Abstract
Based on previous reports on the subscarpal lipo-aponeurotic system (SLAS), the authors describe their experience with biplanar lipo-abdominoplasty. The SLAS was found to serve as an excellent flap for tightening the abdominal wall and improving the abdominal contour. The purpose of this study was to explore the anatomy of the SLAS, its biomechanical properties, and its microscopic composition using tissue analysis and morphometric quantification. Cadaver dissections were performed on preserved cadavers in the anatomic laboratories of the faculty of medicine. Tissue samples were also obtained from abdominoplasty procedures conducted on living patients. These tissues then underwent mechanical analyses and pathological characterization. Average nominal stress–strain curves demonstrated that for each given strain measurement, SLAS samples exhibited significantly higher stress than the subcutaneous samples, at a statistically significant level (P < .001). Average collagen area percent per 100 microscopic fields was found to be 12 ± 1.4 in the subcutaneous fields and 19.7 ± 2.4 in the corresponding SLAS fields (P = .013). Individual variation in information entropy in the tissue samples demonstrated a statistically significant increase in the SLAS sample when compared with the corresponding subcutaneous tissue, with an average of 1.59 ± 0.27 and 1.46 ± 0.25, respectively (P < .0001). SLAS exhibited unique biomechanical, histopathological, and anatomical properties in this study. The unique anatomical qualities of SLAS are important to consider in abdominal wall surgeries.
Level of Evidence: 5 (Therapeutic)
Based on previous reports on the subscarpal lipo-aponeurotic system (SLAS), the authors describe their experience with biplanar lipo-abdominoplasty.1 In a refinement of traditional abdominoplasty, the SLAS was utilized as a flap to decrease tension on the more superficial portion of the abdominal wall and thus improve postoperative tissue perfusion, lymphatic drainage, and the abdominal contour.
Biplanar lipo-abdominoplasty demonstrated positive outcomes when compared with our previous experience with single-plane abdominoplasty. We hypothesized that the SLAS, through its histopathological structure, possessed viscoelastic properties that warranted further investigation.
The SLAS has previously been described in the literature as subscarpal fat, but has not been further studied or examined. Our experience demonstrated its applicability and utilization in abdominal surgery, and thus, we suggested that it should be regarded as an independent layer with characteristics that differ from those of typical fat.
Preliminary description of the SLAS in the previous report1 focused on a macroscopic evaluation of the tissue, and on radiographic studies using contrast enhanced computed tomography scans. The SLAS was described as a circumferential layer under the Scarpa's fascia overlying the anterior rectus sheath and external oblique fascia, and extending posteriorly to the spinous processes.
Morphologically, the SLAS appeared to be more fibrotic than the subcutaneous tissue, with a great number of fibers parallel to the fascia fibers. The ample blood vessels and capillaries in the SLAS appeared larger than those in the overlying subcutaneous layer (Video 1).
In this study, we gained further insights into the histological structure of the SLAS and its mechanical properties. The purpose of this study was to define the anatomical borders of the SLAS in preserved cadavers, histologically assess the morphometric structures and content of the layer, and understand the biomechanical properties that it possesses.
Insights gained from this study augment the current literature on the abdominal wall, including its anatomical structures, and illustrate the usefulness of the SLAS in abdominal wall surgeries.
METHODS
Ethical Approval
This study was conducted after appropriate approval had been received by the Hillel Yaffe Medical Center IRB (0145-21-HYMC).
Cadaver Dissections
A total of 10 preserved cadaver dissections were performed on 7 males and 3 females. The average age of the cadavers was 74.5 years, with an average weight of 68.9 kg, BMI of 23.09, and height of 171.0 cm. Dissections were facilitated using magnification loupes at a 3× magnification level. Photographic documentation was obtained using a Samsung smartphone dual-lens camera with a 12 MP F/1.8, 27 mm (wide), and F/2.2, 123 (ultra-wide) lenses.
Tissue Samples
Equally sized soft-tissue samples were obtained from the resected abdominal wall specimens of 6 female patients who underwent abdominoplasty procedures. Six samples were obtained from each patient according to the locations depicted in Figure 1. Locations were marked as R1 to R3 on the right side and L1 to L3 on the left side of the resected abdominal wall specimens.
Figure 1.
Representation of the areas from which the samples were obtained from each abdominoplasty specimen, marked as R1-3 for the right side and L1-3 for the left side.
The subcutaneous tissue, the hypodermis, and the SLAS were obtained from the same locations in each patient; thus, for each subcutaneous tissue sample, there was a compatible SLAS tissue sample.
Histological Analysis of Pathological Samples
Computerized Morphometry
The tissue sections containing the skin, subcutaneous fat, Scarpa's fascia, and underlying SLAS were stained by routine hematoxylin and eosin staining as well as by Masson trichrome staining to highlight the amount of mature collagen (Type 1) within these sections.
The microscopic images stained by Masson trichrome, which contained the subcutaneous and SLAS samples from each specimen, were scanned using a light microscope (Olympus BX 43, Olympus Corporation, Tokyo, Japan) and digitized at 2× magnification power using a high-resolution camera (Topica) attached to the microscope.
Morphometric Quantification of the Collagen Within the Microscopic Fields
Topica software was used to measure the total tissue within each field. Subsequently, within the same field, the collagenous septa were also measured. The percentage of collagen area relative to the total area of the fatty field was subsequently computed.
Morphometric Analysis of the Structural and Textural Patterns Within the Microscopic Fields
For texture and pattern analysis of the subcutaneous and SLAS-derived microscopic images (containing fat and collagen), we used the Mazda program. The Mazda program was developed by Szczypiński et al at the Lodz University of Technology, and quantifies hidden patterns in images.2
Because each digitized image is represented by a matrix of pixels where each pixel has a certain gray level that ranges from 0 (darkest pixel) to 255 (brightest pixel), the Mazda program was able to analyze and quantify the textures and patterns of these pixel matrices (bitmaps) that composed each microscopic image. The textural and pattern-derived variables that are measured by the Mazda program can be grouped and analyzed through 6 different mechanisms: histogram, gradient, run-length matrix, co-occurrence matrix, autoregressive model, and wavelet transform.3
Information entropy, a measure of information content in an image, was evaluated and quantified for each individual image.
Mechanical Attribute Analyses
We measured the cross section of each sample (height × width) as well as the length of each tissue sample. The length was measured grip to grip. The measurement of the specimen dimensions revealed a lack of uniformity among all the specimens. Therefore, the cross section of the samples that was used in the calculations was considered as the average cross section to obtain representative figures.
All dimensions, including length, width, and thickness, were measured with a ruler and a caliper, respectively, with an estimated experimental error of ±0.5 mm. Representative calculated average dimensions of the specimens were: length: 30 mm, width: 27 mm, and thickness: 7 mm.
The fat tissues were gripped at both ends with specifically designed grips. The grips were comprised of 2 opposite clenches with 10 pins on each side that pierced and held the edges of the specimen. The pins were then inserted into holes on the opposite clench and held in place by 2 bolts. The experimental setup is shown in Figure 2. The specimens did not break adjacent to the grips, and the reported stress–strain curves were not biased by the fixtures.
Figure 2.
The experimental setup specifically designed for mechanical analysis of the tissues.
The tensile machine that we used was an Electropuls E10000 (Instron, Norwood, MA). The tests were conducted under displacement control at a crosshead velocity of 20 mm/min. The typical strain rate of the experiments was 0.67/s, based on a 30 mm representative gauge length.
The measured load-displacement data were translated into nominal stress–strain curves using the original specimen's gauge length and cross section.
RESULTS
Cadaver Dissections
The dissections involved multiple approaches. Starting from the midaxillary line, the dissection extended from the level of the xiphoid process down to the level of the symphysis pubis. A mid-abdominal horizontal incision was made at the level of the umbilicus, and a horizontal dissection was performed at the level of the upper border of the buttocks.
Anteriorly, the skin and subcutaneous tissue were elevated along the level of the Scarpa's fascia, followed by the elevation of Scarpa's fascia itself, thereby exposing the superficial surface of the SLAS. At this stage, the SLAS was traced to its visible borders and dissected from the muscular fascia.
The general configuration of the SLAS was observed to be a circumferential tube-like layer that condensed at the midline, anteriorly blending with the linea alba, and posteriorly merging with other torso layers, namely Scarpa's fascia and Camper's fascia. A 3-dimensional demonstrative model was printed and is shown in Video 2.
Abundant perpendicular fibers were identified running from the SLAS to the subcutaneous tissue and the superficial fascia, predominantly in the midline area, both anteriorly and posteriorly.
Anteriorly, the SLAS covered the rectus abdominis muscles, the external oblique muscles and aponeurosis, and a portion of the serratus anterior muscle. Posteriorly, it covered the latissimus dorsi, the lower part of the trapezius, the lower posterior part of the external oblique, the thoracolumbar fascia, and the inferior lumbar triangle (Petit's triangle; Figures 3, 4).
Figure 3.
Posterior view of a horizontal dissection, at the level of the “Lumbar fat pad,” demonstrating the subscarpal lipo-aponeurotic system, Scarpa's fascia, subcutis, and skin.
Figure 4.
Posterolateral view of the subscarpal lipo-aponeurotic system (SLAS). The skin, subcutaneous tissue, and Scarpa's fascia are elevated above the SLAS.
In the lumbar triangle of Petit, located between the inferior border of the latissimus dorsi muscle and the posteroinferior border of the external oblique, the SLAS thickened to merge with the lumbar fat pad (Figure 3).
The superior border of the SLAS was observed to be at the level of the lower rib cage, the origin of the rectus abdominis, the insertion of the pectoralis major into the lower 3 anterior chest ribs, and the serratus anterior. The lower border was along the inguinal ligament and the symphysis pubis midline, continuing backwards to the iliac crest. Posteriorly, the superior border corresponded to the lower tip of the scapula and the superior border of the latissimus dorsi (Figure 4).
The inferior border was at the level of the iliac crest and the inguinal ligament. The superior border of the SLAS transitioned into the deep soft tissue beneath the superficial fascial system. Anteriorly, it eventually merged with the areolar tissue under the breast and over the pectoralis major. Posteriorly, it blended with the trabeculated thick skin characteristic of the upper back (Figure 5).
Figure 5.
Posterolateral view of the skin and subcutaneous flap, elevated from the Scarpa's fascia which is held by forceps above the subscarpal lipo-aponeurotic system. Perpendicular fibers extending from the flaps are shown inserting into the medial portion of the Scarpa's fascia.
The SLAS appeared thicker in its lower abdominal part, extending laterally to the area of the lumbar fat pad and continuing to the posterior midline. It appeared thinner and more condensed in the upper part above the umbilicus, as well as when covering the upper portion of the latissimus dorsi and trapezius muscles in the back area (Figures 5, 6).
Figure 6.
The subscarpal lipo-aponeurotic system is elevated from lateral to medial, exposing the latissimus dorsi muscle, the lumbar fascia, and condensation of posterior spinous processes.
Histopathological Analysis
Average collagen area percent per 100 microscopic fields was found to be 12 ± 1.4 in the subcutaneous fields and 19.7 ± 2.4 in the corresponding SLAS fields (P = .013). These findings are graphically depicted in Figure 7.
Figure 7.
Graph bars displaying the average collagen percentages in both subcutaneous tissue and subscarpal lipo-aponeurotic system (SLAS) microscopic fields. A significantly higher amount of collagen is seen in the SLAS vs the subcutaneous microscopic fields.
Individual variation in information entropy (a measure of information content in an image) in the tissue samples demonstrated a statistically significant increase in the SLAS samples when compared with the corresponding subcutaneous tissues, with an average of 1.59 ± 0.27 and 1.46 ± 0.25, respectively (P < .0001). The findings are graphically depicted in Figure 8.
Figure 8.
A line graph displays the individual variation of the information entropy between the subcutaneous tissue and subscarpal lipo-aponeurotic system (SLAS) microscopic fields in matched patients. The significant increase in information entropy in SLAS signifies more information and variation observed in SLAS fields compared with the subcutaneous fields in the same patient.
Sample staining with Masson's trichrome of the SLAS and the corresponding subcutaneous tissue is presented in Figure 9.
Figure 9.
Two representative Masson's trichrome stained microscopic images from the subcutaneous tissue and subscarpal lipo-aponeurotic system (SLAS) areas are seen. The image from the SLAS microscopic field shows an increased amount of longitudinally crossing collagenous septa, which are also thicker than those seen in the subcutaneous tissue microscopic field.
Biomechanical Properties
Figure 10 shows the average nominal stress–strain curves for the 2 types of tested tissues. Those results are averaged from a total of 36 specimens per tissue type. Considering the standard deviations of the measurements, it is most noticeable that the SLAS exhibited a much higher degree of variability than the subcutaneous tissue. Inherent variability cannot be avoided given the different origins of the tissues, whereas the variability in specimen dimensions is not likely to be the dominant factor.
Figure 10.
Stress–strain curves of corresponding areas from the subcutaneous tissue and the subscarpal lipo-aponeurotic system tissue.
For a strain level of 0.6 (equivalent to 60% lengthening from the initial length), the applied stress on subcutaneous tissues reached ∼0.6 × 105 (Pa). In stark contrast, under the same strain conditions, SLAS tissues experienced significantly higher stress, ∼1.5 × 105 (Pa) (P < .001; Figure 10).
Consequently, a conservative estimate based on the average values of the stress–strain curves indicated that the strength of the SLAS was approximately twice that of the subcutaneous tissue, with a comparable degree of ductility to failure (the deformation of material before failure or rupture).
To gain further insights into the mechanical differences between the paired tissues, we analyzed their stress–strain curves separately. Most of the examined samples consistently displayed higher stress levels in the SLAS tissues compared with the subcutaneous tissues. These findings indicate that the SLAS tissue exhibits superior mechanical properties, showcasing its ability to withstand greater tensile forces before deformation or failure occurs when compared with the subcutaneous tissue. However, it must be considered that those tissues are bound to each other, and more information would be needed to understand how they withstand the loads, although it is reasonable to assume they are loaded in parallel. In this case, each tissue withstands a different stress level, but both extend by the same measure.
DISCUSSION
The SLAS was previously defined as a lipo-aponeurotic layer, which lies underneath the Scarpa's fascia and overlies the anterior rectus sheath and the external oblique fascia. In this study, we aimed to understand the pathological structure of the tissue and its mechanical attributes. Furthermore, in our study of this novel layer and its defining properties, we also wished to attain insight into the clinical applicability and potential utilization of the SLAS in abdominal surgery.
Morphometric quantification of collagen content and structural patterns in the SLAS demonstrated a substantially higher collagen percentage per field when compared with the corresponding subcutaneous tissue. Moreover, individual variation on information entropy was greater in the SLAS samples, which indicated more structural elements per microscopic field. In the context of collagen analysis, the higher entropy suggests greater collagen content formed in thicker morphometric structures.
To obtain a better understanding of the impact of the morphometric characteristics on the mechanical attributes of the tissue, stress–strain curves were generated from the results of force-displacement tensile experiments. When plotted on stress–strain curves, the SLAS experienced significantly higher stress during equal strain. In simple terms, for an equal degree of lengthening of the tissue from its initial state, the stress required to do so on the SLAS was substantially higher. In addition, the ductility to failure occurred at a comparable degree, signifying that the SLAS is able to withstand greater tensile forces before deformation, failure, or tearing occurs.
The potential clinical significance of the mechanical properties found in the SLAS is worth noting. In the context of abdominal surgeries, SLAS utilization as a flap allows for potential reinforcement of the abdominal wall and can be used to improve not only its contour but also its functionality.
After abdominal surgery, patients are at risk for abdominal wall weakness and herniation of intra-abdominal contents. This outcome, in addition to decreasing the quality of life that patients experience in the postoperative period, can result in incarcerated hernia formation with potential for organ ischemia and hemodynamic compromise.4-8 Therefore, reinforcement of the abdominal wall musculature using the SLAS as a flap can potentially improve the long-term outcomes of abdominal surgery and allow for better postoperative rehabilitation.
We hypothesized, based on our experience, that the SLAS serves a similar role in abdominoplasties as does the facial superficial musculo-aponeurotic system (SMAS), when used as an anchor in biplanar rhytidectomies. The popularity of SMAS utilization in rhytidectomies is attributed to its ability to create greater tension and lateral pull to facial and cervical flaps with less long-term stress-relaxation and tissue creep.9-12
In a biomechanical and viscoelastic study on flaps dissected from 8 fresh cadavers, Saulis et al found that the SMAS layer exhibits significantly less creep compared with a skin flap, a quality that serves aesthetic surgeons well during rhytidectomies.13
Similarly, based on our biomechanical analysis, the uniqueness of the SLAS lies in its ability to withstand greater tension without deformation when compared with the subcutaneous tissue. In addition, we found greater collagen concentration in the SLAS layer compared with corresponding areas of the overlying subcutaneous layer. Similarly, collagen percentage was recognized to be especially high in the SMAS layer, which leads to the praised viscoelastic properties attributed to it.13,14 Consequently, in addition to characterizing the anatomical, biomechanical, and histopathological nature of the SLAS, we also found it to have properties similar to the commonly utilized SMAS layer.
Although we present the results of a thorough and multifaceted study, it is crucial to disclose and discuss limitations of our work. First, the sample size of cadavers used for this study was limited. The sample size was the result of ethical considerations on the use of human cadavers for anatomical studies and the required heterogeneity of patients for reproducible and meaningful results. In addition, samples obtained from abdominoplasty specimens at various locations inevitably resulted in inconsistent sample dimensions, although appropriate measures were taken to ensure the accuracy of the results as best as possible.
Another interesting research path to follow would be to compare the SLAS to the Scarpa's fascia and derive conclusions on notable differences between the layers.
CONCLUSIONS
We describe the biomechanical, histopathological, and anatomic properties of the SLAS, an anatomic layer in the abdominal wall that has previously not been well-described. We believe that its unique anatomic qualities are important to consider in abdominal wall surgeries. Further research is required to better understand the SLAS's embryologic origin, clinical applications, and communications with abdominal wall musculature so that this newly defined anatomical layer's potential clinical used may be ascertained.
Supplementary Material
Supplemental Material
This article contains supplemental material located online at https://doi.org/10.1093/asjof/ojaf034.
Disclosures
The authors declared no potential conflicts of interest with respect to the research, authorship, and publication of this article.
Funding
The authors received no financial support for the research, authorship, and publication of this article.
REFERENCES
- 1.Wolf Y, Weissman O, Dima H, Sandbank J, Fainzilber-Goldman Y. Biplanar lipoabdominoplasty: introducing the subscarpal lipo aponeurotic system. Plast Reconstr Surg Glob Open. 2022;10:e4000. doi: 10.1097/GOX.0000000000004000 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Szczypiński PM, Strzelecki M, Materka A, Klepaczko A. Mazda—a software package for image texture analysis. Comput Methods Programs Biomed. 2009;94:66–76. doi: 10.1016/j.cmpb.2008.08.005 [DOI] [PubMed] [Google Scholar]
- 3.Zhang L, Lyu Q, Ding Y, Hu C, Hui P. Texture analysis based on vascular ultrasound to identify the vulnerable carotid plaques. Front Neurosci. 2022;16:885209. doi: 10.3389/fnins.2022.885209 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Ortega-Deballon P, Renard Y, de Launay J, Lafon T, Roset Q, Passot G. Incidence, risk factors, and burden of incisional hernia repair after abdominal surgery in France: a nationwide study. Hernia. 2023;27:861–871. doi: 10.1007/s10029-023-02825-9 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Gignoux B, Bayon Y, Martin D, et al. Incidence and risk factors for incisional hernia and recurrence: retrospective analysis of the French national database. Colorectal Dis. 2021;23:1515–1523. doi: 10.1111/codi.15581 [DOI] [PubMed] [Google Scholar]
- 6.Huckaby LV, Dadashzadeh ER, Handzel R, Kacin A, Rosengart MR, van der Windt DJ. Improved understanding of acute incisional hernia incarceration: implications for addressing the excess mortality of emergent repair. J Am Coll Surg. 2020;231:536–545.e4. doi: 10.1016/j.jamcollsurg.2020.08.735 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Hrebinko KA, Huckaby LV, Silver D, et al. Predictors of acute incisional hernia incarceration at initial hernia diagnosis on computed tomography. J Trauma Acute Care Surg. 2024;96:129–136. doi: 10.1097/TA.0000000000003994 [DOI] [PubMed] [Google Scholar]
- 8.Martis G, Rózsahegyi M, Deák J, Damjanovich L. Incarcerated and eventrated abdominal wall hernia reconstruction with autologous double-layer dermal graft in the field of purulent peritonitis—a case report. Int J Surg Case Rep. 2017;30:126–129. doi: 10.1016/j.ijscr.2016.12.002 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Trussler AP, Hatef D, Broussard GB, Brown S, Barton FE. The viscoelastic properties of the SMAS and its clinical translation: firm support for the high-SMAS rhytidectomy. Plast Reconstr Surg. 2011;128:757–764. doi: 10.1097/PRS.0b013e3182221354 [DOI] [PubMed] [Google Scholar]
- 10.Hu X, Wang Z, Wang Q, Zhang C, Hu G, Qin H. Are there differences between the upper and lower parts of the superficial musculoaponeurotic system? A preliminary biomechanical study. Aesthet Surg J. 2014;34:661–667. doi: 10.1177/1090820X14528947 [DOI] [PubMed] [Google Scholar]
- 11.Har-Shai Y, Bodner SR, Egozy-Golan D, et al. Mechanical properties and microstructure of the superficial musculoaponeurotic system. Plast Reconstr Surg. 1996;98:59–70; discussion 71-73. doi: 10.1097/00006534-199607000-00009 [DOI] [PubMed] [Google Scholar]
- 12.Angelos PC, Brennan TE, Toriumi DM. Biomechanical properties of superficial musculoaponeurotic system tissue with vs without reinforcement with poly-4-hydroxybutyric acid absorbable mesh. JAMA Facial Plast Surg. 2014;16:199–205. doi: 10.1001/jamafacial.2013.2738 [DOI] [PubMed] [Google Scholar]
- 13.Saulis AS, Lautenschlager EP, Mustoe TA. Biomechanical and viscoelastic properties of skin, SMAS, and composite flaps as they pertain to rhytidectomy. Plast Reconstr Surg. 2002;110:590–598; discussion 599-600. doi: 10.1097/00006534-200208000-00035 [DOI] [PubMed] [Google Scholar]
- 14.Har-Shai Y, Sela E, Rubinstien I, Lindenbaum ES, Mitz V, Hirshowitz B. Computerized morphometric quantitation of elastin and collagen in SMAS and facial skin and the possible role of fat cells in SMAS viscoelastic properties. Plast Reconstr Surg. 1998;102:2466–2470. doi: 10.1097/00006534-199812000-00033 [DOI] [PubMed] [Google Scholar]
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