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