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Customized Finite Element Modelling of the Human Cornea.

Simonini I, Pandolfi A - PLoS ONE (2015)

Bottom Line: Corneal elevation maps of five human eyes were taken with a rotating Scheimpflug camera combined with a Placido disk before and after refractive surgery.Patient-specific solid models were created and discretized in finite elements to estimate the corneal strain and stress fields in preoperative and postoperative configurations and derive the refractive parameters of the cornea.Patient-specific models can be used as indicators of feasibility before performing the surgery.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Matematica, Politecnico di Milano, Milano, Italy.

ABSTRACT

Aim: To construct patient-specific solid models of human cornea from ocular topographer data, to increase the accuracy of the biomechanical and optical estimate of the changes in refractive power and stress caused by photorefractive keratectomy (PRK).

Method: Corneal elevation maps of five human eyes were taken with a rotating Scheimpflug camera combined with a Placido disk before and after refractive surgery. Patient-specific solid models were created and discretized in finite elements to estimate the corneal strain and stress fields in preoperative and postoperative configurations and derive the refractive parameters of the cornea.

Results: Patient-specific geometrical models of the cornea allow for the creation of personalized refractive maps at different levels of IOP. Thinned postoperative corneas show a higher stress gradient across the thickness and higher sensitivity of all geometrical and refractive parameters to the fluctuation of the IOP.

Conclusion: Patient-specific numerical models of the cornea can provide accurate quantitative information on the refractive properties of the cornea under different levels of IOP and describe the change of the stress state of the cornea due to refractive surgery (PRK). Patient-specific models can be used as indicators of feasibility before performing the surgery.

No MeSH data available.


Related in: MedlinePlus

Orthogonal scheme used to compute the normal to the anterior corneal surface.The superscript index refers to meridians, the subscript index refers to circumferences. The circumferential tangent vector tcirc is computed as the unit segment joining two adjacent points on the same circumference. The meridional tangent vector tmer is computed as the unit segment joining two adjacent points on the same meridian. The normal vector na is obtained from the vector product between the two orthogonal unit vectors.
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pone.0130426.g004: Orthogonal scheme used to compute the normal to the anterior corneal surface.The superscript index refers to meridians, the subscript index refers to circumferences. The circumferential tangent vector tcirc is computed as the unit segment joining two adjacent points on the same circumference. The meridional tangent vector tmer is computed as the unit segment joining two adjacent points on the same meridian. The normal vector na is obtained from the vector product between the two orthogonal unit vectors.

Mentions: To construct the posterior surface, we rely on the thickness values measured by the topographer, without corrections for possible optical distortions. The thickness t of a shell is defined as the minimum distance between the anterior and posterior surfaces taken along the normal to the middle surface of the shell. For thin shells, the normal to the middle surface is very well approximated by the normal to the anterior surface. At each point xa of the discretized anterior surface, we compute the normal na using the vector product between two segments tcirc and tmer, both tangent to the anterior surface, constructed from the points surrounding xa in the circumferential and meridian directions, respectively (see Fig 4)tcirc=xji−1−xji+1/xji−1−xji+1/,tmer=xj+1i−xj−1i/xj+1i−xj−1i/,(1)where the superscript index i refers to the circumference and the subscript index j refers to the meridian. The inward normal to the anterior surface at the point derives as


Customized Finite Element Modelling of the Human Cornea.

Simonini I, Pandolfi A - PLoS ONE (2015)

Orthogonal scheme used to compute the normal to the anterior corneal surface.The superscript index refers to meridians, the subscript index refers to circumferences. The circumferential tangent vector tcirc is computed as the unit segment joining two adjacent points on the same circumference. The meridional tangent vector tmer is computed as the unit segment joining two adjacent points on the same meridian. The normal vector na is obtained from the vector product between the two orthogonal unit vectors.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4476710&req=5

pone.0130426.g004: Orthogonal scheme used to compute the normal to the anterior corneal surface.The superscript index refers to meridians, the subscript index refers to circumferences. The circumferential tangent vector tcirc is computed as the unit segment joining two adjacent points on the same circumference. The meridional tangent vector tmer is computed as the unit segment joining two adjacent points on the same meridian. The normal vector na is obtained from the vector product between the two orthogonal unit vectors.
Mentions: To construct the posterior surface, we rely on the thickness values measured by the topographer, without corrections for possible optical distortions. The thickness t of a shell is defined as the minimum distance between the anterior and posterior surfaces taken along the normal to the middle surface of the shell. For thin shells, the normal to the middle surface is very well approximated by the normal to the anterior surface. At each point xa of the discretized anterior surface, we compute the normal na using the vector product between two segments tcirc and tmer, both tangent to the anterior surface, constructed from the points surrounding xa in the circumferential and meridian directions, respectively (see Fig 4)tcirc=xji−1−xji+1/xji−1−xji+1/,tmer=xj+1i−xj−1i/xj+1i−xj−1i/,(1)where the superscript index i refers to the circumference and the subscript index j refers to the meridian. The inward normal to the anterior surface at the point derives as

Bottom Line: Corneal elevation maps of five human eyes were taken with a rotating Scheimpflug camera combined with a Placido disk before and after refractive surgery.Patient-specific solid models were created and discretized in finite elements to estimate the corneal strain and stress fields in preoperative and postoperative configurations and derive the refractive parameters of the cornea.Patient-specific models can be used as indicators of feasibility before performing the surgery.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Matematica, Politecnico di Milano, Milano, Italy.

ABSTRACT

Aim: To construct patient-specific solid models of human cornea from ocular topographer data, to increase the accuracy of the biomechanical and optical estimate of the changes in refractive power and stress caused by photorefractive keratectomy (PRK).

Method: Corneal elevation maps of five human eyes were taken with a rotating Scheimpflug camera combined with a Placido disk before and after refractive surgery. Patient-specific solid models were created and discretized in finite elements to estimate the corneal strain and stress fields in preoperative and postoperative configurations and derive the refractive parameters of the cornea.

Results: Patient-specific geometrical models of the cornea allow for the creation of personalized refractive maps at different levels of IOP. Thinned postoperative corneas show a higher stress gradient across the thickness and higher sensitivity of all geometrical and refractive parameters to the fluctuation of the IOP.

Conclusion: Patient-specific numerical models of the cornea can provide accurate quantitative information on the refractive properties of the cornea under different levels of IOP and describe the change of the stress state of the cornea due to refractive surgery (PRK). Patient-specific models can be used as indicators of feasibility before performing the surgery.

No MeSH data available.


Related in: MedlinePlus