{"id":{"repo_id":"adelaide","oai_identifier":"oai:digital.library.adelaide.edu.au:2440/137159"},"canonical_url":"https://search.dev.ndltd.org/etd/adelaide/oai:digital.library.adelaide.edu.au:2440/137159","repository":{"repo_id":"adelaide","name":"University of Adelaide","base_url":"https://digital.library.adelaide.edu.au/server/oai/request"},"display":{"title":"Posterior Corneal Astigmatism","abstract":"In order to calculate an appropriate intraocular lens (IOL) power for an eye undergoing cataract surgery, the most important measurements are the axial length of the eye and corneal shape. The cornea is a three dimensional structure and so, although it has commonly been considered as a single refractive surface, we must take into account that the anterior and posterior surfaces of the cornea have different astigmatic magnitudes and axes. Total astigmatism of the cornea is determined by the combination of both anterior corneal astigmatism and posterior corneal astigmatism. In order to treat astigmatism of the eye during cataract surgery precisely, total corneal astigmatism needs to be neutralised, and not just the measured anterior corneal astigmatism. Otherwise residual astigmatism will be present and visual quality will be impacted. Although the contribution of posterior corneal astigmatism has been postulated for many decades, our ability to measure it accurately, and incorporate it into a practical surgical plan, has only become possible much more recently. While measuring anterior corneal astigmatism accurately has been relatively straight forward, posterior corneal astigmatism has been less simple to measure due to relative similarity in refractive indices of cornea and the adjacent aqueous humor as well as having a very low magnitude to detect. Our measurement of anterior corneal astigmatism is far from perfect. Measuring a fluid surface accurately and consistently is not easy. When the magnitude of anterior corneal astigmatism is very low, the accuracy and consistency of measures of both magnitude and axis of astigmatism decrease. Measurements of posterior corneal astigmatism are therefore faced with the difficult combination of trying to measure a very low magnitude of astigmatism, and doing so in extremely difficult optical conditions. Modern cataract surgery has seen a shift in IOL calculation methods from using measured anterior corneal astigmatism alone, to incorporating a population statistics based estimation of posterior corneal astigmatism. The research published as part of this thesis has been at the forefront of the logical next step, which is the incorporation of individual measurement of posterior corneal astigmatism into IOL calculation. Despite having been aware of the presence of an optical contribution of posterior corneal astigmatism for a long time, our knowledge about the magnitude and variation of this contribution as well as our ability to measure it has been relatively poorly defined. The main objectives of this thesis are: 1) To assess whether the contribution of posterior corneal astigmatism to total corneal astigmatism in eyes with high magnitude anterior corneal astigmatism becomes so minor that it can be ignored. 2) To assess how measurement of posterior corneal astigmatism using optical coherence tomography (OCT) of the IOLMaster 700 compares to previous estimates. 3) To assess whether IOLMaster 700 measurement of total corneal astigmatism, “total keratometry” (TK) is as accurate as Goggin nomogram adjusted keratometry (GNAK) values.","abstract_html":"In order to calculate an appropriate intraocular lens (IOL) power for an eye undergoing cataract surgery, the most important measurements are the axial length of the eye and corneal shape. The cornea is a three dimensional structure and so, although it has commonly been considered as a single refractive surface, we must take into account that the anterior and posterior surfaces of the cornea have different astigmatic magnitudes and axes. Total astigmatism of the cornea is determined by the combination of both anterior corneal astigmatism and posterior corneal astigmatism. In order to treat astigmatism of the eye during cataract surgery precisely, total corneal astigmatism needs to be neutralised, and not just the measured anterior corneal astigmatism. Otherwise residual astigmatism will be present and visual quality will be impacted. Although the contribution of posterior corneal astigmatism has been postulated for many decades, our ability to measure it accurately, and incorporate it into a practical surgical plan, has only become possible much more recently. While measuring anterior corneal astigmatism accurately has been relatively straight forward, posterior corneal astigmatism has been less simple to measure due to relative similarity in refractive indices of cornea and the adjacent aqueous humor as well as having a very low magnitude to detect. Our measurement of anterior corneal astigmatism is far from perfect. Measuring a fluid surface accurately and consistently is not easy. When the magnitude of anterior corneal astigmatism is very low, the accuracy and consistency of measures of both magnitude and axis of astigmatism decrease. Measurements of posterior corneal astigmatism are therefore faced with the difficult combination of trying to measure a very low magnitude of astigmatism, and doing so in extremely difficult optical conditions. Modern cataract surgery has seen a shift in IOL calculation methods from using measured anterior corneal astigmatism alone, to incorporating a population statistics based estimation of posterior corneal astigmatism. The research published as part of this thesis has been at the forefront of the logical next step, which is the incorporation of individual measurement of posterior corneal astigmatism into IOL calculation. Despite having been aware of the presence of an optical contribution of posterior corneal astigmatism for a long time, our knowledge about the magnitude and variation of this contribution as well as our ability to measure it has been relatively poorly defined. The main objectives of this thesis are: 1) To assess whether the contribution of posterior corneal astigmatism to total corneal astigmatism in eyes with high magnitude anterior corneal astigmatism becomes so minor that it can be ignored. 2) To assess how measurement of posterior corneal astigmatism using optical coherence tomography (OCT) of the IOLMaster 700 compares to previous estimates. 3) To assess whether IOLMaster 700 measurement of total corneal astigmatism, “total keratometry” (TK) is as accurate as Goggin nomogram adjusted keratometry (GNAK) values.","abstract_has_math":false,"creators":["LaHood, Benjamin Richard"],"institution":null,"degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Casson, Robert","Goggin, Michael"],"committee_chairs":[],"committee_members":[],"year":2022,"date_issued":"2022","date_published":"2022","updated_at":"2026-07-24T00:51:02Z","subjects":["Posterior cornea","astigmatism","cataract surgery","toric intraocular lens","posterior corneal astigmatism","cornea"],"languages":["en"],"rights":[],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"https://hdl.handle.net/2440/137159","outbound_label":"Handle","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Casson, Robert","Goggin, Michael"]},{"key":"dc:creator","label":"Author","values":["LaHood, Benjamin Richard"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.issued","label":"Date","values":["2022"]},{"key":"dc:type","label":"Dc Type","values":["Thesis"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Posterior cornea","astigmatism","cataract surgery","toric intraocular lens","posterior corneal astigmatism","cornea"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language.iso","label":"Language (ISO)","values":["en"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://hdl.handle.net/2440/137159"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["In order to calculate an appropriate intraocular lens (IOL) power for an eye undergoing cataract surgery, the most important measurements are the axial length of the eye and corneal shape. The cornea is a three dimensional structure and so, although it has commonly been considered as a single refractive surface, we must take into account that the anterior and posterior surfaces of the cornea have different astigmatic magnitudes and axes. Total astigmatism of the cornea is determined by the combination of both anterior corneal astigmatism and posterior corneal astigmatism. In order to treat astigmatism of the eye during cataract surgery precisely, total corneal astigmatism needs to be neutralised, and not just the measured anterior corneal astigmatism. Otherwise residual astigmatism will be present and visual quality will be impacted. Although the contribution of posterior corneal astigmatism has been postulated for many decades, our ability to measure it accurately, and incorporate it into a practical surgical plan, has only become possible much more recently. While measuring anterior corneal astigmatism accurately has been relatively straight forward, posterior corneal astigmatism has been less simple to measure due to relative similarity in refractive indices of cornea and the adjacent aqueous humor as well as having a very low magnitude to detect. Our measurement of anterior corneal astigmatism is far from perfect. Measuring a fluid surface accurately and consistently is not easy. When the magnitude of anterior corneal astigmatism is very low, the accuracy and consistency of measures of both magnitude and axis of astigmatism decrease. Measurements of posterior corneal astigmatism are therefore faced with the difficult combination of trying to measure a very low magnitude of astigmatism, and doing so in extremely difficult optical conditions. Modern cataract surgery has seen a shift in IOL calculation methods from using measured anterior corneal astigmatism alone, to incorporating a population statistics based estimation of posterior corneal astigmatism. The research published as part of this thesis has been at the forefront of the logical next step, which is the incorporation of individual measurement of posterior corneal astigmatism into IOL calculation. Despite having been aware of the presence of an optical contribution of posterior corneal astigmatism for a long time, our knowledge about the magnitude and variation of this contribution as well as our ability to measure it has been relatively poorly defined. The main objectives of this thesis are: 1) To assess whether the contribution of posterior corneal astigmatism to total corneal astigmatism in eyes with high magnitude anterior corneal astigmatism becomes so minor that it can be ignored. 2) To assess how measurement of posterior corneal astigmatism using optical coherence tomography (OCT) of the IOLMaster 700 compares to previous estimates. 3) To assess whether IOLMaster 700 measurement of total corneal astigmatism, “total keratometry” (TK) is as accurate as Goggin nomogram adjusted keratometry (GNAK) values."]},{"key":"dc:title","label":"Title","values":["Posterior Corneal Astigmatism"]}]}],"canonical_facts":{"dc:contributor.advisor":["Casson, Robert","Goggin, Michael"],"dc:creator":["LaHood, Benjamin Richard"],"dc:date.issued":["2022"],"dc:description.abstract":["In order to calculate an appropriate intraocular lens (IOL) power for an eye undergoing cataract surgery, the most important measurements are the axial length of the eye and corneal shape. The cornea is a three dimensional structure and so, although it has commonly been considered as a single refractive surface, we must take into account that the anterior and posterior surfaces of the cornea have different astigmatic magnitudes and axes. Total astigmatism of the cornea is determined by the combination of both anterior corneal astigmatism and posterior corneal astigmatism. In order to treat astigmatism of the eye during cataract surgery precisely, total corneal astigmatism needs to be neutralised, and not just the measured anterior corneal astigmatism. Otherwise residual astigmatism will be present and visual quality will be impacted. Although the contribution of posterior corneal astigmatism has been postulated for many decades, our ability to measure it accurately, and incorporate it into a practical surgical plan, has only become possible much more recently. While measuring anterior corneal astigmatism accurately has been relatively straight forward, posterior corneal astigmatism has been less simple to measure due to relative similarity in refractive indices of cornea and the adjacent aqueous humor as well as having a very low magnitude to detect. Our measurement of anterior corneal astigmatism is far from perfect. Measuring a fluid surface accurately and consistently is not easy. When the magnitude of anterior corneal astigmatism is very low, the accuracy and consistency of measures of both magnitude and axis of astigmatism decrease. Measurements of posterior corneal astigmatism are therefore faced with the difficult combination of trying to measure a very low magnitude of astigmatism, and doing so in extremely difficult optical conditions. Modern cataract surgery has seen a shift in IOL calculation methods from using measured anterior corneal astigmatism alone, to incorporating a population statistics based estimation of posterior corneal astigmatism. The research published as part of this thesis has been at the forefront of the logical next step, which is the incorporation of individual measurement of posterior corneal astigmatism into IOL calculation. Despite having been aware of the presence of an optical contribution of posterior corneal astigmatism for a long time, our knowledge about the magnitude and variation of this contribution as well as our ability to measure it has been relatively poorly defined. The main objectives of this thesis are: 1) To assess whether the contribution of posterior corneal astigmatism to total corneal astigmatism in eyes with high magnitude anterior corneal astigmatism becomes so minor that it can be ignored. 2) To assess how measurement of posterior corneal astigmatism using optical coherence tomography (OCT) of the IOLMaster 700 compares to previous estimates. 3) To assess whether IOLMaster 700 measurement of total corneal astigmatism, “total keratometry” (TK) is as accurate as Goggin nomogram adjusted keratometry (GNAK) values."],"dc:identifier.uri":["https://hdl.handle.net/2440/137159"],"dc:language.iso":["en"],"dc:subject":["Posterior cornea","astigmatism","cataract surgery","toric intraocular lens","posterior corneal astigmatism","cornea"],"dc:title":["Posterior Corneal Astigmatism"],"dc:type":["Thesis"]},"updated_at":"2026-07-24T00:51:02Z"}