What are Follicular Units (FUs)?
For years it was thought that scalp hair grew as individual strands, but scalp hair actually grows in tiny little groups of 1-4 hairs each. These groups are called “follicular units.” Surprisingly, even after doctors learned about the existence of these groups, they didn’t think to use them in the hair transplant. However, if one clips the hair short (to approx 1-mm) and then looks at the scalp under 30X magnifications, using an instrument called a densitometer, the groups become readily visible. Because FUs are complete anatomic and physiologic structures, keeping them intact during the hair transplant will ensure their maximum survival and growth. FUs are very small structures, however, and only careful stereo-microscopic dissection by a skilled and experienced surgical team can ensure that will be kept whole and will not be damaged during the various phases of the hair restoration process. A major advantage of using follicular units in hair transplant surgery is to be able to keep the recipient sites as small as possible.
The smaller the recipient sites, the closer together they can safely be placed in the scalp. Small recipient sites also allow larger numbers of grafts to be safely transplanted in one hair restoration session without injuring the scalp or compromising its blood supply. Therefore, using these small units maximizes the density of grafts that can be transplanted in one session and the number that can safely be used at any one time. The look of fullness in a hair transplant is not only dependent on the number of grafts but on the number of hairs each graft contains. Since the hairs in the follicular unit are very close together, transplanting them allows the most hair to be placed into the smallest possible recipient sites, giving maximum fullness. Because follicular units represent the way hair grows in nature, it guarantees that these grafts will not appear pluggy. Although using FUs prevents a pluggy look, they must be placed in the proper direction and distribution to ensure a totally natural appearance to the hair restoration. Using grafts larger than follicular units will always compromise naturalness while using grafts smaller than follicular units will always result in sparseness.
Stereo-Microscopic Graft Dissection
One of the most important aspects of FUT Hair Transplant is stereo-microscopic dissection. Successful dissection allows follicular units to be removed from the donor strip without being broken up or damaged. During the dissection, it is critical that the whole FU is kept intact as this will maximize its growth. Intact units will also give the most fullness, as they contain the complete, natural complement of 1-4 hairs. The average donor strip is approximately 1cm wide and of variable length, depending on the number of grafts that are needed for the hair restoration. In the average person’s scalp, there are approximately 90-100 follicular units per square cm of donor tissue, so a 2,000 graft transplant session would require a 1cm wide strip that is slightly over 20 cm in length. The series of stereo-microscopes needed for the follicular unit dissection, are available in the surgical room to allow graft dissection and transplantation to be done simultaneously, saving time.
The recipient sites in all of our hair transplantation procedures are made using lateral slits (also called coronal or horizontal slits). Lateral slits have the advantage of orienting the hair within the follicular unit to match the way it grows in nature. They give the surgeon the highest degree of control over the direction and angle in which the transplanted hairs will ultimately grow. Controlling the depth of incision is also critical in making the recipient sites, as limited depth incisions minimize injury to the deeper blood vessels in the scalp, allow grafts to be placed more closely together, decrease tissue swelling after hair transplant surgery and facilitate healing. We use fine hypodermic needles or a series of custom-made, ultra-fine blades to create the hair transplant recipient sites. The chisel-shaped blades differ in size by increments of one-tenth of a millimetre and range from 0.6mm for single-hair grafts to 1.2mm for 4-hair FUs.
Minimizing the scar from the donor incision is a critical part of a successful hair transplant procedure. A fine donor scar allows a person to keep his/her hair relatively short after the surgery (if one wants to do so) and increases the amount of grafts that can be harvested (removed) in subsequent procedures. A number of techniques have been developed to minimize donor scarring when using a strip excision during FUT Hair Transplant. These include the use of tumescent anesthesia, undermining, absorb-able sutures, buried sutures, staples, and trichophytic closures. The technique of FUE, where grafts are removed directly from the scalp without a linear incision, is covered in another section. Although the length of the donor incision is determined predominantly by the number of grafts required, the width (height) of the donor incision depends upon the patient’s scalp laxity. This is a genetic attribute of the patient’s scalp that must be carefully measured by the surgeon during the initial evaluation. With good scalp laxity, a wider strip may be harvested from the donor area without the risk of scarring (although patients with very loose scalps may be at increased risk of a wide scar. If the scalp is too tight, taking a normal size strip may be impossible.
If the strip width is too narrow, the incision will need to be unnecessarily long to obtain an adequate amount of donor hair. If the strip width is too wide, the risk of having a widened scar will be increased significantly. Expert clinical judgment, acquired over years of experience, is needed for the surgeon to consistently set the appropriate length and width of the donor strip and achieve the minimum possible scar.
Obtaining the Donor Strip Tumescent Anesthesia
A major advance in the removal of the donor strip is the use of tumescent anesthesia. Tumescent simply means to expand by injecting fluid into the tissues. In this technique, very dilute concentrations of anesthetic fluid are injected into the fat layer of the donor region of the scalp. This serves a number of purposes, the first is to decrease bleeding from the pressure of the fluid on small blood vessels (capillaries), the second is to firm the skin so that the incision can be more easily controlled and third, to increase the distance between the follicles and the deeper tissues of the scalp. This helps the surgeon keep the incision superficial so that the larger nerves and blood vessels in the scalp are not injured and so that the fascia is not cut. The fascia is a layer of fibrous tissue that lies just below the fat layer that gives support to the scalp. If this layer is cut, the risk of having a stretched scar is significantly increased.
Trichophytic “Tricho” Closure
In a trichophytic closure, the surgeon makes the initial incision parallel to the hair follicles and then trims away either the top or bottom wound edge so that 1 to 2mm of the top of the hair follicles at that wound edge are removed. During the “tricho” closure, the trimmed wound edge is pulled towards the opposite edge so that the bottom parts of the cut hairs are pointing slightly towards the incision (rather than directly towards the surface of the skin). The goal is that these hairs will eventually grow through the incision and thus decrease the visibility of the scar.
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