Scarring occurs at a critical depth of skin injury: Precise measurement in a graduated dermal scratch in human volunteers

Background: The association between scarring and the depth of dermal injury or burn is clinically recognized but not quantified. The authors tested the I hypothesis that there is a critical depth beyond which a fibrous scar develops. Methods: A novel jig produced a wound that was deep dermal at one end and superficial dermal at the other. Pilot studies in cadaveric and ex vivo breast skin confirmed the depth of injury. Healthy volunteers had a standardized dermal wound made on the lateral aspect of the hip. Digital photography recorded the surface appearance of wound healing and scar development. High-frequency ultrasound demonstrated the depth of the healing wound and subsequent scar in vivo. Results: One hundred thirteen human subjects participated in the clinical study. Mean length of follow up was 28.6 +/- 13.2 weeks. The deep dermal end of the wound healed with a visible scar and the superficial end had no visible residual mark after week 18. The initial length of injury was 51.3 +/- 0.6 mm, which reduced to a scar of 34.9 +/- 1.0 min at 36 weeks (corresponding areas were 196.6 +/- 7.5 mm(2) and 92.7 +/- 9.4 mm(2)). High-frequency ultrasound analysis showed a gradual reduction in scar thickness at the deep end and no detectable scar at the shallow end. The transition point between scar and no scar marked the threshold depth for scarring. This was calculated as 0.56 +/- 0.03 mm, or 33.1 percent of normal hip skin thickness. Conclusions: The dermal scratch provides a well-tolerated, standardized, and reproducible wound model for investigating the healing response to dermal injury of different depths. There is a threshold depth of dermal injury at which scarring develops.