Q: Dear Dr. Bernstein, a full head of hair averages ~100 FU/cm2. To achieve the appearance of fullness with a hair transplant 50% is required. In one of your articles you say that you recommend 25 FU / cm2 to your patients. Is that the density per one session or the final one? If that is final density, then it is far below the 50%. Please explain as I am profoundly confused. — W.N., Easton, C.T.

A: If a person is to become a Norwood Class 6, the hair that we have available for us to transplant is only about 12% of what was there originally. This, of course, will vary from patient to patient depending upon one’s donor density and scalp laxity and a host of other factors.

We make the hair restoration look good by restoring 25-50% in the front, and proportionately less in the back. Logically one cannot restore 1/2 of ones original density to an entire bald scalp with only a thin strip of donor hair – there is just not enough hair, even with multiple sessions.

I transplant 25-35 FU/mm2 in one session, but this is the density created in the front, not overall.

Due to follicular unit graft sorting (placing the larger follicular units in the forelock area) this provides even more density than the actual numbers suggest. If someone is relatively certain to have more limited hair loss, then the numbers can be increased, but it is risky if you underestimate the degree of eventual hair loss.