Topographic detection of photorefractive keratectomy

Topography-Based Screening for Previous Laser In Situ Keratomileusis to Correct Myopia and Hyperopia

PURPOSE

To develop a screening tool based on corneal topography to detect previous myopic and hyperopic laser in situ keratomileusis (LASIK).

METHODS

Clinical data from three private clinics were analyzed in a university setting. A total of 476 topographies (Orbscan II) were randomly selected (1 topography per patient): 338 from unoperated corneas, 81 from corneas that had LASIK to correct myopia, and 57 from corneas that had LASIK to correct hyperopia. The LASIK procedures were performed using a Technolas excimer laser (217C or 217Z) and a Hansatome microkeratome. The first set of algorithms (VESm and VESh) calculated the volume summation between the anterior corneal surface and the best-fit sphere in the central and mid-peripheral regions. The second set of algorithms (DCm and DCh) calculated the mean anterior tangential curvature difference between the central and mid-peripheral regions.

RESULTS

The features VESm and DCm for the screening of a myopic LASIK yield, respectively, sensitivity rates of 92.7% and 95.1% and specificity rates of 94.9% and 96.5%. The features VESh and DCh for the screening of a hyperopic LASIK gave sensitivity rates of 93.1% for both and specificity rates of 94.8% and 97.1%. The performance of those features was superior to clinical assessment using topography color maps.

CONCLUSIONS

Criteria based on Orbscan II corneal topography are proposed for the detection of previous hyperopic and myopic LASIK performed with a Technolas excimer laser (version 217C or 217Z).

Screening of prior refractive surgery by a wavelet-based neural network

PURPOSE

To demonstrate an objective method of screening for previous refractive surgery using corneal topography.

SETTING

Corneal topography research laboratories, LSU Eye Center, New Orleans, Louisiana, USA.

METHODS

Videokeratography (TMS-1, Tomey) examinations from the LSU Eye Center were randomly divided into training and test sets that each included 32 normal corneas and 106 corneas with previous radial keratotomy or photorefractive keratectomy from 1 month up to 10 years after surgery. A set of 1024 axial curvature values were extracted from mires 1 to 25 from each cornea to form a 1-dimensional waveform. Multiresolution wavelet decomposition was performed on this waveform using the s8 Symmlet wavelet. A portion of the resulting wavelet coefficients was input into a backpropagation neural network that was trained to 5% error. After training, the independent test set was passed though the neural net and scored.

RESULTS

The trained network correctly recognized 32 of 32 normal corneas and 105 of 106 refractive surgery corneas for a 99.3% accuracy, 99.1% sensitivity, and 100% specificity for previous myopic refractive surgery detection.

CONCLUSIONS

The 1-dimensional wavelet-based neural network approach was an effective and accurate method of distinguishing eyes that had had myopic refractive surgery from normal eyes. The single error was a result of having too few examples of grossly decentered procedures in the training set.

The specific architecture of the anterior stroma accounts for maintenance of corneal curvature

AIM

To analyse the human corneal stroma in extreme hydration to discover if its structure is responsible for corneal stability.

METHODS

Corneas in several hydration states were used: postmortem control corneas (PM; n=3), corneas left for 1 day in phosphate buffered saline (PBS; n=4), and corneas left for 1 day (n=4), 2 days (n=4), 3 days (n=2), and 4 days (n=4) in deionised water. All corneas were fixed under standardised conditions and processed for light and electron microscopy. In addition, two fresh corneas from the operating theatre were studied which were processed 6 months after storage in sodium cacodylate buffer.

RESULTS

After 1 day in deionised water maximal stromal swelling was reached which did not change up to 4 days. The stroma of deionised water corneas (1400 microm) was much thicker than that of PBS corneas (650 microm) and PM corneas (450 microm). Deionised water treatment led to disappearance of all keratocytes leaving only remnants of nuclei and large interlamellar spaces. In these specimens the distance between the collagen fibres had increased significantly, but the diameter of the collagen fibres did not seem to be affected. A remarkable observation was that the most anterior part of the stroma (100-120 microm) in all deionised water specimens and those stored for 6 months in buffer was not swollen, indicating that the tightly interwoven anterior lamellae are resistant to extreme non-physiological hydration states.

CONCLUSIONS

The rigidity of the most anterior part of the corneal stroma in extreme hydration states points to an important role in maintenance of corneal curvature. Since a large part of this rigid anterior part of the stroma is either removed (PRK) or intersected (LASIK), it is possible that in the long run patients who underwent refractive surgery may be confronted with optical problems.

Correlation Between Refractive and Corneal Topographic Changes After Photorefractive Keratectomy for Myopia

PURPOSE

To compare videokeratographic and refractive data obtained before and after photorefractive keratectomy (PRK) for myopia.

METHODS

Seventy-four eyes underwent PRK for myopia ranging from -2.50 to -17.00 D (mean, -7.76 +/- 3.17 D). All patients had videokeratography with the EyeSys instrument before, and 1 and 6 months after PRK, and the changes in three corneal power measurements (center of the ablation, apex, and effective refractive power) were compared with refractive changes.

RESULTS

Changes obtained in the three corneal power measurements at 1 and 6 months were well correlated with manifest refraction (Pearson's coefficient ranged from 0.71 to 0.84).

CONCLUSION

Power measurements obtained with corneal topography, as described above, are a reliable and objective method for the evaluation and follow-up of PRK, provided addition of an approximate 25% correcting factor.

Significance of corneal topography in predicting patient complaints after photorefractive keratectomy

PURPOSE

To evaluate the sensitivity and specificity of postoperative corneal topography to predict potential patient complaints after photorefractive keratectomy (PRK).

SETTING

Doheny Eye Institute, Los Angeles, California, USA.

METHODS

Postoperative tangential corneal topographic maps, in 0.5 and 1.0 diopter (D) relative scales, were obtained from patients (n = 34) at least 4 months after PRK. Topographies of complaining (n = 18) and noncomplaining patients (n = 16) were analyzed by 6 masked examiners with 2 different experience levels in PRK (experts, n = 2; beginners, n = 4), who assigned the topographies to 1 of the 2 groups.

RESULTS

Topographies of complainers (sensitivity) and noncomplainers (specificity) were correctly classified in 53.2% overall and in 44.0% and 63.5% (P = .06) in complainers and noncomplainers, respectively. Experienced examiners were not significantly more accurate than inexperienced examiners (46.3% and 56.6%, respectively; P = .09). Images of 1.0 D scales received significantly more correct responses than those of 0.5 D scales (56.4% and 50.0% respectively; P = .03). The reproducibility between images for the same patient in both scales was significantly better for the experienced examiners than the inexperienced examiners (kappa coefficient 0.73 and 0.51, respectively; P = .05).

CONCLUSIONS

Subjective analysis of postoperative corneal topography alone is not sufficient to predict potential patient complaints after PRK. Topographic findings should be interpreted only in the context of a complete clinical examination.