Differentiating contact lens induced warpage from true keratoconus using corneal topography.

PURPOSE: Superior corneal flattening associated with inferior corneal steepening is a videokeratoscopic topography pattern that usually describes both keratoconus and contact lens induced warpage. To differentiate these two conditions topographically, we analyzed ten different corneal topographic shape variables and found that two distinct conditions were described. Three unique measurements of corneal geometry: shape factor (SF), irregularity (CIM), and apical toricity (TKM) were analyzed and evaluated as an additional method for differentiating these two conditions. METHODS: A retrospective series of 100 eyes with a medical diagnosis of either contact lens induced warpage or true keratoconus were mapped using the Humphrey Atlas Corneal Topographer (San Leandro, CA) and their individual topographic indices were analyzed in an attempt differentiate the two conditions with videokeratoscopy. Variables included Flat Keratometry Reading, Maximum Axial Curvature, Maximum Tangential Curvature, Corneal Astigmatism, Corneal Shape Factor, Reference Sphere, Corneal Irregularity Measure (CIM), Maximum Elevation, and Mean Reference Toric K (TKM). All patients wore rigid gas permeable contact lenses. RESULTS: True keratoconic eyes had steeperflat K readings (46.9+/-3.7 D vs. 43.04+/-1.18 D; P<6.3E-13) with greater variations in maximum axial curvature (54.1+/-4.76 D vs. 43.6+/-1.5 D; P<2.4E-15) and maximum tangential curvature (55.1 7+/-4.56 D vs. 47.7+/-1.5 D; P<5.4E-16) than did eyes with contact lens induced warpage. The amount of corneal toricity (-3.43+/-2.29 D vs. -1.33+/-0.88 D; P<1.5E-07) was also greater for true keratoconus. True keratoconic eyes had larger and more variable shape factors (0.61+/-0.26 vs. 0.02+/-0.13; P<2.5E-19) and CIM's (2.30+/-1.16 vs. 1.03+/-0.37; P<7.5E-10) than those with contact lens induced warpage. Steeper reference spheres (47.58+/-3.55 D vs. 43.6+/-0.37 D; P<2.2E-10), greater maximum elevation over their respective reference spheres (15.51+/-9.91 microns vs. 6.2+/-3.0 microns; P<8.1E-08) and steeper, more variable toric mean reference spheres (51.88+/-4.01 D vs. 43.82+/-1.82 D; P<3.9E-17) are also present in true keratocon us. True keratoconic eyes can be separated from contact lens induced warpage eyes with a sensitivity of 98%, while identification of contact lens induced warpage demonstrates 94% specificity using corneal topography. CONCLUSIONS: While contact lens induced warpage and true keratoconus exhibit similar corneal topography patterns (superior flattening and inferior steepening), they demonstrate two uniquely different geometric shapes that can be readily differentiated using the various corneal shape indices of videokeratoscopy with a high degree of accuracy and specificity.

RGP quality control: the results of a national survey.

BACKGROUND: Rigid gas permeable (RGP) contact lenses have numerous benefits; however, one of the possible problems of RGP lenses is variable quality that can result in reduced initial comfort and an unsuccessful fit. METHODS: A quality control survey was sent to 200 optometrists, selected at random, to provide information on lens usage, verification, use of reused or modified lenses, use of authorized laboratory, and to evaluate important laboratory characteristics. A total of 86 optometrists responded to the survey. RESULTS: The results indicated that most respondents fit between 10-25 percent of their new contact lens patients with RGPs. In addition, although both of these parameters can be extremely important to patient success, only 56 percent of the practitioners verify edge quality and even less, 38 percent, verify center thickness. Sixty-seven percent of the practitioners indicated that poor initial wettability occurred in, at minimum, 5-10 percent of all new lenses. 75 percent of the practitioners indicated that it would matter to them if the supposed new lenses had previously been used or modified. Finally, optical quality and lens parameter accuracy were considered much more important than turnaround time and material cost. CONCLUSIONS: It appears from these results that variance in quality control can result in compromised lens quality and performance. In addition, it can be reduced by more careful lens inspection by the practitioner. If careful attention is devoted to these recommendations, RGP lens use and success could increase accordingly.

The effect of rigid gas permeable lens cleaners on lens parameter stability.

The introduction of fluoro-silicone/acrylate rigid lens materials has provided wearers with benefits that include, among others, enhanced surface wettability. However, several reports have associated these lens materials with lens power, base curve radius and center thickness changes over time, especially with long-term use of an abrasive cleaner. The purpose of this study was to evaluate the effect of abrasive and non-abrasive cleaners on these specific lens parameters. Twenty-four fluoro-silicone/acrylate lenses, 12 low minus and 12 high minus in power, were cleaned a total of 200 times each. Each lens was cleaned with one of-three different cleaners (1 non-abrasive and 2 abrasive) using one of two cleaning methods; palm of the hand or between the fingers (digital). Lens power was changed on 14/24 lenses (58.3 percent); the primary changes occurred with use of one of the abrasive cleaners. In addition, a slight reduction in center thickness was observed after abrasive cleaner use. Warpage occurred at a much higher rate with the digital cleaning method and with the use of an abrasive cleaner. It was concluded that practitioners should routinely reverify rigid lens power, center thickness and base curve radius; patients should be educated to carefully handle their lenses, notably cleaning them in the palm of the hand.