Understanding the angles at which hair grows is essential to the success of Follicular Unit Extraction (FUE) and CIT (Cole Isolation Technique) hair transplantation procedures.
Precise angle measurement allows surgeons to select the appropriate punch size and angle of insertion, minimizing follicle transection and ensuring optimal graft survival. Despite its importance, measuring hair growth angles remains complex due to inconsistent methodologies, anatomical variability, and oversimplified models in existing research.
A recent study by Rose, Canales, and Zontos titled “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“ attempted to map the internal path of hair follicles to predict punch size. However, several methodological oversights reduce the utility of their findings.
Table of Contents
Methodological Considerations
Measurement Techniques
The distinction between external and internal follicle angles is foundational. While external angles refer to how hair exits the scalp, internal angles describe the follicle’s orientation beneath the skin. Rose et al. measured both but did not standardize measurement depths, nor did they account for hair length—a crucial oversight.
Longer hair creates an illusion of a more acute exit angle, so trimming hair shafts to approximately 1mm is recommended. This length allows accurate assessment without completely masking natural curvature, especially in kinky or curly hair types. Failure to trim consistently introduces error and may affect punch alignment.
Mathematical and Visual Modeling
Rose et al. attempted to use a trigonometric model (tan(q) = x/y) to represent the follicular path. However, their approach oversimplifies the problem. They modeled follicles as straight lines when, in reality, hair often grows along curved paths. A more accurate representation involves modeling the follicle as an arc of a circle (Figure 1). This allows for better prediction of the punch size needed to avoid transection.
Figure 1. Schematic representation of a hair follicle modeled as the length of an arc created at an angle a from the center of a circle with radius r. The value y denotes a chord extending from the intersection of the follicular arc with the scalp surface to the follicular bulb. The minimum punch diameter needed to avoid transection is denoted by x, the distance between chord y and a line parallel to y and tangential to the follicular arc.
Essential variables—such as radius (r) and angle (q)—cannot be directly measured during surgery. Thus, rather than relying on theoretical calculations, it’s more practical to measure the actual lateral deviation (x) of the follicle and select a punch size accordingly.
Anatomical and Ethnic Variability
Anatomical Location Differences
Rose et al. harvested samples from the occipital and lateral donor regions but did not categorize data by location—an important missed opportunity. Patterns of hair growth vary across the scalp:
- On the right donor side, follicles often grow toward the negative x-axis.
- In the center, the direction shifts to neutral.
- On the left side, follicles typically grow toward the positive x-axis.
The slope of hair follicles also changes laterally and vertically. Moving superiorly toward the recipient site, follicle orientation transitions from downward to upward (Figures 2A and 2C). These gradients must inform punch angle adjustments during extraction.
Figure 2. Two-hair scalp hair graft obtained by FUE showing the circular arc of hair from the Caucasian with straight hair.
Ethnic and Hair Type Variability
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.
Figure 3.
Figure 4.
Yet, Rose et al. omitted key demographic data such as hair type percentages and ethnic breakdowns. Including this would have improved the study’s applicability.
Clinical Implications
Complexities in Real-world Extractions
Follicular units often consist of two to four hairs and display diverse configurations. Hair shafts can:
- Exit via one canal and diverge subcutaneously.
- Exit via multiple canals, requiring larger punches.
- Splay out or curve unpredictably.
Failure to consider these complexities risks transection. Rose et al.’s use of single-hair modeling neglects the common reality of multi-hair units and misrepresents punch size requirements.
Figure 5.
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.
Inaccuracies in Modeling Hair Follicle Angle for Minimum Punch Size
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, 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.
Figure 6.
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 the 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.
Limitations of Robotic Extraction
Robotic systems lack the adaptability of manual techniques. When transection occurs, an experienced surgeon can adjust the punch angle or size in real time. Robots, limited by preset parameters, cannot perform dynamic adjustments or ‘lift and look’ evaluations—critical in challenging cases.
Benefits of Manual Adaptability
Manual or manually guided mechanical punches allow real-time correction based on tactile and visual feedback. Surgeons can angle the punch toward the side of the transection, center the punch over the follicular cluster, or adjust the entry angle to accommodate follicle curvature and minimize trauma.
Recommendations for Clinicians
Suggested Sampling Protocol
Before full-scale extraction, perform small sample punches in the donor region to gauge follicular angle and curvature. Measure the actual lateral deviation (x) rather than relying solely on angular models.
Adjustment Guidelines
- Trim hair shafts to ~1mm to observe accurate exit angles.
- Monitor anatomical location and adjust the punch angle accordingly.
- Use larger punch sizes if transections persist despite angle adjustments.
- Align the punch toward the observed transection side to improve outcomes.
Transection Reduction Tips
- Avoid excessive tumescence and traction, which alter hair angles.
- For curly hair, shaving the scalp may help, but take care not to cut too short.
- Use sebaceous glands as markers to determine transection location.
Conclusion
Accurate measurement of hair growth angles is vital for successful FUE and CIT procedures, but overly simplistic models fail to reflect the real-world complexity of hair follicle geometry. The study by Rose, Canales, and Zontos provides useful initial data but lacks crucial anatomical and demographic categorization, making its conclusions less actionable.
Clinicians are encouraged to combine limited modeling with real-time observational techniques, including sampling, trimming protocols, and manual extraction methods, to optimize outcomes and reduce follicle transection. A larger, better-categorized dataset could further refine punch selection guidelines across diverse populations and scalp regions.
References
1. Cole J. Donor harvesting with a multiple blade knife, ISHRS, San Francisco 1999.
2. Cole J. Regional Variation in donor density, ISHRS meeting, Hawaii, 2000
4. Cole J. A method to determine follicle growth angles, ISHRS meeting Puerto Vallarta, Mexico, 2001.
3. Cole J. Follicular extraction punch and method, US patent number 7,172,604.
4. Cole J. The basics of FUE, AAHRS, Bangkok Thailand, July 2010.
5. Cole J. Basics of FUE, Seoul, December 2011.
6. Cole J. The Basics of FUE, Nassau Bahamas, October 2011
7. Cole J. An analysis of follicular punches, mechanics, and dynamics in follicular unit extraction, Facial Plastic Surgery Clinics, 2013.
