Investigation of optimum transplant and extraction density based on the data from the donor area of Chinese androgenetic alopecia patients: a multicenter, retrospective study.

The rise in the incidence of androgenetic alopecia (AGA) in China has increased the inclination toward hair transplantation. To calculate the optimum density of hair follicles needed covering both the hairless area and the safe donor area (SDA), 119 male patients with AGA were recruited into this multicenter investigation. We evaluated the mean diameter and the number of hair follicles (HFs) and follicular units (FUs) in the SDA of AGA. The mean density of HFs was 137.45 ± 30.11 hair/cm and the FUs was 76.82 ± 12.09 FUs/cm. The mean diameter of the hair was 0.97 ± 0.01 mm in the SDA. We obtained the decisive factors in the restoration of the scalp by building a geometric model and summarizing it in a new formula. This study showed that the length and direction of the hair play a vital role in restoring the bald area with hair transplantation. The density of 55.00 ± 10.00 hair/cm (30.00 ± 5.00 FUs/cm) for the transplanted area and no less than 106.88 hair/cm (60.00 FUs/cm) for SDA is regarded as an optimum parameter for hair transplantation of AGA.