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There are two techniques to harvest Follicular Units (FU) from the donor area:

  1. One technique known as Strip, followed by the dissection of the single follicular units under stereo microscope;
  2. Follicular Unit Extraction, better known as FUE, in which follicular units are harvested directly from the donor area using small (<1mm) punches. Unlike Strip, FUE doesn’t leave any linear scar and the healing process is very fast.

By “donor area” we mean the part of the scalp that extends between the occipital and the temporal areas. From this area it is possible to harvest the follicular units that are necessary for the hair transplantation procedure. Most of these follicles are not influenced by androgens, not even in case of androgenic alopecia. The donor area covers a horizontal strip of about 5-6 cm. It has to be carefully identified in each patient with wet hair, examining the scalp and analysing the medical history. By observing the donor area it is possible to understand that not all patients are good candidates for hair transplantation. Each surgeon should examine the donor area taking into account several parameters, such as the total number of hairs and follicular units that can be grafted, the number of hairs per follicular unit, the type of grafts and the degree of miniaturization. The main parameters are indicated in table no.1.

Table 1

Parameters for the evaluation of the donor area:

  • No. of FU/cm2
  • No. of hair/cm2
  • No. of hair / FU
  • Hair diameter Anagen-Telogen ratio
  • Hair and skin colour
  • Hair type: wavy, curly, etc.
  • Scalp laxity and thickness

Hair diameter represents a very important parameter that will give us an idea of the degree of miniaturization, thus influencing the hair transplantation result. Patients with a big hair diameter (> 80 μm) are great candidates; on the other hand, patients with very thin hair (< 50 μm) are not considered good candidates.

Table 2

Hair diameter classification

  • Very thin < 60 μm
  • Thin 60-65 μm
  • Medium-thin 65-70 μm
  • Medium 70-75 μm
  • Medium-thick 75-80μm
  • Thick > 80 μm

By grafting the same number of follicular units (FU) on two patients with different hair diameters we can notice that the patient with a bigger diameter will obtain a higher coverage and a better result.
Indeed, doubled hair density means doubled volume, but doubled hair diameter means quadrupled volume.
That is why it is essential to talk to the patient prior to the procedure in order to make sure expectations meet reality.

Measurement has to be carried out not considering just one hair but at least on 10-20 units in order to obtain an average diameter and a more accurate result.
There are several methods to do it:

  • Digital micrometer,
  • Folliscope,
  • Cross-section,
  • Trichometer,
  • Etc.

Another parameter it is essential to evaluate when analysing the donor area is density.
It is important to differentiate between:
Follicular unit density (FU/cm2);
Hair density (no. of hairs/cm2);
Measured density, that is the average number of hairs per FU.
This last parameter is obtained by dividing the number of hairs in 1 cm2 by the number of FU measured in that cm2.
It is a very important parameter that will determine the final density. A patient with 3-4 hairs per FU will get a better result compared to a patient with the same follicular density but with 1-2 hairs per FU.

The average density of follicular units in the occipital area among white people is approximately 70-100 FU/cm2 and the average hair density is about 260 +/- 30 hairs/cm2.
Due to the different hair density within the donor area, we use a method described by Cole. He suggests that we use 3 measuring points on both sides.
Here is where they are located:

  1. In the occipital protuberance,
  2. 3 cm above the auricular projection;
  3. In the mid-point between 1 and 2.

Another parameter to take into account is the Anagen-Telogen ratio.
Usually about 90% of our hair is in the Anagen phase, while 10% in the Telogen phase.
If the percentage of hair in the Telogen phase is higher it means the donor area doesn’t represent a good source for surgery. The main methods to evaluate this are called Trichoscan and Trichogram.

Thanks to this information and an accurate evaluation of the donor area, that is different in every patient, the surgeon can define the best surgical approach, aiming at a long-term result.

References
Devroye J. Donor harvesting, Hair
Transplantation, Unger, W. Shapiro, R. Unger, R. Unger, M. Unger. 2011: 247-262.
Francisco Jimenez, The Donor Area, HairTransplantation, Marc R. Avram, Nicole E.
Rogers. 2010: chapter 3 Donor Harvesting, Hair Restoration Surgery in Asians, D. Pathomovanich, K. Imagawa. 2010: Part VII