Crab claw pattern on corneal topography: pellucid marginal degeneration or inferior keratoconus?

Purpose To evaluate the topographic, tomographic, and densitometric properties of patients with pellucid marginal degeneration (PMD) and inferior keratoconus. Patients and methods Retrospective, comparative case series. Forty-seven eyes of 32 patients with crab claw patterns were identified from 2751 patients with corneal ectasia. They were divided into two groups, inferior keratoconus and PMD, based on clinical findings. The topographic, tomographic, and densitometric measurements were analyzed. Results PMD was detected in 11 eyes of eight patients (mean age 50.2±11.1 years), and inferior keratoconus was detected in 36 eyes of 24 patients (mean age 34.7±10.1 years). The control group consisted of 40 patients (33.1±4.6 years). The thinnest corneal point and maximum anterior and posterior elevation points were located lower in the PMD than in the inferior keratoconus ( P