ARTICLE
Auteur(s) : Pascale Quatresooz1, Jean-François
Hermanns1, Trinh Hermanns-Le1, Gérald E Pierard1,
Jean-Luc Nizet2
1Department of Dermatopathology, University Hospital
Sart Tilman, Liège, Belgium
2Department of Plastic Surgery, University Hospital Sart
Tilman, Liège, Belgium
accepté le 14 Mai 2008
In dermatologic and plastic surgery, the orientation of scalpel
incisions is of importance in order to reduce the risk of abnormal
scars. For that reason, they are ideally oriented in the direction
of Langer’s lines, or more accurately in parallel with the relaxed
skin tension lines [1]. The overall pattern of these orientations
on the body is commonly depicted in textbooks. However, there are
some inter-individual variations between slim and stout people, and
according to muscle development, body posture and any physical
particularities of the body region under consideration [2, 3].
Tension lines are, in addition, influenced by age, particularly in
younger and older people. At present, there is no recognized simple
and non invasive way to identify the tension lines with the
exception of determining multidirectional changes in the speed of
ultrasound propagation in the skin [2-4].
It is noteworthy that the risk for hypertrophic and keloidal
scars is increased in people of darker skin complexion. It would be
useful to identify those individuals whose Langer’s lines exert
prominent effects on the skin.
Skin of any part of the body is subject to intrinsic mechanical
tensions [1, 5]. It is acknowledged that any force generated by the
skin or applied to it transduces information to cells that may in
turn respond to it [6, 7]. Some effects of mechanobiology may in
particular be evidenced in the functions and tensegrity of
fibroblasts, dermal dendrocytes, keratinocytes and melanocytes
[7-9]. The boosted melanocyte activity is responsible for an
epidermal melanotic hyperpigmentation which can be revealed using
dermoscopy, particularly in scars and striae distensae of people
with a darker skin complexion [10-12].
Subtle differences in skin tensions and in their orientations
can be assessed non-invasively by measuring the speed of
propagation of ultrasound shear waves [2-4, 12-14], which increases
when the skin is under tension. Accordingly, this non-invasive
procedure can be used to reveal the orientation of skin tension
lines, and the skin mechanical anisotropy in a multidirectional
mapping [2, 3, 15].
The aim of the present study was to measure the directional
speed of shear wave propagation in subjects with a darker skin
complexion, and to compare it with the main orientation of the
epidermal melanotic network as revealed by dermoscopy.
Materials and method
The study was performed in accordance with the Declaration of
Helsinki. Healthy skin of darker complexion was examined on the mid
lateral part of the back using dermoscopy (Dermaphot®
Heine Delta 20, Heine Optotechnik, Hersching, Germany) following a
previously described procedure [10-12, 16]. The subjects were 70
adults of both genders (38 M and 32 F) and of sub-Saharan African
ancestry. They were aged from 19 to 49 years. Their body mass index
(BMI) ranged between 19 and 27 kg/m2.
Intrinsic tensile properties of skin were measured on the same
sites. The measuring method relied on the physical principle of
ultrasound shear wave propagation. Resonance running time
measurements (RRTM) were recorded in arbitrary units using the
Reviscometer® RVM 600 (C+K Electronic, Cologne, Germany)
as previously described [2, 3, 12-14]. The probe contained two
piezo-electric transducers. When applied to the skin with
controlled pressure, one transducer transmitted ultrasound shear
waves, and the second served as a receiver. The shear waves
propagated differently according to the skin viscoelasticity. The
time taken by the ultrasounds to travel from the transmitter to the
receiver was inversely proportional to the mechanical tensions
present inside the skin [2, 3]. Measurements were taken in four
controlled directions at each test site. One was along the main
orientation, if any, of the melanotic pattern disclosed by
dermoscopy. The other three axes of measurement were oriented at
angles of 45°, 90° and 135°, respectively. This procedure allowed
us to cover the whole 360° directions at 45° intervals. A ring was
fixed to the skin to hold the probe at exactly the same site while
positioning it at the different angles. For each subject, four RRTM
were recorded in each of the four directions. The minimum, maximum
and mean values of the 16 multidirectional RRTM values were
recorded at each site and in each subject.
The data were grouped according to the patterns of the melanotic
network. As a result of the asymmetrical distribution of the RRTM
data, the medians and ranges were calculated for each series of
data. Differences were tested for their statistical significance
using the Wilcoxon paired rank test and the Mann-Whitney unpaired
test. Regression analysis models were applied to evaluate the
relationships between RRTM values. The linear, logarithmic,
exponential and power correlations were tested on the basis of the
highest value of the coefficient of correlation r in these models.
A p-value < 0.05 was considered statistically significant.
Results
Two main contrasted melanotic aspects were revealed by dermoscopy
of healthy skin of the back, namely the honeycomb and the laddering
patterns. The honeycomb pattern was found in 62.9% (44/70)
volunteers. It was characterized by a regular network of small
rounded structures rimmed by a thin melanotic line (figure 1). The laddering
pattern was found in 37.1% (26/70) volunteers. It was characterized
by parallel melanotic lines (figure 2A and B). The
interval between adjacent lines was quite even and corresponded
approximately to the diameter of the cells of the honeycomb
structure.
The melanotic pattern was not influenced by age and gender. The
melanotic honeycomb pattern was more frequently found in people
with a BMI lower than 23 (figure 3). The laddering
pattern was observed in people with any BMI value. The difference
between the two groups was statistically different (p <
0.01).
Incidentally, striae distensae were observed in two subjects.
These lesions were characterized by a transversal laddering
structure with narrow interline spacing (figure 4). In a few
subjects, the laddering melanotic pattern was clinically associated
with moderate hyperpigmentation.The minimum RRTM values (higher
ultrasound speed) were found in the direction of the melanotic
lines of the laddering pattern. These values were significantly (p
< 0.05) lower than the corresponding values found on skin
exhibiting the melanotic honeycomb pattern (table 1). By contrast, there was no significant
difference in the maximum and mean RRTM values between skins
exhibiting the two different melanotic patterns.
The mechanical anisotropy (difference between the minimum RRTM
and the maximum RRTM) was higher in association with the melanotic
laddering pattern than with a honeycomb pattern. The BMI value
significantly influenced this physiological characteristic, while
the gender and age in the range 19-49 years had no influence.
Table 1 RRTM values: medians (ranges)
|
RRTM values
|
Melanotic pattern
|
|
Honeycomb
|
Laddering
|
|
Minimum
|
247 (188-337)
|
186 (136-260)
|
|
Maximum
|
652 (306-982)
|
627 (310-1015)
|
|
Mean
|
498 (275-655)
|
493 (265-710)
|
Discussion
The variations in the firmness and mechanical anisotropy of skin
can be determined by measuring the velocity of shear wave
propagation, which changes with body posture, gravitation forces
and BMI [3]. For this reason, the RRTM measurements were performed
on the same body region. Although there is evidence that some
mechanical properties of skin are influenced by ethnicity [17-20],
no information is currently available about RRTM differences
between different ethnicities. The present study was focused on
people of dark complexion exclusively as preliminary observations
had failed to reveal melanotic patterns in Caucasians.
The melanotic honeycomb pattern likely reflects the epidermal
rete riges surrounding the dermal papillae. The color difference is
not due to variations in melanocyte density, but rather to the
cumulative amount of melanin in the deeper layers of the epidermis.
On top of dermal papillae, the melanized epidermal layers are
parallel to the plane of the dermoscopic visualization. By
contrast, in the epidermal rete ridges, these layers are almost
vertically oriented. Hence, the observation cumulates the melanin
content on a thicker amount of melanosome-laden keratinocytes.
It is acknowledged that mechanical forces applied parallely to
the skin surface have an impact on the three-dimensional aspect of
the dermo-epidermal junction [21, 22]. The epidermal rete riges
oriented perpendicularly to the force direction become smooth and
even flatten, thus decreasing their dermoscopic color. By contrast,
those rete ridges parallel to the force deepen both their structure
and their dermoscopic color.
The hypothesis of melanocyte mechano-transduction and activation
[8, 10, 12] could further explain the typical laddering melanotic
pattern. Indeed, it has been shown that mechanical pressure and
tension induce melanin formation by human melanocytes both in
culture [8] and in vivo [10, 12]. In the present physiological
conditions, the melanization process could theoretically be boosted
in the epidermal rete ridges oriented in the direction of lower
RRTM values. To be clinically relevant, the tension-related
activation of melanin synthesis should be accompanied by a
facilitated transfer of melanosomes to keratinocytes. As a result,
skin anisotropy becomes readily visible.
The present observation sheds some light on one clinical
expression of natural skin tension lines. Dermoscopy in patients of
darker complexion could identify those with higher intrinsic skin
tension, probably at risk of developing abnormal scarring following
dermatologic and plastic surgery.
Acknowledgments
This work was supported by a grant from the Fonds d’Investissement
de la Recherche Scientifique of the University Hospital of Liège.
No other sources of funding were used to assist in the preparation
of the manuscript. The authors appreciate the excellent secretarial
assistance of Mrs. Ida Leclercq.
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