After four decades of evolution from the large plugs of the late 1950s to the extensive mini and micro grafting of the early 1990s, possibly the central development in hair transplantation today is the recognition that the naturally occurring, individual follicular unit may represent the ideal way in which to transplant hair. The underlying tenet of follicular unit transplantation is that the follicular unit is sacred and should always be transplanted intact. While not all hair transplant surgeons agree on the importance of using follicular units exclusively for the entire transplant, or in every patient, the central role of this previously unrecognized anatomic structure in modern hair restoration surgery is without dispute.

Follicular units are distinct groupings of usually one to four, and occasionally five terminal hairs, surrounded by a circumferential band of collagen called the "perifolliculum." It also includes one, or rarely two, vellus follicles, the associated sebaceous glands, the insertions of the arrector pili muscles, and a neuro-vascular plexus. It has been demonstrated that hairs separated from other hairs of a follicular unit do not grow as well as the same number of hairs transplanted keeping follicular units intact. The follicular unit is thus a physiologic, as well as an anatomic entity.

Follicular unit transplantation offers the surgeon the unique ability to transplant the maximum amount of hair with the minimum amount of non-hair bearing skin. In this way, recipient wounds may be kept small, healing is facilitated, and with proper technique, large numbers of grafts may be safely moved per session. The use of these discrete anatomic units also helps to ensure that the cosmetic result of the transplant will appear completely natural.

In contrast to follicular units, micro grafts (1-2 hairs), and mini-grafts (3-6 hairs), are small grafts cut randomly from donor hair, not specifically as individual intact follicular units. They may consist of partial follicular units, single follicular units, multiple follicular units, or multiple, partial follicular units. In mini-micro grafting, the partial units may be at risk for sub-optimal growth, and the multiple units will contain extra skin that will demand larger recipient sites. This, in turn, causes more wounding to the recipient bed and may limit the number of grafts that can safely be transplanted in a session.

It has often been said that with multiple sessions, mini grafts can look fairly natural in patients with ideal hair characteristics. However, even in these circumstances, on close inspection, mini grafts can look unnatural compared to follicular units. As the public becomes increasingly more discriminating, the future of hair transplantation is therefore likely to involve an increasing demand for procedures that are performed using follicular unit transplantation exclusively.

It is felt by most surgeons who perform follicular unit transplantation routinely, that single strip harvesting and complete stereo-microscopic dissection are required to properly dissect follicular units from the surrounding tissue. The reasons for this are relatively straightforward. Harvesting with a multi-bladed knife will break up follicular units and transect follicles, whereas removing the donor tissue as a single strip will yield the highest proportion of intact follicular units. Once the single strip has been removed, the stereomicroscope, with its 10x magnification and intense illumination, will provide the best visualization for the dissectors to accurately subdivide the strip and to isolate the individual units. Lower power loop magnification does not provide sufficient resolution for precise follicular unit dissection and back-lighting will not penetrate the intact strip.

Although, it is hard to argue the supremacy of the follicular unit in theory, in practice, the follicular unit hair transplant is tedious, demanding on the physician and staff, and requires a relatively high degree of expertise to be properly performed. It is, therefore, reasonable to assume that in situations where follicular unit transplantation is impractical or impossible, the patient might be better served by a more simple technique. In this vein, the standard practice of mini/micro grafting is seen by some as a more practical alternative to follicular unit transplantation.

The advantages of mini / micro grafting are that it is faster and requires less staff. In addition, it is felt that the damage produced by the multi-bladed knife (used in mini / micro grafting) is somewhat offset by the advantage of not having to carefully trim around follicular units, which in itself can be a source of follicular injury (if not done with care). On the other hand, in mini / micro grafting, the slightly larger grafts and concomitantly larger wounds do not permit the total naturalness that is achieved with follicular unit transplantation. In addition, the split follicular units and greater number of hair fragments (produced by the use of the multi-bladed knife and less precise dissecting techniques) may result in less than optimal growth.

The important factors affecting graft survival are still controversial. Graft trauma can take multiple forms. Do longer transplantation procedures lead to greater graft desiccation, donor tissue anoxia (time-out-of-body) and lower yield or does violating the follicular unit microanatomy lead to a lower yield? These important questions lack the controlled studies required for meaningful answers, but the future direction of hair transplantation surgery may, in part, depend upon their outcome!

As we will discuss in subsequent sections, new technology may soon substantially change how both follicular unit transplantation and mini / micro grafting are performed. However, regardless of how the technical parameters of each procedure evolve, the debate of follicular unit transplantation vs. mini / micro grafting will undoubtedly hold the attention of the hair transplant community for years to come. In the meantime, the answer to which procedure is used may unfortunately lie less with the needs of the individual patient, than with the resources and capabilities of the operating surgeon and his staff.

REFERENCES

Bernstein RM, Rassman WR. Dissecting microscope versus magnifying loupes with transillumination in the preparation of follicular unit grafts. A bilateral controlled study. Dermatol Surg 1998; 24: 875-880.

Bernstein RM, Rassman WR, Szaniawski W, Halperin A: Follicular Transplantation. International Journal of Aesthetic and Restorative Surgery 1995; 3: 119-132.

Bernstein RM, Rassman WR, Seager D, Shapiro R, et al. Standardizing the classification and description of follicular unit transplantation and mini-micrografting techniques. Dermatol Surg 1998; 24: 957-963.

Headington JT: Transverse microscopic anatomy of the human scalp. Arch Dermatol 1984;120: 449-456.

Seager D. Binocular stereoscopic dissecting microscopes: should we use them? Hair Transplant Forum Int 1996; 6(4): 2-5.