Simultaneous measurement of refractive index and thickness of multilayer systems using Fourier domain optical coherence tomography, part 2: implementation

We introduce a theoretical method for simultaneous measurement of refractive index and thickness of multilayer systems using Fourier domain optical coherence tomography (FD-OCT) without any auxiliary arrangement. The input data to the formalism are the FD-OCT measured optical path lengths (OPLs) and properly selected spectral components of FD-OCT interference spectrum. The outputs of the formalism can be affected significantly by uncertainty in measuring the OPLs. An optimization method is introduced to deal with the relatively large amount of uncertainty in measured OPLs and enhance the final results. Simulation result shows that by using the optimization method, indices can be extracted with the absolute error ? 0.001 for transparent biological samples having indices < 1.55 .

Simultaneous measurement of refractive index and thickness of multilayer systems using Fourier domain optical coherence tomography, part 1: theory

We introduce a theoretical framework for simultaneous refractive index and thickness measurement of multilayer systems using the Fourier domain optical coherence tomography (FD-OCT) system without any previous information about the item under investigation. The input data to the new formalism are the FD-OCT measured optical path lengths and properly selected spectral components of the FD-OCT interference spectrum. No additional arrangement, reference reflector, or mechanical scanning is needed in this approach. Simulation results show that the accuracy of the extracted parameters depends on the index contrast of the sample while it is insensitive to the sample’s thickness profile. For transparent biological samples with smooth interfaces, when the object is in an aqueous medium and has indices < 1.55 , this method can extract indices and thicknesses with the absolute error ? 0.001 .

Advanced corneal surface ablation efficacy in myopia: changes in higher order aberrations

OBJECTIVE

To evaluate the change of ocular and corneal higher order aberrations (HOAs) after wavefront-guided advanced surface ablation (WF-ASA) for myopia using 4 different epithelial management techniques (AA-PRK, LASEK, Epi-PRK, and Epi-LASIK).

DESIGN

Retrospective single centre excimer database analysis.

PARTICIPANTS

Two hundred and forty eyes equally divided between the 4 WF-ASA techniques, matched for mean and range of required spherical correction.

METHODS

Ocular wavefront aberrations were measured using the wavescan aberrometer and corneal aberrations were obtained from corneal topography elevation maps and calculated by ray-tracing. All data were collected preoperatively and 3 months following treatment. The aberrations were described as Zernike polynomials, and analysis focused on total HOAs and spherical aberration (SA).

RESULTS

Three months postoperatively, there was a statistically significant surgically induced increase in total HOAs and SA both for ocular and corneal analysis (p < 0.001). There was no statistically significant difference in the induced ocular SA and HOAs between the groups, but the differences in induced corneal SA and HOAs were significant (p < 0.010). Ocular changes in SA were weakly correlated to preoperative SA (20.30, p < 0.001) but strongly correlated to applied spherical correction (20.68, p < 0.001). Surgically induced corneal SA was weakly correlated to preoperative corneal SA (20.34, p < 0.001) and applied spherical correction (20.46, p < 0.001).

CONCLUSIONS

Three months postoperatively, all procedures resulted in an increase in ocular and cornealHOAs and SA. Induced aberrations were most strongly correlated to the applied spherical power correction. Modifying the existing ablation pattern to compensate for induced HOAs might improve the outcome.

Effects of advanced surface ablations and intralase femtosecond LASIK on higher order aberrations and visual acuity outcome

BACKGROUND/AIMS

To study the changes in wavefront (ocular) and corneal higher order aberrations (HOAs) and visual acuity (VA) outcome following wavefront-guided advanced surface ablation (ASA) techniques and intralase femtosecond LASIK (iLASIK) in myopia treatment.

METHODS

Ocular aberration and corneal topography of 240 eyes in the ASA techniques (this was equally divided into a flap-on group where the epithelial flap was preserved and reapplied to the photoablated stromal bed and a flap-off group when the epithelial flap was discarded during the procedure), and 138 eyes in the iLASIK group were obtained before and 3 months following treatment. The correlation of aberrations with best spectacle-corrected visual acuity was analyzed.

RESULTS

At 3 months, there was statistically significant (P < 0.001) surgically induced increase in spherical aberration (SA) in each of the techniques for both ocular and corneal analysis. iLASIK induced significantly less ocular and corneal HOAs (P < 0.001). The mean manifest refractive spherical equivalent was closer to attempted correction compared to other groups (P < 0.001). Eighty-three eyes (70%) of flap-on, 80 (67%) flap-off and 94 eyes (68%) in the iLASIK group achieved 20/20 uncorrected VA. Fifteen eyes (11%) accomplished 20/12.5 or better in iLASIK compared to 4 (3%) for flap-on and 7 (6%) for flap-off ASA group. Only the flap-off treatment showed a consistent correlation between the corrected aberrations and visual performance.

CONCLUSION

At 3 months, all procedures resulted in a significant increase in HOAs and SA. All had comparable 20/20 VA and 11% of iLASIK patients achieved 20/12.5 or better level.

Correlation between refractive error, corneal power, and thickness in a large population with a wide range of ametropia

PURPOSE

To determine the correlations between mean keratometry (KM), central corneal thicknesses (CCT), and cycloplegic spherical equivalent (SE) in patients with a wide range of ametropia.

METHODS

Retrospective analysis of the excimer laser surgery database at the University of Ottawa Eye Institute between 1993 and 2008 was performed. This study included 3395 eyes from 1858 subjects. The refractive error ranged from +6.75 to -14.00 D. CCT was obtained either by ultrasound pachymetry or anterior segment tomography. Keratometry was determined using an autokeratorefractometer.

RESULTS

In the myopic group, the SE was observed to be inversely proportional to the KM (correlation coefficient, -0.18; P < 0.01). The KM and CCT were also inversely proportional (-0.11; P < 0.01). In hyperopes, a correlation between the cycloplegic SE and KM was also found (-0.25; P < 0.01), but the CCT did not correlate with either of these metrics. A direct correlation for the myopic group was found between KM and the difference in power of the principal meridians (keratometric astigmatism [KA]) (0.08; P < 0.01). This relationship was not observed for the hyperopic group. Within the myopic group the SE correlated with the refractive astigmatism (RA) (-0.04; P = 0 0.04). In all groups, a strong correlation was observed between RA and KA (0.78; P < 0.01).

CONCLUSIONS

In the myopia group, the KM showed close correspondence with KA and an inverse relationship with SE and CCT. In hyperopes, an inverse correlation between the KM and SE was found, but no correlation with CCT was evident.

A biosynthetic alternative to human donor tissue for inducing corneal regeneration: 24-month follow-up of a phase 1 clinical study

Corneas from human donors are used to replace damaged tissue and treat corneal blindness, but there is a severe worldwide shortage of donor corneas. We conducted a phase 1 clinical study in which biosynthetic mimics of corneal extracellular matrix were implanted to replace the pathologic anterior cornea of 10 patients who had significant vision loss, with the aim of facilitating endogenous tissue regeneration without the use of human donor tissue. The biosynthetic implants remained stably integrated and avascular for 24 months after surgery, without the need for long-term use of the steroid immunosuppression that is required for traditional allotransplantation. Corneal reepithelialization occurred in all patients, although a delay in epithelial closure as a result of the overlying retaining sutures led to early, localized implant thinning and fibrosis in some patients. The tear film was restored, and stromal cells were recruited into the implant in all patients. Nerve regeneration was also observed and touch sensitivity was restored, both to an equal or to a greater degree than is seen with human donor tissue. Vision at 24 months improved from preoperative values in six patients. With further optimization, biosynthetic corneal implants could offer a safe and effective alternative to the implantation of human tissue to help address the current donor cornea shortage.

Collagen-phosphorylcholine interpenetrating network hydrogels as corneal substitutes

A biointeractive collagen-phospholipid corneal substitute was fabricated from interpenetrating polymeric networks comprising 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide and N-hydroxysuccinimide crosslinked porcine atelocollagen, and poly(ethylene glycol) diacrylate crosslinked 2-methacryloyloxyethyl phosphorylcholine (MPC). The resulting hydrogels showed an overall increase in mechanical strength beyond that of either original component and enhanced stability against enzymatic digestion (by collagenase) or UV degradation. More strikingly, these hydrogels retained the full biointeractive, cell friendly properties of collagen in promoting corneal cell and nerve in-growth and regeneration (despite MPC's known anti-adhesive properties). Measurements of refractive indices, white light transmission and backscatter showed the optical properties of collagen-MPC are comparable or superior to those of the human cornea. In addition, the glucose and albumin permeability were comparable to those of human corneas. Twelve-month post-implantation results of collagen-MPC hydrogels into mini-pigs showed regeneration of corneal tissue (epithelium, stroma) as well as the tear film and sensory nerves. We also show that porcine collagen can be substituted with recombinant human collagen, resulting in a fully-synthetic implant that is free from the potential risks of disease transmission (e.g. prions) present in animal source materials.

PEG-stabilized carbodiimide crosslinked collagen-chitosan hydrogels for corneal tissue engineering

Implantable biomaterials that mimic the extracellular matrix (ECM) in key physical and physiological functions require components and microarchitectures that are carefully designed to maintain the correct balance between biofunctional and physical properties. Our goal was to develop hybrid polymer networks (HPN) that combine the bioactive features of natural materials and physical characteristics of synthetic ones to achieve synergy between the desirable mechanical properties of some components with the biological compatibility and physiological relevance of others. In this study, we developed collagen-chitosan composite hydrogels as corneal implants stabilized by either a simple carbodiimide cross-linker or a hybrid cross-linking system comprised of a long-range bi-functional cross-linker (e.g. poly(ethylene glycol) dibutyraldehyde (PEG-DBA)), and short-range amide-type cross-linkers (e.g. 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), and N-hydroxysuccinimide (NHS)). Optimum hybrid hydrogel demonstrated significantly enhanced mechanical strength and elasticity by 100 and 20%, respectively, compared to its non-hybrid counterpart. It demonstrated excellent optical properties, optimum mechanical properties and suturability, and good permeability to glucose and albumin. It had excellent biocompatibility and when implanted into pig corneas for 12 months, allowed seamless host-graft integration with successful regeneration of host corneal epithelium, stroma, and nerves.

Tissue-engineered recombinant human collagen-based corneal substitutes for implantation: performance of type I versus type III collagen

PURPOSE

To compare the efficacies of recombinant human collagens types I and III as corneal substitutes for implantation.

METHODS

Recombinant human collagen (13.7%) type I or III was thoroughly mixed with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide. The final homogenous solution was either molded into sheets for in vitro studies or into implants with the appropriate corneal dimensions for transplantation into minipigs. Animals with implants were observed for up to 12 months after surgery. Clinical examinations of the cornea included detailed slit lamp biomicroscopy, in vivo confocal microscopy, and fundus examination. Histopathologic examinations were also performed on corneas harvested after 12 months.

RESULTS

Both cross-linked recombinant collagens had refractive indices of 1.35, with optical clarity similar to that in human corneas. Their chemical and mechanical properties were similar, although RHC-III implants showed superior optical clarity. Implants into pig corneas over 12 months show comparably stable integration, with regeneration of corneal cells, tear film, and nerves. Optical clarity was also maintained in both implants, as evidenced by fundus examination.

CONCLUSIONS

Both RHC-I and -III implants can be safely and stably integrated into host corneas. The simple cross-linking methodology and recombinant source of materials makes them potentially safe and effective future corneal matrix substitutes.

Innervation of tissue-engineered recombinant human collagen-based corneal substitutes: a comparative in vivo confocal microscopy study

PURPOSE

To compare reinnervation in recombinant human collagen-based corneal substitutes with allografts during a 1-year postimplantation follow-up period in pigs. A retrospective comparison to innervation in porcine collagen-based biosynthetic grafts was also performed.

METHODS

Pigs received a corneal allograft or a substitute made of either recombinant human type-I or -III collagen. In vivo confocal microscopic examination of the central cornea of surgical and untouched control eyes before surgery and at 2, 6, and 12 months after surgery was performed to quantify the number, density, and diameter of nerves at various corneal depths.

RESULTS

By 12 months after surgery, the number and density of regenerated nerves in the anterior and deep anterior corneal stroma recovered to preoperative and control levels in both types of substitute grafts and in the allografts. In the subepithelial and subbasal regions, however, significantly fewer nerves were detected relative to those in control subjects at 12 months, regardless of graft type (P < 0.05), similar to the behavior of porcine collagen-based biosynthetic grafts. An absence of thick stromal nerve trunks (diameter, >10 mum) in all grafts, irrespective of material type, indicated that nerve regeneration in grafts was accompanied by persistent morphologic changes.

CONCLUSIONS

Nerve regeneration in recombinant human collagen-based biosynthetic corneal grafts proceeded similarly to that in allograft tissue, demonstrating the suitability of recombinant human collagen constructs as nerve-friendly corneal substitutes. Furthermore, only minor differences were noted between type-I and -III collagen grafts, indicating an insensitivity of nerve regeneration to initial collagen type.

Vascular factors predict rate of progression in Alzheimer disease

BACKGROUND

While there is considerable epidemiologic evidence that cardiovascular risk factors increase risk of incident Alzheimer disease (AD), few studies have examined their effect on progression after an established AD diagnosis.

OBJECTIVE

To examine the effect of vascular factors, and potential age modification, on rate of progression in a longitudinal study of incident dementia.

METHODS

A total of 135 individuals with incident AD, identified in a population-based sample of elderly persons in Cache County, UT, were followed with in-home visits for a mean of 3.0 years (range: 0.8 to 9.5) and 2.1 follow-up visits (range: 1 to 5). The Clinical Dementia Rating (CDR) Scale and Mini-Mental State Examination (MMSE) were administered at each visit. Baseline vascular factors were determined by interview and physical examination. Generalized least-squares random-effects regression was performed with CDR Sum of Boxes (CDR-Sum) or MMSE as the outcome, and vascular index or individual vascular factors as independent variables.

RESULTS

Atrial fibrillation, systolic hypertension, and angina were associated with more rapid decline on both the CDR-Sum and MMSE, while history of coronary artery bypass graft surgery, diabetes, and antihypertensive medications were associated with a slower rate of decline. There was an age interaction such that systolic hypertension, angina, and myocardial infarction were associated with greater decline with increasing baseline age.

CONCLUSION

Atrial fibrillation, hypertension, and angina were associated with a greater rate of decline and may represent modifiable risk factors for secondary prevention in Alzheimer disease. The attenuated decline for diabetes and coronary artery bypass graft surgery may be due to selective survival. Some of these effects appear to vary with age.

Spectroscopic whole-blood indicators of end-stage renal disease and the hemodialysis treatment

The diffuse reflection spectrum in the 500-1670 nm region for whole blood taken from healthy subjects and end-stage renal disease (ESRD) patients was measured to test the feasibility of optically monitoring ESRD and its treatment by hemodialysis. Spectral regions where optical absorption significantly differed between healthy subjects and ESRD patients were used to form a multiple linear discriminant classification model. With this model a total of 41 whole-blood samples were classified into healthy, pretreatment and posttreatment ESRD classes. 96.7% of original and cross-validated cases and 100% of independent validation cases were correctly classified, indicating ESRD and its treatment exhibit characteristic spectral features in whole blood. Upon comparison of the discriminant model variables with a few key clinical blood parameters, model variables were found to significantly correlate with hematocrit and plasma levels of urea and potassium (P<0.05). The results of this study suggest that the optical signature of whole blood conveys basic clinical status information, and provides a path for investigating improved indices of hemodialysis toxicity, adequacy and patient outcome.

Innervation of Tissue-Engineered Corneal Implants in a Porcine Model: A 1-Year In Vivo Confocal Microscopy Study

PURPOSE

To examine the pattern of nerve regeneration within tissue-engineered corneal substitutes grafted into host porcine corneas over a 1-year postoperative period.

METHODS

Biodegradable corneal substitutes from cross-linked collagen were implanted into the left eyes of 12 pigs by deep lamellar keratoplasty. Regeneration of severed nerves into the central implant region was investigated with in vivo confocal microscopy. Both implant-recipient and control (right) eyes were examined before surgery and 2, 6, 10, and 12 months after surgery, to quantify the number, density, diameter, and branching of nerve fiber bundles at various corneal depths. Transmission electron microscopy was used to confirm the presence of nerve bundles.

RESULTS

Two months after surgery, corneal nerve ingrowth was observed within the deep anterior stroma, with a number and density of regenerated nerves significantly higher than in nonsurgical control eyes (P < 0.01). Nerves within the superficial anterior stroma regenerated by 6 to 10 months after surgery, and the first subbasal epithelial nerves were seen 10 months after surgery. After 1 year, subbasal nerve density recovered to preoperative levels. Nerve fibers in the deep anterior stroma remained significantly thinner relative to control eyes after 1 year (P < 0.001), where both superficial anterior and subbasal nerve diameter did not change relative to control eyes.

CONCLUSIONS

The pattern of reinnervation within tissue-engineered corneal substitutes has been quantified in vivo. Innervation proceeded rapidly in the deep anterior stroma, followed by repopulation of more superficial regions. One year after surgery, nerve density within the tissue-engineered cornea increased or remained unchanged relative to controls in all corneal regions examined.

Hemodialysis monitoring in whole blood using transmission and diffuse reflection spectroscopy: A pilot study

Visible and near infrared transmission and diffuse reflection spectroscopy were used to monitor changes in whole blood resulting from hemodialysis treatment for end-stage renal disease. Blood samples from 8 patients on chronic hemodialysis therapy were measured in the 500- to 1700-nm wavelength range immediately before and after a single treatment. Principal component scores characteristic of each spectrum were derived, and mean pre- and posttreatment scores of the first principal component indicated a significant treatment-dependent change in both optical transmission (P = 0.004) and diffuse reflection (P < 0.001). Significant treatment-induced change persisted (P < 0.05) when the first four principal components were used to account for >97% of the treatment-dependent spectral variation. Some blood spectral changes expressed in terms of difference spectra (posttreatment - pretreatment) were consistent with standard clinical indicators of weight reduction, urea reduction, and potassium change, with probable origins at a molecular level. The results indicate the feasibility of using optical transmission and diffuse reflection spectroscopy to characterize clinically relevant blood changes for the future development of more comprehensive indicators of hemodialysis efficacy and long-term clinical outcomes. Moreover, the optical techniques employed are adaptable for potential online monitoring of blood changes during the hemodialysis treatment.

A Simple, Cross-linked Collagen Tissue Substitute for Corneal Implantation

PURPOSE

To develop a simple corneal substitute from cross-linked collagen.

METHODS

Porcine type I collagen (10%; pH 5), was mixed with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The final homogenous solution was molded to corneal dimensions, cured, and then implanted into rabbits and minipigs by lamellar keratoplasty. The implants were followed for up to 6 months after surgery. Clinical examinations of the cornea included detailed slit lamp biomicroscopy, in vivo confocal microscopy, topography and esthesiometry for nerve function. Histopathologic examinations were also performed on rabbit corneas harvested after 6 months.

RESULTS

Cross-linked collagen (refractive index, 1.35) had optical clarity superior to human corneas. Implanted into rabbit and porcine corneas, only 1 of 24 of the surgical corneas showed a slight haze at 6 months after surgery. All other implants showed no adverse reactions and remained optically clear. Topography showed a smooth surface and a profile similar to that of the contralateral nonsurgical eye. The implanted matrices promoted regeneration of corneal cells, tear film, and nerves. Touch sensitivity was restored, indicating some restoration of function. The corneas with implants showed no significant loss of thickness and demonstrated stable host-graft integration.

CONCLUSIONS

Collagen can be adequately stabilized, using water soluble carbodiimides as protein cross-linking reagents, in the fabrication of corneal matrix substitutes for implantation. The simple cross-linking methodology would allow for easy fabrication of matrices for transplantation in centers where there is a shortage of corneas, or where there is need for temporary patches to repair perforations in emergency situations.

Vitamin D receptor genotype and risk of osteoporotic hip fracture in elderly women of Utah: an effect modified by parity

INTRODUCTION

The associations between vitamin D receptor (VDR) Bsm I and Fok I genotypes, parity, and risk of osteoporotic hip fracture were evaluated in a statewide population-based case-control study in Utah.

METHODS

Women age 50-89 years with hip fracture (n=882) were ascertained via surveillance of 18 Utah hospitals from 1997 to 2001. Age-matched controls were randomly selected (n=897). Participants were interviewed in their homes, and blood samples were collected for genotyping.

RESULTS

In logistic regression analyses that controlled for multiple confounders, Bsm I VDR genotype but not Fok I genotype was associated with risk of osteoporotic hip fracture (OR bb vs. BB genotype: 0.68; 95% CI: 0.50, 0.95). In similar analyses, no overall association was observed between parity status and risk of osteoporotic hip fracture. However, the effect of VDR genotype was modified by parity status. Among nulliparous women (n=140), Bsm I genotype was not associated with risk of hip fracture (OR bb vs. BB: 0.82; 95% CI: 0.28, 2.4); among primiparous women (n=133), bb genotype was associated with increased risk of hip fracture (OR bb vs. BB: 3.30; 95% CI: 0.96, 11.29); among multiparous women (n=1,400), bb genotype was associated with decreased risk of hip fracture (OR bb vs. BB: 0.59; 95% CI: 0.42, 0.84).

CONCLUSION

VDR Bsm I genotype was associated with risk of hip fracture in Utah women, and this effect was modified by parity status. Hormonal or lifestyle factors related to parity may underlie this interaction.

XIAP-mediated neuroprotection in retinal ischemia

Retinal ischemia results in the loss of vision in a number of ocular diseases including acute glaucoma, diabetic retinopathy, hypertensive retinopathy and retinal vascular occlusion. Recent studies have shown that most of the neuronal death that leads to loss of vision results from apoptosis. XIAP-mediated gene therapy has been shown to protect a number of neuronal types from apoptosis but has never been assessed in retinal neurons following ischemic-induced cell death. We injected an adeno-associated viral vector expressing XIAP or GFP into rat eyes and 6 weeks later, rendered them ischemic by raising intraocular pressure. Functional analysis revealed that XIAP-treated eyes retained larger b-wave amplitudes than GFP-treated eyes up to 4 weeks post-ischemia. The number of cells in the inner nuclear layer (INL) and the thickness of the inner retina were significantly preserved in XIAP-treated eyes compared to GFP-treated eyes. Similarly, there was no significant reduction in optic nerve axon numbers in XIAP-treated eyes. There were also significantly fewer TUNEL (TdT-dUTP terminal nick end labeling) positive cells in the INL of XIAP-treated retinas at 24 h post-ischemia. Thus, XIAP-mediated gene therapy imparts both functional and structural protection to the retina after a transient ischemic episode.

Wavefront-guided correction of ocular aberrations: are phase plate and refractive surgery solutions equal?

Wavefront-guided laser eye surgery has been recently introduced and holds the promise of correcting not only defocus and astigmatism in patients but also higher-order aberrations. Research is just beginning on the implementation of wavefront-guided methods in optical solutions, such as phase-plate-based spectacles, as alternatives to surgery. We investigate the theoretical differences between the implementation of wavefront-guided surgical and phase plate corrections. The residual aberrations of 43 model eyes are calculated after simulated refractive surgery and also after a phase plate is placed in front of the untreated eye. In each case, the current wavefront-guided paradigm that applies a direct map of the ocular aberrations to the correction zone is used. The simulation results demonstrate that an ablation map that is a Zernike fit of a direct transform of the ocular wavefront phase error is not as efficient in correcting refractive errors of sphere, cylinder, spherical aberration, and coma as when the same Zernike coefficients are applied to a phase plate, with statistically significant improvements from 2% to 6%.

Functional innervation in tissue engineered models for in vitro study and testing purposes

The biotechnology industry is rapidly expanding and the emerging field of tissue engineering is projected to have a high impact in the near future. Recently the field of cellular, drug, and prosthetic delivery has melded with the field of tissue engineering to make simulated tissues. In addition to their roles as tissue substitutes for transplantation, these simulated tissues may provide more accurate models and environments for toxicology testing and the study of peripheral nerves. The current study demonstrates the importance of innervation, in general, for the function of engineered tissues. We observe that the presence of nerves in a tissue engineered (TE) human cornea model enhances the growth of the epithelium and the formation of its protective mucin layer. Innervation also confers protection to the epithelium from chemical insult, as determined by the level of post-treatment epithelial cell death. We demonstrate differential responses of the nerves to chemical stimuli by changes in intracellular sodium as measured by 2-photon microscopy. The 2-photon imaging techniques also allow for the visualization and study of the fine sensory axon fibers within the 3-dimensional tissue. This work demonstrates a role for innervation in the protective quality and function of the engineered tissue, and the potential to use the nerves themselves as indicators of the severity of an insult. These results are important to consider for the development of any optimized TE models for in vitro study and testing purposes.

Regulation of nestin expression after cortical ablation in adult rat brain

During embryogenesis, transient expression of nestin in proliferating neuroepithelial stem cells signals the commitment of progenitor cells to differentiate. Although adult mammalian brain contains very little nestin, significant upregulation of nestin has been reported following cerebral injury, leading to speculation that nestin may be involved in brain repair. In this study, we assessed the temporal profile of nestin expression following ablation injury of the sensory barrel cortex and investigated the influence of contralateral whisker stimulation on nestin expression. Since the adult mammalian brain contains proliferating neuronal progenitor cells that can be labeled with bromodeoxyuridine (BrdU), we also determined the association of nestin reexpression with BrdU-labeled cells. Nestin reexpression was detected predominantly in the ipsilateral cortex 3 days post-ablation. There was no significant nestin upregulation in the subcortical region. Nestin reexpression was most marked surrounding the lesion, but also extended throughout the entire lateral cortex. Nestin in the ipsilateral cortex subsided by day 7, although perilesional nestin expression was still apparent 28 days post-injury. Western blot analysis of nestin expression 3 days post-ablation confirmed a significant two-fold increase in nestin expression (p<0.05). Double immunofluorescence labeling demonstrated that the majority of nestin expression occurred in astrocytes. We were unable to detect any colocalization with neuronal makers. However, BrdU-labeled cells, which were readily detected in the subventricular zone prior to injury, were readily detected in the perilesional area 3 days post-ablation, concomitant with nestin in this area. Confocal microscopy detected several BrdU-positive cells expressing nestin. Taken together, the data support a potential role for nestin reexpression in brain repair.

A collagen-based scaffold for a tissue engineered human cornea: physical and physiological properties

Stabilized collagen-glycosaminoglycan scaffolds for tissue engineered human corneas were characterized. Hydrated matrices were constructed by blending type I collagen with chondroitin sulphates (CS), with glutaraldehyde crosslinking. A corneal keratocyte cell line was added to the scaffolds with or without corneal epithelial and endothelial cells. Constructs were grown with or without ascorbic acid. Wound-healing was evaluated in chemical-treated constructs. Native, noncrosslinked gels were soft with limited longevity. Crosslinking strengthened the matrix yet permitted cell growth. CS addition increased transparency. Keratocytes grown within the matrix had higher frequencies of K+ channel expression than keratocytes grown on plastic. Ascorbic acid increased uncrosslinked matrix degradation in the presence of keratocytes, while it enhanced keratocyte growth and endogenous collagen synthesis in crosslinked matrices. Wounded constructs showed recovery from exposure to chemical irritants. In conclusion, this study demonstrates that our engineered, stabilized matrix is well-suited to function as an in vitro corneal stroma.

Cellular and nerve regeneration within a biosynthetic extracellular matrix for corneal transplantation

Our objective was to determine whether key properties of extracellular matrix (ECM) macromolecules can be replicated within tissue-engineered biosynthetic matrices to influence cellular properties and behavior. To achieve this, hydrated collagen and N-isopropylacrylamide copolymer-based ECMs were fabricated and tested on a corneal model. The structural and immunological simplicity of the cornea and importance of its extensive innervation for optimal functioning makes it an ideal test model. In addition, corneal failure is a clinically significant problem. Matrices were therefore designed to have the optical clarity and the proper dimensions, curvature, and biomechanical properties for use as corneal tissue replacements in transplantation. In vitro studies demonstrated that grafting of the laminin adhesion pentapeptide motif, YIGSR, to the hydrogels promoted epithelial stratification and neurite in-growth. Implants into pigs' corneas demonstrated successful in vivo regeneration of host corneal epithelium, stroma, and nerves. In particular, functional nerves were observed to rapidly regenerate in implants. By comparison, nerve regeneration in allograft controls was too slow to be observed during the experimental period, consistent with the behavior of human cornea transplants. Other corneal substitutes have been produced and tested, but here we report an implantable matrix that performs as a physiologically functional tissue substitute and not simply as a prosthetic device. These biosynthetic ECM replacements should have applicability to many areas of tissue engineering and regenerative medicine, especially where nerve function is required.

Innervated human corneal equivalents as in vitro models for nerve‐target cell interactions

A sensory nerve supply is crucial for optimal tissue function. However, the mechanisms for successful innervation and the signaling pathways between nerves and their target tissue are not fully understood. Engineered tissue substitutes can provide controllable environments in which to study tissue innervation. We have therefore engineered human corneal substitutes that promote nerve in-growth in a pattern similar to in vivo re-innervation. We demonstrate that these nerves (a) are morphologically equivalent to natural corneal nerves; (b) make appropriate contact with target cells; (c) can generate action potentials; (d) respond to chemical and physical stimuli; and (e) play an important role in the overall functioning of the bioengineered tissue. This model can be used for studying the more general topics of nerve ingrowth or regeneration and the interaction between nerves and their target cells and, more specifically, the role of nerves in corneal function. This model could also be used as an in vitro alternative to animals for safety and efficacy testing of chemicals and drugs.

Increased disk size in glaucomatous eyes vs normal eyes in the Reykjavik eye study

PURPOSE

To evaluate the use of disk diameter as an indicator in the identification of glaucomatous optic neuropathy.

METHODS

We evaluated all available stereofundus photographs for 1,040 right eyes obtained in the Reykjavik Eye Study. Horizontal and vertical disk diameters were determined in a masked manner by a glaucoma specialist (K.F.D.). All disk diameters were corrected for refractive error.

RESULTS

There were significant differences (P <.05) between the corrected vertical disk diameters of normal subjects (0.189 +/- 0.018 inches) and those suspected of having glaucoma (0.202 +/- 0.020 inches) as well as between the normal and the glaucoma groups (0.206 +/- 0.029 inches). The corrected horizontal measurement showed the same pattern.

CONCLUSIONS

In the Reykjavik Eye Study, optic disks meeting structural criteria for glaucoma are significantly larger than normal nerves.

Changes in measured intraocular pressure after hyperopic photorefractive keratectomy

PURPOSE

To investigate the effect of hyperopic photorefractive keratectomy (PRK) on intraocular pressure (IOP) measurements.

SETTING

University of Ottawa Eye Institute, Ottawa Hospital, Ottawa, Canada.

METHODS

In this retrospective cohort study, IOP and central corneal thickness (CCT) were measured preoperatively and at 1, 2, 3, 6, 12, 18, and 24 months in 191 eyes that had hyperopic PRK with the VISX Star excimer laser. All corrections applied were between +1.00 and +6.50 diopters (D) of sphere and less than 3.75 D of cylinder.

RESULTS

At all postoperative examinations, the mean IOP in the hyperopic PRK group was 1.0 to 1.8 mm Hg lower than the preoperative IOP (P <.001). A large range of IOP changes was found across the population; eg, at 6 months, 49% of the eyes had a change in IOP from baseline of at least +/-3 mm Hg. A mean reduction of 19 microm of CCT was found with pachymetry after surgery (P < .001). The change in IOP readings postoperatively was not correlated with age, sex, keratometric readings, or applied correction. Changes in IOP were strongly correlated with preoperative IOP at all time points and with preoperative CCT at 18 and 24 months (P < .001). After hyperopic PRK, the measured IOP was more likely to increase in patients with preoperative IOPs less than 14.5 mm Hg and more likely to decrease in patients with preoperative IOPs above 14.5 mm Hg.

CONCLUSION

Changes in IOP after hyperopic PRK were similar to changes after myopic PRK, despite only minimal changes in the CCT. This suggests that hyperopic PRK results in biomechanical effects that modify the elastic properties of the cornea beyond the changes in rigidity expected from central corneal thinning. There was a strong negative correlation between the measured preoperative IOP and the change in IOP postoperatively that was likely the result of regression of the mean effect.

New Paradigm for the Treatment of Myopia By Refractive Surgery

It is proposed that refractive surgery visual outcomes will be significantly improved when using refractive surgery profiles that maintain preoperative corneal aberrations following surgery. An equation to calculate postoperative corneal shapes is presented.

Acute hyperinsulinemia and very-low-density and low-density lipoprotein subfractions in obese subjects

BACKGROUND

The influence of hyperinsulinemia on concentrations of lipoprotein subfractions in obese, nondiabetic persons has not been clarified.

OBJECTIVE

We analyzed VLDL and LDL subfractions before and after a euglycemic, hyperinsulinemic clamp.

DESIGN

Lipoprotein subfractions were isolated from plasma samples obtained in the basal state and after a 4-h clamp from obese patients, obese patients with type 2 diabetes, and nonobese control subjects.

RESULTS

Hyperinsulinemia tended to reduce concentrations (&xmacr;: 20%) of large, triacylglycerol-rich VLDL(1) in obese patients but had a minor effect on VLDL(2) and VLDL(3). Placing obese patients into insulin-sensitive and insulin-resistant subgroups revealed distinct effects of the degree of insulin sensitivity on VLDL. VLDL(1) concentrations decreased by a mean of 38% (P < 0.05) in insulin-sensitive patients after the clamp, similar to but less marked than the decrease observed in control subjects (&xmacr;: 62%; P < 0.01). VLDL(1) concentrations did not change significantly after the clamp in insulin-resistant patients (and patients with type 2 diabetes), whereas VLDL(3) concentrations decreased in both groups, in contrast with the changes seen in the insulin-sensitive patients and control subjects. Acute hyperinsulinemia modified the LDL subfraction profile toward a greater prevalence of small, dense LDLs in insulin-resistant patients and patients with type 2 diabetes.

CONCLUSIONS

Insulin resistance appears to be the primary determinant of the modifications to VLDL subfraction concentrations. Our results suggest a continuum of impaired insulin action on VLDL, ranging from that in healthy persons to that in patients with type 2 diabetes, in which obese patients occupy a transition state. Insulin resistance may also play a role in detrimental modifications to the LDL profile by allowing the development of hypertriglyceridemia.

Functional human corneal equivalents constructed from cell lines

Human corneal equivalents comprising the three main layers of the cornea (epithelium, stroma, and endothelium) were constructed. Each cellular layer was fabricated from immortalized human corneal cells that were screened for use on the basis of morphological, biochemical, and electrophysiological similarity to their natural counterparts. The resulting corneal equivalents mimicked human corneas in key physical and physiological functions, including morphology, biochemical marker expression, transparency, ion and fluid transport, and gene expression. Morphological and functional equivalents to human corneas that can be produced in vitro have immediate applications in toxicity and drug efficacy testing, and form the basis for future development of implantable tissues.

Positive correlation between Tono-Pen intraocular pressure and central corneal thickness

OBJECTIVE

To examine the relationship between intraocular pressure (IOP) readings taken by the Tono-Pen tonometer (Mentor O&O, Norwell, MA) and central corneal thickness (CCT).

DESIGN

Prospective cross-sectional population study.

PARTICIPANTS

There were 651 eyes of 332 healthy subjects.

MAIN OUTCOME MEASURES

A questionnaire was given to each subject requesting information on gender, age, race, and other factors that can influence IOP. The IOP then was measured using the Tono-Pen followed by measurements of CCT using an ultrasonic pachymeter.

RESULTS

The IOP was found to increase by 2.9 mmHg/100 microns CCT in males and 1.2 mmHg/100 microns in females. For males, CCT was found to be statistically significant in predicting IOP (P < 0.001 in the right and left eyes) and diabetes was of borderline significance (P = 0.012 in the right eye, P = 0.089 in the left eye). For females, CCT was of borderline significance (P = 0.064 in the right eye, P = 0.019 in the left eye). In females, a family history of glaucoma (P = 0.021 in the right eye, P = 0.022 in the left eye) and hypertension (P = 0.010 in the right eye, P = < 0.001 in the left eye) were also significant in the prediction of IOP. Race was found to be a significant predictor of CCT (P < 0.001 in both right and left eyes) for both males and females.

CONCLUSION

Clinicians should be aware that, as with the Goldmann applanation tonometer, the Tono-Pen has a systematic error in IOP readings caused by its dependence on CCT. Tono-Pen IOP readings are positively correlated to CCT in males and, to a lesser extent, in females as well. The CCT measurements should be considered to ensure proper interpretation of IOP measurements in the diagnosis and management of disorders in which the CCT or IOP readings are outside normal limits.

Comparative study of the elevation topography of complex shapes

PURPOSE

To compare the accuracy and precision of the elevation topography from two commercially available instruments using videokeratoscopy or rasterstereography.

SETTING

University of Ottawa Eye Institute, Ottawa General Hospital, Ontario, Canada.

METHODS

Repeated measurements of elevation topography of six calibrated surfaces were done with the PAR Corneal Topography System (CTS) and the Tomey Topographic Modeling System (TMS-1). The shapes simulated normal (A: aspheric, B: spherocylindric) and postsurgical corneas (C: hyperopic, D: myopic, E: central island, F: phototherapeutic keratectomy). Surface shapes were described by parametric equations. Equation parameters associated with each elevation measurement were determined by best-fit analysis. Measurement precision was assessed by the standard deviation of the difference between the fitted and the measured data. Fitted parameters were compared with nominal values obtained from an independent calibration of each surface. The root mean square error (RMSE) of the deviation of the fitted from the nominal surfaces was used to evaluate the accuracy of each instrument.

RESULTS

The accuracy of the CTS exceeded that of the TMS-1 for all surfaces measured. The RMSE values (micron) were (A: 0.1, 6.5), (B: 0.3, 3.8), (C: 1.1, 11.8), (D: 5.0, 43.0), (E: 1.2, 3.2) and (F: 2.2, 17.5) for the CTS and TMS-1, respectively. The differences in the measured data from the fit surface were generally smaller with the TMS-1.

CONCLUSION

Quantitative analysis of elevation measurements showed that the CTS represented surface topography more accurately than the TMS-1.

Correction of intraocular pressure for changes in central corneal thickness following photorefractive keratectomy

OBJECTIVE

To investigate the relation between measurements of intraocular pressure (IOP) and central corneal thickness (CCT) in myopic patients undergoing photorefractive keratectomy (PRK).

DESIGN

Descriptive study.

SETTING

University-affiliated eye care centre in Ottawa.

PATIENTS

A total of 481 consecutive eyes of 318 patients with a mean preoperative refractive error of -6.50 dioptres treated with excimer PRK between March 1993 and December 1996.

OUTCOME MEASURES

IOP measured by Goldmann applanation tonometry and CCT before and 3, 6, 12, 18 and 24 months after surgery.

RESULTS

CCT was a significant predictor of IOP only before PRK and 6 months after PRK (p < or = 0.05). The relation between IOP and CCT suggests corrections for CCT that are not clinically significant (0.81 mm Hg [standard error (SE) 0.33 mm Hg] and 1.00 mm Hg [SE 0.38 mm Hg] per 100 microns of corneal thinning preoperatively and at 6 months respectively). On average, there was a significant decrease in IOP after PRK (0.96 mm Hg and 1.24 mm Hg at 12 and 24 months respectively) (p < 0.05). There was a significant correlation between change in IOP and change in CCT (decrease of 2.1 mm Hg per 100 microns of corneal thinning) (p < 0.05). For a given change in CCT, individual changes in IOP were variable, with increases or decreases of more than 5 mm Hg in some cases.

CONCLUSIONS

There are individual differences in IOP changes following PRK. Until further data are available we propose that the change in IOP between the preoperative visit (or the fellow eye, if healthy or untreated) and the 12-month visit be used as an individual correction factor to be applied to IOP measurements in the operated eye.

Intrauterine growth retardation in Iowa communities with herbicide-contaminated drinking water supplies

In a statewide survey of 856 Iowa municipal drinking water supplies in 1986-1987 the Rathbun rural water system was found to contain elevated levels of triazine herbicides. Rates of low birth weight, prematurity, and intrauterine growth retardation (IUGR) in live singleton births during the period 1984-1990 by women living in 13 communities served by the Rathbun water system were compared to other communities of similar size in the same Iowa counties. The Rathbun communities had a greater risk of IUGR than southern Iowa communities with other surface sources of drinking water (relative risk = 1.8; 95% CI = 1.3, 2.7). Multiple linear regression analyses revealed that levels of the herbicides atrazine, metolachlor, and cyanzinc were each significant predictors of community IUGR rates in southern Iowa after controlling for several potentially confounding factors including maternal smoking and socioeconomic variables. The association with IUGR was strongest for atrazine, but all three herbicides were intercorrelated and the independent contributions of each to IUGR risk could not be determined. We conclude that communities in southern Iowa with drinking water supplies contaminated with herbicides have elevated rates of IUGR compared to neighboring communities with different water supplies. Because of the limitations of the ecologic design of this study, including aggregate rather than individual measures of exposure and limited ability to control for confounding factors related to source of drinking water and risk of IUGR, a strong causal relationship between any specific water contaminant and risk of IUGR cannot yet be inferred. The association between the water supplied to the Rathbun communities and the increased risk of IUGR should be considered a preliminary finding that needs to be verified by more detailed epidemiologic studies.