Selection of safe permanent donor area
If you have ever seen a man with Class VII balding, and we all have, you have seen a graphic representation of the limits and confines of the donor area. This is the hair zone that is considered permanent. With rare exceptions, this rim of hair remains even in the most advanced cases of male pattern baldness. The boundaries of this zone extend from in front of the ears, around the temples, and to the back of the head. The hair at the temples may recede back toward the ear, and the balding area of the crown may dip quite low into the occipital area, at the back of the head. We must always assume that any man considering hair transplant surgery will eventually advance to this Class VII level for balding; it’s easy to understand why. Visible scars may be revealed if the baldness advances, and donor tissue has been taken too high, too low, or too far in front of the ears
Minimizing Donor Hair Wastage
We can directly measure not only the FU density, but the hair density as well, by using a tool called a densitometer or digital microscope (Proscope HR). With small areas of hair clipped short, a fixed area is observed under bright light and magnification. Then, we can calculate the appropriate numbers, for example: we can look at the density of FU’s and hair in the center of the back of the head, over the ear, and halfway in between. If the numbers average 1FU/mm2, and 2 hairs/FU, and the patient will be receiving 1500 grafts, then we can calculate that we will need to harvest about 15 square centimeters (cm2) from the back and side of the head to give us our required grafts. We can also assume that, given a 20% occurrence of single hair grafts in those with average density, our 20cm2 donor strip will provide us with about 300 single hair grafts, which should be enough for our hairline transition zone.
Moreover, we can use calculations to plan for the future. The average, non-balding person has about 100,000 hairs on the scalp. This would translate to 50,000 follicular units (FU’s). The "permanent zone" comprises about 25% of the total scalp; therefore, there would be one-fourth, or 25% of these total 50,000 FU’s in the permanent zone, which would equal 12,500 FU’s. We know that about half of the hairs in an area must be lost before there is any appearance of balding, so we could safely harvest up to half of the permanent zone FU’s, or 6,250 FU’s. This gives us an idea of the viable, reasonable donor reserves that a given patient has for current and future transplantation.
It is important to realize that the amount of coverage and density that a given person achieves with FU transplantation will vary not only according to their donor density and scalp laxity, but also according to their hair characteristics. This is another point where art meets science in the field of hair transplant surgery.