MMP13 and keratoconus: These factors may contribute to the pathogenesis of keratoconus; furthermore, studies have shown that post-rubbing tear samples from normal eyes exhibit high concentrations of IL-8, MMP-13, IL-6, TNF-α, and epithelial growth factor compared to contralateral control eyes (Kallinikos and Efron, 2004; Balasubramanian et al., 2013).