In their manuscript, “Examination of the Exit Angle of Hair at the Skin Surface versus the Internal Angle of Hair as It Relates to the FUE/CIT Harvesting Method,” Rose, Canales, and Zontos, attempted to map the path hair follicles take below the scalp surface in an effort to predict the minimum punch size that can be used during follicular unit extraction without risk of transection. Although such information could hold a high degree of clinical significance, the authors’ insufficient data categorization with respect to anatomical location, exclusion of patient demographic information, apparent failure to standardize the depth at which angles were measured in all test subjects, oversimplification of hair follicle growth patterns, and over simplified mathematical modeling undermine the value of what should constitute an impressive and original data set (i.e., the internal and external angles of hair follicles at the scalp surface).
Rose, Canales, and Zontos evaluated hair angles in the central occipital and lateral regions of full thickness strips that were harvested from 13 patients; however, they did not categorize their results with respect these sites. Site- and side-specific data would be useful since noted patterns in the external orientation of hair follicles have been established and are presented here in Figure 1.
Specifically, follicles on the right side of the donor region tend to grow in the direction of the negative x-axis (i.e., towards the patient’s left side) and often retain this orientation across the midline of the donor area. Left of midline, the direction of hair follicle growth relative to the x-axis shifts rather abruptly to a neutral position, while on the left side of the donor region, hair follicles grow in the direction of the positive x-axis. Moreover, the slope of hair follicles changes along the lateral plane of the scalp. The absolute value of the positive slope of follicles decreases in the direction of the right periauricular area of the donor region, while the absolute value of the negative slope decreases in the direction of the left periauricular area of the donor region (see Figure 1 and Figure 2B).
Moving superiorly along the scalp towards the recipient area, the downward orientation of follicles gradually transitions to an upward orientation (See Figure 2A and 2C). Interestingly, as also shown in Figure 2, individuals may present with unique deviations from the more common patterns; consequently, as the surgeon moves from one location to another in the donor area, the angle at which the punch is inserted must be altered to accommodate the orientation of hair follicles growing in the targeted region.
In addition to the anatomical variation in external growth angles discussed above, the degree to which the internal growth angle deviates from the external growth angle is known to differ drastically among various ethnicities and hair types. African hair and beard hair, for example, tend to grow along a tighter curve, resulting in a larger change in internal and external angle measurements. See Figures 3 and 4, respectively.
While the authors acknowledged that the path of curly or wavy hairs underwent more dramatic angle changes than straight hairs, they did not provide the percentage of the total hair population that each hair type represented and did not offer a summary of ethnicities included in the patient population, both of which would have improved the impact of their work. Moreover, the authors did not report using a standardized protocol for trimming hair before measuring internal and external angles. Since the perceived angle of hair growth becomes more acute as the hair length external to the scalp increases, inclusion of the overall height of hair above the scalp in addition to the location along the hair shaft where the exit angle was measured needed to be controlled in the study and, if it was, then the procedure should be described in the manuscript. The curvature of lengthy hair follicles is the rationale for trimming the hair shaft prior to FUE; the actual angle of growth can only be appreciated in full when the hair shaft is trimmed to a length of approximately 1mm or slightly shorter. The hair shaft should not be cut too short, however, because this can also mask the true angle of growth. Furthermore, shorter hair facilitates placement of the punch circumferentially around the hair follicles, in particular with a continuously rotating punch. Excessive tumescence can obliterate the angle of hair growth if the hair is cut too short, while both tumescence and traction can alter the angle of hair growth causing erroneous modification to the angle of approach. The notable exception is with kinky hair; it is often preferable to shave the scalp.
The most common follicular units on the scalp occur in groups of between two and four hairs (see Figure 5), but naturally occurring single hairs are rare. Still, the authors elected to present only a single hair model and, while noting that factors such as splay were commonly observed, neglected to discuss the impact of such confounding factors on the selection of punch size. Additionally, in the trigonometric relationship described by Rose, Canales, and Zontos, the parameters are not presented clearly, and, more importantly, the relationship does not account for the significant curvature of hair follicles below the skin. The authors define q as the change in angle below the skin surface relative to that above the skin surface, which implies that the value of q inserted into tan (q) = x/y is the result of the following subtraction process: (q= external hair angle – internal hair angle); however, q, as presented in their Figure 5, is depicted as the absolute value of the internal hair angle. Furthermore, since the growth path of the hair follicle is not linear throughout its entire depth, the location of the follicular bulb is not accurately captured by the authors’ overly simplistic tan(q) relationship, and a twofold increase in this value does not constitute a valid computation for minimum punch size. This method of representing the hair follicle as the hypotenuse of a right triangle has been used previously to visualize the relationship between hair growth and surgical intervention,1,2,3,4,5,6,7 but in order to be accurate, the hair follicle should be modeled as an arc length from a circle of unknown radius and the minimum punch size determined from the length of x in Figure 6. The length of x could be computed from the following two equations if the values of r and q could be measured experimentally.
However, since the radius r and q cannot be measured directly, the more appropriate route is simply to measure the length of x and select a punch with a diameter of equal or greater value. Although we can appreciate the relative ease with which clinicians could theoretically apply the trigonometric relationship during a surgical procedure, we still advocate that evaluating a few extracted samples and adjusting approach angle or punch size as needed constitutes a more efficient and efficacious protocol. Irrespective of the above critique, the average angle change values as presented have too many significant figures and no error is reported.
Further support for using a few sample extractions in lieu of the tan(q) expression provided by Rose, Canales, and Zontos to determine the most appropriate punch size for a given donor area arises from the variability in the paths each hair within a multi-hair unit takes below the scalp surface. Hairs often converge at the surface of the skin, exit in a single follicular canal, and progressively widen their distance apart as they penetrate the skin. Hairs that are convergent below the surface of the skin may distance themselves from one another in a circular pattern that requires a smaller punch size or a linear fashion that requires a larger punch size. Follicles that exit from multiple follicular canals may need larger punch sizes still, and since some outer root sheaths can only remain intact by increasing the depth of incision, they may necessitate a more voluminous punch. The sebaceous gland is almost always on the acute angle side of the follicles and, therefore, can serve as a landmark for determining where any follicle transection occurs (right, left, superior, inferior) concerning individual grafts. The first approach to correcting transections should always be to adjust the punch alignment by angling it towards the transection. However, when such angling maneuvers do not eliminate transection, the surgeon may be forced to consider using a larger punch or focusing on smaller clusters of hair follicles. Alternatively, the surgeon can attempt to align the center of the follicular group with the center of the punch. With acutely angled follicles, for example, move the obtuse angle of the punch closer to the cluster of follicles and follow a functionally simpler, more obtuse angle of approach (an explanation of this is beyond the scope of this response). In this way, if the precise angle of penetration is missed, the offset from the traditional center-on-center alignment will provide a margin of error.
These final points emphasize the advantages of a completely manual or manually directed mechanical extraction over robotic surgery. Anytime manual extraction is employed, the physician may evaluate the response to his removal and even perform an open, lift and look, technique followed by adjustments to overcome obstacles in the harvesting process. Robotic surgery, on the other hand, is limited by the pre-configured parameters programmed into the device that limit the complexity of cases they can successfully perform at this time. Consequently, a surgeon with decades of experience can often harvest grafts from cases that experience robotic failure.
Upon thorough evaluation of the research article, “Examination of the Exit Angle of Hair at the Skin Surface versus the Internal Angle of Hair as It Relates to the FUE/CIT Harvesting Method,” we concluded that the authors’ simplistic approach to modeling changes in follicular angle devalues the interesting data that was accumulated. Critical factors such as splay, multi-hair units, and changes to apparent growth angle caused by anatomical location of the follicle or length of the hair outside the skin surface should have been addressed, but instead they were neglected. Finally, we have demonstrated that the use of mathematical equations to predict the minimum punch size is not practical since two of the parameters needed to satisfy the correct model cannot be measured by a clinician. Instead, we recommend that the data from this publication be compiled with that of a much larger patient data set and sorted based on race-ethnicity, hair type, scalp region of extraction, etc. to build a profile of average punch approach angles.