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We respond with great pleasure to the question brought up by our colleagues, Drs. Gosselink and Ctercteko, from Australia. Studying in situ hair orientation with scanning electron microscopy, they found that hair orientates itself root first into the sinus tract in most patients. They can be congratulated for these results. These results are particularly interesting, as the effect of hair scales had been previously neglected because of the small size in proportion to the hair diameter.1 Nevertheless, scale effects have thus far not been studied extensively with modern techniques. Hair movement into the sinus tract has been largely attributed to friction of gluteal muscles during walking.2 Patey and Scarff started examining the orientation of sinus hair,3 as Weale did the same year.4 The conclusive hint was given by Page in 1969,5 when he inserted 2 hairs into a single sinus tract of a young woman, 1 root first and 1 root last. The root-first hair was marked with a knot. After 1 hour of having the young lady walk around, the root first–orientated hair had been drawn significantly further into the sinus as compared with the root-last hair had at the same time (Fig. 1).
So, independent of suction or friction being the main force of hair movement inward, hair orientation plays a major role in smaller tracts. In larger-diameter sinus tracts, scale effects may be diminished because of reduced scale-to-wall contact surface. In a recent study, we examined the hair in the sinus.6 We found that more than two thirds of the hair in the sinus was without root. The average length of the <400 hair particles in the sinus was 0.9 ± 0.7 cm. We did not count the one side cut hair to both side cut hair, but it can be estimated that between half and two thirds of hair has been cut at both ends.6 Because of the missing root and the small diameter differences in short cut hair fragments, we often could not tell the orientation of distal and proximal hair ends.7
Hair of larger diameter and larger axial strength is most likely to exert larger scale effects, promoting forced hair insertion. Because most hairs are sharp cut on both sides, the initial injection into the skin surface is possible on both sides. However, only the now-rootless proximal side (root first) will be the ruthless one and will be propelled much deeper and faster. Drs. Gosselink and Ctercteko can be congratulated. They add another important piece to the puzzle that fits convincingly into the pathomechanism of pilonidal sinus.
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