Hair Mass Index and Hair Mass Transferred

We are quite lucky that our creator used cylinders to cover our spherical scalp. Archimedes pointed out that the sphere is "inscribed" in the cylinder. It's north pole just touches the top of the cylinder; the south pole just touches the bottom. And the cylinder and sphere just barely make contact all along the equator. If the cylinder were the least bit shorter or skinnier, the sphere would not fit inside. If r = the radius of the sphere and the radius of the cylinder, then 2r = the height of the cylinder, V_c = volume of the cylinder =(¼r²)(height) = 2¼r³, and V_s = volume of the sphere = (4/3)¼r³. Therefore, V_c/V_s = 3/2, the ratio Archimedes was most proud of. The can will hold 50% more soda pop than the ball. It is interesting to note that the ratio of the cylinder's area to the sphere's area is the same as the ratio of their volumes: A_c = area of top + area of bottom + area of side = ¼r² + ¼r² + (2¼r)(2r) = 6¼r² and A_s = 4¼r², so A_c/A_s = 3/2. It takes 50% more paint to decorate the can than to decorate the ball. It follows that it requires 50% less cylindrical hair to cover the spherical scalp. Interestingly, these follicular groups are arranged in mathematical spirals, another complex calculation developed by Archimedes, but this is beyond the scope of our discussion.

The term mass is actually a measure of volume. A mass of 1 gm is a cubic centimeter of water. It follows that hair mass and hair volume are essentially the same. In 1998, Dr. John Cole introduced Hair Mass Transferred and Total Hair Volume Transferred as a predictor of hair transplant coverage. The problem with the Cole method was it relied on the mean hair diameter, a variable that required tedious measurements with potentially costly equipment. In May 2001 Dr. James Arnold presented Hair Mass Index (HMI) as an ingenious means to quickly and inexpensively asses Hair Mass. He measured HMI in both the donor area and recipient areas of his patients. The Arnold method may not be as precise as the Cole method and does not evaluate the actual mass of hair that is transferred at the time of surgery. Rather, it assesses the individual’s hair mass before surgery and after re-growth of the transplanted grafts. Interestingly, he noted a lower than expected HMI in the recipient area than was predicted based on the number of grafts and hairs he transferred in many of his former patients. This was the forefather to objective efficiency evaluations. Dr. Frank Neidel wrote a chapter about HMI in the most recent edition of Hair Transplantation of Dr. Walter Unger and Dr. Ron Shapiro. HMI measures the hair mass in the donor area and recipient area, but it does not measure what is actually transferred.

Cole has stated that scalp hairs may be classified according to the following table:

Very fine	Fine	Medium-Fine	Medium	Medium-Coarse	Coarse
<60um	60-65um	65-70um	70-75um	75-80um	80um>

It should be noted that hair on other parts of the body has a much different diameter than scalp hair. This characteristic offers great potential coverage from coarse hair sources such as from the chest.

Neidel notes that HMI can be classified according to table below:

Optical effect of fine hair	Optical effect of normal hair	Optical effect of thick hair
0.18-0.32	0.32-0.5	0.5-0.72

Methods:

Measurement of mean hair diameter requires a sample size of 20 hairs. We have found in multiple evaluations that this sample size results in a more precise average hair diameter. The hair is measured with the help a micrometer. The measurements are very precise with an error of about 1 micron. Mean hair diameter is equal to the sum of 20 hair diameters divided by 20. Hair diameters are measured in micrometers. Cole recognizes the extreme variability of hair diameters. For this reason, he believes that when one measures only 20 hairs, you should not include hair diameters that are particularly small or extremely large so that you get a more accurate estimate of the average diameter.

This measurement allows us to calculate the mean surface area and mean hair volume for any patient provided the length of hair is known. Hair length varies from one individual to another. Therefore, we introduce the term hair mass transfer index (HMTI) to compensate for hair length. HMTI assumes a standard length of 1 cm. It is easy to compensate for any length of hair as we will show. While cutting the grafts, assistants examine and write down the number of hairs contained in each graft. The grafts may be individual follicular units, double follicular units (DFU), multiple follicular units, (MUG). In addition, you may include fractionated follicular units that may include a variety of combinations to include three hair follicular units fractionated into three one hair grafts. The objective is to assess the total number of hairs that are transferred of a particular graft type or size. On a statistical point of view, it is worth using this method for the first 200 grafts of any particular type (FU, MUG, etc.) and then it is possible to proceed to an extrapolation. Single hair grafts that are obtained by fractionation should always be counted in their entirety. This sum is the Total Hairs Transferred (THT). Mean hair radius is the quotient of the mean hair diameter divided by 2.

Mean hair volume index (MHVI) is the product of the square of the mean hair radius, p, and a hair length of 1 cm.

MHVI = ( r 2 ) ( p ) ( 1 )

Hair mass transferred index is the total hairs transferred (THT) and mean hair volume index (MHVI).

HMTI = (THT) ( MHVI)

The expected actual hair mass you have transferred (HMT) may be easily calculated based on any hair length. One simply multiplies the HMTI by the actual length of hair on the patient. Now one has the capacity to evaluate efficiency of their hair transplant. One can calculate the hair mass index in the recipient area and compare it to the actual HMT calculated at the time of surgery. This method allows one to evaluate efficiency on multiple regions of the scalp.

These are two old and two recent examples of predicted Hair Mass Transferred:

Example no.1:
A. Before
B. After

Example no.2:
A. Before
B. After

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