| The follicular unit density (FUD)
is important because it helps you predict the number of FU you will transfer
from a given amount of excised donor tissue. To determine the FUD, count the
number of FU in the surface area of your densitometer. Extrapolate these numbers
to a square centimeter. This value gives you an estimate of the number of FU per
square centimeter at that specific reference point.
If your strip is limited to the mid-occipital area, you will need to know
only the FUD in the mid-occipital area. Should your strip extend from the
mid-occipital region to the supra-auricular area, you should take the average of
the three FU densities at the three reference points referred to earlier. This
number will give you a reasonably safe estimate of the number of FU per square
centimeter in your donor strip. Now you are prepared to calculate the number of
FU in the tissue you propose to excise. Suppose your donor strip extends from 3
cm superior to the right auricle at the external auditory meatus to the
occipital protuberance with the head in the Franklin position and the length of
your strip is 15 cm (see below). You measure the density of hair in the
mid-occipital, mid-mastoid, and supra-auricular regions as 260 hairs per square
cm, 210 hairs per square cm, and 140 hairs per square cm, respectively. The FU
density at each reference point is 100 FU/ cm2 in the mid-occipital
region, 80 FU/ cm2 in the mid-mastoid region, and 60 FU/
cm2 in the supra-auricular region. The calculated densities in the
mid-occipital, mid-mastoid, and supra-auricular areas are 2.6 hairs (H) per FU,
2.63 hairs (H) per FU, and 2.33 hairs (H) per FU, respectively.
The average number of FU or groups in this 15 cm area is the average of the
three FU densities (100 + 80 + 60) / 3 or 80 FU per square cm, and a 15 cm will
yield approximately 80 FU per square cm. A 15 cm strip excised with a
2-bladed scalpel set at 1 cm between the blades will therefore yield
approximately 1200 FU. If you take the average of the three calculated densities
(2.6 H/FU + 2.63 H/FU + 2.33 H/FU / 3) or 2.52 H/FU, you may estimate the number
of hairs you will be moving. This formula involves multiplying the average
number of H/FU by the total number of FU in the donor strip. In this case, the
number of hairs is safely estimated at (2.52 H/FU X 1200 FU) or 3024 hairs.
Suppose you want to obtain 300 single hairs from this tissue. If all single
hairs are removed from the most medial region of the scalp, and the average
number of hairs/FU in that area is 2.6 H/FU, you may divide the number of
desired single hairs by mid-occipital area calculated density (300 / 2.6 H/FU =
115) to obtain the number of FU required to produce this number of single hair
grafts by dividing all FU into single follicles. You must subtract
this number of FU from the predicted total and add the number of single hair
grafts to determine the total number of grafts in the donor strips. In this
case, (1200 FU – 115 FU + 300 single hair grafts) or 1385 grafts can be expected
from the excised donor tissue. To this number you must also add the number of
grafts and hairs from the tapered ends. Generally, we find approximately 30 to
50 additional FU in each tapered end and these ends are usually mirror
images of each other. If you use the trapezoid closure (described later in this
Chapter) the two triangular-shaped ends can be combined to form a 1
cm2 (Figure #). In the example we have been using, the average number
of FU from the two ends is {(100 FU + 60 FU) / 2} or 80
FU. Therefore, we could anticipate approximately 1465 grafts (1385 + 80 FU), or 3226 hairs (1200 FU x 2.52 hairs/FU) + (80 FU
x 2.52 hairs/FU).
The following formulas are useful in understanding the three types of density
and estimating the total number of hairs transferred:
Density = D = The total number of hairs in a given surface
area or (Hair/ cm2)
FUD = FD = total number of follicular units in a
given surface area or (FU) / (cm2)
Calculated Density = CD = {(D) / (FD)} = Hairs / FU
Total Hairs Transferred = THT = (Mean CD) (Mean FD)
(cm2 excised)
Which simplifies to:
Total Hairs Transferred = THT = (mean density) (cm2
excised)
Counting hairs is much more difficult with the Rassman densitometer and other
type of densitometers than with a microscope. Therefore, it is more accurate to
count the number of hairs/FU (CD) after excising the tissue from the
donor area. Counting larger entities such as follicular clusters or FU is much
easierprecise with a densitometer. It follows that the most accurate means of
estimating the total number of hairs transferred would result from noting the FU
density with a densitometer and the calculated density with a microscope.
The CD is noted for up to three reference points along the strip. FU density
and hair density estimates, after donor excision, are not accurate because the
donor tissue shrinks an estimated 5 to 10% following its removal from the donor
area. A 5% reduction in the surface area of the aforementioned strip would
increase the density of the 1200 FU to 89 FU/cm2
from 80 FU/cm2 (an 11% increase). A 10% reduction in
surface area would increase the FU density to 99
FU/cm2 (a 24% increase). Therefore, the densities increase
proportionately (11 to 24 %, and sometimes more) after donor tissue removal.
In this instance, the mean FD is noted in the three reference points with a
densitometer, whose surface area is known. The reference points are then circled
with a marker. Once the strip is excised, the reference points are noted. Within
each reference point the total number of hairs from a given number of FU is
counted with the aid of a microscope set at 10x to 20x. The total number of
hairs is divided by the sum of the FU investigated. The quotient is the CD. This
FD and CD are plugged into the formula for THT to arrive at a more accurate
estimate.
Summary:In summary, hair density quantifies the number of hairs per
unit of area. It suggests how many hairs and grafts are available from part or
all of the donor area. A high CD suggests that FU grafts may be larger simply
due to more hairs and their associated perifolliculum while grafts containing
multiple FU may be smaller if a fixed number of hairs/grafts is desired or if
such grafts are "cherry-picked". HairCalculated density also suggests how many
hairs are present in any size graft. As noted earlier, despite all this
potentially useful information, currently, few physicians measure hair density
during the donor area examination. The previous discussion is intended to
provide a template for maximum accuracy and for investigators, but many
clinicians would find such measurements and calculations cumbersome and too time
consuming (although they could be assigned to a trusted assistant). An easier
method to estimate the total number of hairs moved is to simplify the equation
by taking the average of the three densities and multiplying this by the surface
area removed from the donor region. In our example case, the surface area is 15
cm2 (plus the tapered ends) and the average density is 2.033 hairs /
mm2. This corresponds to 203.3 hairs per square cm. Hence, the
surface area would yield an estimated 3253 hairs [ (15 cm2 + 1 cm2 for the tapered ends)
(2.03 hairs/cm2)] or only 27 more
hairs than the more accurate calculations. Finally, it is very useful to record
(a) the number of h, FU, and grafts obtained after graft preparation is
completed and (b) the length and width of the excised strip. In subsequent
sessions, one can then maintain or alter these numbers by taking the new donor
strip directly adjacent to the scar from the earlier harvest(s), and by excising
a strip that has the same, less or more length and/or width. |
