1. Introduction

Figure 1. Open and closed port entry are shown in this figure. Panel (a) displays an example of Hasson port entry. Image from Figure 1.3, Ref. [3]. Panel (b) demonstrates entry using the closer (Veress needle) technique and image from Figure 1.4, Ref. [3].

Figure 1. Open and closed port entry are shown in this figure. Panel (a) displays an example of Hasson port entry. Image from Figure 1.3, Ref. [3]. Panel (b) demonstrates entry using the closer (Veress needle) technique and image from Figure 1.4, Ref. [3].

2. Materials and Methods

2.1. Extraction of Abdominal Wall Tissues

Approaches for tissue dissection were developed with reference to Grant’s Dissector [13] and Gray’s Anatomy [14]. Tissues were extracted unilaterally from a site devoid of previous surgery. A skin incision was made along the midclavicular line extending from the inferior costal margin to approximately 10 cm inferior to the umbilicus. Camper’s and Scarpa’s fasciae were identified and divided sharply to expose the superficial surface of external oblique muscle. The plane superficial to the external oblique was entered bluntly and extended medially to the semilunar line, and laterally to raise a 10 cm flap of skin and subcutaneous fascia. The external oblique was divided sharply 1 cm lateral to the semilunar line and separated from the underlying internal oblique by blunt dissection. Blunt dissection was continued laterally to isolate approximately 10 cm of the external oblique muscle. The external oblique was divided superiorly and inferiorly such that it could be reflected laterally to allow access to the internal oblique. The above dissections were repeated for the internal oblique and transversus abdominus muscles. Following isolation of the anterolateral muscles, a 5x5 cm tissue sample was extracted from each. Finally, a 5x5 cm section of peritoneum (with transversalis fascia) was extracted from the lateral abdominal wall deep to transversus abdominus. In vivo orientation muscle samples were labeled and prepared for subsequent analysis.

Next, the anterior rectus sheath was divided medial to the semilunar line and above the umbilicus. Fibrous attachments to the tendinous intersections were divided sharply, and the anterior rectus sheath was reflected medially towards the linear alba. A section of rectus abdominus (between tendinous intersections) was lifted off the posterior rectus sheath. The anterior rectus sheath, rectus abdominus, posterior rectus sheath, musculofascial composites and linear alba were sampled.

All tissues were kept in their in vivo orientation and cut into rectangular samples in the transverse and longitudinal directions (Figure 3). Where possible, multiple samples of the same tissues were collected. In some instances (e.g., peritoneum or posterior rectus sheath), there were adhesions, which made it difficult to extract individual tissue layers. Table 2 summarizes the total number of samples taken from each tissue type.

Figure 1. Example of the external oblique muscle extracted in its in vivo orientation. The longitudinal and transverse axes demonstrate the orientation in which samples were cut for mechanical testing.

Figure 1. Example of the external oblique muscle extracted in its in vivo orientation. The longitudinal and transverse axes demonstrate the orientation in which samples were cut for mechanical testing.

Table 1. Summary of the tissues collected from each specimen (1-15). The samples highlighted in blue indicate FNF specimens.

Table 1. Summary of the tissues collected from each specimen (1-15). The samples highlighted in blue indicate FNF specimens.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	TOTAL
Anterior Rectus Sheath	2	2	2	2	2	2	4	2	4	2	2	4	4	4	4	42
Posterior Rectus Sheath	—	2	—	—	2	2	—	—	1	2	2	3	2	2	2	20
Linea Alba	—	2	2	2	—	2	2	2	2	2	2	—	—	—	4	22
Peritoneum	—	2	2	2	2	1	3	2	1	2	1	1	3	—	4	26
Rectus Abdominis	4	2	2	2	2	2	2	—	2	1	1	1	4	3	4	32
External Oblique	3	2	2	2	2	2	—	2	4	2	2	3	4	—	4	34
Internal Oblique	—	2	2	2	2	2	2	2	2	2	2	—	4	—	4	28
Transversus Abdominis	—	2	1	—	2	2	2	2	2	2	2	3	4	—	4	28
Composite	—	2	—	—	2	—	—	—	—	—	—	—	3	—	—	7
TOTAL	9	18	13	12	16	15	15	12	18	15	14	15	28	9	30	239

Tissue samples were adhered to the smooth side of 320 Grit Paper (3M, Maplewood, Minnesota, USA) using cyanoacrylate (Loctite ®, Henkel, Düsseldorf, Germany) to prevent slippage within the grips of the testing machine (Figure 4).

After samples were adhered to Grit Paper, a vernier calipers was used to measure sample gauge length, width, and thickness (± 0.02 mm). Tissue measurements are summarized in Table 3. All mechanical tests were performed at room temperature (18–21 °C) in the GAF, and tissues were kept hydrated with a specimen wetting solution prepared by the UQ GAF staff.

Table 2. Median (interquartile range [IQR]) of length, width, and thickness are summarized in the table below.

Table 2. Median (interquartile range [IQR]) of length, width, and thickness are summarized in the table below.

Tissue	Sample Size	Length (mm)	Width (mm)	Thickness (mm)
Anterior Rectus Sheath	42	14.25 (12.43)	7.22 (3.73)	0.59 (0.38)
Posterior Rectus Sheath	20	12.17 (7.98)	5.95 (2.38)	0.32 (0.39)
Linea Alba	22	7.87 (4.57)	5.90 (2.52)	0.88 (0.76)
Peritoneum	26	12.29 (7.31)	5.82 (2.35)	0.39 (0.29)
Rectus Abdominis	25	13.14 (4.66)	6.66 (2.48)	1.82 (0.74)
External Oblique	33	14.02 (7.31)	7.78 (3.96)	2.26 (1.28)
Internal Oblique	28	12.78 (6.74)	6.64 (2.22)	2.07 (0.92)
Transversus Abdominis	26	12.47 (6.57)	6.52 (3.47)	0.93 (0.53)
Composite	7	8.90 (2.50)	7.36 (1.36)	2.40 (0.48)

2.2. Tensile Testing

Tensile testing was conducted using a ST-1001 Universal Testing Machine (Salt, Incheon, Republic of Korea). The Universal Testing Machine (UTM) has a maximum load cell capacity of 5 kilonewtons (kN) (± 0.0333 newton [N] resolution), and data were recorded with a sample rate of 194 Hz. Samples were positioned in screw grips (Side-Action Grips 5kN, Salt Incheon, Republic of Korea) and tightened to “finger-tightness.”

Since the goal of this research is to select synthetic materials that match tissue properties, tissues were not pre-conditioned [8]. Strain rate was selected based on similar work [15], and was conducted at a quasi-static strain rate of 5 mm/min. Data collection began once tissues were pre-stressed to 0.2 N and were tested to failure. The point of failure was visually inspected and noted as being either upper third, middle third, bottom third, or within the grips (Figure 5). Tissue failure was defined as not occurring in the grips and as either of the following conditions: 1) a 90% drop in the applied load or 2) when the applied load fell to zero.

Figure 2. Example of muscle tissue failure within the UTM. The figure also shows the visual approximations of tissue failure. In Figure 5, the sample was noted to have failed in the middle third of the sample.

Figure 2. Example of muscle tissue failure within the UTM. The figure also shows the visual approximations of tissue failure. In Figure 5, the sample was noted to have failed in the middle third of the sample.

After testing was completed, load (N) and displacement (mm) outputs from the Light-Salt testing software (Salt, Incheon, Republic of Korea) were collected to generate stress-strain curves.

2.3. Tensile Data from Stress-Strain Curves

Using load and displacement outputs from the Light-Salt testing software, stress and strain were calculated. An assumption of incompressibility was made, meaning that the materials were not considered to be porous, and volume was conserved [16]. Therefore, engineering stress (σ) was calculated by dividing force (F) over the original cross-sectional area (A₀) (Equation 1).

$$\sigma =\frac{F}{A_0}$$

(1)

The original cross-sectional area (A₀) was assumed to be rectangular and calculated by multiplying sample width (w) by thickness (t) (Equation 2).

$$A_0=w\times t$$

(2)

Engineering strain (ε) was calculated by dividing the change in specimen length (Δl) by the gauge length (l₀) (Equation 3).

$$\epsilon =\frac{\mathsf{\Delta }\ell }{{\ell }_0}$$

(3)

Once calculated, stress-strain curves for each sample were plotted using MATLAB (R2022b, The Mathworks, Natick Massachusetts, USA). Once plotted, elastic modulus, ultimate tensile strength (UTS), and strain at failure were extracted from the stress-strain curves. Considering that stress-strain curves of biological tissues exhibit nonlinear or hyperelastic curves [16], the elastic modulus was calculated as the first linear portion of the stress-strain curve (Figure 6). In biological materials, the toe region (Region I) is the initial, nonlinear portion of the stress-strain curve where there is a small amount of stiffness due to crimped collagen fibers [17]. Following the toe region, is the linear region (Region II) of the curve [16]. In this study, the elastic modulus was calculated from Region II by selecting two points and calculating the slope between the two points (Figure 6). The ultimate tensile strength (UTS) was calculated as the maximum stress in the portion of the curve in Region III (Figure 6). Strain at failure was selected from the strain value at failure of the material (Figure 6).

After plotting stress-strain curves, graphical quality of the curves was assessed. The rating system (developed by the authors) is described in Table 4. Curves that were too noisy (rating of 4) could not reliably provide tensile properties and were excluded from the dataset. A total of 10 stress-strain curves were excluded because of this. All excluded data were muscle samples: rectus abdominis (7), external oblique (2), and transversus abdominis (1).

Table 3. Stress-Strain curve rating system, descriptions, and curve examples.

Table 3. Stress-Strain curve rating system, descriptions, and curve examples.

Rating	Description	Example
1	Very little noise Tensile properties such as toe region, linear elastic region, and ultimate tensile strength are easily discernable
2	Some noise Tensile properties such as toe region or linear elastic region are visible, but not as easy to discern
3	Quite a bit of noise Linear elastic region can still be estimated
4	Lots of noise Too difficult to discern tensile properties with a high degree of reliability

As mentioned in Section 2.3, location of failure was noted for tissue specimens. The elastic modulus for specimens that failed in the grips was calculated. However, because the specimens did not meet failure criteria, UTS and strain at failure could not be calculated.

3. Results

Figure 3. Representative stress-strain curves of abdominal wall tissues.

Figure 3. Representative stress-strain curves of abdominal wall tissues.

3.1. Inter- and Intra-Cadaveric Differences in Tensile Properties

As described in section 2.3, the elastic modulus, ultimate tensile strength, and strain at failure were obtained from the stress-strain curves for each sample. A swarmchart (Figure 8) shows the spread of the mechanical properties between and among cadavers.

Figure 4. Swarmcharts for tensile properties (elastic modulus, ultimate tensile strength, and strain at failure) are shown for the various tissue types of the anterolateral abdominal wall. Each cadaveric specimen is represented by the same color. Points of the same color indicate multiple tissue samples from the same cadaver. The points of the swarmchart are jittered on the x-axis for ease of visualization [19].

Figure 4. Swarmcharts for tensile properties (elastic modulus, ultimate tensile strength, and strain at failure) are shown for the various tissue types of the anterolateral abdominal wall. Each cadaveric specimen is represented by the same color. Points of the same color indicate multiple tissue samples from the same cadaver. The points of the swarmchart are jittered on the x-axis for ease of visualization [19].

As seen in Figure 8, there is a larger spread of elastic moduli and ultimate tensile strength values for the fascia (anterior/ posterior rectus sheaths and linea alba) and peritoneum, whereas the tensile properties of the various muscles seem to be less variable. The intra-cadaveric tissue variability of fascia (anterior rectus sheath) versus muscle (external oblique) is again illustrated in Figure 9. Figure 9 demonstrates the broader range of values for tensile properties from anterior rectus sheath samples (Elastic modulus: 0.95–6.88 Megapascals [MPa]; UTS: 0.82–1.79 MPa; Strain at Failure: 52.79%–234.45%) compared to samples from the external oblique muscle (Elastic modulus: 0.21–0.59 MPa; UTS: 0.23–0.42 MPa; Strain at Failure: 93.28%–251.95%).

Figure 5. Four anterior rectus sheath and four external oblique muscle samples were taken from the same cadaver and tested in tension. The above figure displays the stress-strain curves from each sample.

Figure 5. Four anterior rectus sheath and four external oblique muscle samples were taken from the same cadaver and tested in tension. The above figure displays the stress-strain curves from each sample.

3.2. Fresh-Never-Frozen (FNF) and Fresh-Frozen (FF) Tensile Properties

Tensile property data from different cadaveric preservations (i.e., FNF and FF) were collated by tissue type (Table 5). As seen in Table 5, the median values for the elastic moduli of fresh frozen tissues were generally stiffer than those of FNF cadavers. The three exceptions were posterior rectus sheath, peritoneum, and transversus muscle. The UTS was generally higher for FF tissue except for the posterior rectus sheath and transversus muscle. Values for strain at failure were higher in FNF cadavers for five of the tissue types (anterior rectus sheath, posterior rectus sheath, peritoneum, internal oblique muscle, and composite).

Tissue tensile properties by cadaveric preparation were Mann-Whitney U test. The median elastic modulus was significantly higher in the external oblique of FF (0.57 vs. 0.24 MPa, p = 0.009), and the median ultimate tensile strength was significantly higher in the external oblique of FF (0.28 vs. 0.16 MPa, p = 0.015). A significant difference in medians was also detected for UTS of the rectus abdominis (FF significantly greater than FNF [0.14 vs. 0.06 MPa, p = 0.014]). However, it should be noted that in the case of the rectus abdominis comparisons, there was a small sample size for FNF tissues.

Table 4. Median (Interquartile range [IQR]) are displayed below for separate tissues and as a composite. Values are given for fresh-never-frozen (FNF) and fresh-frozen (FF) cadavers. Outputs from the Mann-Whitney U test are included in the footer of this table.

Table 4. Median (Interquartile range [IQR]) are displayed below for separate tissues and as a composite. Values are given for fresh-never-frozen (FNF) and fresh-frozen (FF) cadavers. Outputs from the Mann-Whitney U test are included in the footer of this table.

Cadaveric Preparation	Tissue	Elastic Modulus (MPa)	Ultimate Tensile Strength (MPa)	Strain at Failure (%)
FNF	Anterior Rectus Sheath	6.02 (10.14)	1.54 (3.02)	132.92 (139.00)
	Posterior Rectus Sheath	14.32 (57.62)	4.14 (13.34)	134.92 (116.00)
	Linea Alba	2.67 (6.87)	0.87 (1.28)	195.96 (126.00)
	Peritoneum	6.79 (6.09)	1.80 (3.99)	145.54 (118.00)
	Rectus Abdominis	0.19*	0.06*§	92.21*
	External Oblique	0.24 (0.37) †	0.16 (0.16) ‡	196.07 (86.00)
	Internal Oblique	0.42 (0.68)	0.20 (0.22)	185.67 (87.00)
	Transversus Abdominis	2.75 (5.24)	0.79 (0.99)	141.61 (169.00)
	Composite	1.64*	0.89*	233.43*
FF	Anterior Rectus Sheath	8.29 (15.76)	3.45 (4.51)	124.54 (84.00)
	Posterior Rectus Sheath	7.31 (67.69)	3.02 (14.04)	133.33 (190.00)
	Linea Alba	3.92 (3.80)	1.40 (2.88)	225.44 (138.00)
	Peritoneum	4.42 (14.15)	2.24 (3.66)	135.71 (108.00)
	Rectus Abdominis	0.30 (0.88)	0.14 (0.32)	135.60 (152.00)
	External Oblique	0.57 (1.27)	0.28 (0.48)	215.06 (243.00)
	Internal Oblique	0.58 (1.11)	0.40 (0.41)	185.46 (127.00)
	Transversus Abdominis	0.81 (2.44)	0.49 (0.94)	182.43 (118.00)
	Composite	1.12 (2.79)	0.53 (1.22)	195.55 (68.00)

* Sample size is 3. † U = 196.00, z = 2.62, p = 0.009. ‡ U = 191.00, z = 2.43, p = 0.015. § U = 46.00, z = 2.46, p = 0.014.

3.3. Composite Tissue vs. Separated Tissues

The composite contained the tissue layers in the following order: anterior rectus sheath, rectus abdominis muscle, posterior rectus sheath, parietal peritoneum. As previously detailed in Table 2 (Section 2.1), a total of 7 composite samples were extracted from 3 different cadavers. The tensile properties of the composites of each cadaver were compared to the tensile properties of the tissues that comprise the composite (Figure 10). Interestingingly, when comparing the composite tissue to that of its individual parts, the tensile properties do not seem to have an easily discernable pattern.

Figure 6. The bar graphs show comparisons of composite samples and their individual tissues (anterior rectus sheath, rectus abdominis, posterior recuts sheath, parietal peritoneum) for elastic modulus (a–c), ultimate tensile strength (UTS) (d–f), and strain at failure (g–i). Samples were taken from three different cadavers: Specimen 2 (blue), Specimen 5 (green), and Specimen 13 (yellow). The x-axes for all bar charts are in the following order: composite, anterior rectus sheath, rectus abdominis muscle, posterior rectus sheath, and peritoneum.

Figure 6. The bar graphs show comparisons of composite samples and their individual tissues (anterior rectus sheath, rectus abdominis, posterior recuts sheath, parietal peritoneum) for elastic modulus (a–c), ultimate tensile strength (UTS) (d–f), and strain at failure (g–i). Samples were taken from three different cadavers: Specimen 2 (blue), Specimen 5 (green), and Specimen 13 (yellow). The x-axes for all bar charts are in the following order: composite, anterior rectus sheath, rectus abdominis muscle, posterior rectus sheath, and peritoneum.

4. Discussion

5. Conclusions

References

1. Krishnakumar:, S.; Tambe, P. Entry complications in laparoscopic surgery. Journal of Gynecological Endoscopy and Surgery 2009, 1, 4–11. [Google Scholar] [CrossRef] [PubMed]
2. Kostis, S.; Chung, K.C. Application of the "see one, do one, teach one" concept in surgical trianing. Plastic Reconstructive Surgery 2013, 131, 1194–11201. [Google Scholar]
3. Davis, K.G.; Coviello, L. Tools. In Laparoscopic colectomy: A Step-by-Step Guide; Stein, L.R., Ed.; Springer Nature: Switzerland, AG, 2020; pp. 1–14. [Google Scholar]
4. Last, R.J. Last’s anatomy: regional and applied; Ninth, ed, McMinn, R., Eds.; Elsevier Australia: Chatswood, New South Wales, AU, 2020; pp. 375–404. [Google Scholar]
5. Goldberg, I.; Docimo, S. Normal radiographic anatomy of anterior abdominal 2all. In Fundamentals of Hernia Radiology; Docimo, S., Blatnik, J.A., Pauli, E.M., Eds.; Springer Nature: Switzerland AG.
6. Coles, T.R.; Meglan, D.; John, N.W. The role of haptics in medical training simulators: A survey of the state of the art. IEEE Trans Haptics 2011, 4, 51–66. [Google Scholar] [CrossRef] [PubMed]
7. Favier, V.; Subsol, G.; Duraes, M.; Captier, G.; Gallet, P. Haptic fidelity: The game changer in surgical simulators for the next decade? Frontiers in Oncology 2021. [Google Scholar] [CrossRef] [PubMed]
8. Preis, A.; riedle, H.; Benke, E.; Franke, J. Matching of mechanical properties of biological tissues and technical materials for the fabrication of anatomical models by material jetting. In Proceedings of the 14^th Joint Conference on biomedical engineering systems and technologies (SCITEPRESS - Science and Technology Publications 2021); p.p. 189-94.
9. Transplantation and Anatomy Act 1979. Queensland Health. Available online: https://www.legislation.qld.gov.au/view/html/inforce/current/act-1979-074 (accessed on 12 September 2023).
10. Transplantation and Anatomy Regulation 2017. Queensland Health. Available online: https://www.legislation.qld.gov.au/view/html/inforce/current/sl-2017-0148 (accessed on 12 September 2023).
11. Criminal Code Act 1899. Queenlsand Government. Available online: https://www.legislation.qld.gov.au/view/html/inforce/current/act-1899-009 (accessed on 12 September 2023).
12. Work Health and Safety Act 2011. Queensland Government. Available online: https://www.legislation.qld.gov.au/view/html/inforce/current/act-2011-018 (accessed on 12 September 2023).
13. Tank, P.W. The Abdomen. In Grant’s Dissector; Lippincott Williams & Wilkins: Philadelphia, PA US; pp. 89–100.
14. Gray, H. Muscles and fasciae of the abdomen. In Gray’s Anatomy; Barnes & Noble, Inc.: New York, NY, US, 2010; pp. 329–341. [Google Scholar]
15. Cardoso, M.H.S. Experimental study of the human anterolateral abdominal 2all: Biomechanical Properties of Fascia and Muscles. Master’s Thesis, Faculty of Biomedical Engineering, University of Porto, Porto, Portugal, 2011. [Google Scholar]
16. Roeder, R.K. Mechanical characterization of biomaterials. In Characterization of biomaterials; Bandyopadhyay, A., Bose, S., Eds.; Elsevier Inc.: Waltham, Massachussetts, US, 2011; pp. 49–104. [Google Scholar]
17. Galbusera, F.; Innocent, B. Ligament and Tendon Biomechanics. In Human orthopedic biomechanics: fundamentals, devices, and applications; Academic Press: London, UK, 2022; pp. 137–149. [Google Scholar]
18. Fung, Y. Chapter 7: Bioviscoelastic solids. In Biomechanics: Mechanical properties of living tissues; Springer: New York, New York, US, 1993; pp. 242–320. [Google Scholar]
19. Swarmchart. The MathWorks, Inc.: Natick, MA US. Available online: https://au.mathworks.com/help/matlab/ref/swarmchart.html (accessed on 12 September 2023).
20. Ben Abdelounis, H; Nicolle, S.; Ottenio, M.; Beillas, P.; Mitton, D. Effect of two loading rates on the elasticity of the human anteiror rectus sheath. Journal of the Mechanical Behavior of Biomedical Materials 2013, 20, 1–5. [Google Scholar] [CrossRef]
21. Kirilova, M.; Stoytchev, S.; Pashkouleva, D.; Kavardzhikov, V. Experimental study of the mechanical properties of human abdominal fascia. Medical Engineering and Physics 2011, 33, 1–6. [Google Scholar] [CrossRef] [PubMed]
22. Horwood, A.; Chockalingam, N. Principles of tissue stress. In Clinical biomechanics in human locomotion: origins and principles; Elsevier: London, England, UK, 2023. [Google Scholar]
23. Hollinsky, C.; Sandber, S. Measurement of the tensile strength of the ventral abdominal wall in comparison with scar tissue. Clinical Biomechanics 2007, 22, 88–92. [Google Scholar] [CrossRef] [PubMed]
24. Sherman, V.R.; Yang, W.; Meyers, M.A. The materials science of collagen. Journal of the Mechanical Behavior of Biomedical Materials 2015, 52, 22–50. [Google Scholar] [CrossRef] [PubMed]
25. Rath, A.M.; Zhang, J.; Chevrel, J.P. The sheath of the rectus abdominis muscle: an anatomical and biomechanical study. Hernia 1997, 1, 139–142. [Google Scholar] [CrossRef]
26. Cooney, G.M.; Iake, S.P.; Thompson, D.M.; Castile, R.M.; Winter, D.C.; Simms, C.K. Uniaxial and biaxial tensile stress-stretch response of human linea alba. Journal of the Mechanical Behavior of Biomedical Materials 2016, 63, 134–140. [Google Scholar] [CrossRef] [PubMed]
27. Hernandez, B.; Pena, E.; Pascual, G.; Rodriguez, M.; Calvo, B.; Doblare, M.; Bellon, J.M. Mechanical and histological characterization of the abdominal muscle. A previous step to modelling hernia surgery. Journal of the Mechanical Behavior of Biomedical Materials 2011, 4, 392–404. [Google Scholar] [CrossRef] [PubMed]
28. Hwang, W.; Carvalho, J.C.; Tarlovsky, I.; Boriek, A.M. Passive mechanics of canine internal abdominal muscles. Journal of Applied Physiology 2005, 98, 1829–1835. [Google Scholar] [CrossRef] [PubMed]
29. Forstemann, T.; Trzewik, J.; Holste, J.; Batke, B.; Konerding, M.A.; Wolloscheck, T.; Hartung, C. Forces and deformations of the abdominal wall - a mechanical and geometrical approach to the linea alba. Journal of Biomechanics 2011, 44, 600–606. [Google Scholar] [CrossRef] [PubMed]