63 Motorized Sharp Punch Follicular Unit Extraction
Summary
Keywords: sharp motorized FUE rotation oscillation cutting edge tangential force axial force follicle splay hair growth angle
Key Points
•Mechanical rotation of sharp punches minimizes axial force during punch insertion, reducing intradermal follicle distortion and transection.
•It is essential to use extremely sharp punches and depth limitation in sharp motorized follicular unit extraction (FUE).
•The optimal technique and adjustment of technical variables must be determined for each patient and donor region.
63.1 Introduction
Sharp motorized follicular unit extraction (FUE) refers to using a sharp punch and mechanical rotation or oscillation to remove follicular groupings. The goal is to allow the punch to follow the path of the hair follicles with as little damage or transection as possible. Sharp motorized FUE (SMF) is faster than manual or blunt techniques and can produce excellent results in skilled hands. However, every patient is different, so one must adjust the technique based on each patient’s hair and skin characteristics. Technical choices such as punch size, punch depth, punch angle, rotation settings, and more will vary based on the patient’s skin and hair characteristics such as curl, degree of splay, graft size (number of follicles), exit angle, hardness of the skin, etc. Not only do these variables change from patient to patient, but they also change in different areas of the donor in the same patient. With sharp motorized dissection, it is important to constantly reevaluate the results and modify the approach based on the quality of the grafts. With time, one can develop a “soft touch” and the ability to “feel” when minor adjustments in angle, depth, etc., are needed.
63.2 The Physics of Sharp Motorized FUE: Key to Success
An understanding of the physics of SMF is helpful to better understand how skin and hair characteristics can determine modifications of the procedure to limit follicular trauma.1,2
63.2.1 Axial and Tangential Forces
Both Axial force (forward-directed force) and tangential force (rotational/oscillation) are used to cut through skin structures. Skin behaves as a fluid and hair follicles are small pliable structures existing in this fluid. Fluid dynamics apply and ensure that any force on the skin surface will translate into force on the hair follicles, causing movement that alters the location of the hair follicle. This movement in response to forces on the skin can rapidly move the follicle outside the center of a punch, resulting in follicle transection. In simple terms, this means that when you press on the skin with an axial force, the hair can bend forward, and move into the path of the cutting edge, increasing follicular transection.
Thus, it is important to allow the punch to penetrate the skin using more tangential force than axial force. Sharper punches with smaller surface area require the least axial force to penetrate skin.1,3 Ideally, one should have minimal resistance and see minimal skin dimpling as the punch is inserted. However, if dimpling is seen, limiting the depth of incision will cause less movement outside the lumen of the punch, thus reducing the risk of follicle transection (Fig. 63.1).
Fig. 63.1 The impact of maximum axial force on follicular movement as a function of depth of its application. (a) When maximum axial force is applied at a shallow depth, the shaft of the hair follicle experiences dramatic bend toward the superior edge of the punch and the follicular bulb upward in the direction of the inferior punch edge. (b) Alternatively, when the maximum axial force is not applied until the punch is fully inserted, follicular movement is greatly reduced. Reactionary responses to the maximum axial force, F, are shown in red, starting location of the follicle in dashed line, and the final location of the follicle in solid lines.
63.2.2 Friction and Torsion
Friction is another force that can cause follicular damage and transection. Resistance between skin and the surface of the punch during rotation increases with punch size and depth. When friction reaches a sufficient level, torsion results. Torsion may cause direct follicular trauma through twisting or displacement of the follicles outside the cutting edge, increasing the risk of transection. Torsion may be overcome by using a stepped approach of multiple limited-depth incisions for each graft, changing punch or rotation parameters as needed to reduce resistance. Modified punches like the Window punch (Cole Instruments), which decreases surface area, and the Hex blunt punches (Colorado Hair Science Center), which has a slippery polished inner surface, decrease friction (Fig. 63.2).4
Fig. 63.2 The Window Punch (Cole instruments) is a variation of a standard punch that has been modified to have a window above the cutting edge.
63.3 Hair Characteristics and Sharp Motorized FUE Results
Exit angle of hair emergence is predominately in the x– and y-axes. An acute slope and a larger value for x in either the negative or the positive direction increases excision difficulty (Fig. 63.3).
Fig. 63.3 Schematic illustration of hair follicles emerging from the scalp surface at acute angles with increasingly negative or positive x-values. As the absolute x-value increases, the follicle becomes more difficult to extract.
Tethering is the term for the attachment between the surrounding connective tissue and the outer root sheath (ORS).5 A firm attachment between the ORS and the connective tissue requires a deeper incision and increases the risk of transection. Strong tethering also increases the force needed to extract graft, which can also cause follicular damage.
Graft length influences follicle transection. Longer hair usually has a deeper tethering attachment and a longer path over which transection can occur. Incising deeper increases the risk of follicle transection. The goal is to keep follicle transection under 3% and graft transection under 12%. Minimal depth reduces transection rates and improves follicle regeneration when combined with a topical extracellular matrix product such as Acell.1 Deeper incisions increase the risk of follicle amputation. It is essential that the surgeon locate the optimal depth in each patient and each donor region.
Follicle splay or curl increases the risk of follicle transection and may require a larger punch or a shallower incision.
63.4 Skin Characteristics and Sharp Motorized FUE Results
Skin characteristics can have a major influence on SMF results. Skin can be hard, rubbery, and difficult to penetrate versus soft, nonrubbery, and easy to penetrate. Skin can be flexible or loose, allowing for a lot of movement when subjected to external force, versus firm or stiff, allowing for less movement.
63.4.1 Hard, Rubbery Skin
Hard, rubbery skin dulls a punch more quickly, while soft skin allows a punch to stay sharper longer. As a punch dulls with hard skin, it requires more axial force, causing more follicular displacement and increased transection. You can overcome a dulling punch by increasing the rotation per minute (RPM; which increases the tangential force). Start a procedure at 1,500 to 2,500 RPM. Hard skin may require a higher initial RPM. When the punch dulls and transection increases, increase the RPM in increments (e.g., 1,800, 2,500, 3,000, 4,000, etc.). At some point, increasing the RPM is no longer effective at reducing transection and the physician should change the punch. Sometimes when transection increases, changing to a new punch sooner or increasing the punch size is a better solution than increasing the RPM. Sometimes very high RPMs cut clean, but at other times they can rip the tissue. No one solution works equally well in all patients. However, devices with limited or fixed rotation speeds are not as versatile. There are no absolutes, but typically the lowest effective RPM is optimal with SMF. Try not to start above 2,500 RPM with a fresh, sharp punch.
Slide is another consequence that can occur with hard skin, especially at high RPM. Exceedingly hard skin can cause a punch to deviate (or slide) off its intended path, both when initially touching the skin (engagement) and as one moves the punch deeper, thus increasing the risk of follicle transection. The sliding movement will be in the direction of rotation. Slide is like a rotating drill bit placed on a piece of metal that lacks a premade hole. Instead of penetrating along the desired path, the drill bit slides in the direction of rotation. Typically, the skin is hardest in the most superficial epidermal–dermal 1 mm of depth, but when the hardness extends deeper, punch slide is more common.
As described earlier, torsion and friction also occur more frequently with hard skin due to the necessity to increase RPMs to penetrate the skin. Overcoming slide, friction, and torsion can be accomplished by penetrating the skin more gradually. This can be done using incremental, increasing depths of incision, at a lower RPM or perhaps using oscillation. Allow the inferior edge to penetrate the skin first, followed by the upper side of the punch gradually using “soft hands” (Fig. 63.4). Once the circumference of the punch enters the skin, one may still need to continue this process through the firmest part of the dermis. One can bounce the punch in or cut to depth of 1 mm, remove the punch, and then reinsert the punch again in the appropriate angle to free the graft. This is referred to as a two-step punch technique. With soft skin that breaks quickly, a single, swift penetration is often possible, but with firm, rubbery skin and deep incision, a multistep method may be required.
Fig. 63.4 Schematic illustration of a follicular unit extraction (FUE) punch making contact with the scalp surface. The acute emergence angle of most hair follicles makes the inferior punch edge the leading edge that contacts the scalp surface first and penetrates to a greater depth.
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