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Biomedical subjects

K Naeser

Publications and source records attributed to K Naeser.

At least 37 records · Page 2Linked to original sources
PubMed1994

Quantitative assessment of corneal astigmatic surgery: expanding the polar values concept.

The purpose of astigmatic corneal surgery is to flatten the steeper meridian of the preoperative cylinder, to steepen the flatter meridian, or both. Therefore, it may be useful to quantitate the surgical effect by calculating the equivalent dioptric value of the postoperative cylinder in these principal meridians. In this study, the dioptric value projected on the preoperatively steeper meridian is termed the with-the-power (WTP) component, the portion projected on the flatter meridian, the against-the-power (ATP) component. Consider a preoperative net astigmatism of the power N in the meridian a. After astigmatic corneal surgery, the postoperative corneal cylinder is M in the meridian b. For the postoperative cylinder, the WTP component = M x sin2([b + 90]-a). The ATP component = M x cos2([b + 90]-a). The astigmatic polar value is defined as the difference between these magnitudes: AKP = M x (sin2[(b + 90)-a] - cos2[(b + 90)-a]). By calculating the astigmatic polar value, the surgeon immediately knows the outcome of the surgical procedure (i.e., whether the preoperative astigmatism has been undercorrected, overcorrected, or perfectly corrected). We describe the theory behind this new formula and discuss its applications and limitations.

Astigmatism↗
PubMed1993

Calculation of the thickness of an intraocular lens.

Lens manufacturers do not usually supply information about the central thickness of an intraocular lens optic. This paper describes a method to calculate the central lens thickness from variables normally supplied by the manufacturer, i.e., the total lens power in situ, the edge thickness, the optic diameter, and some knowledge of the optic architecture. The formulas are universal and valid for any lens, regardless of lens design and material. There is a correlation between calculated values of optic thickness and values supplied by manufacturers. Individually estimated lens thickness may increase the accuracy of intraocular lens power calculation, pseudophakic anterior chamber depth estimation, and pseudophakic axial length calculation.

Anterior Chamber↗
PubMed1993

Epidemiology of retinal detachment following extracapsular cataract extraction: a follow-up study with an analysis of risk factors.

We reviewed the medical records of 1,726 eyes consecutively operated on with extracapsular cataract extraction to examine the incidence of and the risk factors for retinal detachment (RD). In Denmark, both diagnoses and deaths are registered centrally; therefore, a complete follow-up was achieved in each case and actuarial statistical methods were used in the risk assessment. Seven eyes (0.41%) developed RD and 345 eyes (20.0%) had YAG-laser capsulotomy in the observation period, which averaged 39 months. We identified the following risk factors for RD: age less than 70 years at the time of surgery, intraoperative complications, axial length > or = 25 mm. Nonsignificant factors included male gender and postoperative complications. One eye (0.29%) developed RD following YAG-laser capsulotomy. We concluded that the high risk of RD is concentrated in a small group of myopic patients who have cataract surgery when they are less than 70 years of age. Our results do not suggest a causal relationship between YAG-laser capsulotomy and subsequent RD.

Adult↗
PubMed1993

Retinal detachment following intracapsular and extracapsular cataract extraction. A comparative, retrospective follow-up study.

In a retrospective study we reviewed the post-operative incidence of retinal detachment in a consecutive series of 762 eyes operated on with intracapsular cataract extraction and a consecutive series of 1351 eyes operated on with extracapsular cataract extraction. Follow-up time was 2 1/2-4 1/2 years in both series. The incidence of retinal detachment was 0.79% after intracapsular cataract extraction and 0.44% after extracapsular cataract extraction. The difference was not statistically significant. Age below 70 years was not statistically significantly correlated to retinal detachment.

Adolescent↗
PubMed1991

Intraocular pressure 2 1/2 years after extracapsular cataract extraction and sulcus implantation of posterior chamber intraocular lens.

The intraocular pressure in 50 eyes of 50 patients was measured 2 1/2 years after planned extracapsular cataract extraction with intended implantation of a posterior chamber lens in the ciliary sulcus. The results were compared to the preoperative values, and the pressure observed after 4 months. Although a significant elevation was found from the fourth month (P = 0.015), the intraocular pressure at 2 1/2 years was found to be significantly lower than the preoperative value (P = 0.002). No significant difference was found in two subgroups in which transillumination defects in the iris or pigment in the lower anterior chamber angle was present.

Aged↗
PubMed1990

Vaulted posterior chamber lenses and the posterior capsule.

Sixty-eight patients who had extracapsular cataract surgery with intraocular lens implantation in the bag received one of two types of lenses that had been designed to create a gap between the posterior surface of the optic and the posterior capsule. Two to four months after surgery, we found that the central spacing between the posterior lens surface and the posterior capsule measured by optical pachymetry averaged 0.25 mm. The mean of the spacing for the Pharmacia LSP laser ridge was 0.25 mm (range 0.1 to 0.6) and that of the 3M LE meniscus lens was 0.24 mm (range 0 to 0.8). The amount of adherence of the posterior capsule to the posterior lens surface was less with the laser ridge design than with the meniscus lens.

Aged↗
PubMed1990

Biometry of the posterior lens capsule: a new method to predict pseudophakic anterior chamber depth.

The distance between the anterior corneal vertex and the posterior lens capsule was measured in 60 eyes before and three months after extracapsular cataract extraction with implantation of a posterior chamber lens. The preoperative capsule position ranged from 6.15 mm to 8.60 mm and averaged 7.64 +/- 0.47 mm (+/- SD). All posterior lens capsules had moved anteriorly following surgery. The postoperative capsule position ranged from 4.31 mm to 5.53 mm and averaged 4.93 +/- 0.29 mm. The postoperative capsule position could be predicted by the following optimized multiple linear regression equation: 2.4 + 0.011 x patient age + 0.171 x anterior chamber depth + 0.051 x axial length. The multiple correlation coefficient was 0.48. The postoperative anterior depth could be predicted with a correlation coefficient of 0.61 by subtracting both the averaged value of the clinical "laser space" and the exact central implant thickness from the estimated position of the posterior lens capsule. We suggest that this principle for predicting the postoperative anterior chamber depth may be useful for a number of lenses with different designs.

Adult↗
PubMed1990

Conversion of keratometer readings to polar values.

Corneal astigmatism is a complex entity that has direction and magnitude. This study reports a new method to describe corneal astigmatism within the with- and against-the-rule concept. Each net astigmatism of the maximal power M in the meridian alpha may be divided into two dioptric components: a with-the-rule astigmatism projected on the 90-degree meridian and an against-the-rule component projected on the 180-degree meridian. The former figure has the dioptric value M X sin2 alpha, the latter M X cos2 alpha. The polar value is defined as the difference between these magnitudes: M X (sin2 alpha - cos2 alpha). The polar value calculates the balance between the with- and against-the-rule components for any given net astigmatism. The entire model allows an exact description of surgically induced with- or against-the-rule astigmatism following cataract extraction. The advantage of the model is that a corneal astigmatism may be expressed by a single figure. The system enables each surgeon to evaluate the contribution of the preoperative astigmatism, incision type, suture technique, and postoperative treatment on the final astigmatism. This in turn allows the surgeon to estimate a number of different surgical techniques empirically. By disposing of and choosing between several known techniques the surgeon may be able to minimize final astigmatism even in cases of significant preoperative astigmatism.

Astigmatism↗
PubMed1990

Visual outcome and complications following intracapsular and extracapsular cataract extraction. A prospective, controlled follow-up study.

We followed two comparable groups of patients operated on for cataract immediately before and after a transition from intracapsular extraction with an anterior chamber lens to extracapsular technique with a posterior chamber lens. Both groups consisting of 69 eyes of 69 patients were invited to follow-up examinations 4 months and 2 1/2 years after surgery. The visual acuity was similar in the two groups at the two follow-up examinations. Significantly more eyes in the intracapsular group showed either a rise in intraocular pressure or received anti-glaucomatous medications at the latest follow-up. YAG-laser capsulotomy had been performed in 14.5% of the 69 extracapsularly operated eyes. There was a trend towards more intra-operative and cumulative post-operative complications in the intracapsular group, but this difference was not statistically significant. We conclude, that both intracapsular and extracapsular cataract extraction are followed by such low incidences of post-operative complications, that it will require a very large number of eyes to confirm or reject a possible statistically significant difference in post-operative complication rate.

Aged↗
PubMed1990

Morphological changes 2 1/2 years after extracapsular cataract extraction with implantation of a posterior chamber lens. A prospective re-examination.

Morphological changes and lens position were examined in 51 eyes of 51 patients 25 to 37 (mean 31) months after extracapsular cataract extraction with implantation of a posterior chamber lens in the ciliary sulcus. Ocular morphology was compared to a previously published examination of the same eyes performed 4 months after surgery. The anterior chamber depth, lens position and the position of the posterior lens capsule had remained unchanged. 'Iris bulging', i.e. a gonioscopically visible slight impression of the iris overlying the lens haptics, had increased from 0 to 42%, iridal transillumination defects from 34 to 46%, and pigment dispersion in the chamber angle from 46 to 72%. Twenty-four percent of the eyes had developed significant, capsulotomy-requiring opacification of the posterior lens capsule. We draw the following conclusions: The posterior chamber lens and the posterior lens capsule have reached their permanent positions by the fourth post-operative month. Implantation of a posterior chamber lens in the ciliary sulcus constitutes a constant stimulus for structural changes of the iris with subsequent dispersion of pigment on neighbouring ocular structures.

Aged↗
PubMed1989

Axial length following implantation of posterior chamber lenses.

We used ultrasound to measure axial length in 66 eyes before and three months after extracapsular cataract extraction with implantation of a posterior chamber lens. An ultrasound velocity of 2,718 m/s for poly(methyl methacrylate) was assumed. The intraindividually measured difference (postoperative--preoperative axial length) ranged from -0.4 mm to 0.6 mm and averaged 0.11 mm. The lengthening of the eye was statistically significant (P less than .001; two-tailed, paired t-test). Statistical analysis of the regression equation between the preoperative and the postoperative axial lengths disclosed that this correlation was not more accurate than simply substituting the two measurements. Assuming no real change in axial lengths, our data suggest an ultrasound velocity of 1,679 m/s for the natural lens or an average velocity of 1,560 m/s for the phakic eye. We conclude that the preoperatively measured axial length may be used as a fairly accurate estimate of the postoperative distance in intraocular lens power calculation formulas.

Biometry↗
PubMed1988

Epidemiology of aphakic retinal detachment following intracapsular cataract extraction: a follow-up study with an analysis of risk factors.

We reviewed 604 eyes in 521 patients who had intracapsular cataract extraction. The follow-up period averaged 39 months. The incidence of rhegmatogenous aphakic retinal detachment (ARD) was 1.3% in the whole group. The ARD incidence was 1.0% in eyes without surgical complications and 5.4% in myopic eyes (myopia defined as an aphakic refraction less than or equal to +9.0 diopters). The log-rank test was used to estimate the statistical significance of various ARD predictors. Significant predictors were age at surgery below 70 years (P = .0004) and myopia (P = .001). Our results indicate that the high risk of ARD is concentrated in a small group of myopic patients operated on at a relatively early age. During the follow-up period, 128 patients died. Compared with the mortality rate of the entire Danish population, this was not an above average mortality rate. Thus, our results do not support the hypothesis that senile cataracts reflect general systemic deterioration rather than local eye disease.

Adult↗
PubMed1988

Prediction of pseudo-phakic anterior chamber depth from pre-operative data.

Multiple linear regression analysis of pre-operative data were used to construct formulas to predict the anterior chamber depth 3 months after implantation of two different posterior chamber lenses. The resulting formulas differed widely. For lens I (27 eyes) the formula had a correlation coefficient of 0.78 and a standard deviation of +/- 0.2 mm. The corresponding figures for lens II (41 eyes) were 0.52 and +/- 27 mm. For lens I the predictors of significance were axial length (P less than 0.0001) and anterior corneal curvature (P = 0.07). For lens II significant predictors were patient age (P = 0.0008) and pre-operative anterior chamber depth (P = 0.04). We suggest that post-operative anterior chamber depth may not solely depend on the dimensions of the eye, but also on the lens architecture and the 'memory' of the lens haptics. We conclude that formulas to predict post-operative anterior chamber depth should be derived empirically for each lens type separately.

Age Factors↗
PubMed1987

A prospective study of intraocular pressure four months after extracapsular cataract extraction with implantation of posterior chamber lenses.

Sixty-six consecutive cases of extracapsular cataract extraction (ECCE) and sulcus implantation of a posterior chamber lens had intraocular pressure (IOP) measurements recorded four months postoperatively. These results were compared to the fellow unoperated eyes, as well as to a group of 64 patients, whom we reported previously, who had intracapsular cataract extraction (ICCE) and implantation of an anterior chamber lens. The ECCE and posterior chamber lens group demonstrated a significant reduction in the IOP of the operated eye after four months (P less than 0.001), with none of the patients having an IOP greater than 20 mm Hg. No significant IOP change could be demonstrated in the fellow eye (0.1 less than P less than 0.2). We found a significant difference between the ECCE and ICCE groups in the IOP of the operated eye (P less than 0.001).

Aged↗
PubMed1987

Prospective study of intraocular pressure 2 1/2 years after intracapsular cataract extraction and implantation of a semiflexible anterior chamber lens.

Fifty-one patients available for a 2 1/2 year follow-up after intracapsular cataract extraction and implantation of the semiflexible McGhan/3M, style 70 anterior chamber lens were evaluated for intraocular pressure (IOP). No significant change in the IOP could be demonstrated despite progression of iris-like tissue adhesions between the iris and lens haptics at or close to the trabecular meshwork. Four eyes (7.8%) had IOPs greater than or equal to 22 mm Hg without medication, and four other eyes used timolol eyedrops with subsequent IOPs less than or equal to 18 mm Hg. Eight eyes (15.7%) demonstrated an IOP increase greater than or equal to 25% over baseline pressures.

Anterior Chamber↗