Histopathological Differential Diagnosis of Keloid and Hypertrophic Scar

Distinguishing hypertrophic scar (HS) from keloid histopathologically is sometimes difficult because thickened hyalinized collagen (keloidal collagen), the hallmark of keloid, is not always detectable and alpha-smooth muscle actin (alpha-SMA), a differentiating marker of HS, is variably expressed in both forms of scar. The aim of this study was to investigate additional distinguishing features to facilitate differentiation between keloid and HS. We compared various histologic features and the expression of alpha-SMA in 40 specimens of keloid and 10 specimens of HS. The features more commonly seen in keloids were: (a) no flattening of the overlying epidermis, (b) no scarring of the papillary dermis, (c) presence of keloidal collagen, (d) absence of prominent vertically oriented blood vessels, (e) presence of prominent disarray of fibrous fascicles/nodules, (f) presence of a tongue-like advancing edge underneath normal-appearing epidermis and papillary dermis, (g) horizontal cellular fibrous band in the upper reticular dermis, and (h) prominent fascia-like fibrous band. The last three features were found in keloid specimens only, including the ones lacking detectable keloidal collagen. Our study confirmed the diagnostic value of keloidal collagen, but it was only found in 55% of keloid specimens. Alpha-SMA expression was found in both HS (70%) and keloid (45%), thus it would not be a differentiating marker. In scars with no detectable keloidal collagen, the presence of the following feature(s) favors the diagnosis of keloid: non-flattened epidermis, non-fibrotic papillary dermis, a tongue-like advancing edge, horizontal cellular fibrous band in the upper reticular dermis, and prominent fascia-like band.