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Density is a function of the incision size. There is a simple formula. The distance between the grafts in millimeters is equal to the a constant, 10.7, divided by the square root of the density in sq. cm. The distance between grafts at 80 per sq. cm is 1.12 mm. The diameter of a single hair is 0.42 mm on average. Two hairs average 0.8 mm and three hairs 1.2 mm. In order to pack 80 per sq. cm, you need to make very tiny incisions and place very small grafts that are aggressively trimmed. In other words, there is a trade off between trauma, size of the graft, the incision size, and the comfort of the fit. Is it acceptable to compress a graft too tightly? Does the yield go down? Is it better or worse to force a graft into an incision size that is too small? No one knows these answers.
There are other concerns, as well. When doing body hair transplants, I am always amazed at how many new hairs show up in the same area that I picked clean the year before. There are always hairs that are very hard to see because they are early anagen hairs or exogen hairs that are so microscopic that you cannot see them with the naked eye. This occurs with scalp follicular units, as well. Aggressive trimming risks loosing these hairs and tossing them in the waste.
Most strip harvested grafts have fat on them. This additional fat takes up volume. With FUE, we have no fat so we do not have the extra volume. This extra volume can cause a couple of problems. One is ridging. The other is that they take up space in an already small incision site. Therefore, fat in a small incision site can make the placement even more traumatic.
Even with no fat, I find the most comfortable density for my full size grafts is 40 to 50 per sq. cm. I go higher, with single hair grafts, or well trimmed 2 hair grafts, but with full size follicular units, I average 45 per sq. cm. With body hair, I average 70 per sq. cm. Numbers are critical with body hair. We need higher densities to achieve the fullness we are looking for with body hair. Most of my FIT cases come in for multiple days. One of the tremendous advantages here is that I am able to go back into the area the next day or several days later and increase the density so full that it is not possible to squeeze in another graft without overlapping incisions.
My scalp grafts average 2.5 hairs per follicular unit. Strip surgery averages only 2.3 hairs per follicular unit. What happened to the other 8%, you may ask. With body hair we average less than 2 hairs per graft. Therefore, smaller incision sites are more practical and we get the higher densities we need. The smaller the incision, the harder it is to place the multi-hair graft and the more traumatic it is to place the graft.
It is easy to say you pack at 80 per sq. cm. It is entirely different to actually do it and to achieve good growth.
There are other factors to consider. Many of my patients come to me for scar repair or hair transplant repair surgery. Lets face it. There are almost 50 years of hair repair surgery cases out there. I don’t know anyone doing what we are doing with hair repair surgery. This is an entirely different situation that requires specialized solutions that are often customized. Grafting into these areas may result in a lower yield if you go for a super dense packing. One must always individualize the case and customize the procedure so that it works in the best interest of the patient. We are not interested in setting any world records or marketing ourselves based on the current flavor of the day. What our patients are interested in is results. Results are how we promote techniques. We’ve done that for over 15 years
Other factors control yeild. One of these is the pH of the storage solution. We constantly monitor our storage solutions and change them every hour. Most physicians use normal saline. this has a much lower pH than the body. This pH decreases as grafts metabolize outside the body in an anaerobic medium. as they do this, the cells make hydrogen atoms that reduce the pH further. We start with a solution that is about 7.4 in pH or about the same as the human body. we monitor the solution and change it every hour. This is a much more healthy environment for hair and we are convinced it produces a better yeild in the long run.
More hair per graft, a higher yeild, and a better overall result. This is our mantra. |
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