The Development and Application of the Hair Diameter Index (HDI)

Introduction: A study was presented in New York in 2003 investigating a proposed measurement system, the Hair Volume Index (HVI), for evaluating visual hair density. This was based on the idea that scalp coverage was related to the measured volume of hair in a given area. The study results did not support this contention but rather that visual density was correlated with the number of hairs and the hair shaft diameter, which led to the development of the Hair Diameter Index (HDI). This article describes the study results and the application of the HDI in hair restoration surgery for follicular unit excision (FUE) donor area planning and graft implantation density.