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发表时间:2015-12-08 浏览次数:541次
Introduction
Wound contraction is a normal physiological phenomenon reducing the area
of a skin defect and therefore expediting its closure. This contraction
is based on scar contraction and myofibroblast activity; all originate
from granulation tissue that develops during the 1 st week of
the inflammatory process, part of the normal wound-healing course. The
application of skin grafts to fresh skin defects has been proven to
reduce wound contraction and hypertrophic scarring compared with
full-thickness wounds that have been left to granulate and heal by
secondary intention alone. [1],[2]
However, skin grafts can also contract, resulting in a compromised
esthetic outcome and restricted mobility of the joints involved.
Skin
graft contraction occurs in two stages: primary and secondary
contraction. Primary contraction refers to the immediate reduction in
size of the skin graft, directly after it has been harvested from its
donor site. Primary contraction is due to passive recoil of the elastin
fibers in the dermis and is, therefore, dependent upon the thickness of
the graft. Full-thickness skin grafts (FTSGs) contain large volumes of
elastin-containing dermis and consequently exhibit the greatest degree
of primary contraction. Due to the reduced volume of dermis included,
spilt-thickness skin grafts (STSGs) exhibit less contraction, whereas
pure epidermal grafts do not contract. [3]
Secondary contraction is due to a wound bed contraction. This secondary
contraction reduces both the size of the graft at the interface with
its recipient bed and the circumference of the graft at its periphery. [1],[4]
Traditionally, it is accepted that the degree of secondary contraction
is inversely related to the thickness of the graft of FTSGs to minimize
the extent of secondary contraction. [5]
Studies have shown that a granulating recipient bed, burn size, young
age of the patient, anatomical area and grafting over mobile tissues may
prompt skin graft contraction. [1],[5],[6],[7]]
Skin grafting is a major element of reconstructive surgery. It is,
therefore, important that every aspect of its practice is thoroughly
investigated and evaluated. Primary contraction was first described and
assessed by Davis and Kitlowski in 1931. [3]
The authors of that pioneering study used human specimens, a practice
that has some limitations regarding the number, size and shape of skin
grafts. Furthermore, although > 80 years have elapsed, there have not
been any studies that further looked further into the development and
cause of primary contraction. In this preliminary in vivo porcine
study, we assessed the degree of primary skin graft contraction and
investigated whether the shape of skin grafts affects the degree of
contraction.
Methods
The study was conducted at an accredited animal research facility (CRO,
Lahav Research Institute, Lahav, Israel) following national and
institutional guidelines for the care and use of laboratory animals. [8]
The animals were anesthetized female domestic pigs (Susscrofa),
weighing approximately 30 kg. The study was approved by the
Institutional Animal Care and Use Committee.
The study consisted
of two parts. In the first part, we compared the contraction of FTSGs
and STSGs. Four animals participated in the study, and a total of 67
specimens of skin grafts were harvested from the back of the pigs, 41
and 26 of FTSGs and STSGs, respectively.
For the study of the FTSG contraction, circles of 4 cm diameter were drawn with a permanent marker on the skin of each pig [Figure 1]. The area of the circles marked was calculated using the formula: area = πr2 , where r = radius of the circle and π = 3.14, resulting in the area value of 12.56 cm2.
The skin was excised with a No. 15-blade in a circle shape and
meticulously defatted. The FTSGs were placed on a flat surface 15 min
after skin graft harvesting to facilitate complete primary contraction;
their diameter was measured in 3 axes (with an axis-to-axis angle of
120°) and the average diameter was used to calculate the radius of the
circle [Figure 1]. The mean value of the diameter was computed, and the surface was again calculated using the formula: area = πr2.
For the study of STSG contraction, a rectangle stripe of 4 cm height and
40 cm width was drawn with a permanent marker on the pig. Additional
lines were drawn vertically to produce 10 squares of equal 4 cm sides [Figure 1]. The area of each square was calculated using the formula: area = s2 , where s = side of the square and the initial area value of 16 cm2
were recorded. All STSGs were harvested with a dermatome adjusted to
0.014 inch skin thickness, corresponding to moderate to thick skin
grafts. The stripe was cut into squares according to our drawings, and
the grafts were placed on a flat surface and left there for 15 min
before measuring [Figure 1].
Due to the primary contraction, the initial squares changed into
rectangles. Each side of the contracted STSGs was, therefore, measured,
and the surface was calculated using the formula: area = h × w, where h = height and w = width.
In
the second part, we examined whether the shape of the skin graft
affected the degree of contraction. For that purpose, we took a total of
27 specimens, harvested from the back of a single pig [Figure 2].
Nine of these specimens were circle-shaped FTSGs, 6 were square-shaped
FTSGs, 8 circle-shaped STSGs and 4 square-shaped STSGs. The harvesting
and calculation of the area of FTSGs and that of square-shaped STSGs
were made in the same way as in the first stage of our study, with the
initial surface area being 12.56 cm2 and 16 cm2 for the circle- and square-shaped specimens, respectively. For
harvesting circle-shaped STSGs, we used a dermatome to create a "skin
belt" graft, which was subsequently cut into a circle-shaped skin graft
with scissors. All STSGs were harvested with a dermatome adjusted to
0.014 inch skin thickness [Figure 2].
Statistical analysis was performed using Microsoft Excel 2003® (Microsoft, Redmond, Washington, USA) and SPSS® version 14 (IBM-SPSS Inc., New York, USA). Statistical tests used Pearson's Chi-square test and Student's t-test.
Results
In the first part of the study, comparison was made between the contractions of FTSGs and STSGs. The initial surface area of the FTSG specimens was 12.56 cm2. After excision and defatting, the area values of the contracted skin grafts ranged from 12.6 cm2 (0% shrinkage) to 9.3 cm2 (25.6% shrinkage). The mean area value of the FTSGs after primary contraction was 11.1 cm2 and the median was 11.0 cm2. The mean percentage of graft shrinkage was 12.0% and the median was 12.2% [Table 1]. The initial surface area of the STSG specimens was 16 cm2. After harvesting with the dermatome, the area values of the contracted skin grafts ranged from 16 cm2 (0% shrinkage, 1 specimen recorded) to 13.30 cm2 (17% shrinkage). The mean area value of the STSGs after primary contraction was 14.9 cm2 and the median was 15.2 cm2. The mean percentage of graft shrinkage was 6.9% and the median was 5.0% [Table 1]. FTSGs presented greater primary contraction than STSGs at a statistically significant level (P = 0.0011) [Table 1].
In the second part, the role of the skin graft shape on primary skin
contraction was investigated. The initial surface area of the
circle-shaped specimens of both FTSGs and STSGs was 12.6 cm2 and the initial surface area of the square-shaped specimens was 16.0 cm2. After excision and defatting, the circle-shaped FTSGs had a calculated surface range from 12.3 cm2 (2.0% shrinkage) to 11.3 cm2 (9.8% shrinkage). The mean area value of the circle-shaped FTSGs was 11.8 cm2 and the median was 11.7 cm2. In this group, the mean percentage of graft shrinkage was 5.8% and the median was 6.9% [Table 2]. On the other hand, the square-shaped FTSGs ranged from 15.6 cm2 (2.5% shrinkage) to 14.8 cm2 (7.4% shrinkage). The mean area value of the square-shaped FTSGs was 15.3 cm2 and the median was 15.4 cm2. In the square-shaped FTSGs group, the mean percentage of graft shrinkage was 4.2% and the median was 3.8% [Table 2].
The comparison of primary contraction values between square- and
circle-shaped FTSG specimens was not statistically significant (P = 0.14). The circle-shaped STSG specimens demonstrated a primary contraction ranging from 12.6 cm2 (0% shrinkage) to 11.9 cm2 (5.0% shrinkage). The mean area value of the circle-shaped STSGs was 12.4 cm2 and the median was 12.6 cm2. In the circle-shaped STSGs group, the mean percentage of graft shrinkage was 1.1% and the median was 0% [Table 2]. The square-shaped specimens showed primary contraction ranging from 16.0 cm2 (0% shrinkage) to 15.8 cm2 (1.3% shrinkage). The mean and median area values of the square-shaped STSGs were both 16.0 cm
2. In the square-shaped STSGs group, the mean percentage of graft shrinkage was 0.31% and the median was 0% [Table 2]. The different shrinkage rates between square- and circle-shaped STSG specimens were not statistically significant (P = 0.33).
Discussion
Skin graft contraction is a common problem resulting in significant
morbidity with restriction of joint mobility and cosmetic complications,
often requiring multiple corrective operations. Secondary contraction
has received the most research emphasis due to the fact that it is
clinically more important than primary contraction. Secondary
contraction often results in severe effects on body function or patient
appearance. Studies on the cellular activity underlying skin graft
contraction support a most probable theory that the contraction occurs
secondary to the differentiation of fibroblasts to myofibroblasts with
expression of α-actin filament bundles which exert an inward pull on the
wounds edges. [1],[9],[10]
The myofibroblasts have contractile properties similar to smooth muscle
cells and organize their actin cytoskeleton along the lines of greatest
skin tension. [1],[11]
As the myofibroblasts are adherent both to one another and to the
fibronectin-rich wound bed, the entire mass of granulation tissue
contracts. [1]
Keratinocytes may also play a distinct role at the early stages of
contraction, since studies have shown that keratinocytes are capable of
inducing collagen gel contraction in vitro. [12],[13],[14],[15]
The actinfilament organization within keratinocytes at the wound margin
appears to be responsible for the epidermal "purse-string phenomenon". [14]
In addition, cytokines and growth factors such as ransforming growth
factor-β1, insulin-like growth factor and fibroblast growth factors have
also been found to play a major role in secondary contraction. [15]
Unlike secondary contracture, which is the result of a prolonged
biological process, primary skin graft contraction is mainly an
immediate physical change in graft dimensions mediated by the tough
fibrous layer of the dermis, which is primarily composed of collagens,
glycosaminoglycans and elastins.
Davis and Kitlowski [3]
were the first to study the primary contraction of skin grafts. The
authors used skin grafts from patients of various age and donor sites
and recorded the percentage of skin contracture in relation to the
thickness of the graft. Their results showed that, regarding the "whole
thickness skin grafts" (FTSGs), the mean amount of shrinkage was 43.6%
with little variations according to the donor site. The "half thickness
skin grafts" (mid thickness split thickness STSGs) were presented with a
mean shrinkage of 24.86% and the "thick Ollier-Thiersch grafts" (small
grafts with thinner periphery and thicker-centered STSGs) with a mean
shrinkage of 11.26% and 11.95% for abdominal and thigh donor sites,
respectively. The very thin "true Ollier-Thiersch grafts" (thin
thickness STSGs) demonstrated a greatly reduced primary contraction of
1.24%. [3]
According to the authors, the shrinkage observed was in direct relation
to the amount of dermis included in the harvested skin grafts. Using
specimens from humans, however, had the limitation that the grafts and
their donor sites could not be standardized according to the site, size
and shape of the examined grafts. Homogeneity of the samples was further
compromised due to variables like gender and age of the studied
subjects.
Other authors had previously referred to the etiology
of skin graft contraction, coming to the conclusion that the network of
elastic fibers of the dermis is responsible for its ability to stretch
under the movement of the underlying tissues, as well as for the
shrinkage of the skin graft. [16]
In Ragnell's study on the secondary contracting tendency of free skin
grafts, the elasticity of circular pieces of rabbit skin was estimated
using a manometer device. The author concluded that rabbit skin
presented uniform elasticity, but no further studies on primary skin
graft contraction were performed. [16]
Skin is a very complex, integrated, dynamic organ that has many
functions. In mammals, the primary functions of the skin include
insulation and temperature regulation, although the role of the skin as
an endocrine organ and a critical component of the immune system cannot
be ignored. [17]
Species differences in all of these functions may dramatically alter
skin behavior regarding its mechanical characteristics or drug
absorption. When barrier, pelage, vascular, endocrine and immunological
properties are considered en masse, pigskin is very similar to human skin. [17]
Pigskin resembles human skin in both structure and function, having
similar sparse hair coating, a relatively thick epidermis, similar
turnover kinetics, lipid composition, carbohydrate biochemistry, lipid
biophysical properties, and - what is most relevant to the present study
- a similar arrangement of dermal collagen and elastic fibers. [9],[18] All these similarities establish the pig to be an essential model in cutaneous research. Since in vivo
experiments on primary skin grafts would require grafts of different
shapes, minimum dimensions of 2 or 3 cm, symmetrical locations of the
grafts and suitable controls, human experimental material is not
available.
In our study, the skin grafts were harvested from
pigs, resulting in standardized specimens in terms of size, shape and
location of the donor site and at the same time, skin behavior close to
that of human skin. The substantial differences between our results and
the results reported by Davis and Kitlowski, [3]
however, point out the different primary contraction behavior of human
and porcine skin. Although similar in many ways, different thicknesses
and possibly different elastic properties between human and porcine skin
may lead to different contraction behaviors of skin grafts.
Furthermore, the rate of primary skin contraction probably depends on
donor site characteristics. Clinical experience shows that skin
harvested from the backs of patients presents limited contraction when
compared with skin grafts from other sites. Furthermore, there was one
specimen from the STSG group of the first part of the study that
surprisingly showed 0% shrinkage. Since 0% primary skin contraction
before is rather unusual, we believe that this behavior was related to a
specific donor site and will be further investigated in upcoming
studies. Another possible reason for such a discrepancy in skin graft
shrinkage could be technical difficulties: harvesting very thin skin
grafts with small amounts of dermismay have been responsible for graft
contraction.
The mean percentage of primary skin graft
contraction has found to be different in the two parts of the study,
with mean values for both FTSG and STSG contraction showing
inconsistencies between the study series. These unexpected differences
were probably due to the small number of specimens in the second part,
which did not yield statistically significant results. Furthermore, the
use of only one animal in the second part of the study may have
magnified the role of biological variation, a matter that will be
further investigated in future larger studies.
To the best of our
knowledge, primary skin contraction in relation to the shape of the
skin graft has never been investigated before. In our study the mean
graft shrinkage was 5.8% and 4.2% in circle-shaped and square-shaped
FTSGs, respectively [Table 2].
The difference was more notable in the STSGs, where the circle-shaped
specimens showed graft shrinkage of 1.1%, whereas the square-shaped
present a mere 0.31% [Table 2].
The difference recorded could be due to the relation of the line of
contraction with the skin tension lines. Square specimens have one
contraction vector that runs parallel to the direction of the skin
tension lines, whereas the round-shaped specimens have multiple
contraction vectors positioned at various angles to the skin tension
lines. Theoretically, the projection of the skin tension lines to the
radii of the circle-shaped specimens could possibly add to their total
graft shrinkage. Due to the small number of specimens, however, further
studies should be conducted in order to determine potential
statistically significant findings.
Mean percentage of primary
contraction for square STSGs was found to be 3.8 times higher than round
STSGs (4.2% and 11% for square and circular STSGs, respectively). The
recorded difference for the FTSGs, however, is not that prodigious.
Since this is only a preliminary report with a small number of specimens
involved, we believe that future studies will help to clarify the
issue.
Limitations regarding human specimens necessitate the use
of animal models; further studies are required in order to investigate
whether pigskin is suitable for the study of primary graft contraction.
The study cohort is limited and a larger series for all arms is needed
for a better understanding of these phenomena.
Skin grafts are
widely used and any information regarding their characteristics is
valuable. Our preliminary report reveals an expected increased shrinkage
of FTSGs compared to STSGs and in a limited number of specimens, the
shape of the skin graft seems to affect primary contraction of the
STSGs. Although it is difficult to dramatically change the shape of skin
grafts, if this feature is ultimately found to alter primary
contraction, the results could possibly be applied in clinical practice.
References
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