1 Crab claw or butterfly patterns on the sagittal topographic map of anterior corneal curvature reveal steepening of the inferior corneal periphery and flattening of the cornea along the vertical meridian. In PMD, the thinning extends 1–2 mm away from the inferior limbus, and the corneal protrusion occurs above the area of thinning. Keratoconus and pellucid marginal degeneration (PMD) are bilateral, progressive, ectatic corneal disorders characterized by thinning of the cornea. Tomographic and densitometric evaluations may facilitate the differential diagnosis. Therefore, analyzing only the anterior corneal surface is not sufficient in differential diagnosis. There is a higher probability of a patient with crab claw pattern on the topography of having inferior keratoconus than having PMD.
The densitometry values of PMD were significantly higher than those of the controls in all zones and layers ( P<0.01) and significantly higher than the densitometry values of inferior keratoconus in the 6–10 and 10–12 mm zones ( P<0.05). In the PMD, all deviation indices were higher than the controls ( P<0.01), whereas the deviation indices, except Dt ( P=0.960), were lower than the inferior keratoconus ( P<0.01). The thinnest corneal point and maximum anterior and posterior elevation points were located lower in the PMD than in the inferior keratoconus ( P<0.01). The control group consisted of 40 patients (33.1±4.6 years). PMD was detected in 11 eyes of eight patients (mean age 50.2☑1.1 years), and inferior keratoconus was detected in 36 eyes of 24 patients (mean age 34.7☑0.1 years). The topographic, tomographic, and densitometric measurements were analyzed. They were divided into two groups, inferior keratoconus and PMD, based on clinical findings. Forty-seven eyes of 32 patients with crab claw patterns were identified from 2751 patients with corneal ectasia. To evaluate the topographic, tomographic, and densitometric properties of patients with pellucid marginal degeneration (PMD) and inferior keratoconus.
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