Stress Distribution Patterns In Keratoconus Eyes: Longitudinal Data

To investigate whether stress distribution patterns can predict biomechanical progression in keratoconus (KCN) eyes using longitudinal data. Biomechanical properties of the cornea were calculated using the corneal contribution to stress, CCS=r/2t, on the Hoop stress formula without intraocular pressure. Here r is radius of curvature and t is corneal thickness, thus CCS exhibits greater stress with lower corneal thickness or flatter curvature.

Single-center, retrospective study at the University Eye Clinic, Maastricht University Medical Center, Maastricht, the Netherlands. In collaboration with The Ohio State University College of Medicine, Columbus, Ohio, United States.

We calculated CCS from Pentacam curvature and thickness maps (Oculus, Wetzlar, Germany) and investigated the difference in the magnitude of stress between the 2-mm zones of the CCS_min and CCS_max, ΔCCS, and the distance between these 2 zones, ΔCCS_distance. We included patients with diagnosed KCN and healthy controls. Exclusion criteria were use of contact lenses, previous corneal surgery, including corneal crosslinking, corneal scar, other corneal diseases and bad quality of Pentacam images. We investigated if ΔCCS and ΔCCS_distance changed over time and used a mixed linear model approach to determine if ΔCCS at baseline could predict keratoconus progression.

A total of 111 eyes of 67 KCN patients and 31 eyes of 31 healthy controls were included with a mean age of 25±7 and 24±4 years, respectively. KCN patients had a mean follow-up time of 566 days (range 28 to 1720 days). At baseline, in KCN eyes mean greatest stress was 8.3±1.0 (range 5.8 to 11.3) and lowest stress was 6.6±0.6 (range 4.9 to 8). For healthy eyes mean values were 7.4±0.5 (range 6.4 to 8.4) and 6.5±0.5 (range 5.6 to 7.6) respectively. ΔCCS significantly increased over time (p<0.001) and correlated significantly with zonal K_max (r=0.82, p<0.001) (Fig. 1). ΔCCS at baseline predicted progression over time of zonal K_max (p=0.007). ΔCCS_distance correlated significantly with ΔCCS (r=-0.33, p<0.001), but did not change in time (p=0.42).

The difference between the minimum and maximum stress contribution, ΔCCS, changes in time and its baseline values predict progression in keratoconus patients. We did not find a change in the distance between the highest and lowest stress area over time.