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OPD: corneal scar

The OPD is useful to quantify and qualify the optical aberrations of the eye; It can be used to specify the origin (corneal topography and OPD acquired jointly through the OPD SCAN 3).  The calculation of the corneal Wavefront allows treatments to target "corneoplastique", as shown in the following example.

A patient presents traces of corneal wound of the right eye (laceration). His vision is measured right at 4/10 without correction and 8/10 with + 1 (-3 × 60 °). He complains of a vision constantly blurred with sensation of "sail", and a reduction of the perception of contrasts, especially when the brightness is reduced.

Examination to the biomicroscope shows an oblique and superficial corneal pillowcase.

corneal scar

Corneal scar: aspect biomicroscopique. Superficial pillowcase without neovascularization.

 

The corneal thickness is kept. In this context, we must try to establish if low vision is related to the reduction of transparency (corneal opacity) or the induction of aberrations from top of gre)astigmatism irregular).

 

Corneal topography (left: anterior curvature, axial mode) shows the present of an irregular toricite (irregular, asymmetric corneal astigmatism). SRI (Surface Regularity Index) and SAI (Surface Asymetry Index) indices are abnormal; they contribute to classify this as suspicious cornea of pellucid marginal degeneration by the software "corneal navigator" (this diagnosis is wrong, because the software is designed for the automated analysis of corneas free of trauma).

The analysis of map of the refraction in the pupil (OPD card, to the right) objective the "irregular" nature of refractive astigmatism and corneal originated.

Topographic maps and aberrometrique (OPD SCAN 3) corneal scar

The topographic (anterior cornea) highlights the irregularity of the corneal curvature (left). The OPD (refractive fluctuations within the pupil) map reveals the optical consequences. Note the parallelism between the corneal deformation and the OPD map: areas where the cornea is more arched logically are responsible for an increase in optical power.

The OPD map, which is a representation of the variations of the local refraction within the pupil objective local power fluctuations. They are quite marked, and explain the Visual symptoms: light rays focus not at the same point, regardless of the fixed bezel. Local fluctuations of refraction are the consequence of the aberrations of high degree, they even induced the deformation of the corneal surface.

map local refraction OPD in the pupil corneal scar irregular astigmatism

OPD map: local refraction in the pupillary area. The local asymmetric distribution of the refractive power is related to the presence of optical aberrations of high level (corneal deformation-induced)

The following map shows the type and rate of optical aberrations of high degree: the aberrometric allows the collection of high level of the entire eye aberrations. The software also provides a representation of the corneal Wavefront, based on information collected by the surveyor at the level of the front of the cornea. By subtraction, it is possible to calculate the internal wave front (contribution of the eye dioptres other than the front of the cornea: posterior cornea and crystalline face).

total, corneal wave fronts, internal OPD horny scar

Wave fronts: entire eye (left), corneal (in the Center) and internal (on the right). Maps colors baby-sitter to the representation of phase shifts (in microns). The polynomials of Zernike decomposition is represented in each of the wave fronts. Observed that the front of the cornea contributes most of the optical aberrations of the entire eye. Most are the aberrations of high degree "odd" (they characterize the optical consequences of corneal asymmetry)

The use of optical aberrations of high degree allows to draw a map of simulated Visual acuity:

simulated Visual acuity (convolution) horny scar

Simulation of the retinal acuity (convolution): note the degradation of the image, and the reduction of the contrast of the optotype (letter)

In this patient, the Visual quality degradation is induced by corneal deformation; He must regularize the anterior surface of the cornea, making him a more harmonious shape. It's the price of this "corneoplastie" one can hope to improve the optical quality.

The use of a custom excimer laser photoablation is logical; corneal thickness is normal, and the photoablation can also improve the transparency of the cornea. The profile of ablation is established from the refractive and corneal wave front (corneal wavefront). The topographic information is used to establish a profile of specific ablation for the irregularity (see box).

Profile of ablation guided by corneal wave front, to aiming corneoplastique

Representation of the profile of ablation (excimer laser) which is based on the topographic information (calculation of the corneal Wavefront). The irregularity is being a specific sequence of treatment.

A the ocular Wavefront-guided treatment is also possible, but his collection might be flawed, because of the importance of corneal deformation and anomalies of transparency. In this context, it is preferable to opt for a photoablation guided by corneal topography (topolink).

 

After issuance of the photoablation custom (technical PKR) surface guided by topography, and a phase of healing of a few months, there has been a clear reduction of the irregularity of the corneal surface, and improving joint of the optical quality of the cornea. Especially note the standardization of indexes for corneal regularity.

Topographic maps and aberrometrique postoperative after cornéoplastie(OPD SCAN 3)

Topographic maps and aberrometrique acquired several months after treatment personal laser. There is the regularization of the corneal profile (axial mode). Refraction (OPD card, to the right) fluctuations are substantially mitigated.

 

The regularization of the anterior corneal curvature allows to reduce the amplitude of refractive changes in the pupil (they now range from-1 to + 1 D-6 and + 6 before the laser treatment).

Wavefront maps objectify the reduction in the rate of optical aberrations of high degree:

map local refraction OPD in the pupil postoperative treatment customized corneoplastie

OPD map: variations of the refraction in the pupil of entry are significantly reduced after regularization of the anterior corneal surface (the rate of the high degree of original corneal aberrations reduction)

The study of ocular wave-front full, corneal and internal confirms the reduction in the rate of optical aberrations of high degree of corneal origin.

total, corneal wave fronts, postoperative internal after laser photoablation custom topo guided treatment

Wave front: total, corneal, internal after regularization of the anterior corneal surface. Upstairs: maps of optical dephasing, downstairs rates RMS of optical aberrations (polynomials of Zernike decomposition)

The comparison between the simulated Visual acuity and of Modulation Transfer Function (MTF) preoperative and postoperative cards to objectify the gain in quality of the retinal image and contrast of the transfer.

Visual acuity and MTF convolution after corneo plastics

Comparison between the simulated Visual acuity (obtained by convolution with the PSF) and cards MTF between pre and postoperative.

In conclusion, this example illustrates the benefits of the OPD for the understanding and the objectification of visual disturbances from trauma to the cornea. In addition, the possibility of establishing a record of aberrations for eye components to establish a targeted therapy (corneoplastie by issuance of a corneal topography - calculation of the corneal Wavefront-guided treatment).

 

 

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