Sharp punch

The sharp punch was designed to lower friction and axial force by reducing the contact surface area between the skin surface and the punch blade. This allows the surgeon to perform harvesting at a very high speed while minimizing the axial force. However, the punching depth (PD) must be shallow to minimize graft transection, which is a major disadvantage of this punch type.

Blunt punch

The blunt punch, represented by Harris’ SAFE (surgically advanced follicular extraction) system, reduces follicular damage owing to the blunt surface of the punch tip edge.[7] This punch can punch deeper than 4 mm. The ARTAS hair restoration robotic system is equipped with a dual punch[8]: initial scoring with the inner sharp punch is followed by deeper dissection with the outer blunt punch.

Flared punch

With the sharp and blunt punches, the cutting axis has the same vector as the punching axis. The cutting end point exists between the inner and outer walls of the punch. With the flared punch, the cutting vector is directed more outward than the sharp and blunt punches and the cutting end point is located further outward than the outer diameter [Figure 3]. This structure reduces follicular injury. Various punches designed by Umar, Trivellini, and Devroye fall in the flared category.[910]

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Figure 3

Cutting vector and cutting end point of various punches

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Edge-out punch

The edge-out punch was introduced by Trivellini and allows slight pressure to be applied at the end of the punch through a thicker wall. In addition, the cutting vector is directed slightly outward.[11] However, the cutting end does not face outward beyond the outer diameter. In this case, the outer diameter and the cutting end coincide, and only the cutting vector is directed slightly outward. This type of punch represents an intermediate form between the sharp and flared punches.

Ring punch

Trivellini also introduced an octagonal ring punch based on Harris’ concept.[12] The octagonal shape of the ring increases vibration and facilitates separation of the graft from the wall of the punch. In addition, the distal edge of the punch is blunt and has several notches, which allows excision of both long and shaved hairs using the same punch.

Punch angle

According to Rose et al.,[13] the angle of the punch upon skin incision should be approximately 45º on average. As this angle becomes more vertical, the wound surface decreases as does the incidence of follicular injury.[14] The hair exit angle can easily be converted into less acute angles simply by applying upward traction to the scalp or using tumescent injections.[15]

When donor hairs have an extremely acute exit angle, the lower and upper edges of the punch contact the skin at different times, which can cause tissue distortion [Figure 4]. Because of the 360 degree-nature of the punch blade, when the lower edge is dissecting the skin surface, the upper edge still hangs in the air; therefore, the skin is asymmetrically incised only on one side of the punch, depending on the skin characteristics and hair follicle depth. In this case, using a sharp punch or making changes in the punch setting or insertion speed may help prevent follicular damage by reducing the time between lower edge skin contact and upper edge skin contact.

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Figure 4

The lower and upper edges of the punch contact the skin at different times according to the punching angle

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When the punch insertion angle is extremely acute or the skin is extremely elastic, use of a punch that is too blunt or flared causes a large skin scoring time difference between the lower and upper ends. This may increase the tendency for graft distortion. Therefore, the cutting time between the two ends should be shortened. The screw effect of the multi-wave punch, which will be described later, plays a role here.

Surgically advanced follicular extraction punching depth

In general, as the PD increases, the amount of perifollicular tissue increases, enabling harvesting of “chubbier” grafts. Conversely, when PD decreases, the amount of perifollicular tissue decreases, as does incidence of graft injury.[16]

If the PD is too shallow, capping or plucking occurs, and the follicle is not extracted. Minimal depth can be reached by advancing slightly deeper, where hair follicles can be successfully extracted. Cole[5] introduced a technique for minimal depth punching using a sharp punch. Park[17] preferred a deeper PD that was able to safely acquire healthy donor grafts atraumatically; he named this depth the safe PD. Advancing deeper than this increases the chance of follicular damage [Figure 5].

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Figure 5

Concept of safe punching depth

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Safe PD is not an absolute value and differs according to the surgeon’s skill, punch type, punch motion, patient characteristics, and punching location in the same patient. Even at the same PD, the vibratory energy generated from the multi-wave punch is transmitted to the surrounding tissue, which widens the gap between the target graft and the surrounding tissue and facilitates graft extraction. In other words, graft harvesting is plausible at a shallower PD.

Punch wall thickness and sharpness

Thickness and sharpness of the punch wall lumen are important punch tip characteristics. In general, the wall thickness of the punch lumen ranges from 0.08 to 0.125 mm; 0.1 mm is most commonly used. The thinner the wall, the faster the tip abrasion; the thicker the wall, the more resistance encountered. The cutting angle of the punch edge can be created at an obtuse or acute angle. At a more obtuse angle, the cutting point remains the same but the cutting vector is directed more laterally, which is advantageous. At a more acute angle, tissue cutting is easier, less axial force is applied, and operation time is reduced; however, the graft transection rate may increase.

The scalp and follicular unit excision punches

A punch tip sequentially penetrates the epidermis, dermis, and subcutaneous layer of the scalp. The galea and pericranium are not penetrated. Because each of these layers has a different density and thickness, the punch tip interacts differently with each one. The outermost epidermis is encountered by the punch tip first. Next, is the dermis, which has a high density. Last is the subcutaneous later, which contains fat and is loosely organized.

Hairs do not exit perpendicular to the skin. The average punch advancing angle in FUE is approximately 45º.[8] Therefore, as the punch tip enters the skin, the layer introduced to its upper edge would never coincide with that introduced to the lower edge. The lower edge first contacts the skin while the upper edge is still hanging in the air without any contact. As the punch tip dissects the skin surface, it will simultaneously stay in three different layers, making a simple act a very complex phenomenon.

The ideal punch would comprise three different parts to efficiently target each of the three scalp layers penetrated. First is the cutting edge, which comprises the first 0.25 mm of the tip and affects the changes and phenomena that occur when the punch first comes into contact with the skin. Second is the leading edge, which rests in the 1.5 mm-long section next to the cutting edge. Its characteristics affect the advancement of the cutting edge through the 1.5–2 mm of dermis. Third is the main body, which is adjacent to the leading edge. This part is responsible for advancement of the cutting and leading edges into the subcutaneous layer.

Likewise, the process of graft dissection can be divided into three sections. Optimally designing the punch shape to generate appropriate changes in motion by each section allows the surgeon to obtain high-quality grafts while minimizing follicular damage.