The Role of the Follicular Unit in the Hair Restoration Procedures

Introduction - What is a Follicular Unit?

Scalp hair grows in follicular units, rather than individually. This most easily observed by densitometry, a simple technique where scalp hair is clipped to approximately 1 mm in length and observed through a high-magnification instrument. From this vantage point, it can also be appreciated that follicular units are relatively compact, but are surrounded by substantial amounts of nonhair-bearing skin. The actual proportion of nonhair-bearing skin is probably about 50%, so that its inclusion in the dissection can have a substantial effect upon the outcome of the surgery. When multiple follicular units are used in a hair transplant, and the skin is included, these effects are magnified.

The advantage of a Follicular Unit Transplant over mini-micrografting and other hair restoration techniques is that in the former procedure the excess skin between follicular units is trimmed away rather than transplanted. Therefore, a two-hair follicular unit will consist of two closely spaced follicles and a single sheath of circumferential connective tissue. In contrast, a two-hair micrograft comprised of two distinct one-hair follicular units will contain both the circumferential sheaths of each unit and the connective tissue between them, significantly increasing the bulk of the graft and the subsequent wound that must accommodate it. The same argument can be made for grafts containing three or more hairs that are not pure follicular units.

A great benefit then, of using individual follicular units, is that the wound size can be kept to a minimum, while the amount of hair that can be placed into it is maximized. Having the flexibility to place up to four hairs in a tiny recipient site has important implications for density, the design, and the overall cosmetic impact of the surgery.
Follicular Unit Sorting

In a Follicular Unit Transplant the numbers of grafts present in any given size donor strip is determined by nature, since each graft represents one follicular unit. It follows that if only the naturally occurring individual units are to be used, practitioners are limited by their normal distribution in the scalp. Using larger sessions, greater numbers of each type of unit will be generated, adding flexibility to the surgery. This will enhance the cosmetic outcome of the procedure when the surgeon seeks to create soft transition zones using single-hair units or when creating central density with three- and four-hair follicular unit grafts.


Clinical Characteristics of Follicular Units

Follicular unit constant

One of the interesting aspects of transplanting with follicular units is that nature was kind to space them at approximately one per mm2. This has been observed clinically using densitometry and stereomicroscopic dissection and histologically using transverse microscopic sections of the skin (this was first described by Headington in 1984). Not only does a follicular unit constant of 1 unit/mm2 make the math easy, but it makes accurately estimating the donor harvest possible, and gives a logical basis for planning the density and distribution of the grafts.

The constancy of follicular unit density has been studied, but inconsistent study designs prevent accurate conclusions. There is general agreement on the following:

1. Follicular unit density generally decreases as work moves laterally from the mid-occiput at the level of the occipital protuberance towards the temples.

2. Follicular unit density varies among races such that the darker skinned races tend to have the lowest follicular unit densities and light-skinned races the highest.

3. Within the same race and same location on the scalp, there is significant person-to-person variation in follicular unit density.

4. Given points 1-3 above, the constancy of the follicular unit density is still rather striking so that patients with high hair density have more hairs per follicular unit rather than follicular units spaced closer together and those with low hair density for the most part have fewer hairs per follicular unit than units spread further apart.

Although the follicular unit density is not exactly 1/mm2, it is close enough to this number that it can be extremely useful in the surgery. Once it is realized that the follicular unit density is relatively constant and that hair density varies, it follows that the number of hairs per follicular unit largely determines hair density. The implications of this in hair transplantation can be summarized as follows:

> Since the follicular unit density is relatively constant, the same number of follicular units should generally be used to cover a specific-sized bald area regardless of the hair density of the patient.

> With low hair density, using the same number and spacing of follicular units as in a patient with high density will help to ensure that there is proper conservation of donor hair for the long term.

> Hair density is a characteristic of the follicular unit specific to each individual, and together with hair shaft diameter, color and wave, will determine the cosmetic impact of the transplant.

Hair characteristics

The results of a hair transplantation procedure are determined as much by the characteristics of the individual hairs as the number of hairs per follicular unit. Since hair counts are easily measurable, the more subjective hair characteristics, or the ones more difficult to measure, are often discounted. Many characteristics including hair shaft diameter, hair color, texture (wave, curl, kink) and other factors such as the emergent angle of the hair, static, oiliness, or sheen, all contribute to the appearance of fullness, however, hair shaft diameter probably has the greatest impact.

The total volume of hair determines the visual impact. Doubling the hair density doubles the hair volume, while doubling the hair diameter quadruples the hair volume (remember pi multiplied by r-squared). Hair density varies from 1.5-3.0 hairs/mm2, while hair shaft diameter varies from 0.06-0.14 mm. Therefore, hair shaft diameter plays a greater role than absolute hair numbers in determining overall cosmetic impact.


Conclusion - Hair Transplants with a Natural Look

The introduction of follicular unit transplantation created a literal revolution in hair restoration surgery, with doctors discarding their multi-bladed knives, buying stereomicroscopes, and training their staff in the dissection of individual follicular units. However, the belief that in order to get the best results, only naturally occurring individual follicular units should be transplanted has not been universally accepted.

With the new harvesting method of extracting follicular units directly from the donor area, new technical issues are challenging the wisdom of always keeping follicular units intact. In particular, if the object is to minimize the number of recipient wounds when performing follicular unit extraction, it may pay to split larger units into single hairs to create a soft hairline rather than making separate punch holes to harvest each one-hair graft. This and other technical issues of follicular unit extraction, where only partial units are removed, require fresh thinking about what should constitute follicular unit transplantation.