A Calculated Look At the Donor Area - Page 2

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.

You will first note that the surface area of the 4 mm circle is 12.57 square millimeters. You can see from this data that the mean surface area of the 4mm punch graft is greater than 12.57 mm² in all four regions. The reason for this discrepancy results from the angle of hair growth. You will recall that the length of an incision of a two bladed knife is equal to the width between the blades divided by the sin of the angle of insertion. This fact makes the length of the 4mm plug greater than the width, an occurrence that results in an elliptical surface. Recall that Headington and Whiting performed their research by evaluating plugs, as well⁴. Neither took into account these geometric principals. Unfortunately, this makes their data inaccurate. Analysis of Whiting's data shows that he found an average of 13 follicular units in the 4 mm plug (fig 4.). This corresponds to a density of 1.03 FU per square millimeter. Using the Principal of Jimenez and Ruifernandez (PJR)we calculate the distance between follicular units at 1.05 mm or ½ mm greater than purported by Headington, Whiting, and Bernstein⁵. If you take into account that the average 4mm plug resulted in a surface area greater than 12.57 mm², the density of follicular units is actually less than 1 per mm². Extrapolating my mean surface area for the crown to Whiting's findings would result in an average density of 0.87 FU/ mm². Pluging this data into PJR results in a distance of 1.15 mm between follicular units or 0.15 mm greater than the distance of Headington, Whiting, and Bernstein. This suggests that the density of follicular units and hair is less at the mid-dermal level than at the surface of the skin. It implies that our skin structure is

capable of creating more density on the surface of the skin than actually exists in the human body.

This also suggests we can define two distances between the follicular units. L_SA stands for the distance between the follicular units on the surface area and L_MD represents the distance between the follicular units at the mid-dermis. We see that as the density of follicular units decreases the distance between the follicular units increases and that as the density of follicular units increases, the distance between the follicular units decreases. Similary, if the distance between follicular units increases (L_MD), the density of follicular units must decrease if k remains constant as compared to L_SA where the distance between follicular units decreases as the density of follicular units increases. In other words, the skin structure appears generate more density from less. This principal is supported by my findings from the study of regional variation of density (fig 5.).

Fig 5. Surface Density of Follicular Groups per Square Millimeter

From this chart we see that the corrected mean surface density of follicular groups (number of follicular groups / the measured surface area) is 110 FG per cm² in the crown. This is certainly greater than the extrapolated value of Whiting's crown density of follicular units at the mid-dermal level (87 FU per cm².). The mean L_SA of the crown is calculated at 1.02mm between follicular groups.

Ron Shapiro and Walter Unger among others have stated they are able to create the illusion of more fullness when they incorporate grafts containing more than one follicular unit. Grafts containing more than one follicular unit have a decreased L_SA and consequently, a higher density than the preoperative natural density of the donor region. This powerful formula of Jiminez and Ruifernandez may shed some mathematical support to their assertions. Could it be that decreasing L improves the illusion of coverage? Indeed, there may be greater value to the larger graft than is possible from the use of pure follicular groups alone. It may be that as hair transplant surgeons, we are able to create "more from less" by incorporating a combination of grafts.

The calculated density may be assessed in different ways. First, you can preoperatively count the number of follicular groups and hairs in a specific surface area. You should exercise care to use adequate lighting and magnification during this process. I have found the Rassman Densitometer, also known as the 30X illuminated microscope (cat. No 63-851) at Radio Shack to note the follicular and hair densities and subsequently, calculate the CD. Dr. Devorye pointed out that better lighting and higher magnification improve the physician's ability to record hair counts. Therefore, the second way to obtain the CD is to count the number of hairs you see in a series of grafts and then divide the total number of hairs by the number of follicular groups you assessed. The third way to obtain the CD is to count the total number of hairs produced by the surgery staff and divide this by the number of grafts created.

In 1996 I began collecting data from graft dissection. In all 107,000 grafts were evaluated. For the purpose of this evaluation I asked my surgery staff to maintain the integrity of each follicular group and to accurately as possible record the total number of hairs in each graft produced in this manner. The dissection included the use of the Meji EMT microscope and 5X loops. I first calculated the density for each patient by dividing the total number of hairs produced by the total number of grafts. I then quantified the number of 1, 2, 3, 4, 5, and 6 hair grafts produced for that patient. I then placed all patients with the same calculated density in a single group. I then determined the mean ratio of one, two, three, four, five, and six hair follicular groups as a percentage of 100. I found that the post operative calculated densities could be applied to the pre-operative calculated density to achieve a reasonable prediction of the expected number of a particular size graft. (Fig 6 and Fig 7). The average calculated density is about 2.3 hairs per mm².

Fig 6. Comparison of the ratio of different size follicular units as a function of the calculated density.

Page 2 of 3 All Pages