1. General introduction 10 Keloids The word keloid is derived from the Greek word ‘chele’, meaning crab’s claw because of its typical sideways growth into normal skin.1 Keloids are fibroproliferative scars that cause reduced quality of life, due to pain, itch, social stigma and restriction of movement.2 The mean Dermatology Life Quality Index (DLQI) in patients with keloids is 7.8 5.1, meaning that keloids have a ‘moderate effect’ on patients quality of life, comparable to patients with mild to severe psoriasis vulgaris.3 Keloids can develop due to inflammation such as inflammatory acne and folliculitis or after (minor) trauma.4 Keloid formation may be caused by a dysregulation in at least one of the four phases of wound healing: hemostasis, inflammation, proliferation and remodeling.5,6 The inflammatory response in wound healing may play a crucial role in keloid formation. In keloids proinflammatory factors including interleukin-1α, -1 -6 and tissue growth factor-beta 1 (TGF-1) are upregulated.7 The upregulation of these proinflammatory factors suggests that the formation of keloids can be considered as an inflammatory disorder of the reticular dermis.7 Moreover, recent studies have proposed that keloids possess features of an auto-immune disease, since anti-hnRNPA2B1 autoantibodies, Immunoglobulin A and M, and Complement components C1Q and C3 depositions were found in keloid tissue.8,9 Keloids vs. hypertrophic scars Keloidal scars differ from hypertrophic scars because keloids will expand beyond the borders of the original wound while hypertrophic scars do not.10 However, keloids also differ from hypertrophic scars in other aspects; keloids are histologically different, often cause more symptoms, and are more difficult to treat (Figure 1A-1D).11,12 On histological examination, keloids show thicker collagen bundles, without a distinct pattern, while hypertrophic scars show relative thinner collagen bundles in a regular pattern.13 The prevalence of keloids strongly differs among different ethnic populations. Keloids occur most frequently in the African, Asian and Hispanic population (prevalence up to 16%),
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