In most cases, myopia is characterized by an increase in axial length of the eye, but the exact mechanisms for the axial elongation are still unknown. Higher intraocular pressure (IOP) has been associated with myopia and could be involved in eye enlargement. Also, some investigators have argued that the mechanical stresses generated by the ocular muscles during near work cause the eye to stretch out of shape. The purpose of this work was to investigate the effect of IOP on eye geometry, build a finite element model of the corneo-scleral shell of a chick eye, and verify if the eye could elongate due to increased IOP and hence become myopic. In the present study, myopia was induced in the right eye of chicks using -15 dioptre (D) goggles. The in-vitro pressure-volume curves of normal and myopic chick eyes were obtained using a computer controlled syringe pump and a digital pressure gauge. The axial length and horizontal equatorial diameter of the chick eyes as pressure increased were measured from digital photographs. To build the finite element model, normal chick eyes were frozen, sliced using a microtome, and photographed. The image sequence was aligned using MATLAB software and imported into SolidWorks® and Mimics software packages for three-dimensional (3D) reconstruction. Three 3D models were constructed and imported into Abaqus/CAE® software for finite element analysis. Isotropic, homogeneous, linear elastic and exponentially stiffening material properties were used for the finite element models. The results of the finite element models were compared with the experimental data. The results showed that normal chick eyes elongated in the axial direction and initially contracted in the horizontal equatorial direction as IOP increased. Myopic chick eyes did not elongate as much in the axial direction and did not contract as much in the horizontal equatorial direction compared with normal eyes. The volumetric deformation of myopic eyes was similar to normal eyes suggesting that growth and remodelling of the ocular tissues was involved in experimental myopia in the chick. The 3D reconstructed geometry of a chick eye was similar to a real eye although small geometric inaccuracies were present. The finite element model with isotropic, homogeneous, and exponentially stiffening material properties agreed well with the experimental strains in spite of the fact that the anisotropy of the ocular tissues was not included in the finite element model. This suggests that the oblate geometry of the chick eye was the main parameter dictating its deformation under increased IOP. The natural tendency of chick eyes is to elongate in the axial direction as IOP increases and this suggests that IOP could play a role in myopia onset and progression. The way chick eyes deformed as pressure increased is probably due to their oblate geometry. The finite element model could be used in the future to study the effect of different parameters such as ocular muscle forces and ocular tissues material properties on eye geometry. This way, an eye configuration more prone to eye elongation and myopia could be determined.

Overview: Optical concepts as they relate to the ophthalmologic correction of vision in corneal laser vision correction and intraocular lens design was examined. Purpose: The interaction between the excimer laser and residual corneal tissue in laser vision correction produces unwanted side effects. Understanding the origin of these artifacts can lead to better procedures. Furthermore, accommodating intraocular lenses offer a potential for eliminating presbyopia. Understanding the properties of a new accommodating intraocular lens incorporating a deformable interface may lead to advances in cataract surgery. Introduction: Corneal surface irregularities following laser refractive procedures are commonly seen. They regularly result in a patient’s decreased best corrected visual acuity and decreased contrast sensitivity. These changes are only seen in biologic tissue and the etiology has been elusive. A thermal response has been theorized and was investigated in this research. In addition, intraocular lenses using a mechanically deforming interface to change their power in order to duplicate natural accommodation have been developed. The deforming interface(s) induce optical aberrations due to irregular deformations. Design efforts have centered on minimizing these deformations. Both of the ophthalmic applications have been analyzed using finite element analysis (FEA) to understand their inherent optical properties. Methods: FEA modeling of thermal theory has been applied to verify that excimer laser induced collagen contraction creates corneal surface irregularities and central islands. A mathematical model which indicates the viability of the theory was developed. The modeling results were compared to post ablation changes in eyes utilizing an excimer (ArF 193 nm), as well as non-ablative thermal heating in eyes with a CO₂ laser. Addition modeling was performed on an Intraocular lens prototype measuring of actuation force, lens power, interface contour, optical transfer function, and visual Strehl ratio. Prototype verified mathematical models were utilized to optimize optical and mechanical design parameters to maximize the image quality and minimize the required force. Results: The predictive model shows significant irregular central buckling formation and irregular folding. The amount of collagen contraction necessary to cause significant surface changes is very small (0.3%). Uniform scanning excimer laser ablation to corneal stroma produces a significant central steepening and peripheral flattening in the central 3mm diameter. Isolated thermal load from uniform CO₂ laser irradiation without ablation also produces central corneal steepening and paracentral flattening in the central 3mm diameter. The iterative mathematical modeling based upon the intraocular lens prototype yielded maximized optical and mechanical performance through varied input mechanical and optical parameters to produce a maximized visual Strehl ratio and a minimized force requirement. Conclusions: The thermal load created by laser irradiation creates a characteristic spectrum of morphologic changes on the porcine corneal stromal surface which correlates to the temperature rise and is not seen inorganic, isotropic material. The highly similar surface changes seen with both lasers are likely indicative of temperature induced transverse collagen fibril contraction and stress re-distribution. Refractive procedures which produce significant thermal load should be cognizant of these morphological changes. The optimized intraocular lens operates within the physiologic constraints of the human eye including the force available for full accommodative amplitude using the eye’s natural focusing feedback, while maintaining image quality in the space available. Optimized optical and mechanical performance parameters were delineated as those which minimize both asphericity and actuation pressure. The methodology combines a multidisciplinary basic science approach from biomechanics, optical science, and ophthalmology to optimize an intraocular lens design suitable for preliminary trials.

In vivo, biomechanical stress plays an important role in tissue physiology and pathology, affecting cell and tissue behavior. Even though conventional outflow tissues in the eye are constantly exposed to dynamic changes in intraocular pressure (IOP), the effects of such biomechanical stressors on outflow tissue function have not been analyzed. In particular, changes in IOP with each heartbeat have been measured in human eyes approximating 2.7 mmHg/sec. The purpose of this dissertation is to determine the effect(s) of ocular pulse on conventional outflow tissue regulation and the effect that contractility plays in this mechanical stress-mediated response. The central hypothesis directing this research is that cyclic intraocular pulsations (i.e. ocular pulse) play a significant role in conventional outflow facility.In order to address our hypothesis we studied the effect of biomechanical stressors on conventional outflow physiology using three different strategies: (1) by comparing conventional outflow endothelial cells to blood and lymphatic capillary endothelia, we gained a better understanding of the effects of biomechanical stress on conventional outflow tissue physiology, (2) by modifying the anterior segment perfusion model, we were able to measure the effect of ocular pulse on conventional outflow facility, and (3) by exposing trabecular meshwork cell monolayers to cyclic biomechanical pressure oscillations in the presence of compounds known to affect trabecular meshwork contractility, we were able to analyze the effect of rho-kinase-mediated contractility on the ocular pulse-associated response.Perfused human and porcine anterior segments showed a significant ocular pulse-mediated decrease in outflow facility; in addition, perfused trabecular meshwork monolayers showed an increase in intra-chamber pressure when exposed to cyclic pressure oscillations. This effect was blocked by Y27632 inhibition of rho-kinase-mediated contraction.In conclusion, the work shown in this dissertation demonstrates for the first time that trabecular outflow tissues are capable of responding to a physiologically-relevant cyclic biomechanical stress. This response can be observed as an increase in outflow resistance that translates to lower baselines in outflow facility of anterior segments and lower hydraulic conductivity of trabecular meshwork monolayers. In addition, we concluded that the observed ocular pulse-mediated response of trabecular meshwork cells is regulated by rho-kinase-induced contractility.

In most cases, myopia is characterized by an increase in axial length of the eye, but the exact mechanisms for the axial elongation are still unknown. Higher intraocular pressure (IOP) has been associated with myopia and could be involved in eye enlargement. Also, some investigators have argued that the mechanical stresses generated by the ocular muscles during near work cause the eye to stretch out of shape. The purpose of this work was to investigate the effect of IOP on eye geometry, build a finite element model of the corneo-scleral shell of a chick eye, and verify if the eye could elongate due to increased IOP and hence become myopic. In the present study, myopia was induced in the right eye of chicks using -15 dioptre (D) goggles. The in-vitro pressure-volume curves of normal and myopic chick eyes were obtained using a computer controlled syringe pump and a digital pressure gauge. The axial length and horizontal equatorial diameter of the chick eyes as pressure increased were measured from digital photographs. To build the finite element model, normal chick eyes were frozen, sliced using a microtome, and photographed. The image sequence was aligned using MATLAB software and imported into SolidWorks® and Mimics software packages for three-dimensional (3D) reconstruction. Three 3D models were constructed and imported into Abaqus/CAE® software for finite element analysis. Isotropic, homogeneous, linear elastic and exponentially stiffening material properties were used for the finite element models. The results of the finite element models were compared with the experimental data. The results showed that normal chick eyes elongated in the axial direction and initially contracted in the horizontal equatorial direction as IOP increased. Myopic chick eyes did not elongate as much in the axial direction and did not contract as much in the horizontal equatorial direction compared with normal eyes. The volumetric deformation of myopic eyes was similar to normal eyes suggesting that growth and remodelling of the ocular tissues was involved in experimental myopia in the chick. The 3D reconstructed geometry of a chick eye was similar to a real eye although small geometric inaccuracies were present. The finite element model with isotropic, homogeneous, and exponentially stiffening material properties agreed well with the experimental strains in spite of the fact that the anisotropy of the ocular tissues was not included in the finite element model. This suggests that the oblate geometry of the chick eye was the main parameter dictating its deformation under increased IOP. The natural tendency of chick eyes is to elongate in the axial direction as IOP increases and this suggests that IOP could play a role in myopia onset and progression. The way chick eyes deformed as pressure increased is probably due to their oblate geometry. The finite element model could be used in the future to study the effect of different parameters such as ocular muscle forces and ocular tissues material properties on eye geometry. This way, an eye configuration more prone to eye elongation and myopia could be determined.

The discovery of the ocular hypotensive effect of topically applied prostaglandins (PGs) has raised a number of questions about the mechanisms of action involved. The aim of the present thesis was to answer some of these questions. PGs reduce the intraocular pressure (IOP) by increasing uveoscleral flow through the ciliary muscle, but the exact mechanism is not known. Morphological changes may be involved. PGs are also involved in the inflammatory response. In the first study the aim was to investigate the effect of latanoprost, a prostaglandin F2 a-analogue, on the blood-aqueous barrier and the IOP restoration after long-term treatment. 26 glaucoma patients were treated with latanoprost (50 pg/ml) once daily for 6-12 months. Aqueous protein concentration was followed with a laser flare meter in 16 patients throughout this period. No change was observed. IOP increased slowly after withdrawal of treatment. It was concluded that latanoprost has no clinically significant effect on the permeability of the blood-aqueous barrier and that the IOP will return to pretreatment levels within a few weeks, indicating that any changes in the ciliary muscle morphology are reversible. In 20 healthy volunteers it was attempted to prevent the ocular hypotensive effect of latanoprost by inhibiting uveoscleral flow by a pronounced ciliary muscle contraction. For this purpose a high dose of the cholinergic agonist, physostigmine (1 drop 8 mg/ml alternate hours) was used. However, the effects on IOP of the two drugs were mainly additive most likely due to a short-lasting effect of physostigmine on the ciliary muscle. The progressive IOP reduction by physostigmine in the second study raised the question as to whether the drug reduces aqueous flow apart from enhancing outflow. On the contrary, in the third study repeated administrations of physostigmine, in 20 normal subjects, increased aqueous flow, measured with fluorophotometry, by about 25%. From studies of patients it is known that latanoprost twice daily has less ocular hypotensive effect than once daily. This was the subject of the two remaining studies. The possibility that latanoprost causes a short-lasting increase in aqueous flow was examined in 18 healthy volunteers. Application of a second drop in the morning would blunt some of the early IOP lowering effect of latanoprost. Once or twice daily applications had similar effect on aqueous flow, a tendency to an increase without any difference between the dose regimens. The next study confirmed the difference in effect on IOP between once and twice daily applications in 40 normal subjects. The difference remained even when one of the two applications was omitted after two weeks’ treatment. The results indicate that applying latanoprost twice daily induces a modest receptor desensitisation. / <p>Diss. Umeå : Umeå universitet, 1997, härtill 5 uppsatser.</p> / digitalisering@umu

Purpose An age-cohort was studied with long-term follow-up. The population was born in 1915, living in the municipality of Skellefteå in 1981. The purpose was to investigate the prevalence and incidence of pseudoexfoliation (PEX), its influence on intraocular pressure (IOP) and development of open-angle glaucoma (OAG). Another purpose was to estimate the value of screening for glaucoma by comparing the screened group within the cohort with the remaining unscreened group. Methods In 1981, 339 (40%) of the 856 individuals in the cohort underwent an eye examination. This screened group was re-examined at seven-year intervals until 2002. At each visit the presence of PEX was registered, IOP was measured and the presence of glaucoma was assessed. After the 21-year follow-up period, glaucoma cases were also searched for in the medical records of the remaining unscreened individuals in the cohort. Proportions of glaucoma were compared between the two groups. Results The prevalence of PEX was 23% (95% confidence interval (CI): 20-26%) at the age of 66 and increased to 61% (CI: 50-71%) at the age of 87. The annual incidence of PEX was 1.8% (CI: 1.3-2.4%). The prevalence of OAG increased from 2.1% (CI: 0.8-4.3%) at the age of 66 years to 25 % (CI: 16-35%) at 87 years. The overall annual incidence of OAG was 0.9% (CI: 0.6-1.3%) and for OAG with PEX 2.1% (CI: 1.2-3.3%). PEX increased the risk of developing glaucoma at least four-fold. The incidence of diagnosed OAG in women was higher in the screened group than in the unscreened group (incidence rate ratio (IRR)=1.94, p=0.035). A corresponding difference could not be verified for men (p=0.58). The mean, agedependent, increase in IOP during the 21-year observation period was 0.05 mmHg/year. Conclusion The prevalence of PEX in this study population was the highest reported, and it increased with age. The presence of PEX increased the risk of developing OAG four times. In this study a higher proportion of OAG was revealed by screening among women but not among men. The age-related IOP increase was clinically insignificant.

Diss. (sammanfattning) Stockholm : Karol. inst., 2004. / Härtill 5 uppsatser.

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2005. / Härtill 6 uppsatser.

Η χειρουργική καταρράκτη διαμέσου μικρής τομής με τη χρήση αναδιπλούμενων ενδοφθάλμιων φακών είχε ως αποτέλεσμα την εμφάνιση μιας νέας επιπλοκής˙ της θόλωσης των φακών. Αρκετοί παράγοντες έχουν ενοχοποιηθεί για την αιτιολόγηση του φαινομένου. Μεταξύ αυτών αναφέρονται η μετεγχειρητική φλεγμονή, η χρήση διαλυμάτων πλύσης και ιξωδοελαστικών υλικών, η σιλικόνη, τα λιπαρά οξέα που εμπεριέχονται στο υδατοειδές υγρό. Προηγούμενες αναφορές έχουν αποδώσει την αιτία του φαινομένου της θόλωσης των ενδοφακών στην ασβεστοποίηση. Για τη μελέτη του φαινομένου της ασβεστοποίησης έχουν κατά καιρούς χρησιμοποιηθεί in vivo μοντέλα. Στα μοντέλα αυτά γίνονται απλοποιήσεις ώστε να προσομοιώνονται όσο το δυνατόν καλύτερα οι φυσικοχημικές συνθήκες των βιολογικών ρευστών που είναι σε επαφή με τους αντίστοιχους ιστούς. Ακριβείς θερμοδυναμικές μετρήσεις και μελέτες της κινητικής στις συνθήκες αυτές είναι δυνατόν να δώσουν χρήσιμες πληροφορίες όσον αφορά στην εμφάνιση και εξέλιξη με το χρόνο του φαινομένου της ασβεστοποίησης διάφορων ιστών ή οργάνων. Στην παρούσα εργασία, και με στόχο να προσομoιωθούν οι συνθήκες του προσθίου θαλάμου κατά την επαφή του φακού με το υδατοειδές υγρό, κατασκευάσθηκε από πολυαμίδιο διπλότοιχος θερμοστατούμενος αντιδραστήρας συνολικού όγκου 10 ml. Ο πυθμένας και το άνω μέρος του αντιδραστήρα ήταν από γυαλί, έτσι ώστε το σύνολο να είναι δυνατόν να διευθετηθεί στο έδρανο οπτικού μικροσκοπίου διερχομένου φωτός, προκειμένου να είναι δυνατή η συνεχής παρατήρηση των δοκιμίων. Το συνθετικό υδατοειδές υγρό παρασκευάστηκε με τριπλά απεσταγμένο νερό στο οποίο διαλύθηκαν συγκεκριμένες ποσότητες κρυσταλλικών αλάτων έτσι ώστε η τελική σύσταση να αντιστοιχεί στη σύσταση του υδατοειδούς υγρού, σύμφωνα με την βιβλιογραφία. Η ροή του συνθετικού υδατοειδούς υγρού προς τον αντιδραστήρα γινόταν με τη βοήθεια αντλίας σύριγγας ρυθμιζόμενης παροχής. Ο ρυθμός ροής ήταν 0.2ml/h όπως και στην περίπτωση της ροής του υδατοειδούς υγρού στον πρόσθιο θάλαμο του φακού. Εντός του αντιδραστήρα τοποθετήθηκαν σε ειδικό δειγματοφορέα τρεις υδρόφιλοι ακρυλικοί ενδοφθάλμιοι φακοί (Α, Β και Γ) με περιεκτικότητα σε νερό 26% κ.β. Η μελέτη του συστήματος έγινε σε συνθήκες αντιπροσωπευτικές του oργανισμού, δηλαδή pH=7.4 και θερμοκρασία=37˚C. Η παρακολούθηση των φακών με τη βοήθεια του οπτικού μικροσκοπίου, το οποίο ήταν εφοδιασμένο με βιντεοκάμερα, γινόταν καθημερινά και λαμβάνονταν φωτογραφίες για περαιτέρω ανάλυση. Οι φακοί Α, Β και Γ απομακρύνθηκαν από τον αντιδραστήρα στους πέντε, εννιά και δώδεκα μήνες αντίστοιχα, με σκοπό να μελετηθεί τόσο η επιφάνειά τους όσο και το εσωτερικό του πολυμερικού υλικού. Τα μορφολογικά χαρακτηριστικά των εναποθέσεων εξετάσθηκαν και μελετήθηκαν με τη βοήθεια ηλεκτρονικού μικροσκοπίου σάρωσης ενώ η χημική σύσταση των κρυσταλλιτών ταυτοποιήθηκε με μικροανάλυση με φασματοσκοπία ενεργειακής διασποράς ακτίνων Χ (EDX). Περαιτέρω ταυτοποίηση έγινε με φασματοσκοπικές μεθόδους (φασματοσκοπία Raman) και με περίθλαση ακτίνων Χ. Η τελευταία τεχνική, λόγω της μικρής αναλογίας κρυσταλλικό στερεό/πολυμερές, δεν έδωσε αποτελέσματα και ως εκ τούτου δεν είναι δυνατόν να χρησιμοποιηθεί ως τεχνική ταυτοποίησης. Η χρήση της προϋποθέτει διαφορετική γεωμετρία δείγματος-ανιχνευτή. Ο υπερκορεσμός είναι η κινητήρια δύναμη για την έναρξη της πυρηνογένεσης και της εν συνεχεία ανάπτυξης των σταθερών πυρήνων σε κρυσταλλίτες φωσφορικού ασβεστίου. Δεδομένου του ότι, η συγκέντρωση ασβεστίου του υδατοειδούς υγρού είναι χαμηλή, περίπου η μισή της αντίστοιχης του πλάσματος, υπετέθη, ότι κάθε αιτία τοπικής αύξησης του ασβεστίου και του φωσφόρου, εντός του υδατοειδούς υγρού, μπορεί ενδεχομένως να καταλήξει σε δυστροφική ασβεστοποίηση των φακών. Η μελέτη των φακών και η αξιολόγηση του φαινομένου στο πειραματικό μοντέλο που δημιουργήθηκε στην παρούσα εργασία έγιναν σε συνθήκες σταθερού υπερκορεσμού, δεδομένου του ότι υπήρξε συνεχής ανανέωση του υδατοειδούς υγρού εντός του αντιδραστήρα, κατά τρόπο κατάλληλο ώστε να επιτευχθεί ικανοποιητική προσομοίωση των in vivo συνθηκών. Τα περισσότερα βιολογικά ρευστά, περιλαμβανομένου του υδατοειδούς υγρού είναι υπέρκορα ως προς διάφορες φάσεις αλάτων φωσφορικού ασβεστίου, οι οποίες κατά σειρά μειωμένης διαλυτότητας είναι το διένυδρο φωσφορικό ασβέστιο (CaHPO42H2O, DCPD), το φωσφορικό τριασβέστιο (Ca3(PO4)2, TCP), το φωσφορικό οκτασβέστιο (Ca8H2(PO4)65 H2O, OCP) και ο υδροξυαπατίτης (Ca10(PO4)6(OH)2, HAP). Η διαλυτότητα των κρυσταλλικών αυτών φάσεων και ο υπερκορεσμός των διαλυμάτων καθορίζονται από παράγοντες όπως η θερμοκρασία, το pH κτλ. Η τάση για καταβύθιση και σχηματισμό συγκεκριμένης φάσης κρυσταλλικού ασβεστίου εντός του υδατοειδούς υγρού, μπορεί να καθοριστεί από το διάγραμμα διαλυτότητας και εξαρτάται από το pH του διαλύματος και από τη θερμοκρασία. Σε υψηλές τιμές υπερκορεσμού και σε διαλύματα με υψηλό pH, έχει ταυτοποιηθεί ο σχηματισμός και η σταθεροποίηση πρόδρομων φάσεων, ενώ σε συνθήκες χαμηλού υπερκορεσμού και μειωμένου pH σχηματίζεται απ’ ευθείας ΗΑΡ. Η διαδικασία της ασβεστοποίησης των ενδοφακών επηρεάζεται από παράγοντες όπως η δομή και η κατεργασία του πολυμερούς υλικού βάσης (μήτρα), η παρουσία πόρων και η περιεκτικότητά του σε νερό. Τα ακρυλικά πολυμερή διαθέτουν επιφανειακές ιονιζόμενες καρβοξυλικές ομάδες, η παρουσία των οποίων σε ιονισμένη μορφή (-COO-, pH>4) ευνοεί τη συμπλοκοποίηση με ιόντα ασβεστίου του υδατοειδούς υγρού. Τα επιφανειακά αυτά σύμπλοκα είναι πιθανόν να λειτουργούν ως ενεργά κέντρα για την πυρηνογένεση και περαιτέρω ανάπτυξη των κρυστάλλων του φωσφορικού ασβεστίου. Επιπλέον, η ασβεστοποίηση φαίνεται ότι είναι έντονη στους υδρόφιλους φακούς, λόγω της υψηλότερης ενυδάτωσης των υδρόφιλων πολυμερών, η οποία έχει ως αποτέλεσμα την υψηλότερη επιφανειακή συγκέντρωση των ιονισμένων ομάδων (ομάδες / m2) και συνεπώς και των επιφανειακών συμπλόκων- ενεργών κέντρων για την κρυσταλλική ανάπτυξη. Στην παρούσα εργασία και στο in vitro μοντέλο με τη χρησιμοποίηση ενός υδρόφιλου φακού, διαπιστώθηκε ότι η διαδικασία της ασβεστοποίησης λαμβάνει χώρα στο εσωτερικό των φακών, στην πολυμερική μήτρα. Οι σχηματισμοί στο πειραματικό μοντέλο εντοπίσθηκαν στο εσωτερικό των ενδοφακών και με την πάροδο του χρόνου, διαπιστώθηκε ότι μετατοπίζονταν προς τα επιφανειακά στρώματα των φακών. Επίσης, οι εναποθέσεις εμφανίσθηκαν ως γραμμικό μέτωπο παράλληλο προς την γραμμή της επιφάνειας των ενδοφακών. Τόσο η μορφολογική εξέταση των εναποθέσεων στο εσωτερικό όσο και η φασματοσκοπική τους ταυτοποίηση, έδειξε ότι η σύστασή τους ήταν εξ’ ολοκλήρου ΗΑΡ, χωρίς να αποκλείεται και ο σχηματισμός πρόδρομης φάσης OCP, δεδομένου του ότι βρέθηκαν κρυσταλλίτες οι οποίοι μορφολογικά παρουσιάζουν μεγάλη ομοιότητα στη φάση αυτή. Η ερμηνεία της διαπίστωσης του γεγονότος ότι η ασβεστοποίηση λαμβάνει χώρα στο εσωτερικό των ενδοφακών, συνίσταται στο ότι η διάχυση των ιόντων Ca2+ και PO43- εντός της πολυμερικής μήτρας, προχωρεί μέχρι τέτοιο βαθμό ώστε να δημιουργούνται στο εσωτερικό του πολυμερούς συνθήκες τοπικού υπερκορεσμού, κατάλληλες για τη δημιουργία των αντίστοιχων κρυστάλλων. Με την υπόθεση ότι οι τιμές των συντελεστών διάχυσής των ιόντων Ca2+ και PO43- εντός της πολυμερικής μήτρας, είναι της αυτής τάξεως μεγέθους είναι δυνατόν να εξηγηθεί και η εμφάνιση μετώπου συγκέντρωσης των. Η συσσώρευση των ιόντων τα οποία αποτελούν τα δομικά συστατικά του ΗΑΡ συνεχίζεται μέχρις ότου επιτευχθεί μία κρίσιμη τιμή υπερκορεσμού. Σε αυτό το σημείο σχηματίζονται οι πυρήνες και λαμβάνει χώρα η κρυσταλλική ανάπτυξη με τον ερχομό και συσσώρευση επιπρόσθετων ιόντων. Η ασβεστοποίηση των υδρόφιλων ενδοφακών είναι μία προοδευτική και συνεχώς εξελισσόμενη διαδικασία μετά την εμφύτευσή τους και όσο περισσότερο αφήνεται να εξελιχθεί τόσο αυξάνεται η πυκνότητα των εναποθέσεων στο εσωτερικό των φακών. Η επιφάνεια μπορεί να προσβληθεί μόνο σε όψιμες φάσεις και αρκετά χρόνια μετά την εμφύτευση των φακών. Στο σημείο αυτό κρύσταλλοι μπορεί να αναπτυχθούν και σε σημεία, στα οποία έχουν δημιουργηθεί στην επιφάνεια του πολυμερούς σχισμές και χαρακιές. Το θέμα της έναρξης της ασβεστοποίησης (εσωτερικό ή επιφάνεια) χρειάζεται περαιτέρω διερεύνηση. Σύμφωνα με κάποιες αναφορές στη βιβλιογραφία, προτείνεται ότι η έναρξη της ασβεστοποίησης γίνεται στην επιφάνεια των ενδοφακών. Στην παρούσα εργασία, σε συνθήκες χαμηλού υπερκορεσμού, ανάλογες με τις αντίστοιχες υγιούς υδατοειδούς υγρού η ασβεστοποίηση έλαβε χώρα στο εσωτερικό της πολυμερικής μήτρας του ενδοφακού και σε χρόνο και με τρόπο ο οποίος μπορεί να εξηγηθεί από τη (βραδεία) διάχυση των δομικών ιόντων του ΗΑΡ στο εσωτερικό της πολυμερικής μήτρας. Τα ευρήματα και τα συμπεράσματα από την ολοκλήρωση της πειραματικής διαδικασίας με τη χρήση υδρόφιλων ενδοφακών με υδρόφοβη επικάλυψη δείχνουν ότι το θέμα της έναρξης της ασβεστοποίησης˙ εσωτερικό ή επιφάνεια φαίνεται ότι εκτός από τον υπερκορεσμό, υψηλός ή χαμηλός, εξαρτάται και από τη φύση του πολυμερούς του φακού˙ υδρόφιλη ή υδρόφοβη. Τα ευρήματα δείχνουν ότι η υδρόφιλη φύση ευνοεί τη διάχυση, ενώ το υδρόφοβο υλικό την έναρξη της ασβεστοποίησης στην επιφάνεια του φακού. Τα ευρήματα δείχνουν ότι η υδρόφιλη φύση ευνοεί τη διάχυση, ενώ το υδρόφοβο υλικό την έναρξη της ασβεστοποίησης στην επιφάνεια του φακού. Συμπερασματικά, η όψιμη μετεγχειρητική ασβεστοποίηση των ενδοφακών συνιστά σοβαρή επιπλοκή και αιτία μείωσης της όρασης. Λόγω του ότι η διαδικασία και η εκδήλωση του φαινομένου έχει καθυστερημένη έναρξη, είναι σημαντική η μακροχρόνια και προσεκτική παρακολούθηση αυτών των ασθενών. Αρκετοί οφθαλμίατροι δεν είναι ενήμεροι ως προς αυτήν την κλινική οντότητα και αναγνωρίζοντάς την βοηθούν ώστε να μην υποβάλλουν τους ασθενείς σε ανώφελες επεμβάσεις. Η χειρουργική εξαίρεση του φακού συνιστά τη μοναδική θεραπευτική προσέγγιση, καθώς ασθενείς με ασβεστοποιημένους φακούς παρουσιάζουν σταδιακή μείωση της οπτικής τους οξύτητας και καμία περίπτωση αυτόματης υποστροφής δεν έχει παρατηρηθεί. / Small incision cataract surgery with foldable IOL implantation resulted in a new postoperative complication, IOL opacification. Various factors implicated in the phenomenon have been suggested, including inflammation, irrigation solutions and viscosurgical devices, silicone and fatty acids contamination. There is increasing evidence that IOL opacification is due to calcification. Modeling in vivo processes by reliable and reproducible in vitro methods is of key importance to understand the underlying mechanisms. Precise thermodynamic calculations of equilibrium speciation in combination with kinetics measurements at conditions simulating the eye environment are expected to yield mechanistic information concerning the formation of calcium phosphate deposits. In the present contribution we have developed an experimental approach for the investigation of the mechanism of calcification of hydrophilic acrylic IOLs. A double walled thermostated reactor was constructed, volume totaling 10 ml made of polyamide. The reactor had glass windows on top and bottom to allow for the direct observation of the IOL specimens in situ using an optical microscope combined with an image analysis system. In the external wall of the reactor, water supplied from a thermostat was circulated in order to maintain the temperature at 37.0  0.2˚C, while in the interior of the reactor constant flow of a simulated aqueous humor solution (SAH) was ensured with the help of a syringe pump. The SAH solution was introduced in the reactor in an once flow mode at a flow rate of 0.2ml/h, simulating the in vivo flow in the anterior chamber, where aqueous humor is fully renewed within 2 hours. Hydrophilic acrylic IOLs (A, B and C) in triplicate, made of Poly-HEMA with 26% water content were placed in a special holder. The observation of IOLs was done in situ daily by optical microscopy, for the assessment of the opacification progress. Five months after the initiation of the experiment, Lens A was removed in order to be inspected, both at the surface and in the interior. The morphology of the deposits was examined using Scanning Electron Microscopy (SEΜ). The composition of the deposits was identified by microanalysis with Energy Dispersive x-ray Spectroscopy (EDS). Lens B was removed on the ninth month, while Lens C was inspected one year after the onset of the experiment. Similar studies including SEM and EDS analysis were used for the investigation of both those lenses. Investigation showed deposits of calcium phosphate crystallites in the interior of opacified IOLs. These deposits however, were not observed on the IOL’s surface. The thermodynamic driving force for the formation of a salt from solution is the difference between the chemical potentials of the salt in solution from the equilibrium. Taking into account that the calcium concentration in normal aqueous humor is low, about half of the respective value in blood serum, it may be assumed that any cause of localized increase in calcium and phosphorus might result in dystrophic calcification. The experiments in the present work were done at conditions of practically constant supersaturation, since the solution in the experimental reactor was flown once through thus providing a reasonable simulation of the in vivo conditions. Heterogeneous nucleation, an almost ubiquitous phenomenon, is initiated at impurity sites and foreign surfaces in contact with supersaturated solutions. The induction time preceding the formation of nuclei of solids which grow to crystals depends on several factors, including temperature, pH, ionic strength, solution composition which determine the thermodynamic driving force for the formation of the solid phase. Biological fluids like blood serum or aqueous humor are supersaturated with respect to a number of different phases of calcium phosphate salts, in the order of decreasing solubility: Calcium phosphate dehydrate (CaHPO42H2O, DCPD), Tricalcium phosphate (Ca3(PO4)2, TCP), Octacalcium phosphate (Ca8H2(PO4)65 H2O, OCP) and Hydroxyapatite (Ca10(PO4)6(OH)2, HAP). HAP is the thermodynamically stable phase while the rest of the crystalline phases are precursors which may be formed and/or stabilized in supersaturated solutions depending on the conditions in the supersaturated solutions. The tendency for particular calcium phosphate phases to form in supersaturated solutions may be estimated from the solubility phase diagrams of calcium phosphates. Unstable precursor phases, if formed, they convert through hydrolytic processes to the thermodynamically most stable. The process of biomaterial calcification seems moreover to be influenced by factors such as polymer structure, polymer porosity and water content. The ionizable surface hydroxyl groups (present in the carboxyl functional groups) available on the surface of the acrylic polymers may act as sites for nucleation and growth of mineral phase, through surface complexation with calcium ions. The presence of larger numbers of –COOH groups on the polymers accelerate the process of Ca-P overgrowth. The higher extend of hydration in hydrophilic materials leads to higher ionization of the surface functional groups, thus promoting calcification through the formation of complexes with Ca2+ ions. The higher calcification incidents observed in IOLs with higher water content has been attributed to this fact. The polar functional groups (-COO) on the surface of the polymeric matrix of IOLs result in the significant increase in electron density and the subsequent reduction of the interfacial energy between the polymer and aqueous solution. The energy barrier to diffusion of calcium and phosphate species from the bulk solution to the substrate is also reduced, through the formation of surface complexes, which favor the accumulation of Ca2+ and PO43- ions, thus promoting calcification. Our in vitro model experiments have shown that IOL’s calcification is a process initiated at the interior of the IOLs tested. SEM investigation in combination with EDS microanalysis, confirmed the presence of HAP crystallites with sizes less than 100nm. Raman spectroscopy analyses of the opacified lenses corroborated the findings for HAP formation in the interior of the polymeric matrix. X-Ray diffraction measurements failed to identify the presence of minerals apparently because of the low content (by mass) of solid in the polymeric matrix. Supersaturation of the aqueous humor, with respect to calcium phosphate, is the driving force a necessary condition for nucleation of Ca-P salts. The formations observed in our experimental set-up which simulated closely in vivo conditions, were found exclusively in the interior of the IOLs. The solid deposits formed linear fronts parallel to the lens surfaces, advancing with time. It may be suggested therefore that the deposits fronts is the result of diffusion of Ca2+, PO43- and OH- ions through the polymer matrix in contact with the polymer. Assuming similar values for the diffusion coefficients of Ca2+, PO43- and OH- ions in the gel (bulk polymer) material the formation of linear deposits fronts may be explained from the formation of the supersaturated solutions at a depth in which a critical supersaturation was reached. At this point calcium phosphate nuclei form and grow with the arrival of additional ions. IOL’s calcification is an ongoing process after IOL implantation, and the longer the process proceeds, the density of the deposits in the interior of the IOL increases. In all of the cases surface was free of deposits and the distortions that were observed are thought to be due to changes in polymer structure in the IOL’s interior. Surface can be affected only in late phases of calcification and many years after IOL implantation. At this stage crystals may outgrow especially at places in which the polymer’s surface has developed fissures. The issue of calcification’s process initiation (interior or surface) needs further investigation. Reports in the literature, suggest that calcification is initiated on the surface of the IOL. In our experiment where low supersaturation conditions have been achieved, diffusion was favored resulting in calcification at the interior of the polymeric matrix. Investigation and analysis of IOLs with hydrophobic surface confirmed that the issue of calcification`s process initiation is more over influenced by factors others than supersaturation conditions, such as the hydrophilic or hydrophobic nature of the IOLs surface. The experimental analysis proved that hydrophilic IOLs favor ions diffusion while hydrophobic material limits calcification process on the surface. In conclusion, late postoperative IOL opacification causes a severe loss of visual acuity. Because calcification process appears to be of delayed onset it is important to be vigilant in the long-term follow-up of these patients. Many ophthalmologists are not aware of this clinical problem and recognizing this phenomenon will help prevent patients from undergoing useless procedures. IOL exchange is the only therapeutic approach, in such patients, as patients with calcified IOLs have gradual deterioration of their visual acuity and no case of spontaneous recovery has been observed.

A mensuração da pressão intraocular (PIO) é essencial para o exame oftálmico, pois permite o diagnóstico de doenças oftálmicas como uveíte e glaucoma, além de auxiliar no monitoramento das cirurgias intraoculares. Objetivou-se comparar a tonometria de rebote com a tonometria de aplanação em equinos. Além disso, determinar a reprodutibilidade dos valores obtidos em ambos os aparelhos. A PIO foi aferida em 30 equinos da raça Crioula, machos ou fêmeas, com idade entre um e 16 anos utilizando o tonômetro de rebote (TonoVet®) e o tonômetro de aplanação (Tonopen Avia®). Previamente a aferição da PIO, todos os animais foram submetidos ao exame oftálmico incluindo, avaliação dos reflexos pupilares, teste da lágrima de Schirmer, prova da fluoresceína e biomicroscopia com lâmpada de fenda portátil. Foram utilizados apenas bulbos oculares hígidos. A análise estatística foi feita com ANOVA e teste t de Student. A reprodutibilidade foi avaliada pelo cálculo do coeficiente de correlação intraclasse (ICC). A média da PIO foi de 26,16 ± 5,13 mmHg utilizando o tonômetro de rebote e de 22,55 ±7,32 mmHg com o tonômetro de aplanação. A média da PIO de acordo com o avaliador utilizando o tonômetro de rebote foi de 25,27 ± 4,9 mmHg para o examinador A, 25,80 ± 5,2 mmHg para o examinador B e de 27,40 ± 5,1 mmHg para o examinador C. Com o tonômetro de aplanação as médias obtidas foram de 20,03 ± 4,5 mmHg com o examinador A, 19,67 ± 6,0 mmHg com o examinador B e de 27,97 ± 10,3 mmHg com o examinador C (p<2,3). O ICC foi de 0,18 para o tonômetro de aplanação e de 0,41 para o tonômeto de rebote. Os valores da pressão intraocular média obtidos com o tonômetro de rebote são superestimados quando comparados aos valores obtidos com o tonômetro de Tonopen Avia® em equinos saudáveis. Comparativamente ao Tonopen Avia® os valores obtidos com o Tonovet® demonstraram maior reprodutibilidade. / The measurement of intraocular pressure (IOP) is essential for the ophthalmic examination, as it allows the diagnosis of eye diseases such as uveitis and glaucoma, as well as aid in the monitoring of intraocular surgery. This study aimed to compare the mean intraocular pressure (IOP) values in horses obtained by three different examiners using the rebound and applanation tonometer’s. Each examiner measured the IOP of 30 “Crioulo” horses, males or females, aged one to 16 years old, using rebound tonometer and applanation tonometer. Previous to the experiment an ophthalmic examination was performed in horses. Only animals without alterations were selected. Three veterinarians measured IOP with rebound (Tonovet®) and applanation tonometer’s (TonoPen Avia®). Comparisons were made using ANOVA and student t test. Intraclass correlation coefficient (ICC) was calculated for reproducibility. IOP measurements resulted in mean values of 26.16 ± 5.13 mmHg for rebound tonometer and 22.55 ±7.32 mmHg for applanation tonometer. These values were different on statistical analysis. Rebound tonometer mean values, according with the examiner, were: A 25.27 ± 4.9 mmHg, B 25.80 ± 5.2, C 27.40 ± 5.1 mmHg. For applanation tonometer the mean IOP for examiner A (20.03 ± 4.5 mmHg), B (19.67 ± 6.0 mmHg), C (27.97 ± 10.3 mmHg). With the Tonopen Avia® tonometer examiner C measured higher IOP’s than examiners A and B. Intraclass correlation coefficient was 0.41 for rebound tonometer was 0.18 for applanation tonometer. It was observed that the values obtained with the Tonovet® were overestimated compared to Tonopen Avia® in healthy horses. When compared with Tonopen Avia® values obtained with the Tonovet® showed a higher reproducibility.