Donor supply is critical in performing hair restoration procedures and repairing bad hair transplants. It is true that many of the cosmetic defects created by poor techniques can be partially or completely reversed by meticulously removing and re-implanting unsightly grafts. However, the main factor that generally prevents the surgeon from achieving all of the patient's restoration goals is a limited donor supply. A depleted donor supply can be the result of wasted hair during a bad hair transplant procedure, or due to the patient's own genetic limitations.
Hair wastage due to poor surgical techniques, as discussed above, is usually the main cause of donor supply depletion. The early telltale signs of hair wastage may be a transplant that appears too thin for the number of grafts used, poor growth manifested as gaps at the hairline, or uneven density in areas where the coverage should be uniform. The fact that donor hair was wasted might be surmised from a donor incision that was longer than expected for a given number of grafts, or an abnormally low density in the donor area near the donor scar. Unfortunately, it is very difficult to ascertain the exact underlying causes after the fact and, by the time surgeon is aware that he has run out of usable donor hair, the damage to the patient has been done.
Because an adequate donor supply is so critical to a successful repair, accurately assessing the amount of hair available becomes paramount. When performing a hair transplant procedure on a virgin scalp, quantifying the donor supply is rather straightforward, as density and scalp laxity are relatively uniform in the donor area. In repairs, however, additional factors come into play. Even though there may appear to be enough hair in the donor area, it may not be surgically accessible. Factors that limit the available donor hair include:
> Low donor density
> Fine hair caliber
> Poor scalp mobility
> Scarring
Low Donor Density -
Donor hair density (donor density) can be measured using a simple hand-held device called a Densitometer. This instrument is invaluable for the assessment of donor density, follicular unit composition, and miniaturization. Patients with high hair density have more hairs per follicular unit, rather than having follicular units spaced more closely together. The converse is also true. A person with naturally low hair density would have fewer hairs per follicular unit, but with the same spacing between the units (i.e., 1 follicular unit/mm2). At very low densities, this rule is less applicable.
The scarring produced by the traditional punch-graft method, that used the open-donor technique to harvest the hair, is a visible marker of the amount of surgery performed. One can easily estimate the amount of donor hair used by comparing the area of open-donor scarring to the remaining virgin donor scalp. In strip-harvesting, however, the linear scar gives little indication of the strip's original size, since it only reflects the length of the excised skin and not the width. With this method, the actual amount of tissue removed cannot be easily ascertained.
The percent decrease in follicular unit density will provide an indication of how much tissue was removed and more important, how much remains to harvest. In general, a person's follicular unit density can be decreased to approximately 0.5 units/cm2, before the donor area will become too thin and no more hair should be harvested. Therefore, if the follicular unit density in the area of previous donor harvests measures 0.75/mm2, approximately half of the potential donor hair has been used and approximately half of the usable hair remains. In the example given above, a 12.5% decrease in follicular unit density means that 25% of the available donor hair in that area was used in the prior procedure.
The value in measuring both follicular unit size (hairs/follicular unit) and follicular unit density (follicular units/mm2) is that the former gives the surgeon information about the patient's original hair density, and the latter about how much hair has been used in previous surgeries, even when the patient's original hair density had not been measured.
Fine Hair Caliber -
Although not affected by the transplant, hair shaft diameter is an extremely important contributor to hair volume and thus available hair supply. Hair shaft diameter is less often mentioned than the actual number of hairs because it is more difficult to measure. However, its importance in both the virgin transplant procedure -- and in repairing a bad hair transplant -- cannot be overemphasized.
The range in terminal hair shaft diameter is approximately 2.3 fold (0.06 mm for very fine Caucasian hair to 0.14 mm for coarse Asian hair). This represents a variation in x-sectional area of approximately 5.4 fold, since area = ~r2 or ~(1/2d)2. In contrast, the range in hair density in patients that we transplant is from 150 hairs/cm2 in those of low density, to approximately 300 hairs/cm2 for those with the highest, a 2-fold difference. If we compare this to the 5.4 fold range in hair cross-sectional area, we see that, in theory, variations in hair shaft diameter should have a 2.7 times greater impact on the appearance of fullness (visual density) than the absolute number of hairs.
The importance of this in a repair is that, for a given degree of "plugginess," fine hair will provide less camouflage than coarser hair. Therefore, fine hair must be transplanted in greater numbers or in multiple sessions to achieve the same degree of camouflage. When this quantity of hair is not available, compromises must be made in the repair.
Poor Scalp Mobility -
Donor density and hair shaft diameter are not the only factors affecting the available donor supply. In order for an adequate amount of hair to be harvested by the strip method, there must be sufficient scalp laxity to close the wound after the donor strip is removed. In the face of a low donor density, having adequate scalp laxity is especially important because a widened scar may be visible through the thin hair.
The location of the donor incision greatly affects scalp mobility. The ideal position for the donor incision is in the mid-portion of the permanent zone that lies, in most individuals, at the level of the external occipital protuberance and the superior nuchal line. The muscles of the neck insert into the inferior portion of this ridge, so that an incision placed below this anatomic landmark will be impacted by the muscle movement directly beneath it. A stretched scar in this location is extremely difficult to repair since re-excision, even with undermining and layered closure tends to heal with an even wider scar. To compound the problem, one is more likely to cut through fascia with a low donor incision; once the fascia has been violated, the risk of having a widened scar is greatly increased.
Incisions placed significantly above the occipital protuberance have a greater incidence of widened scars, but this risk is not as great as it is when incisions are placed below the occipital protuberance. The main risks of placing scars too high on the posterior scalp are lack of permanence of the transplanted hair (it may be subject to androgenic alopecia) and future visibility of the scar if the donor fringe were to narrow further.
In "Follicular Unit Extraction," a technique using very small punches (~1mm) to remove individual follicular units directly from the donor area without using a linear incision, the problem of a tight scalp may be circumvented. This procedure is somewhat analogous to the old punch technique except that, in addition to merely keeping the punches aligned parallel to the hair shafts, the punches are now fitted over individual follicular units. When using this procedure for repairs, follicular transection can be a significant problem, because the alignment of the follicles is often distorted by scar tissue.
Scarring -
Scarring in the donor area limits the amount of hair accessible to the surgeon for a number of reasons. The most obvious is that a larger donor strip must be removed to harvest the same amount of hair. The second, mentioned above, is that scarring decreases scalp laxity by destroying elastic tissue and obliterating the subcutaneous space. This causes the dermis to be bound down to the fascia below. The third is that the scars themselves may be visible and require more hair to be left in the donor area to cover the scarred area than would be necessary to cover normal scalp.
With the objective of avoiding transection, but in the face of poor visualization, the surgeon using the traditional punch technique often passed completely through the sub-cutaneous space to ensure that he was below the level of the hair follicle. Because many individual punches were needed, this could result in significantly more obliteration of the sub-cutaneous space than produced by a linear incision. The very nature of secondary intention healing, on which the open donor technique depended, caused a significant amount of scarring in the donor area.
The presence of open donor scars may give the surgeon a false sense of security thinking that, because an excision with a primary closure was not performed, the patient's donor laxity has not been compromised. This logic may lure the unwary surgeon into harvesting a donor strip that is too wide, often with disastrous consequences. When the surgeon attempts to close the donor wound, the tight closure requires more tension on the sutures. The sutures, however, tend to tear the scarred wound edges, since scar tissue is significantly less elastic and more friable than normal scalp. An attempt to redistribute the tension by undermining runs the risk of causing additional trauma to a greater area and may result in an even larger wound.
