Biomedical subjects

Anja Viestenz

Publications and source records attributed to Anja Viestenz.

15 recordsLinked to original sources

PubMed2006

Ray tracing through a schematic eye containing second-order (quadric) surfaces using 4 x 4 matrix notation.

Ray tracing is used in ophthalmology for evaluation of the optical properties of the eye. We demonstrate an algebraic method for tracing a bundle of rays through the optical system of an eye containing aspheric surfaces. Restricting to second-order surfaces (quadric surfaces) such as ellipsoids, paraboloids or hyperboloids, a surface is described by a 4 x 4 matrix. In this case, the normal vector can be derived analytically and the ray-surface intersection is calculated by solving a quadratic equation. We applied this straightforward matrix-based strategy to the spherical 4-surface Le Grand schematic eye, and the Le Grand eye modified by Kooijman containing four aspheric surfaces. We calculated the spot diagram for the focal plane as well as a pre- and post-focal plane for both model eyes, and found that the optical quality of the aspheric model characterized by the ray scatter in the spot diagram at the focal plane is much better than that of the spherical model. This calculation strategy may be helpful for evaluating the image distortion of decentred or tilted spherical or aspheric artificial intra-ocular lenses.

Cornea↗

PubMed2005

Intraocular lens power prediction for triple procedures in Fuchs' dystrophy using multiple regression analysis.

PURPOSE: To develop a correcting term for intraocular power (IOLP) prediction for penetrating keratoplasty combined with simultaneous extracapsular cataract extraction and posterior chamber lens implantation (triple procedure). METHODS: As part of a prospective clinical study, triple procedures were performed in 42 eyes with Fuchs' dystrophy. Only eyes with readable preoperative K-values were included in this study. Differences (DEV) between achieved and target refraction (TR) depending on the values of the theoretical-optical formula according to HAIGIS were investigated using multiple regression analysis in a linear anova model: DEV = a + b CP + c AL + d IOLP + e TR. CP represents central corneal power, AL represents axial length. RESULTS: Spherical equivalent after suture removal was - 1.39 +/- 2.86 D (TR: - 1.64 +/- 1.72 D). A multiple regression formula was developed for correction of conventionally calculated IOL power. CP (b = - 1.391, p = 0.028), AL (c = - 4.733, p = 0.007), IOLP (d = - 1.301, p = 0.009) and TR (e = - 1.804, p = 0.005) correlated significantly with DEV (a = 198.684). CONCLUSION: Proposed correcting multiple regression formula for IOL power prediction may help to improve the postoperative refractive outcome in patients undergoing triple procedures.

Adult↗

PubMed2003

Corneal shrinkage induced by nonmechanical Q-switched erbium:YAG laser trephination for penetrating keratoplasty in porcine eyes.

PURPOSE: To assess the degree of corneal diameter shrinkage induced by Q-switched mid-infrared laser corneal trephination for penetrating keratoplasty in an experimental model. METHODS: Corneal trephination was performed in 80 enucleated porcine eyes fixed in a holder centered on an automated globe rotation device, by Q-switched (2.94 microm) Er:YAG laser along open masks. Four types of masks were used to protect the underlying corneal tissue: metal masks (donor and recipient) and ceramic masks (donor and recipient). Two spot diameters (0.65 mm and 0.96 mm) were combined with two energy settings (40 mJ/pulse and 50 mJ/pulse) for each of the masks used. Repetition rate was fixed at 5 Hz. Diameters of donor buttons/recipient beds (horizontal and vertical) were measured immediately after the trephination and compared to the given mask size. RESULTS: Minimum corneal shrinkage was found in the recipient metal mask group (mean +/- SD=0.3+/-0.4%) with 50 mJ pulse energy and 0.65 mm spot diameter (in the horizontal diameter), while the maximum shrinkage (5.3+/-2.8%) was found in the donor metal mask group with 50 mJ pulse energy and 0.96 mm spot diameter. Corneal shrinkage was less pronounced in recipient beds than in donor buttons (P<0.01). The differences in shrinkage between the use of ceramic and metal masks were insignificant (P>0.05). Mean induced corneal diameter discrepancies between the donor button and the recipient bed (with metal and ceramic masks) were 2.5% and 2.5% in vertical diameter and 3.4% and 2.4% in horizontal diameter. CONCLUSIONS: The Q-switched Er:YAG laser experimental corneal trephination for penetrating keratoplasty may induce minor degrees of corneal diameter shrinkage in donor buttons and recipient openings. Oversizing of donor masks by 0.25-0.35 mm (i.e. 3-4% of graft size) may be a valid option to avoid refractive consequences.

Animals↗

PubMed2003

Nonmechanical Q-switched erbium:YAG laser trephination for penetrating keratoplasty: experimental study on human donor corneas.

OBJECTIVE: To assess the alterations in human donor corneal tissue induced by Q-switched erbium (Er):YAG laser corneal trephination. METHODS: Thirty human corneoscleral donor buttons unsuitable for transplantation were placed in an artificial chamber on an automated rotation device. Corneas were trephined with a Q-switched Er:YAG laser (wavelength, 2.94 microm; pulse duration, 400 nanoseconds) along (donor and recipient) aluminum silicate (ceramic) open masks. A spot diameter of 0.65 mm, energy setting of 50 mJ/pulse, and repetition rate of 5 Hz were used. Corneal thermal damage and cut regularity were quantitatively assessed in 24 corneas processed for light microscopy and by transmission and scanning electron microscopy. RESULTS: The stromal thermal damage was the highest (mean [SD], 8.0 [2.7] microm) at a 150-microm cut depth and decreased downward. Cut regularity was very good and did not significantly differ between donors and recipients. Scanning electron microscopy confirmed that the cuts were highly regular; transmission electron microscopy revealed 2 distinctive subzones within the stromal thermal damage zone. CONCLUSIONS: Thermal damage induced by Q-switched Er:YAG nonmechanical corneal trephination was low, and the regularity of the cuts was very good. CLINICAL RELEVANCE: The Q-switched Er:YAG laser may have the potential to become an alternative to the excimer laser for nonmechanical penetrating keratoplasty.

Cornea↗

PubMed2003

[Vigabatrin-associated bilateral simple optic nerve atrophy with visual field constriction. A case report and a survey of the literature].

CASE REPORT: A 70-year-old male with optic disc pallor and visual field constriction up to 35 degrees bilaterally was previously treated with vigabatrin. The cumulative dosage was 3.7 kg. The b-wave was reduced in photopic and scotopic electroretinography and the patient had delayed visual evoked potentials. The retinal nerve fiber thickness was pathologically reduced in the Heidelberg retinal tomography (HRT) and the retinal nerve fiber layer analyzer (GDx). CONCLUSION: Vigabatrin is an antiepileptic drug and may increase the intraretinal GABA level due to an inhibition of the enzyme GABA-transaminase and may induce an ascending simple optic nerve atrophy. Vigabatrin-associated visual field defects may be irreversible. A visual field control is recommended before starting the treatment with vigabatrin and at regular 6-month intervals thereafter. In cases of visual field constriction, the therapy should be stopped. If the cumulative dosage is more than 3 kg of vigabatrin, the visual field controls should be performed more frequently because of the dosage-toxicity relationship.

Aged↗

PubMed2003

[Impact of an Er:YAG laser beam control on cut quality of non-mechanical corneal trepanation for penetrating keratoplasty].

BACKGROUND: Results of non-mechanical corneal trepanation using the excimer laser enhance the morphological and functional results for penetrating keratoplasty. Searching for alternative laser sources we assessed the impact of an automatic laser beam control for the Er:YAG solid-state laser on the cut performance and thermal damage zone in non-mechanical corneal trepanation. METHODS: We compared the cut quality of A) a manually guided laser beam, B) a semiautomatically guided laser beam and C) a fully PC-controlled laser beam positioning system (q-switched, repetition rate 5 Hz, pulse energy 65 mJ, spot size 0.7 mm) along slit aperture masks on 28 rabbit eyes using macroscopic images and histological sections (PAS staining). RESULTS: The manually guided laser beam control (A) induced the broadest thermal damage zone in the corneal stroma (19.3+/-8.7 microm) compared to the semi-automatic mode (B) (8.8+/-3.0 microm, p=0.03) and the PC-controlled laser beam control (C) (7.0+/-3.0 microm, p=0.016). CONCLUSION: The fully automatic PC-controlled laser beam positioning system for the Er:YAG solid-state laser with a small spot size and fixed low repetition rate allows a precise laser beam guidance and a significant enhancement of the cut performance compared to a manual laser beam control via micromanipulator in experimental nonmechanical corneal trepanation.

Animals↗

PubMed2003

Evaluation of corneal flap dimensions and cut quality using the SKBM automated microkeratome.

PURPOSE: To evaluate flap dimensions and cut quality with repeated blade use of the automated Summit Krumeich-Barraquer microkeratome (SKBM [LadarVision]). SETTING: Department of Ophthalmology, University Erlangen-Nuremberg, Erlangen, Germany. METHODS: The SKBM (160 microm plate, intended flap diameter 9.0 mm) was used to perform a corneal hinged flap in 35 pig cadaver eyes. Seven blades were reused 5 times each. The flap diameter was measured by planimetry, and the thickness was assessed by ultrasonic pachymetry. Scanning electron microscopy (SEM) of blades and stromal beds was performed. RESULTS: With single use of the blade, the mean central flap thickness was 145 microm +/- 25 (SD). The vertical/horizontal flap diameter was 9.0 +/- 0.03 mm/8.6 +/- 0.03 mm. No thickness gradient was observed from the incision (138 +/- 31 microm) to the flap hinge (130 +/- 30 microm). If the blade was used more than 2 times, the flap was thinner at the incision (157 +/- 34 microm versus 124 +/- 20 microm; P =.003) and the hinge (143 +/- 24 microm versus 122 +/- 31 microm; P =.04), but the central thickness remained unchanged. With multiple use of the blade, SEM analysis showed increasing cut irregularity, more tissue remnants on the blade surface, and a progression in blade irregularities (up to 9.3 microm). CONCLUSIONS: Reproducible flap size and thickness can be obtained with single use of stainless steel blades in the SKBM. With multiple use, the quality of the blades and the stromal bed deteriorates and the peripheral thickness of the flaps decreases. Thus, single use of blades is recommended.

Animals↗

PubMed2003

[Assessment of the optical image quality of the eye using raytracing technique of corneal topography data].

BACKGROUND: Optical aberrations in the optical system may downgrade image quality and cannot be fully compensated by spherocylindrical glasses. The subjectively evaluated visual acuity may be significantly reduced. The purpose of this study was to calculate the image forming properties of the eye using a spotlight source or alternatively extended objects. METHODS: A convex and first derivative continuous (C1) surface from the rough height data of the anterior corneal surface (TMS-1, Tomey, Erlangen) or the anterior and posterior corneal surface (Orbscan, Orbtec, USA) was calculated by means of an interpolating subdivision scheme (modified Butterfly algorithm). The characteristics of the residual refractive surfaces were used according to Navarro's eye model. The focal distance was calculated from the exact raytracing calculation (Snellius' law) of the point-spread function by minimising the variance of the point-spread function. The diffraction property of the aperture stop was implemented with a transmission characteristic according to a radially symmetrical Bessel function within the entrance pupil. The algorithm was realised with a C code on the LINUX platform and applied to a normal eye (example 1, TMS-1), an eye with severe keratoconus (example 2, TMS-1) and an eye with corneal scars (example 3, Orbscan). RESULTS: The focal distance in example 1 (22.5 mm, 22.6 mm, and 22.8 mm) increased with the pupil diameter (2 mm, 3 mm, and 5 mm). The variance of the approximately radially symmetrical point-spread function in the focal plane attained a minimum value with a pupil size of 3 mm (0.164, 0.104, and 0.230). In example 2, the focal distance changed inconclusively (21.1 mm, 21.0 mm, and 21.3 mm) with the pupil size (2 mm, 3 mm, and 5 mm). The variance of the markedly asymmetrical point-spread function in the focal plane was systematically higher compared to the values of example 1 and reached a minimum value with a pupil size of 3 mm (0.255, 0.224, and 0.371). The imaging of the sinus-modulated pattern is anisotropic due to the asymmetry of the point-spread function. In example 3, the focal distance (22.3 mm, 22.3 mm, and 22.5 mm) did not change systematically with the pupil size (2 mm, 3 mm, and 5 mm). The variance of the nearly radially symmetrical point-spread function changed only marginally between pupil sizes of 2 mm and 3 mm (0.231, 0.239, and 0.338). CONCLUSIONS: Raytracing of corneal topography height data based on refined eye models with the option of auto-focussing has the potential to trace the optical resolution of the eye for arbitrary objects. Further studies on contrast sensitivity and the conversion of the real image to a perceived image by the retina and brain are required for complete modeling of subjective visual acuity.

Adult↗

PubMed2003

[Cataract and keratoplasty--simultaneous or sequential surgery?].

BACKGROUND AND PURPOSE: Since the introduction of the triple procedure (simultaneous penetrating keratoplasty [PK], extracapsular cataract extraction [CE] and implantation of a posterior chamber intraocular lens [PCIOL]) in the mid-seventies, there is an ongoing discussion among corneal surgeons concerning the best approach for combined corneal disease and cataract. METHODS: Besides the classical triple procedure (1), two alternative microsurgical approaches are feasible: (2) CE + PCIOL prior to PK and (3) CE + PCIOL after PK. For the refractive results after TRIPLE some intraoperative details are crucial: Trephination of recipient and donor from the epithelial side without major oversize (Guided Trephine System or Nonmechanical Excimer Laser Trephination) should preserve the preoperative corneal curvature. Graft and the PCIOL placed in the bag after continuous curvilinear capsulorhexis should be centered along the optical axis. If possible, performing the capsulorhexis under controlled intraocular pressure conditions prior to trephination may help to minimise the risk of capsular ruptures. RESULTS: The major advantage of the TRIPLE is the faster visual rehabilitation and less efforts for the mostly elderly patients. However, two intraocular interventions with approach (2) and (3) bear an increased risk of infection and suprachoroidal haemorrhage. Approach (2) requires a cornea that is still transparent enough to perform cataract surgery, and the risk of intraocular pressure rise after PK seems to be increased. Approach (3) has the potential of a simultaneous reduction of astigmatism during CE (appropriate location of the incision, simultaneous refractive keratotomies or implantation of a toric PCIOL). Disadvantages may include the loss of graft endothelial cells and the theoretically increased risk of immunological allograft reactions. After TRIPLE, major deviations from target refraction have been reported. However, individual multiple regression analysis may help to minimise this problem with appropriate methods of trephination. Since suture removal after PK may result in major individual changes of the corneal curvature, IOL power calculation for approach (3) requires all sutures to be removed at the time of CE. However, even after complete suture removal the abnormal proportions between anterior and posterior curvatures and/or the irregular topographies after PK may be responsible for marked IOL power miscalculations in the individual case. CONCLUSIONS: The postulated better refractive outcome and better uncorrected visual acuity after the sequential approach is opposed by a markedly delayed visual rehabilitation. For this reason, we consider the TRIPLE procedure including CE via open sky in general anesthesia as the method of choice for combined lens and corneal opacities. Because of the often rapidly progressive nuclear cataracts after PK, we recommend the simultaneous approach in elderly patients with Fuchs' dystrophy even with incipient lens opacities.

Aged↗

PubMed2003

[Impact of intracameral pressure on donor cut angles in nonmechanical Er:YAG laser trephination for penetrating keratoplasty].

BACKGROUND AND PURPOSE: Congruent cut surfaces are a predisposition for good apposition of donor and recipient during penetrating keratoplasty (PK). The purpose of this study was to assess the impact of the intracameral pressure during nonmechanical donor trephination from the epithelial side on the cut angles for experimental human PK. METHODS: With a Q-switched 2.94 micro m Er : YAG laser a 6 mm sized corneal donor trephination was performed subtotally in 30 human corneas using an artificial anterior chamber device allowing different intracameral pressures (10, 20, and 40 mm Hg). The cut angles were measured immediately after the trephination by ultrasound biomicroscopy (UBM) at four quadrants: between trephination cut and corneal epithelium (angle 1 = A1-UBM) and between trephination cut and horizontal plane (angle 2 = A2-UBM). The positions of the measures were marked, the corneas were fixed in a buffered 10 % paraformaldehyde solution, and the same positions were analyzed by histology. The histological cuts were digitized, the images printed, and the cut angles measured in paper (A1-histology). RESULTS: Mean angles were 111.6 degrees /113.5 degrees /126.6 degrees (A1-UBM), 88.4 degrees /93.5 degrees /101.8 degrees (A2-UBM) and 120.4 degrees /125.1 degrees /119.3 degrees (A1-histology) with 10/20/40 mm Hg, respectively. The A2-UBM showed a significant increase of divergence with increasing intracameral pressure (p < 0.03). For A1-UBM cut angles with 10 and 20 mm Hg were significantly smaller compared to those with 40 mm Hg (p < 0.002), but there was no difference of divergence between 10 and 20 mm Hg. The A1-histology cut angle showed no correlation with intracameral pressure (p > 0.09). CONCLUSIONS: Increasing intracameral pressure using an artificial anterior chamber during donor trephination from the epithelial side for nonmechanical PK using Er : YAG laser results in increasing divergence of cut angles. This may disturb the congruence of the cut angles in donor-recipient apposition. To achieve standardised cut angles for a good donor recipient apposition, similar normotonic intracameral pressures for donor and recipient trephination should be attempted. The UBM has the potential to analyse the cut angle immediately after subtotal trephination preserving the attempted intracameral pressure in the artificial anterior chamber. Histological analysis of the cut angles seems to lack methodological validity.

Anterior Chamber↗

PubMed2003

Q-Switched 2.94-microm Er:YAG laser trephination with convergent and divergent cut angles for penetrating keratoplasty.

PURPOSE: To study the morphologic properties of perpendicular (P), convergent (C), and divergent (D) cut angles using different speeds of rotations during donor and recipient nonmechanical trephination for experimental penetrating keratoplasty. METHODS: With a Q-switched 2.94-microm Er:YAG laser corneal trephination was performed in 150 enucleated porcine eyes using ceramic open masks with 8 "orientation teeth/notches" and an automated globe rotation device allowing different cut angles [0 degree (P), 10 and 20 degrees (C and D)] toward the optical axis and variation of the rotation speed [3, 7, and 11 rotations per minute (rpm)]. The regularity of the cut (I, regular; II, slightly irregular; III, irregular) was assessed by light microscopy. The area of thermal damage and the number and size of "spikes" in the stroma at the superficial, intermediate and deep level of the excision were analyzed using digital images and the Optimas image processing software. RESULTS: The regularity of the cut was classified as I in 42%/22% of donor/recipient and as II in 41%/56%, respectively. The thermal damage was least expressed with D20 degree cut angle and donor mask (P < 0.01). With all cut angles and speeds of rotation, thermal damage at the deep level of excision was significantly smaller (P < 0.01). With different speeds of donor rotations, the thermal damage showed no significant difference. With recipient trephination, the thermal damage at the deep level was greatest with 7 rpm (P < 0.01). The number and size of spikes of thermal damage with donor and recipient masks were significantly smaller in the deep stroma (P < 0.01). CONCLUSIONS: Q-switched Er:YAG laser trephination with appropriate settings results in low thermal damage zones at the cut margin. Different cut angles and speeds of trephination may affect the cut performance and quality of the excision. In our study, low rotation speed and divergent donor cut angles showed the best results. The cut quality and the small thermal damage with the Q-switched 2.94-microm Er:YAG laser seem to be tolerable for corneal trephination. Therefore, this modality may be a low-cost, easy-to-handle alternative for nonmechanical corneal transplantation in humans.

Cornea↗

PubMed2002

[Bilateral nevus of the caruncle].

BACKGROUND: Pathologic processes of the caruncle are relatively rare with a predominance of papillomas, pyogenic granulomas and nevi. PATIENT: A 22 year old Caucasian male patient presented with bilateral hyperpigmentations of the caruncle. They had first been observed at an age of seven and showed slight progression in size and colour with time. Examination revealed hyperpigmentation with clear pseudocystic spaces. The tarsal or bulbar conjunctiva was not involved. An excision of both processes was performed. HISTOLOGY: Light microscopy revealed nests of nevus cells with melanin granules and round basophilic nuclei as well as gland ducts and sebaceous glands, beneath and within the multilayered keratinizing and nonkeratinizing squamous epithelium. The sections revealed a bilateral melanocytic compound nevus of the caruncle. DISCUSSION: To our knowledge, there is no report on bilateral nevi of the caruncle among the Caucasian population. The possibility of a common precursor cell during the embryologic development causing the bilaterality is discussed.

Adult↗

PubMed2002

[Ocular Manifestation of Mucopolysaccharidosis I-S (Scheie's Syndrome)].

BACKGROUND: Bilateral stromal corneal opacifications are a differential diagnostical challenge to identify associated systemic diseases. CASE-REPORT: A 47-year old civil engineer (height 167 cm) with bilateral stromal corneal clouding presented with visual loss for the last 27 years: VA 20/100 OD and 20/50 OS (following penetrating keratoplasty OS). The cornea showed milky-whitish, cloudy, diffuse stromal deposits without a separate lipoid arc. The posterior segment showed tapetoretinal degeneration. Scotopic ERG was decreased. A suspicious stiffness of interphalangeal joints on both hands was observed. There was an aortic and mitral insufficiency grade I. Serum levels of LDL, HDL and triglycerides were normal. The biomicroscopical diagnosis of Scheie's syndrome (mucopolysaccharidosis I-S) was confirmed by a deficiency of alpha-L-iduronidase in leukocytes (0.02 nmol/min/mg protein, normal range: 0.3 - 1.5). CONCLUSION: The differential diagnosis of bilateral corneal stromal opacification includes in addition to the mucopolysaccharidoses HDL-deficiency diseases (LCAT deficiency, Tangier disease, Fish eye disease), Schnyder's crystalline stromal dystrophy, cystinosis, gout and mucolipidoses. MPS I-S may easily be detected by alpha-L-iduronidase deficiency in leukocytes and increased mucopolysaccharides in the urine. Furthermore, patients with MPS I-S need general medical care because of cardiovascular abnormalities, joint stiffness and myopathies.

Corneal Opacity↗

PubMed2002

Superficial corneal effects of experimental nonmechanical penetrating keratoplasty using a Q-switched Er:YAG laser.

PURPOSE: To assess thermal effects of Q-switched Er:YAG laser trephination to corneal epithelium and superficial stroma using different mask types and materials for experimental penetrating keratoplasty. METHODS: Laser trephination was performed in 20 freshly-enucleated porcine eyes (repetition rate 5 Hz, pulse energy 65 mJ, spot size 0.7 mm). We used flat, open-metal and ceramic masks for donor and recipient trephination placed directly onto the corneal surface. Main outcome measures as assessed by light microscopy after PAS staining of 8-microm paraffin sections included: extension of tissue thermal damage at the cut edge in the superficial and basal epithelial layers, the basement membrane and subepithelial stroma, and depth and width of epithelial/stromal involvement in the area of the donor mask contact. RESULTS: The thermal damage in the superficial epithelium was more pronounced in donor (mean extension 61.6 +/- 15.6 microm) than in recipient (29.4 +/- 24.9 microm, p= 0.05) trephination. In donor trephination, thermal damage zone of the superficial epithelial layer was significantly smaller with ceramic than with metal masks (21.0 +/- 23.0 versus 61.6 +/- 15.6 microm, p= 0.014). In contrast, differences at basal epithelial layer (p= 0.44), basement membrane (p= 0.79), and subepithelial stroma (p= 0.2) were not statistically significant. Superficial donor involvement of the cornea adjacent to the paracentral donor mask contact zone was seen neither with ceramic nor with metal masks. CONCLUSION: Superficial corneal alterations adjacent to the mask-cornea contact zone may be minimized by using the Er:YAG laser in a Q-switched mode. Ceramic masks, in contrast to metal masks, further reduce superficial thermal alterations at the cut edge.

Animals↗

PubMedDate not supplied

Conoidal fitting of corneal topography height data after excimer laser penetrating keratoplasty.

PURPOSE: To demonstrate a mathematical method for approximation of discrete corneal topography height data with a biconic model surface for better appreciation of the optical performance of the graft after penetrating keratoplasty. METHODS: In this retrospective study we included 50 eyes of 50 patients (30 keratoconus; 20 Fuchs' dystrophy) undergoing nonmechanical excimer laser penetrating keratoplasty. Conventional keratometry, corneal topography (TMS-1), subjective refraction, and best spectacle-corrected visual acuity (BSCVA) were assessed preoperatively, 3 and 6 months postoperatively, and before/after suture removal. A biconic model surface was fitted to the topographic raw data (8.0-mm region of interest) minimizing the root mean square error and a set of parameters (meridional power, axis, conic constant, and approximation error) was determined. The refractive cylinder was correlated with keratometric power readings, the Simulated Keratometry (SimK) of the topography system, and the respective parameters of the model surface. RESULTS: Keratometric/SimK astigmatism increased from preoperatively (3.40/4.30 D) to 3 months (4.30/4.80 D) and decreased to 3.40/3.90 D after suture removal. Refractive cylinder/cylinder of the biconic increased slightly from 2.10/2.60 D preoperatively to 3.20/3.30 D after suture removal. The topographic cylinder SimK yielded the highest and the refractive cylinder the lowest values at each follow-up examination. Central keratometric power readings were stable before suture removal and decreased (about 1 D) due to suture removal. The conic constants in both meridional cross-sections changed from a prolate to a spherical shape in the early time course after penetrating keratoplasty and reconverted to a prolate shape after suture removal. Regarding cylinder axis, there was a significant correlation of the model surface with the refractive cylinder at all examination (P < .05) but only a mild correlation of the keratometric and SimK cylinder axis to the refractive cylinder axis at some examination stages. CONCLUSION: Approximation of corneal topography height data with a biconic model surface renders reconstruction of clinically relevant corneal topography parameters including corneal asphericity with marked data compression. The correlation of amount/axis of refractive cylinder was best represented with the model surface parameters.

Aged↗