Distribution of human hair in follicular units. A mathematical model for estimating the donor size in follicular unit transplantation.

BACKGROUND: Human hair emerges from the scalp in groupings known as follicular units. In follicular unit transplantation, the follicular unit is the exclusive element to be moved in the transplant.
OBJECTIVE: To study the distribution of follicular units in the human occipital (donor) scalp.
METHODS: Using digital photography, we counted in 50 patients the hair density, follicular unit density, and the proportion of 1-, 2-, and 3-hair units per square centimeter. We measured the median distance between the follicular units. All the data obtained was statistically analyzed.
RESULTS: In the occipital (donor) scalp the number of follicular units per square centimeter ranges between 65 and 85, and the hair density ranges between 124 and 200. The proportions of the different hair groupings change according to the patient's hair density. We have developed a mathematical model that can predict the number and the most probable distribution of 1, 2, and 3 hair groupings, based on the patient's hair density. The distance between follicular units ranges from 1.0 mm to 1.4 mm.
CONCLUSION: Hair transplant surgeons can now predict the total number of follicular units to be obtained from any given donor strip. In addition, the proportion of 1-, 2-, and 3-hair follicular units can also be anticipated. Some variance is to be expected due to the lack of uniform density in the donor area.