The following is how one plans for this surgically:

Defer transplanting patients who are very early in the balding process, i.e. those who are content with the way they look now but are more concerned about future hair loss. A good rule of thumb is to wait unit the patient needs a minimum of approximately 600-800 follicular units before considering surgery. Often medical therapy, rather than surgery, would be appropriate for these patients.
When considering surgery, define the boundaries to be transplanted via densitometry as well as by gross visual inspection. Densitometry, is a more sensitive indicator of miniaturization.
Transplant through (rather than around) an area that is highly miniaturized, since it is likely that this area will be lost by the time the transplant has grown in. Two examples of this would be a "forelock" composed of wispy miniaturized, rather than strong terminal hair, or the "bridge" of a Norwood class 5 patient that is beginning to break down.
Plan to use enough follicular units so that, if possible, the volume of transplanted hair is greater than the volume of hair that will likely be lost from telogen effluvium. Remember, we are never replacing "hair for hair" in the surgery. We are, in effect, replacing a large number of fine, miniaturized hairs, with a much smaller amount of permanent, full-thickness terminal hairs.
In areas, of extensive miniaturization, it may be appropriate to transplant follicular units in the same density as though the area were totally bald.

Economizing The Donor Supply

As we mentioned in the introduction, the concern over the donor supply being finite is a rather recent development in hair restoration surgery, but since it is the ultimate limiting factor in all transplantation surgery, every possible effort should be made to insure the maximum yield. We would briefly like to review the logic of using large sessions with regard to the surgical wound. The importance of proper harvesting techniques and precise follicular dissection in ensuring maximum donor yield will be covered in later sections.

The donor supply is more sensitive to donor density than one might think. In fact, for every unit change in donor density, there is a 2-fold change in the amount of movable hair.9 Although not immediately obvious, the logic of this is illustrated in table 1. As will be discussed in the section "A Mathematical Look at Balding, " a person may loose 50% of his/her hair volume before it is clinically noticeable. Although we commonly think of this in terms of the balding scalp, it applies to the permanent zone as well.5 Therefore, in the average person with a density of 1 follicular unit/mm2 (2 hairs/mm2), the follicular unit density can be reduced to approximately 0.5 units/mm2 (1hair/mm2), before the donor area appears too thin. In those with high hair density, a greater percentage may be removed (see table 1). Therefore, a patient with a hair density of 2.5 hairs/mm2 would have 50% more movable hair than the average patient with a hair density of 2.0 hairs/mm2, although his hair density was only 25% more. The amount of movable hair will also depend upon other characteristics of the patient’s follicular units (see section "Characteristics of the Follicular Unit") and upon his scalp dimensions and laxity.

The density will obviously be affected by each hair transplant. If a person has a hair transplant procedure(s) that decreases his donor density by 25%, then half of his movable hair will be exhausted, since his follicular unit density will be reduced to 0.75 units/mm2 (1.5 hairs/mm2). If that same patient, began with 25% less hair density i.e. 1.5 hairs/mm2 (remember, the follicular unit density is constant and would still be 1unit/mm2), then the same transplant(s) would reduce the follicular unit density to 0.75 units/mm2 and would leave a hair density of 1 hair/mm2 (0.75 units/mm2 x 1.5 hairs/unit), too thin to permit further transplantation. These numbers serve to underscore the importance of trying to conserve donor hair in every aspect of the procedure.

Table 1. THEORETICAL EFFECTS OF CHANGES IN DONOR DENSITY ON TRANSPLANTABLE HAIR*

A Donor Hair Density (hairs/mm2) 3.0 2.5 2.0 1.5 1.0
B Follicular Unit Density (units/mm2) 1.0 1.0 1.0 1.0 1.0
C Total Hair in Permanent Zone 37, 500 31, 250 25, 000 18, 750 12, 500
D Follicular Units in Permanent Zone 12, 500 12, 500 12, 500 12, 500 12, 500
E Hairs that must Remain in Permanent Zone 12, 500 12, 500 12, 500 12, 500 12, 500
F Movable Hairs (C-E) 25, 000 18, 750 12, 500 6, 250 0
G Average Hairs per Follicular Unit (G=A) 3.0 2.5 2.0 1.5 1.0
H Transplantable Follicular Units (F/G) 8, 333 7, 500 6, 250 4, 167** 0

* These numbers serve to illustrate the effects of changes in donor density on hair supply. The actual number of grafts that can be harvested depends upon a multitude of factors including donor dimensions, scalp laxity, hair characteristics (such as hair shaft diameter and wave), and skin/hair color contrast. It also assumes that the efficiency of the harvest is 100%, and that this can be maintained between procedures (see discussion below).

** Although the patient with a donor density of 1.5 hairs/mm2 has ½ the available follicular unit grafts as a patient with a density of 3.0 hairs/mm2 (4, 167 grafts Vs 8, 333 grafts), each of his grafts, on the average, have only ½ the hair content of the patient with the density of 3.0, so that his transplant will appear only 1/4th as full. (4, 167 grafts averaging 1.5 hairs per graft Vs 8, 333 grafts averaging 3 hairs per graft).

Regardless of how impeccable the surgical technique, each time an incision is made in the donor area, and each time sutures are placed, hair follicles are damaged or destroyed. This damage can be minimized (but not eliminated), by keeping the sutures very close to the wound edges so that they don’t encompass much hair. In subsequent procedures the damage can be reduced by using the previous scar as the upper or lower boarder of the new excision. In this way the amount of distortion and possible damage to existing hair is limited to only one free edge. Some physicians advocate the use of staples, feeling that this method of closure is quick, causes less trauma to the skin, and produces less damage to surrounding hair follicles (resulting in a superior scar). Others feel that staples produce unnecessary post-op discomfort and actually produce greater scarring due to the less controlled approximation of the wound edges. Studies still need to be performed to compare these two techniques and provide a definitive answer.

There are other more subtle effects of the surgery. In all healing, even with primary intention closures, collagen is laid down and reorganized. This distorts the direction of the hair follicles and increases the risk of transection in subsequent procedures. In addition, the fibrosis makes the scalp less mobile for subsequent surgeries, thus decreasing the amount of additional donor tissue that can be harvested. It should be clear that each time there is surgery these factors come into play, so that transplanting in large sessions, which minimizes the total number of individual procedures, will conserve on total donor hair.

Complexion of Follicular Units
A final issue regarding the use of large sessions, is their ability to enhance the complexion of the follicular units generated from the donor strip. The logic behind this is very straightforward. In follicular unit transplantation the numbers of grafts present in any given size donor strip is determined by nature, since each graft represents one follicular unit. In contrast, in mini-micrografting techniques, the numbers are determined by the surgical team who cut the grafts "to size" depending upon how many of each size the surgeon feels are needed. For example if the "mini-micrografter" needed 200 single-hair grafts, he might divide up 100 two-hair grafts to produce 200 ones. If one felt he needed 100 four-hair grafts, he might combine 200 two-hair grafts to satisfy his needs. As we have discussed in earlier sections, this is strictly taboo in follicular unit transplantation, since the splitting of units risks damage and poor growth, and the combining of units produces unnecessarily large wounds and results that are not totally natural.

It follows that if we are to use only the naturally occurring individual units we are then limited by their normal distribution in the scalp and with larger sessions greater numbers of each type of unit will be generated. For example, in a scalp of average hair density (2.1 hairs/mm2), a donor strip of 1 cm x 20 cm would contain approximately 2, 000 follicular units of the following distribution:

400 1 hair implants
1000 2 hair implants
500 3 hair implants
100 4 hair implants

In the same patient, a 5 cm strip of the same width would obviously contain 500 follicular units in similar proportions yielding:

100 1 hair implants
250 2 hair implants
125 2 hair implants
25 4 hair implants

In the average patient, it takes approximately 250 single hairs to create the soft transition zone of the frontal hairline, so in the smaller procedure the number of single hair grafts would be inadequate if one wanted to complete the procedure in one session. At the other end of the spectrum, one might need 500 three- and four-hair grafts placed in the "forelock" part of the scalp to give the patient a full, rather than diffusely thin look frontal. The smaller strip would only generate 250 of the larger three and four hair grafts, an inadequate number for this purpose. Clearly, then the logic of using larger procedures is that they will offer the surgeon the greatest flexibility in designing the transplant without having to combine or split follicular units.

Figure 7: Size of follicular units as hair density varies

As can be seen in figure 7, the patient’s absolute hair density will greatly effect the proportion of each of the 1-, 2-, 3-, and 4-hair follicular units found in the scalp. In patient’s with low hair density, a substantial proportion of follicular units will contain only a single hair and therefore the 1-hair grafts needed to construct a frontal hairline will be plentiful. In patients with high density, the higher proportion of the larger 3- and 4-hair units will provide the "natural resources" to create significant fullness in certain areas. How the different size follicular units are utilized will greatly affect the cosmetic outcome of the transplant and deciding their density and distribution is an "art" in itself.14

THE LOGIC OF THE FOLLICULAR UNIT CONSTANT
One of the interesting aspects of transplanting with follicular units is that nature was kind in spacing them at approximately one per square millimeter. Not only does this make the math easy, but it makes estimating the donor harvest easy, and gives us a logical basis for planning the density and distribution of the grafts. The relative constancy of the follicular unit density has been observed after performing densitometry on thousands of patients, 9 and has been observed histologically by Headington as early as 1982.13

The follicular unit density is not exactly 1/mm2, but it is close enough to this number in most Caucasian and Asian scalps that it can be extremely useful in the surgery. In is significantly less in the black races, averaging around 0.6/mm2, and will decrease in everyone’s scalp as one moves laterally from the densest part over the occiput, towards the temples (see table 2). It also tends to remain relatively constant with age.9 Finally, it is important to differentiate follicular unit density which is relatively constant, from hair density which can vary significantly from 1.5 hairs/mm2 to 3, or more, hairs/mm2 in the general population.3

Table 2. RACIAL VARIATIONS IN THE FOLLICULAR UNIT

Caucasians Asians Africans
Follicular Units/mm2 1 1 0.6
Average Density (hairs/mm2) 2.0 1.7 1.6
Predominant Hair Grouping Two Two Three

(From Bernstein RM, Rassman WR: The Aesthetics of Follicular Transplantation. Dermatol Surg 23:785-799, 1997; with permission.)

Once one realizes 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. In other words, patients with high hair density have more hairs per follicular unit rather than follicular units spaced closer together, and those with low hair density have fewer hairs per follicular unit, rather than follicular units spread further apart. One can easily see this relationship in the three videografts shown in figure 2. The implications of this in hair transplantation are enormous and 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 size 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.
Traditionally, hair restoration surgeons have "sold" the hair transplant procedure to patients by promising the high density of larger grafts. In reality, the results are determined by the hair characteristics of the patient, rather than by the promises of the physician. In a patient with low hair density (or poor hair characteristics), each follicular unit has less cosmetic value, so the results will appear less full. On the other hand, in patients with high hair density and greater hair shaft diameter, the same number of follicular units will provide fuller coverage. Since the follicular density in each patient’s donor area is approximately the same, if we try to give the patient with fine hair and low density a "thick" look by combining them, we will simply run out of hair. Not to mention that combining the units will also produce a pluggy, unnatural appearance.

For most patients, the limitations of the donor supply compared to the demands of the recipient area are such that trying to transplant hair in a way that approaches, or equals, the donor density will limit the ability to properly distribute the hair on the long-term. Fortunately, it takes a surprisingly little amount of hair to make a difference in the appearance of a bald individual. Even in those individuals with thinning hair, the addition of even limited amounts of healthy terminal hair can radically change their appearance. Logic might question this assumption, but clinical observations in thousands of patients have proven, over and over again, that a limited amount of hair, properly distributed, can radically improve the appearance of a balding man.

A Mathematical Look at Balding

To put things in perspective, let’s look at some aspects of the balding process mathematically.7 The normal hair density is approximately 2 hairs/mm2 or one follicular group/ mm2. The average person can loose 50% of his hair population without being detectably thin. That means that one needs to restore only one follicular unit every 2 mm2 in the hairline for a person’s density to appear normal from a frontal view. In areas behind the hairline, where layering of the hair can add value, significantly less than 50% of the original density may suffice to produce fullness. For example, with modest styling considerations, significantly less than 1/8th of the original density can appear to look full if placed behind a well constructed hairline.

In a typical patient with 50, 000 follicular units on his scalp, the permanent donor area represents approximately 25% of this total number, or 12, 500 units, with the remaining 37, 500 at risk to be lost. Of the 12, 500 units in the donor area, approximately half are available for harvesting (i.e. 6, 250). We therefore have a total of 6, 250 units to cover an area that originally had 37, 500. We thus can replace only one sixth (6, 250/37, 500) of what we had to begin with. There are many creative ways to distribute the grafts so that the transplant has the appearance of being much fuller (see reference 7), but the point is that combining units to create more density is not one of them. This will only make the ratios worse.

For example, if we use only individual follicular units, the average spacing between units, once they have been transplanted into the recipient area, is six times further apart than their original spacing in the donor area (or six times further apart than in the pre-balding scalp). If we were to combine follicular units, i.e. combine three units into one, then the spacing increases to eighteen times as much. Visualizing the transplant process in this way, one can easily see that there is no logic in combining grafts to give more density. It only results in larger spaces, but never more hair. Fortunately, the patient with the thin looking donor area, will look appropriately balanced and natural with a thin looking transplant. The surgeon should promise no more.

Now that we realize that we can’t combine grafts to produce more fullness, how can we use the follicular constant to design the transplant and maximize the cosmetic impact? The issue is always one of long-term planning. Unfortunately, the patient doesn’t usually present with the final balding pattern. Therefore, when transplanting a patient early on, the density and distribution must be similar to how we would have transplanted him if he were further along in the process. Thus, if a patient has temporal recession at age 25, we shouldn’t give him any more density in this area than we would if he were 45. If we do, then when he is forty-five he will look unnatural.

Here is where an understanding of the follicular density comes in handy. If we have only one sixth the follicular density overall to work with and we want to use ½ the donor density in a certain area (i.e. 3x the average), then we can only use 1/18 the donor density (1/3 the average) in another area (given that these areas are of equal size) or we will run out of hair. For example, if we plan to eventually replace 50% of the patient’s original density in the forelock area, then some other region of the scalp must "give." This might be accomplished by transplanting less on top of the scalp or transplanting the crown very lightly, or not at all. In the example of the 25 year-old above, if we decide that the final density of the lateral aspects of the frontal hairline should be only ½ the density of the central "forelock, " then once we achieve this density, we must resist transplanting additional hair in that area, or the long-term distribution will be inappropriate.

The same would apply to the early treatment of the crown. If a patient presents with only crown balding, but because of his density, age, or family history he is expected to be very bald, one must place a limit on how much hair should be placed in this area. For example, it we assume that when he is completely bald and totally transplanted, the crown should have a density that is no greater than 1/20 the density of the donor area, then that is all that should be placed from the outset. Too often, a young patient, with a small area of balding is "packed" with hair to approximate the surrounding density and then later on he is left with a distribution so unnatural it can not be repaired.

Procedures which Defy Logic: Scalp Reductions and Flaps
The logic in using the follicular constant applies equally to other forms of hair restoration surgery. When analyzed in this manner, it becomes clear why flaps and scalp reductions cause so many long-term cosmetic problems2. In a flap, follicular units are moved from the donor to the recipient area in a one to one ratio, i.e. in a density that is six times what we have just shown to be appropriate. As a result, the flap will consume vast amounts of the donor supply to treat a relatively small portion of the balding scalp, and produce a transplanted density that always looks unnaturally high.

In scalp reductions, the donor skin that is moved is actually being stretched, so the density transfer is somewhat less than with a flap. For example, if two inches of bald scalp are removed from a series of scalp reductions (after any stretch-back has occurred), and three inches of donor fringe from each side have participated in this movement, then, in effect, six inches of hair-bearing scalp have been distributed over an area of eight inches. Assuming that the distribution is relatively uniform, the previously bald scalp now has a follicular density of approximately 75% of the donor density (6 divided by 8). On first blush one might think that this is a miracle. Especially since scalp reductions are presented as a technique which "…effectively conserves a significant donor area for future use.", 23 we seemingly have gotten something for nothing. When viewed in the context of our previous discussion, where the crown was only being transplanted to 1/20 of the donor density to conserve hair for future transplants in the front, one wonders where all of this hair is suddenly coming from.

Remember, we said that approximately one half of the donor supply could be used for transplantation before it appeared too thin? Well, we have just used up half of that 50% with the scalp reduction. In other words, if we can normally remove donor hair so that a follicular density of 1 unit/mm2 may be reduced to 0.5 follicular units/mm2 before it appears too thin, then our scalp reductions have already taken us to 0.75 units/mm, 2 or half way there (see preceding paragraph). And what have we gained by using up half of our donor supply? We’ve covered only two inches of bald area in the back of the head, thinned the scalp, and altered the normal hair direction. Furthermore, additional donor hair will be needed to cover the resulting scalp reduction scars and additional hair will be needed to address any new cosmetic problems as the balding in the crown progresses. In sum, scalp reductions have the unfortunate effect of simultaneously decreasing donor density and scalp laxity, and thus limiting the amount of hair available for the cosmetically important areas of the scalp.

Characteristics of the Follicular Unit
Before leaving this section, we just want to stress that when considering the cosmetic impact of the hair restoration procedure, it is important to consider all of the patient’s hair characteristics, as they can be of equal, or even greater importance, than the absolute number of hairs. For example, a close match of hair color and skin color will significantly contribute to the appearance of fullness, as will wavy hair. The follicular unit can actually be characterized by the following features:

Hairs per follicular unit
Hair shaft diameter
Hair color
Texture (wave, curl, kink)
Other factors (emergent angle, static, oiliness, sheen, etc.)
It would seem logical to assume that the number of transplanted hairs is the major determinant in the cosmetic impact of the transplant. In reality, hair shaft diameter plays a more significant role than the absolute number of hairs. Coarse hair can be over twice the diameter of fine hair, so that when the area (p r2) of the hair shafts are compared, coarse hair has more than five times the cross-sectional area (and thus over 5 times the cosmetic value) of fine hair.3 If we compare this variance in hair shaft size to the natural variation in hair density, we can see that the impact of the hair shaft diameter (and thus volume) is over 2 ½ times as significant as the absolute number of hairs. Table 3 shows the range of hair density and hair shaft diameter commonly seen in the population of patients that are candidates for hair transplantation surgery and summarizes the relationship between the two. After reviewing this data, it should be apparent that an understanding of both the aesthetic and mathematical elements of transplantation are needed to predict the outcome of the surgery.

Table 3. RELATIVE SIGNIFICANCE OF HAIR DENSITY AND HAIR SHAFT DIAMETER

Range Variance
A Hair Density 1.5 – 3.0 hairs/mm2 2
B Hair Shaft Diameter 0.06 – 0.14 mm 2.3
C Cross Sectional Area 0.0028 – 0.0154 mm2 5.4
D Area/Density (C/A) ------ (2.7)

* Patients with a donor density less than 1.5 hairs/mm2 or hair shaft diameter less than 0.06mm are rarely candidates for hair transplantation.

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