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Page 1 of 3 A CALCULATED LOOK AT THE DONOR AREA
John P. Cole, MD
Jean Marc Devroye, MD"We work basically only with donor hair."1 Surprisingly, it has taken us considerable time to place emphasis on this portion of the procedure. Conversely, two decades ago the foundation for soft natural hairlines began with the employment of single hair grafts for the hairline. This development was followed by the introduction of large numbers of natural follicular groups to the single hairs in an irregular manner to fashion a natural appearance to the recipient area. We have spent considerable effort assessing and improving the aesthetics of the hair transplant. More recently factors influencing survival and donor harvesting techniques have surfaced as areas of significant interest to hair restoration surgeons.2 Understanding these factors and assessing the yield of donor harvesting technique requires an appreciation for the contents of the donor region. The practioner must also possess an efficient means of recording the results of his dissection. He may then compare his results to the known quantities in the donor region to access the efficiency of his dissection.
There three densities, which should be evaluated with each procedure. The first is the hair density or the number of hairs per square centimeter. The second is the follicular density, which is the number of follicular groups (FG) per square centimeters. The third is the number of hairs per follicular group, which is a calculated value. Because it is calculated, I call this the calculated density. The calculated density determines the percentage of FG containing a specific number of hairs. Therefore, once we know the calculated density, we can predict the proportion of FG of any particular size. For instance, for any particular calculated density you can estimate how many three hair grafts, how many two hair grafts, and how many natural one hair grafts should result from the donor dissection. How do we know this?
First, lets discuss the follicular group. What is it? Hairs exit the skin in clusters. Some of the hairs exit from the same follicular canal, while others exit from separate but closely neighboring canals. Many in our field call these clusters follicular units. This is unfortunate because the term follicular unit is a bastard term born out of wedlock and has little more than historical significance to the average hair transplant surgeon. The follicular unit is a histological term first described by Headington in 19843. In Headington’s paper he stated that the follicular unit is the pilosebaceous unit as disclosed at the mid-dermal level. Headington noted that the density of the follicular unit was about one per square millimeter. He pointed out that he did not study regional or racial variation in this density. Headington stated nothing about the surface anatomy of hair. It is the surface anatomy that we deal with in hair restoration surgery.
There is a wide variation in the follicular density from one region of the scalp to another and from one individual to another. Furthermore, there is a wide variation in the calculated density from one patient to another and from one region of the scalp to another. Individuals with a high follicular density have a reduction in the distance between the follicular groups, which results from packing more groups in the same surface area. As the distance between the groups decreases it becomes more difficult to assess whether a cluster of hair is composed of one follicular unit or more than one follicular unit (fig 1a and 1b). Another problem in assessment arises with the high calculated density. Here it becomes difficult to determine whether the large follicular group is composed of one or more than one follicular unit. In the situation where there is a high follicular density and a high calculated density, the problem is even more marked. For these reasons, it is completely inappropriate to call our natural clusters of hair bearing grafts follicular units. Rather, they should be termed follicular groups, follicular clusters, or bundle grafts.
Fig 1a. 5 hair follicular group.
Follicular groups exist in symmetrical patterns, although there is a somewhat irregular arrangement of the follicular groups within the pattern. Jimenez and Ruifernandez first noted the formula L = k / sq rt of n, where L is the density of hairs in square millimeters, k is a constant depending on the geometric spacing of the follicular units, and n is the density of hair in square centimeters. I have evaluated their formula and found it to be accurate. The geometric arrangement of follicular units follows a triangular pattern (fig 2). k is 10.7 for a triangular pattern. In this case the density of follicular units must equal 114.5 per square centimeters for the distance between the follicular units to equal 1 mm. The distance between the follicular units should be measured from the center of the follicular unit..
Fig. 2.
Jimenez and Ruifernandez are to be commended for the introduction of this powerful formula. With this formula we can make many predictions and question old beliefs. Last year I presented a paper on the regional variation of hair density and diameter. In this paper I noted that the 4 mm punch graft is elliptical in shape, not circular. For this reason the surface area is greater than that of the 4 mm circle. The variation in this surface area is noted in fig. 3.
Fig. 3.
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