08 Sep

An Analysis of Follicular Punches, Mechanics, and Dynamics in FUE

Key Points

  • Follicular unit extraction punches are made from a variety of metals.
  • The degree of sharpness varies significantly from one punch manufacturer to another.
  • Sharp punches require less axial and tangential force to penetrate the skin and dissect hair follicles.
  • Minimizing axial and tangential forces helps to reduce the fluid movement of hair follicles during the
  • dissection process.
  • Force compression testing allows the degree of sharpness of any punch to be determined.
  • Follicular groups consist of between 1 and 6 hair clusters. The frequency of cluster size varies from
  • one person to another.
  • The degree of hair splay varies from one person to another.
  • Variation in punch size and incision depth allows the dissection of grafts to be customized to the
  • individual patient and follicle transection to be minimized.

Introduction

Follicular unit extraction (FUE) is the latest major technical advancement in surgical hair restoration. 1–5 The methodology of FUE evolved from the basic principles of circular graft extraction that were introduced more than a half century ago. However, unlike macroscopic plug hair restoration, which used large circular punches, FUE is a refined procedure that requires high-power magnification and uses small circular trephine punches to isolate and extract individual follicular units (Fig. 1). The driving force for developing this technique was elimination of the linear donor scar that accompanies traditional strip harvesting. Shorter hair styles and a greater awareness for cosmesis in the donor site stimulated this movement.

The follicular unit is a delicate structure that is vulnerable to several types of injury during the extraction process. Transection is perhaps the most common injury observed with FUE (Fig. 2). Success with FUE depends on being able to predictably dissect excellent-quality grafts from the donor region. A high-quality trephine punch is mandatory for the accurate isolation of individual follicular units. The hair restoration surgeon must have a thorough understanding of the FUE punch and the nuances of FUE surgical technique to ensure consistent graft quality.

Fig1

Fig2

Punch Metallurgy

Surgical instruments are made of a variety of types of stainless steel. The type of steel depends on the function of the surgical instrument. FUE is a minimally invasive procedure dependent on a strong material with a sharp edge, thin wall, and small volume. Modern FUE punches are made of a variety of elements including Fe, C, Cr, Ni, Mn, Mo, Si, P, and S. Chromium makes stainless steel corrosion resistant. The other elements enhance other properties of the steel. Some materials, like 303 and 304 stainless steel, cannot be hardened by heat treatment. Other materials, like 17-4, 420, and 465, can be heat treated to harden them to a higher level. Hardness is a measure of the resistance to deformation or indentation. The hardness of steel is often measured using the Rockwell scale, which is a measure of indentation depth under load or indentation hardness.

Punch Handle and Trephine

Many physical and technical factors affect punch cutting dynamics and their related tendency to produce tissue distortion and graft damage:

  • Punch diameter
  • Cutting edge location
  • Punch wall thickness
  • Punch metal type
  • Punch edge sharpness
  • Punch edge shape (smooth vs contoured)

There are 3 important diameters with respect to an FUE punch:
1. Internal diameter (distance between the internal margins of the punch)
2. External diameter (distance between the outer margins of the punch)
3. Cutting diameter (distance between the cutting edges of the punch) (Fig. 3)
The cutting edge is located in one of 3 important locations and the bevel is situated according to each: 1. Inside margin cutting edge with an outside bevel (type 5 punch)
2. Middle margin cutting edge with a middle bevel (type 4 punch)
3. Outside margin cutting edge with an inside bevel (type 3 punch)
The distance between the cutting edges determines the physical location of the incision on the skin. The location of the cutting edge also affects the fluid dynamics on the tissue during the punching process. An outside diameter (inside bevel) is designed to minimize the effect of the fluid dynamics during the punching process as the blunt bevel abuts tissue that has already been cut. The author has found that this design improves tissue cutting, reduces follicle trauma, and is best for high-quality extractions. The inside diameter (outside bevel) punch cuts a narrow hole in which the blunt outside bevel is forced into a hole that is smaller than the outer diameter of the punch. This punch style seems to have the greatest deleterious impact on follicle fluid dynamics and could increase the risk of a lower-quality graft. The middle diameter or middle bevel punch has the cutting edge somewhere between the external and internal punch margins, and seems to have an intermediate impact on adverse follicle fluid dynamics and graft quality. Tissue distortion is also influenced by the thickness of the punch wall and by the rate of punch insertion. In all instances, a thin punch wall reduces the resistance as the punch is introduced into the skin. A titanium nitride (TIN) coating helps to improve the life of the cutting edge on punches made of soft steel. However, these punches lack the degree of sharpness that modified hardened steel punches possess. TIN-coated punches are perhaps the most popular of all the sharp punches

Fig3

and they are the least expensive. The punches range from 0.6 mm up to 1.5 mm in diameter depending on physician preference. The internal diameters of these punches decrease after a few millimeters (Fig. 4). The internal diameters of punches vary, depending on the arbitrary discretion of the manufacturer and the vendor. However, some manufacturers and vendors label their punches with inaccurate internal diameters. Hardened steel punches have a razor sharp edge that minimizes friction as the punch enters the skin and results in a reduced axial force. There are multiple handles available that can accept a variety of stainless steel punches. A handle designed to precisely limit cutting depth is available to further boost the accuracy of the hardened steel punches.

CContoured Surface Punches

Two contour surface punch designs are available. The triple wave punch is made of 303 stainless steel and titanium nitride coating. It is designed with elevated waves that reduce friction by limiting the total surface area of the punch in contact with the skin. The second is the Serrounded punch (Cole Instruments), which is made of hardened steel (Fig. 5). This punch decreases the surface area in contact with the skin, thereby reducing friction and minimizing the axial force required to

Fig4

Fig5

penetrate the skin. The Serrounded punch has a thinner wall, has more cutting edges, and is sharper than the triple wave punch. The author typically prefers the contoured punch for mechanical rotation and the standard Cole Instrument (CI) punch for manual extraction. The standard CI punch often seems to work better with oscillation rather than continuous rotation. Sometimes the Serrounded punch yields higher quality grafts using oscillation rather than continuous rotation.

Analysis of Follicular Dissection

The analysis of tissue cutting in hair transplant surgery is complex, especially for a follicular isolation procedure. The complexity results mainly because soft tissue is composed of layers with different modulus and elastoplastic properties. The mechanical characteristics of homogeneous materials are not applicable to skin. Skin has no unique, single Young modulus (a measure of the stiffness of an elastic material), or shear modulus, because such properties for skin vary depending on the strain applied. Biological materials typically have stress-strain diagrams with an elastic part, which can be linear or nonlinear. In addition, the mode of load application is time dependent and, with a round punch acting at some angle to the surface of the tissue, is complex. The bevel on the punch adds even greater complexity to the cutting procedure. One approach to help understand punch cutting dynamics is to simplify the properties and analyze some individual interactions, such as the interaction of the punch and tissue at initial contact. For example, with a rotating or oscillating punch, a torque is applied to the punch by hand or by a mechanical means. Torque is the rotational equivalent of a force. Just as a force can do work by being applied through some distance, torque can only do work by being applied through some angle. The rotation per minute (rpm) is the rate at which that angle changes, or the rate of rotation. In the case of a motor power, the torque results from the motor horsepower and the speed.

Fig5.2

The tangential force at the punch cutting edge thus can be determined from the two relations. Just as a force is a push or a pull, a torque can be thought of as a twist. During dissection, a tangential force is applied on the contact surface and an axial force is applied in the direction of the axis of the punch. The tangential force results from a torque applied to the rotating punch. The torque on the punch produces a force at the peripheral point of the punch. The direction of the force depends on the direction of rotation of the punch. At the initial contact of the punch to the tissue, the punch applies a force in the direction of rotation and the tissue applies resistance to the rotation. In such a case, the following may take place.

    
  • Because of the friction between the punch edge and the tissue surface, a friction force opposing the applied force results. The friction force depends on the applied tangential force, normal or axial force, and the friction coefficient between the two surfaces.
  • 
  • If the applied force exceeds the friction or resistance force, either the tissue is pulled in the direction of rotation or the punch tends to rotate over the tissue in the direction of rotation.
  • If the punch is prevented from rolling, the force results in a shear stress in the tissue. A shear stress is a stress state in which the stress is parallel to the surface of the material. When the shear stress exceeds the shear strength of the tissue, the tissue cracks or fails. Although this is a basic fact, it remains a simplistic analysis because the resulting stresses and the property of the material at contact are complicated.
  • 
  • The shear stress t developed is directly proportional to the tangential force Ft and Fig. 5. The surrounded punch cutting edge showing multiple cutting edges. Follicular Punches, Mechanics, and Dynamics in FUE 441 inversely proportional to the area of contact A (t 5 Ft/A). If the contact area is small, as in the case of a sharp punch, the resulting shear stress is high. Thus, the shear strength of the tissue is exceeded more quickly and the tissue fails or is cut more quickly. A dull punch has a larger contact area, which results in a lower shear stress compared with an equivalent-diameter sharp punch. To exceed the shear strength of the tissue, a higher force is needed with the dull punch. A higher force means a higher torque. A punch with a higher torque results in a higher twist or distortion of the tissue and is also difficult to control and to center about the follicle axis. These variables are the main causes of transection.

FUE requires an understanding of fluid dynamics and the physics involved in the removal of the grafts. A full understanding of these variables is beyond the scope of this article. However, a force applied to the skin results in a reaction by both the various skin layers and the hair follicle(s) comprising a follicular unit. The cutting of skin with a dull instrument requires more force than does cutting with a sharp instrument. The term dull punch is a misnomer in that this device is a punch that is not very sharp (ie, a punch that is incompletely dull). Although it cuts the skin with less force than a completely dull instrument, it can transect hair follicles despite its nonsharp edge. A dull punch or one that is not very sharp requires significant mechanical energy, and the excessive force limits manual control. Any punch driven by a force great enough to cut epidermis and dermis can also cut a hair follicle because the outer root sheath (ORS) and follicle are more delicate than the epidermis and dermis. Excision of the follicle requires 2 forces: an axial (penetrating force) and a tangential force (rotation or oscillation). Because hair exits the body at an acute angle, the inferior margin of the punch makes first contact with the skin (Fig. 6). A large axial force along this inferior margin results in an inferior displacement of the follicle (Fig. 7), which can lead to follicle amputation if the displacing force moves the follicles outside the lumen of the punch. A predominantly tangential force incises into the skin without displacing the follicles as long as torsion of the graft does not occur. A sharp punch minimizes torsion while maximizing the benefits of a tangential force. If a predominantly axial force is used, the surgeon must attempt to achieve an equal force along the perimeter of the punch so as to minimize follicle deflection.

Fig6 7

Friction is a resistive force that curtails movement of the punch through the skin. It is a function of the circumference of a punch and the depth of punch insertion (Fig. 8). Large punches and deep extractions increase resistance during follicular unit dissection.

Force Compression Testing

Fig8

This sharpness testing procedure is needed to evaluate punch cutting edges, to establish reference punch sharpness data, and to monitor the consistency of the quality of the manufactured punch cutting edges. Sharpness testing measures the force required to penetrate through a specimen with known material properties and parameters. A compression force gauge may be used to determine the punching force as a measure of sharpness. The punch is mounted with the axis in a vertical position and the cutting edge facing downwards. A test media of silicon rubber with thickness of 0.8 mm (1/32 in) is placed on a horizontal support surface or substrate. The test media used has a minimum wearing effect on the cutting edge and the support material should give a much lower punching force reading to avoid confusion and prevent punch edge damage. We lowered each test punch on the test media slowly at constant velocity. If a manual testing machine was used, the feed rate was kept as constant as possible. During testing, the punch cuts through the rubber media into the softer support material. The punching force was plotted against the time or distance of punch travel as it cut through the media thickness. The cutting force increased with increasing travel or time and decreased abruptly when the punch cut through the medium. The punch travel was stopped after seeing the decrease in force value. The maximum force reading on the force-time plot of the force gauge was used as the measure of sharpness. At the beginning of each new test and after each cut through the test media, the medium was moved to a new position and a new punch was replaced. Using these parameters and a Dillon Model GTX force gauge, we assessed a variety of different punches manufactured by different vendors and plotted the force reading for each punch (Fig. 9).

Fig9

Punch Size Variation

In 2003, the author conducted a study evaluating the benefits of punches 0.75 mm in diameter. The healing from these extraction sites appeared to be the same as the healing with 1.0-mm extraction sites when intact follicular units were extracted. However, the follicle transection or amputation rate was significantly greater with the smaller punches. After this, the author evaluated punches of 1.25 mm in diameter and discovered that the healing was identical to that of 1.0-mm punches when the intact follicular unit was extracted. Based on this knowledge, the author began to vary punch diameter based on the follicle transection rate. If the rates were high, the author increased the size of the punch. Variation in punch size along with the use of sharper punches resulted in a decline in the mean follicle transection rate from 8% in 2003 to less than 3% by 2006. Some follicular groupings are larger than others, and some follicular groups have more follicular splay. When large groups or significant splay is present, the author discovered that larger punches allowed a lower follicle transection rate with identical healing. However, follicle splay is sometimes so great that even a larger punch size does not overcome all the potential follicle transection caused by the splay.

punch incision geometry based on punch size and hair growth angle

Length of Incision

The incision length created by a punch is based on the angle of punch insertion, which depends on the angle of hair growth. Incising the skin at an angle with a circular punch creates an elliptical opening with a long axis length that is equal to the diameter of the punch divided by the sine of q where q is the angle of hair emergence from the skin (Fig. 10).

Fig10


Punch Length
The incision length increases as the diameter of the punch increases and as the angle of hair growth decreases.

Depth of Incision

The punch typically enters the skin along the axis of hair growth. When the punch enters the skin at the acute angle along the axis of hair growth, the inferior margin of the punch always enters the skin deeper than the superior margin of the punch (Fig. 11). The acute angle of hair growth creates a disparity in the incisional depths of the superior and the inferior aspects of the punch as it incises down the follicle along the axis of hair growth. This increased depth is equal to the diameter of the punch divided by the tangent of the angle that the punch enters the skin

Fig11

The cutting depth of the inferior punch margin increases as the diameter of the punch increases and as the angle of hair growth decreases. At approximately 27° to the skin, the inferior margin of the punch incises to a depth twice as deep as the superior margin.

Factors affecting follicle dissection in FUE

To best appreciate graft quality problems associated with FUE, the surgeon needs to have a thorough understanding of microscopic follicle and follicular unit anatomy. Each individual follicle is a complex miniorgan formed by multiple mesenchymal and epithelial cell layers. More than 20 distinct cell populations contribute to the formation of individual mature follicles, many of which are arranged in concentric layers that extend out from a central core (Fig. 12). The physical properties of each individual cell line vary with respect to cell size, cell strength, layer thickness, and adhesive bond to the neighboring cell layer(s). Variations in these physical properties influence each patient’s predisposition to complications relating to the cutting, rotating, and traction forces imposed in and around each follicle. Follicular groupings tend to exit the scalp similarly to flowers out of the neck of a vase. The follicles are closer together on the surface of the skin, but progressively farther apart as they move toward their position in the adipose (Fig. 13). This movement away from one another is termed follicle splay and individual variation is possible. In some, splay is minimal, whereas in others it can be significant. In a few, most follicles follow a similar direction, whereas 1 or 2 follicles in the same surface grouping diverge in a different direction. The degree and type of splay can affect the transection rate of follicles. Using a larger punch often accommodates splay. At other times, the physician might use a smaller punch and target only a few hairs within the larger follicular cluster. Elastic skin often complicates the extraction process. One way to overcome elasticity is to excise the grafts slowly with minimal axial force. Another is to apply tension on the donor area so that elasticity is reduced. Torsion is common with rotating mechanical extractors, but does not occur with oscillating manual extraction. Limiting the depth of the incision minimizes torsion. Another way to minimize torsion is to cut slowly down the direction of hair growth in progressive steps. With each shallow incision, the skin is allowed to relax before incising deeper. Torsion may also be limited by use of an oscillating extractor; however, follicle transection is often higher with oscillating extractors. Tethering of grafts is a reference to the attachment of the ORS to the adipose. When the attachment of the ORS to the adipose is strong, a deep incision is required to remove the graft from the adipose. When the attachment is normal, the author may limit his incision to 2 mm and refers to this as a zero.

Fig13

When the attachment is strong the incision may require an incision up to 3 mm and the author calls this a 12. When the attachment is weak, the incision may be limited to less than 2 mm, and the author calls this a 1. Other factors that play a role include the strength of the attachment of the ORS to the inner root sheath (IRS). If this attachment is unusually weak, there is a tendency for the ORS to separate from the IRS, in which case the follicle may be removed without the inferior aspect of the ORS and the dermal papilla (Fig. 14). A deeper incision is often required to remove the graft intact when this attachment is weak. Less commonly, the dermal sheath including the ORS and IRS are friable and easily damaged during punch insertion. In such instances, 1 or more follicles within some of the follicular units are missing their ORS and IRS during the extraction process. In this instance, greater caution must be exercised, but there is no specific protocol other than a gentle technique that can prevent the problem.

Fig14

Summery

Excellent graft quality requires technical expertise, familiarity with punch design, and a thorough knowledge of punch and soft tissue dynamics. With FUE, physicians find that no single punch or method works equally well for all patients, and that versatility is key to ensuring consistently high graft quality from patient to patient. The surgeon should learn both manual and mechanical methods of graft removal because the transection rate is too high when only mechanical devices are used for all patients. The author’s success with FUE is a result of variations in punches and incision depth (Fig. 15). It is logical that variation in the use of mechanical and manual extraction techniques will also have an effect on individual patient transection rates. By using these strategies it is often possible to achieve transection rates of less than 3%.

Fig15

References

1. Available at: http://www.engineersedge.com/stainless_steel.htm. Accessed February 15, 2013.
2. Available at: http://en.wikipedia.org/wiki/Surgical_stainless_steel. Accessed February 15, 2013.
3. Available at: http://www.wpiinc.com/index.php/vmchk/Surgical-Instruments.html. Accessed February 15, 2013.

Friction and force representation

4. Meriam JL, Kraige LG. Engineering mechanics, SI version: statics. 6th edition. John Wiley & Sons; 2008. Technology & Engineering.
5. Stephenson DA, Agapiou JS. Metal cutting theory and practice. CRC Press; 2006.

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