Hypertrophic scars and keloids: Overview of the evidence and practical guide for differentiating between these abnormal scars

Abstract

Although hypertrophic scars and keloids both generate excessive scar tissue, keloids are characterized by their extensive growth beyond the borders of the original wound, which is not observed in hypertrophic scars. Whether or not hypertrophic scars and keloids are two sides of the same coin or in fact distinct entities remains a topic of much debate. However, proper comparison between the two ideally occurs within the same study, but this is the exception rather than the rule. For this reason, the goal of this review was to summarize and evaluate all publications in which both hypertrophic scars and keloids were studied and compared to one another within the same study. The presence of horizontal growth is the mainstay of the keloid diagnosis and remains the strongest argument in support of keloids and hypertrophic scars being distinct entities, and the histopathological distinction is less straightforward. Keloidal collagen remains the strongest keloid parameter, but dermal nodules and α-SMA immunoreactivity are not limited to hypertrophic scars alone. Ultimately, the current hypertrophic scars-keloid differences are mostly quantitative in nature rather than qualitative, and many similar abnormalities exist in both lesions. Nonetheless, the presence of similarities does not equate the absence of fundamental differences, some of which may not yet have been uncovered given how much we still have to learn about the processes involved in normal wound healing. It therefore seems pertinent to continue treating hypertrophic scars and keloids as separate entities, until such a time as new findings more decisively convinces us otherwise.

Keywords: diagnosis; histopathology; hypertrophic; keloid; scar.

FIGURE 1

Scar spectrum. Watercolour illustration of a normotrophic, a (linear) hypertrophic and a (major) keloid scar

FIGURE 2

Natural progression of hypertrophic and keloid scars over time. Graph in figure taken from Ref.[2] reproduced with permission from publisher. Hypertrophic and keloid scars show distinct clinical behaviour. The bottom graph illustrates the differential cellular and matrix arrangement of hypertrophic and keloid scars, together with their contrary biological behaviour. Unlike normotrophic and hypertrophic scars, keloids rarely mature, but mild and severe subtypes exist with gross morphologic differences. Hscar: hypertrophic scar. Kscar: keloid scar

FIGURE 3

A, Histopathology decision tree: hypertrophic or keloid scar? This decision tree can be used to distinguish between hypertrophic scars and keloids, * after other differential diagnoses (eg dermatofibroma, dermatofibrosarcoma protuberans) have already been excluded or deemed highly unlikely based on clinical findings. Legend; white text boxes: in vivo findings; blue text boxes: ex vivo findings; associated references are listed to the right of each text box; bold font: strongest discriminating features; +: present, normal expression or values; +/↑: variable expression, both normal and increased expression observed; +/−: variable expression, both presence and absence of expression observed; ↑: increased; −: absent; ↓: decreased; ±: %: percentages of. Abbreviations listed in alphabetical order; α‐SMA: alpha smooth muscle actin; RD: reticular dermis; SPB: suprabasal expression. B, Haematoxylin and eosin staining of a normotrophic scar, a hypertrophic scar and a keloid. Area with keloidal collagen is marked with an asterisk (*) under the dotted line in the keloid panel. Scale bar = 200 μm