONCLUSIONS: The change in central corneal curvature takes place within minutes after corneal incision. As a single parameter, intraoperative keratometry cannot be used for titrating surgery. Information from the results of first eye radial keratotomy surgery with intraoperative keratometry is, however, predictive for radial keratotomy in the second eye. These findings suggest that a large source to refractive variability after radial keratotomy is related to individual patient factors, such as corneal biomechanics and wound healing.

Adult↗
PubMed1997

Comparison of objective methods for quantifying the refractive effect of photo-astigmatic refractive keratectomy using the MEL-60 excimer laser.

PURPOSE: To investigate the accuracy and precision of automated keratometry, automated refractometry, and computerized corneal topography in estimating the subjective refractive outcome of photo-astigmatic refractive keratectomy six months postoperatively. METHODS: Photo-astigmatic refractive keratectomy (Aesculap-Meditec, MEL-60 Excimer Laser) was performed on 26 eyes with a preoperative myopia ranging from -4.0 to 7.6 dioptres, and a naturally occurring astigmatism from 0.75 to 5.0 dioptres. Six months postoperatively refractive outcome was evaluated by automated keratometry, automated refractometry (Nikon NRK-8000), computerized topography (TMS-1), and subjective refraction. Estimate errors were computed as the difference between the change in subjective refraction and the change in automated keratometry, automated refractometry, and surface topography, respectively. Astigmatic changes were evaluated by the second harmonic component in the Fourier series analysis. RESULTS: Subjective spherical as well as cylindrical values were reduced significantly six months postoperatively. The estimate error (mean +/- one standard deviation) for automated keratometry was -1.26 +/- 0.72 dioptres for the spherical equivalent and -1.36 +/- 1.02 dioptres for the cylinder; for automated refractometry it was -0.78 +/- 0.91 dioptres for the spherical equivalent and -0.66 +/- 0.92 dioptres for the cylinder. The best estimates of subjective changes were obtained when the average of ring 2 and 3 of the topographic data was used: -0.15 +/- 0.82 dioptres for the spherical equivalent and -0.78 +/- 0.80 dioptres for the cylinder. CONCLUSIONS: The computerized topographer with the Fourier analysis was superior to automated keratometry and automated refractometry in estimating the subjective spherical refractive outcome and comparable to automated refractometry in estimating the subjective cylinder refractive outcome after photo-astigmatic refractive keratectomy.

Adult↗
PubMed1997

Topography of corneal grafts before and after penetrating keratoplasty.

PURPOSE: Refractive error after penetrating keratoplasty is a major clinical problem. The purpose of the present study was to investigate whether the topography of the donor cornea influence the topography of the graft after transplantation. METHODS: Twenty-five donor corneas were measured with a video-keratograph (TMS-1): in situ and before and after organ culture. Clinical video-keratographic images of the transplanted grafts were subsequently obtained one week, 1, 3, 6, 12, and 24 months after surgery. The central spherical equivalent power and corresponding regular and irregular astigmatic powers were computed. RESULTS: A statistically significant correlation between spherical equivalent central donor power and spherical equivalent central graft power after keratoplasty was found at all times up to two years after surgery. Only 13-50% of the variation in post-keratoplasty spherical graft power could, however, be explained by the donor graft power. Corresponding 95% confidence limits for prediction of post-keratoplasty power from donor graft power were approximately +/- 6.5 diopters. Post-keratoplasty regular or irregular corneal astigmatism did not correlate with astigmatism in the donor graft. CONCLUSION: Corneal donor graft spherical equivalent power does influence the spherical equivalent corneal power after keratoplasty, especially during the first months after surgery. The dependency is, however, not very strong and until other determinants of post-keratoplasty corneal shape are known and controllable, 'power-typing' of donor corneas appears to be of limited clinical use.

Adolescent↗
PubMed1996

Regional elastic performance of the human cornea.

The regional mechanical performance of the cornea and limbus was studied in vitro by pressure loading of 18 intact human eyes. The pressure-induced (2-100 mmHg) meridional and circumferential tangential mechanical strains of the epi-and endothelial side of the cornea were measured at the centre, the para-centre, the periphery and the limbus. Strains were computed from digital measures of distances between tiny mercury droplet markers fixed on the corneal surfaces. Corresponding in-plane membrane stresses were calculated from measurements of the regional meridional and circumferential radius of curvature and from measurements of the corneal thickness of the four regions. Young's moduli of elasticity were computed for each region in the meridional and circumferential direction assuming orthotropic elastic behaviour of the corneal stroma. A power function was used to fit the non-linear elastic stress-strain relationships. The pressure-induced meridional strains were smallest at the corneal paracentre and periphery, and largest at the limbus. The circumferential strains varied less between regions with the para-centre straining most. In the meridional direction, Young's modulus of elasticity was highest at the central and para-central corneal regions, whereas the highest circumferential elastic modulus was found at the limbus. This study supports the notation of circumferentially orientated reinforcing structures in human limbal tissue. The para-central region of the human cornea was found stiffer in the meridional direction compared with the circumferential direction, suggesting a meridionally orientated reinforcement of the para-central parts of the human cornea. These findings may have important implications for mechanical modelling of keratorefractive procedures.

Aged↗
PubMed1996

Evaluating surgically induced astigmatism by Fourier analysis of corneal topography data.

PURPOSE: To evaluate surgically induced astigmatism using Fourier harmonic series analysis of corneal topography data. SETTING: Aarhus Kommunehospital, Aarhus University, Denmark. METHODS: We evaluated the results of 46 phacoemulsifications with a 4 or 6 mm scleral tunnel sutureless incision based on the axis of the steepest meridian. We performed conventional keratometry and corneal topography before and up to 1 month after surgery. Using Fourier analysis, the corneal topographic images were broken into spherical power, regular astigmatism, and nonregular astigmatism for individual or aggregate analysis of surgically induced astigmatism. The induced refractive change (average of the difference between preoperative and postoperative corneal topographies) was analyzed and normalized according to the surgical meridian and to right/left eye. RESULTS: Regular astigmatism calculated by Fourier analysis of mires from the keratometer zone correlated well with conventional keratometry readings. Surgery induced a localized flattening in the superior region and a with-the-rule regular astigmatism component in the central area. CONCLUSION: Surgically induced corneal topography changes can be analyzed by Fourier series harmonic analysis, allowing aggregate data to be broken into optically meaningful quantities.

Adult↗
PubMed1996

[Biomechanical study of corneal stability after radial keratotomy].

BACKGROUND: Radial Keratotomy is one of the most frequent refractive surgical procedures performed worldwide. It results in considerable alterations of the corneal structure. What does this mean for the mechanical properties of the cornea? METHODS: Intraocular pressure was increased via a 180 degrees tilt. Before and during this procedure, corneal topography was measured by photokeratoscopy. We examined 36 patients after RK and 25 controls who had not undergone any surgical procedure. RESULTS: The corneal center flattened by 0.523 +/- 0.054 dpt (p < 0.01) in the RK patients and by 0.187 +/- 0.045 dpt (p < 0.05) in the control group. CONCLUSION: Even several years after radial keratotomy, corneal stability is still decreased.

Adult↗