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Measurement of the tensile strength of the ventral abdominal wall in comparison with scar tissue
Clinical Biomechanics 22 (2007) 88–92 www.elsevier.com/locate/clinbiomech
Measurement of the tensile strength of the ventral abdominal wall in comparison with scar tissue C. Hollinsky *, S. Sandberg Kaiserin Elisabeth Hospital, Department of Surgery, Huglgasse 1-3; A-1150 Vienna, Austria Received 22 February 2006; accepted 6 June 2006
Abstract Background. Paramedian laparotomies lead to incisional hernias in approximately 30% of cases. In contrast, incisional hernias occur very rarely in the linea alba or the ventral abdominal wall. In this setting we investigated the difference between scar tissue and the nonincised abdominal wall tissue. Methods. At the post mortem examination of 66 recently deceased individuals, accurately measured pieces of resected tissue from the linea alba, the anterior and the posterior rectus sheath, and scar tissue following median laparotomy, were exposed to tensile loads. Findings. In the epigastric region the tissue ruptured at a mean horizontal load of 10.0 (SD 3.4) N/mm2 in the linea alba and 6.9 (SD 2.5) N/mm2 in scar tissue (P < 0.001), and at a mean vertical load of 4.5 (SD 2.0) N/mm2 in the linea alba and 3.3 (SD 1.6) N/mm2 in scar tissue (P < 0.05). In the hypogastric region as well, scar tissue was significantly less resistant in the main direction of load. Interpretation. Scar tissue has a significantly lesser loading capacity than the intact ventral abdominal wall and therefore poses a permanent risk for herniation. For this reason, closure of the abdominal wall should be given due consideration and subjected to further investigation. Specifically, sustained reinforcement of scar tissue by means of suture techniques or non-absorbable sutures warrants further study. When constructing meshes for reinforcement of incisional hernias, the two-fold tensile load on the midline in horizontal direction as opposed to the craniocaudal direction must be taken into account. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Linea alba; Scar tissue; Rectus sheath; Tensile strength; Incisional hernia; Tensiometry
1. Introduction The problem of incisional hernia is an extensively discussed and highly debated subject. Occurring at a rate of 11–15% in the first postoperative year, it is a poorly controlled complication of primary abdominal wall closure (Wissing et al., 1987). According to Mudge and Hughes (1985), the high initial incidence of incisional hernia is doubled in the following nine years. It may be assumed that nearly every third abdominal wall closure is unable to resist loads and culminates in tissue rupture. Scars constitute a weak site of the abdominal wall. Therefore, it is important to identify the differences *
Corresponding author. E-mail address: [email protected] (C. Hollinsky).
0268-0033/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2006.06.002
between this tissue and the load-bearing, tear-resistant linea alba. Furthermore, correction of incisional hernia is associated with high recurrence rates of up to 24% despite the widespread use of mesh as reinforcement (Cassar and Munro, 2002; Flum et al., 2003; Luijendijk et al., 2000; Rios et al., 2001). The suture technique and the mesh material are primarily selected by the surgeon, based on his own experience, published reports of other surgeons, and the results of evidence-based medicine. No attempts have been made so far to compare loads and the load-bearing strength of the linea alba and scar tissue in order to draw conclusions about closure of the abdominal wall or the treatment of incisional hernias. The interaction between the muscles of the abdominal wall and their effect on the linea alba and scar tissue result from the load-bearing capacity of these structures in
C. Hollinsky, S. Sandberg / Clinical Biomechanics 22 (2007) 88–92
89
various directions of stress. The purpose of the present study was to compare the stress resistance of the linea alba and of scar tissue, and to provide evidence of the poor tensile strength of scar tissue. The results of the investigation led to significant conclusions about primary abdominal wall closure and the treatment of incisional hernias. Tension =
2. Methods In 66 recently deceased patients with an average age of 77 years (range: 17–94 years), tensile load tests on the ventral abdominal wall were performed in the course of post mortem investigations within 24 h after death. The linea alba was subjected to horizontal and vertical tensile load in the epigastrium and the hypogastrium. In cases of preexisting median laparotomies, which had been performed on average 12 years previously (range: 0.5–41 years), the scar tissue was subjected to tensile loads in the same manner and compared with the values obtained for the intact midline. Similarly, the anterior and the posterior rectus sheath were tested for their stability when subjected to horizontal and vertical tensile loads above and below the navel. In the posterior rectus sheath, all tests were performed above the arcuate line, since an adequate quantity of evaluable connective tissue was not available below this line. The tissue specimens were spread on a plate and cut to a standard size with a scalpel, using a metal template (Fig. 1). The thickness was measured accurately and the surface area of the rupture site was calculated in square millimeters (Fig. 2). The piece of tissue was then stretched in the Tensiometer 80035 (Pesola AG, Baar, Switzerland) with a maximum permissible deviation at full load of 0.3%. The tissue was subjected to tensile load at uniform speed of 10 mm/min. until the point of rupture. We selected this speed, which is rather slow for a test of material, because the tested samples were biomaterials with viscoelastic prop-
Template
1 cm
3 cm
1 cm
2 cm
Fig. 1. The metal template used to cut the tissue samples.
Internal strength Cross-section surface
Unit of tension =
N mm2 b a
Fig. 2. The surface of the stretched samples was measured accurately in mm2 and subjected to tensile loads. When a sample tore out of the fixation or if tissue ruptured outside the measured area, the result was excluded from the analysis.
erties. The tension (tensile load/surface) at the time of rupture was measured in N/mm2. If the tissue tore or ruptured at a different site than the pre-determined rupture site (in 22 tests), the test result was not taken into account. The results are presented in means and standard deviation. A Mann–Whitney U-test was used to identify statistical differences between the experimental groups. 3. Results Of the 66 recently deceased persons, 27 had a median laparotomy scar while 39 had an intact linea alba. Thirty-six female and 30 male cadavers aged on average 77 years (range: 17–94 years) were investigated. Of these, three cadavers were less than 50 years of age (17, 39, 48 years) and 7 cadavers were between 50 and 60 years of age. All others were older than 60 years. The stability of the linea alba in the epigastrium during horizontal tensile load was significantly superior (mean 10.0, SD 3.4 N/ mm2) to that of scar tissue (mean 6.9, SD 2.5 N/mm2) (P < 0.001). Likewise, in the same direction of tensile load, the linea alba in the hypogastrium resisted a significantly greater load than did scar tissue, with mean 8.4 (SD 3.1) N/mm2 vs. 6.1 (SD 2.3) N/mm2 (P < 0.001) (Table 1). When subjected to vertical load as well, the linea alba was significantly superior to the scar tissue in the epigastrium with mean 4.5 (SD 2.0) N/mm2 vs. 3.3 (SD 1.6) N/ mm2 (P < 0.05), but no significant difference was registered in the hypogastrium (Table 1). The anterior rectus sheath resisted a horizontal load of mean 8.1 (SD 2.1) N/mm2 in the epigastrium and was thus markedly more stable than the posterior rectus sheath, which resisted a load of mean
90
C. Hollinsky, S. Sandberg / Clinical Biomechanics 22 (2007) 88–92
5.0 (SD 2.5) N/mm2 (P < 0.001) (Table 1). Greater stress resistance was also registered in the hypogastrium and in vertical direction, as shown in Table 1. Comparison of the groups in terms of age and gender showed that higher tensile loads in the midline were tolerated by men and younger individuals than by women and elderly individuTable 1 Measurement of the ventral abdominal wall and of scar tissue Investigated tissue
Direction of load
N
Newton/ mm2
SD
Significance P value
Epigastric linea alba Epigastric scar tissue
Horizontal
39 27
10.0 6.9
3.4 2.5
<0.001
Hypogastric linea alba Hypogastric scar tissue
Horizontal
39 25
8.4 6.1
3.1 2.3
<0.001
Epigastric anterior rectus sheath Epigastric posterior rectus sheath
Horizontal
36
8.1
2.1
<0.001
34
5.6
2.0
Hypogastric anterior rectus sheath Hypogastric posterior rectus sheath
Horizontal
24
8.5
2.5
18
5.0
2.5
Epigastric linea alba Epigastric scar tissue
Vertical
39 22
4.5 3.3
2.0 1.6
<0.05
Hypogastric linea alba Hypogastric scar tissue
Vertical
27 20
4.1 3.3
2.5 1.7
n.s.
Epigastric anterior rectus sheath Epigastric posterior rectus sheath
Vertical
35
3.4
1.6
<0.001
32
1.9
1.5
Hypogastric anterior rectus sheath Hypogastric posterior rectus sheath
Vertical
24
3.4
2.0
16
2.1
1.5
<0.001
n.s.
Tensile loads on the tissue were imposed in vertical and horizontal direction. N = All evaluable tests; SD = standard deviation; Significance = Mann–Whitney U-Test (equivalent to Wilcoxon’s rank sum test); n.s. = The two samples are not significantly different (P P 0.05, two-tailed test).
als; however, the difference was not significant. Similarly, the investigation of BMI showed no significant difference between underweight individuals and those of normal weight or obese persons (Table 2). The tensile load experiments also showed that scar tissue achieves approximately 70% of the stability of the linea alba in the first year; this level of stability was not altered even after several decades (Fig. 3). 4. Discussion 4.1. Scar tissue In the present study we found that the load-bearing capacity of scar tissue is approximately 30% less than that of the linea alba in nearly all directions; the maximum difference was 3 N/mm2 in horizontal direction in the epigastrium. We were thus able to obtain a quantitative measure of the strength of scar tissue compared to that of the linea alba. This provides an approximate value of the stability required for the treatment of incisional hernia. Furthermore, we found that this low resistance capacity persists for several decades after the operation (Fig. 3). Therefore, patients with median laparotomies are at risk for incisional hernias even several decades after surgery. Reinforcement of primary closure of the abdominal wall, such as that reported by Gutierrez de la Pena et al. (2003), appears to be very meaningful for the reduction of incisional hernias and should be considered especially in patients at risk (Hoer et al., 2002; Sorensen et al., 2005). 4.2. Load resistance of the midline Our results are in concurrence with the data reported from Korenkov et al. (2001); Seidl et al. (1974); in the linea alba the authors registered a value of 10 N/mm2 for horizontal loads and 5 N/mm2 for vertical loads. These results are easily explained by Axer et al. (2001) and by Urquhart et al. (2005), since the large majority of connective tissue
Table 2 Measurement of the ventral abdominal wall differentiated by gender, age and BMI Investigated tissue
Gender, age, BMI
Direction of load
N
Newton /mm2
SD
Significance P value
Epigastric midline
Male Female
Horizontal
30 36
9.6 8.3
3.1 2.4
0.14
Hypogastric midline
Male Female
Horizontal
29 35
8.1 6.6
2.1 2.4
0.10
Epigastric midline
Age < 80 Age P 80
Horizontal
28 38
9.7 8.7
2.7 2.8
0.26
Hypogastric midline
Age < 80 Age P 80
Horizontal
28 36
8.4 7.1
3.0 2.8
0.18
Epigastric midline
BMI < 23 BMI P 23
Horizontal
30 36
9.2 9.0
2.9 2.9
0.94
Hypogastric midline
BMI < 23 BMI P 23
Horizontal
29 35
7.8 7.4
2.2 2.5
0.59
C. Hollinsky, S. Sandberg / Clinical Biomechanics 22 (2007) 88–92
91
12
10
Newton
8
6
4
2
0
0
5
10
15
20
25 years
epigastric scar
30
35
40
45
50
hypogastric scar
epigastric linea alba
hypogastric linea alba
Fig. 3. Tensile strength of the epigastric and hypogastric median laparotomy scar in horizontal direction 1–40 years after the operation in comparison with the epigastric and hypogastric linea alba, shown in N/mm2. (– - – - –) Tensile strength of the epigastric linea alba (............) Tensile strength of the hypogastric linea alba.
fibers are oriented horizontally while others are oriented cross-wise in the linea alba. Due to the absence of vertically oriented fiber bundles, the strength of the linea alba in vertical direction is reduced by 50%; this difference in the tensile strength of the midline should also be taken into account when designing meshes. Currently we have meshes that possess nearly equal stability in all directions. Foreign tissue could be reduced to 50% without increasing the risk of recurrence, purely on the basis of static and dynamic features. Based on our results, it would be meaningful to devise new types of meshes in linear form that neutralize horizontal forces, while avoiding the potential complications of a large scar, restriction of abdominal wall motility (Junge et al., 2001) and chronic pain (Machairas et al., 2004). 4.3. Anterior and posterior rectus sheath Our results concerning the stability of the two rectus sheaths showed significantly greater stability for the anterior rectus sheath in horizontal and vertical direction compared to the posterior rectus sheath. Thus, the results obtained by Rath et al. (1997) were confirmed in the present study. Such greater resistance capacity was seen in the epigastrium as well as in the hypogastrium, with all measurements performed above the arcuate line. Below this limit, adequate stable tissue from the posterior rectus sheath was not available for performing tensile load tests. Thus, it may be concluded that an increase in intraabdominal pressure (such as that resulting from laparoscopic procedures) is of secondary importance for the structures of the abdominal wall. Otherwise the anatomy of the abdominal wall would be different, with the stronger structures oriented towards the inner side. These concepts are
confirmed by the structure of the linea alba and the posterior rectus sheath: practically no force-absorbing structures are present below the arcuate line in the posterior rectus sheath. Notably, these findings show that the selection of the position of a mesh when using the onlay technique or the sublay technique cannot be based on the argument proposed by Schumpelick et al. (2002), namely, that intraabdominal pressure requires reinforcement of the abdominal wall from within. Rather, one is inclined to believe that tensile forces in the abdominal wall are caused by abdominal muscles, which play a very important role in the emergence of incisional hernias. Tension in the oblique muscles of the abdominal wall (e.g. when coughing, laughing or during defecation) causes very high tensile loads in horizontal direction on the Linea alba because the forces are passed on to the anterior and posterior rectus sheath. Due to the orientation of its fibres the Linea alba is able to withstand these loads. However, the scar tissue is unable to resist the load and this may lead to hernias. Therefore, when the abdominal wall is reinforced, the position of the mesh on the anterior or posterior rectus sheath is of secondary importance, because recurrences following mesh-reinforced abdominal walls may be caused by the mesh shearing off or being torn from the fixation (Hollinsky and Hollinsky, 1999). It is much more important to achieve better fixation of the mesh in the abdominal wall, since nearly all recurrent hernias occur at the point of transition between the mesh and tissue. 5. Conclusion It may be concluded that scar tissue is unstable in terms of its stress resistance and therefore poses a high risk of
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hernia for patients with median laparotomies. The fact that the horizontal load on the linea alba is twice as high as the vertical load should be taken into account during primary abdominal wall closure and also during mesh construction. This field calls for extensive research in the future. References Axer, H., Graf v Keyserlingk, D., Prescher, A., 2001. Collagen fibers in linea alba and rectus sheaths. J. Surg. Res. 96, 127–134. Cassar, K., Munro, A., 2002. Surgical treatment of incisional hernia. Br. J. Surg. 89, 534–545. Flum, D.R., Horvath, K., Koepsell, T., 2003. Have outcomes of incisional hernia improved with time? A population-based analysis. Ann. Surg. 237, 129–135. Gutierrez de la Pena, C., Medina Achirica, C., Dominguez-Adame, E., Medina Diez, J., 2003. Primary closure of laparotomies with high risk of incisional hernia using prosthetic material: analysis of usefulness. Hernia 7, 134–136. Hoer, J., Lawong, G., Klinge, U., Schumpelick, V., 2002. Factors influencing the development of incisional hernia. A retrospective study of 2983 laparotomy patients over a period of 10 years. Chirurg 73, 474–480. Hollinsky, C., Hollinsky, K.H., 1999. Static calculations for mesh fixation by intraabdominal pressure in laparoscopic extraperitoneal herniorrhaphy. Surg. Laparosc. Endosc. Percutan. Tech. 9, 106–109. Junge, K., Klinge, U., Prescher, A., Giboni, P., Niewiera, M., Schumpelick, V., 2001. Elasticity of the anterior abdominal wall and impact for reparation of incisional hernias using mesh implants. Hernia 5, 113–118. Korenkov, M., Beckers, A., Koebke, J., Lefering, R., Tiling, T., Troidl, H., 2001. Biomechanical and morphological types of the linea alba and
its possible role in the pathogenesis of midline incisional hernia. Eur. J. Surg. 167, 909–914. Luijendijk, R.W., Hop, W.C., van den Tol, M.P., de Lange, D.C., Braaksma, M.M., Ijzermans, J.N., Boelhouwer, R.U., de Vries, B.C., Salu, M.K., Wereldsma, J.C., Bruijninckx, C.M., Jeekel, J., 2000. A comparison of suture repair with mesh repair for incisional hernia. N. Engl. J. Med. 343, 392–398. Machairas, A., Misiakos, E.P., Liakakos, T., Karatzas, G., 2004. Incisional hernioplasty with extraperitoneal onlay polyester mesh. Am. Surg. 70, 726–729. Mudge, M., Hughes, L.E., 1985. Incisional hernia: a 10-year prospective study of incidence and attitudes. Br. J. Surg. 72, 70–74. Rath, A.M., Zhang, J., Chevrel, J.P., 1997. The sheath of the rectus abdominis muscle: an anatomical and biomechanical study. Hernia 1, 139–142. Rios, A., Rodriguez, J.M., Munitiz, V., Alcaraz, P., Perez, D., Parrilla, P., 2001. Factors that affect recurrence after incisional herniorrhaphy with prosthetic material. Eur. J. Surg. 167, 855–859. Schumpelick, V., Junge, K., Rosch, R., Klinge, U., Stumpf, M., 2002. Retromuscular mesh repair for ventral incision hernia in Germany. Chirurg 73, 888–894. Seidl, W., Tauber, R., Hoffschulte, K.H., 1974. Messungen zur Festigkeit der Bauchdeckennaht. Chirurg 45, 266–272. Sorensen, L.T., Hemmingsen, U.B., Kirkeby, L.T., Kallehave, F., Jorgensen, L.N., 2005. Smoking is a risk factor for incisional hernia. Arch. Surg. 140, 119–123. Urquhart, D.M., Barker, P.J., Hodges, P.W., Story, I.H., Briggs, C.A., 2005. Regional morphology of the transversus abdominis and obliquus internus and externus abdominis muscles. Clin. Biomech. 20, 233–241. Wissing, J., van Vroonhoven, T.J.M.V., Schattenkerk, M.E., Veen, H.F., Ponsen, R.J.G., Jeekel, J., 1987. Fascia closure after midline laparotomy. Br. J. Surg. 74, 738–741.
Measurement of the tensile strength of the ventral abdominal wall in comparison with scar tissue C. Hollinsky *, S. Sandberg Kaiserin Elisabeth Hospital, Department of Surgery, Huglgasse 1-3; A-1150 Vienna, Austria Received 22 February 2006; accepted 6 June 2006
Abstract Background. Paramedian laparotomies lead to incisional hernias in approximately 30% of cases. In contrast, incisional hernias occur very rarely in the linea alba or the ventral abdominal wall. In this setting we investigated the difference between scar tissue and the nonincised abdominal wall tissue. Methods. At the post mortem examination of 66 recently deceased individuals, accurately measured pieces of resected tissue from the linea alba, the anterior and the posterior rectus sheath, and scar tissue following median laparotomy, were exposed to tensile loads. Findings. In the epigastric region the tissue ruptured at a mean horizontal load of 10.0 (SD 3.4) N/mm2 in the linea alba and 6.9 (SD 2.5) N/mm2 in scar tissue (P < 0.001), and at a mean vertical load of 4.5 (SD 2.0) N/mm2 in the linea alba and 3.3 (SD 1.6) N/mm2 in scar tissue (P < 0.05). In the hypogastric region as well, scar tissue was significantly less resistant in the main direction of load. Interpretation. Scar tissue has a significantly lesser loading capacity than the intact ventral abdominal wall and therefore poses a permanent risk for herniation. For this reason, closure of the abdominal wall should be given due consideration and subjected to further investigation. Specifically, sustained reinforcement of scar tissue by means of suture techniques or non-absorbable sutures warrants further study. When constructing meshes for reinforcement of incisional hernias, the two-fold tensile load on the midline in horizontal direction as opposed to the craniocaudal direction must be taken into account. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Linea alba; Scar tissue; Rectus sheath; Tensile strength; Incisional hernia; Tensiometry
1. Introduction The problem of incisional hernia is an extensively discussed and highly debated subject. Occurring at a rate of 11–15% in the first postoperative year, it is a poorly controlled complication of primary abdominal wall closure (Wissing et al., 1987). According to Mudge and Hughes (1985), the high initial incidence of incisional hernia is doubled in the following nine years. It may be assumed that nearly every third abdominal wall closure is unable to resist loads and culminates in tissue rupture. Scars constitute a weak site of the abdominal wall. Therefore, it is important to identify the differences *
Corresponding author. E-mail address: [email protected] (C. Hollinsky).
0268-0033/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2006.06.002
between this tissue and the load-bearing, tear-resistant linea alba. Furthermore, correction of incisional hernia is associated with high recurrence rates of up to 24% despite the widespread use of mesh as reinforcement (Cassar and Munro, 2002; Flum et al., 2003; Luijendijk et al., 2000; Rios et al., 2001). The suture technique and the mesh material are primarily selected by the surgeon, based on his own experience, published reports of other surgeons, and the results of evidence-based medicine. No attempts have been made so far to compare loads and the load-bearing strength of the linea alba and scar tissue in order to draw conclusions about closure of the abdominal wall or the treatment of incisional hernias. The interaction between the muscles of the abdominal wall and their effect on the linea alba and scar tissue result from the load-bearing capacity of these structures in
C. Hollinsky, S. Sandberg / Clinical Biomechanics 22 (2007) 88–92
89
various directions of stress. The purpose of the present study was to compare the stress resistance of the linea alba and of scar tissue, and to provide evidence of the poor tensile strength of scar tissue. The results of the investigation led to significant conclusions about primary abdominal wall closure and the treatment of incisional hernias. Tension =
2. Methods In 66 recently deceased patients with an average age of 77 years (range: 17–94 years), tensile load tests on the ventral abdominal wall were performed in the course of post mortem investigations within 24 h after death. The linea alba was subjected to horizontal and vertical tensile load in the epigastrium and the hypogastrium. In cases of preexisting median laparotomies, which had been performed on average 12 years previously (range: 0.5–41 years), the scar tissue was subjected to tensile loads in the same manner and compared with the values obtained for the intact midline. Similarly, the anterior and the posterior rectus sheath were tested for their stability when subjected to horizontal and vertical tensile loads above and below the navel. In the posterior rectus sheath, all tests were performed above the arcuate line, since an adequate quantity of evaluable connective tissue was not available below this line. The tissue specimens were spread on a plate and cut to a standard size with a scalpel, using a metal template (Fig. 1). The thickness was measured accurately and the surface area of the rupture site was calculated in square millimeters (Fig. 2). The piece of tissue was then stretched in the Tensiometer 80035 (Pesola AG, Baar, Switzerland) with a maximum permissible deviation at full load of 0.3%. The tissue was subjected to tensile load at uniform speed of 10 mm/min. until the point of rupture. We selected this speed, which is rather slow for a test of material, because the tested samples were biomaterials with viscoelastic prop-
Template
1 cm
3 cm
1 cm
2 cm
Fig. 1. The metal template used to cut the tissue samples.
Internal strength Cross-section surface
Unit of tension =
N mm2 b a
Fig. 2. The surface of the stretched samples was measured accurately in mm2 and subjected to tensile loads. When a sample tore out of the fixation or if tissue ruptured outside the measured area, the result was excluded from the analysis.
erties. The tension (tensile load/surface) at the time of rupture was measured in N/mm2. If the tissue tore or ruptured at a different site than the pre-determined rupture site (in 22 tests), the test result was not taken into account. The results are presented in means and standard deviation. A Mann–Whitney U-test was used to identify statistical differences between the experimental groups. 3. Results Of the 66 recently deceased persons, 27 had a median laparotomy scar while 39 had an intact linea alba. Thirty-six female and 30 male cadavers aged on average 77 years (range: 17–94 years) were investigated. Of these, three cadavers were less than 50 years of age (17, 39, 48 years) and 7 cadavers were between 50 and 60 years of age. All others were older than 60 years. The stability of the linea alba in the epigastrium during horizontal tensile load was significantly superior (mean 10.0, SD 3.4 N/ mm2) to that of scar tissue (mean 6.9, SD 2.5 N/mm2) (P < 0.001). Likewise, in the same direction of tensile load, the linea alba in the hypogastrium resisted a significantly greater load than did scar tissue, with mean 8.4 (SD 3.1) N/mm2 vs. 6.1 (SD 2.3) N/mm2 (P < 0.001) (Table 1). When subjected to vertical load as well, the linea alba was significantly superior to the scar tissue in the epigastrium with mean 4.5 (SD 2.0) N/mm2 vs. 3.3 (SD 1.6) N/ mm2 (P < 0.05), but no significant difference was registered in the hypogastrium (Table 1). The anterior rectus sheath resisted a horizontal load of mean 8.1 (SD 2.1) N/mm2 in the epigastrium and was thus markedly more stable than the posterior rectus sheath, which resisted a load of mean
90
C. Hollinsky, S. Sandberg / Clinical Biomechanics 22 (2007) 88–92
5.0 (SD 2.5) N/mm2 (P < 0.001) (Table 1). Greater stress resistance was also registered in the hypogastrium and in vertical direction, as shown in Table 1. Comparison of the groups in terms of age and gender showed that higher tensile loads in the midline were tolerated by men and younger individuals than by women and elderly individuTable 1 Measurement of the ventral abdominal wall and of scar tissue Investigated tissue
Direction of load
N
Newton/ mm2
SD
Significance P value
Epigastric linea alba Epigastric scar tissue
Horizontal
39 27
10.0 6.9
3.4 2.5
<0.001
Hypogastric linea alba Hypogastric scar tissue
Horizontal
39 25
8.4 6.1
3.1 2.3
<0.001
Epigastric anterior rectus sheath Epigastric posterior rectus sheath
Horizontal
36
8.1
2.1
<0.001
34
5.6
2.0
Hypogastric anterior rectus sheath Hypogastric posterior rectus sheath
Horizontal
24
8.5
2.5
18
5.0
2.5
Epigastric linea alba Epigastric scar tissue
Vertical
39 22
4.5 3.3
2.0 1.6
<0.05
Hypogastric linea alba Hypogastric scar tissue
Vertical
27 20
4.1 3.3
2.5 1.7
n.s.
Epigastric anterior rectus sheath Epigastric posterior rectus sheath
Vertical
35
3.4
1.6
<0.001
32
1.9
1.5
Hypogastric anterior rectus sheath Hypogastric posterior rectus sheath
Vertical
24
3.4
2.0
16
2.1
1.5
<0.001
n.s.
Tensile loads on the tissue were imposed in vertical and horizontal direction. N = All evaluable tests; SD = standard deviation; Significance = Mann–Whitney U-Test (equivalent to Wilcoxon’s rank sum test); n.s. = The two samples are not significantly different (P P 0.05, two-tailed test).
als; however, the difference was not significant. Similarly, the investigation of BMI showed no significant difference between underweight individuals and those of normal weight or obese persons (Table 2). The tensile load experiments also showed that scar tissue achieves approximately 70% of the stability of the linea alba in the first year; this level of stability was not altered even after several decades (Fig. 3). 4. Discussion 4.1. Scar tissue In the present study we found that the load-bearing capacity of scar tissue is approximately 30% less than that of the linea alba in nearly all directions; the maximum difference was 3 N/mm2 in horizontal direction in the epigastrium. We were thus able to obtain a quantitative measure of the strength of scar tissue compared to that of the linea alba. This provides an approximate value of the stability required for the treatment of incisional hernia. Furthermore, we found that this low resistance capacity persists for several decades after the operation (Fig. 3). Therefore, patients with median laparotomies are at risk for incisional hernias even several decades after surgery. Reinforcement of primary closure of the abdominal wall, such as that reported by Gutierrez de la Pena et al. (2003), appears to be very meaningful for the reduction of incisional hernias and should be considered especially in patients at risk (Hoer et al., 2002; Sorensen et al., 2005). 4.2. Load resistance of the midline Our results are in concurrence with the data reported from Korenkov et al. (2001); Seidl et al. (1974); in the linea alba the authors registered a value of 10 N/mm2 for horizontal loads and 5 N/mm2 for vertical loads. These results are easily explained by Axer et al. (2001) and by Urquhart et al. (2005), since the large majority of connective tissue
Table 2 Measurement of the ventral abdominal wall differentiated by gender, age and BMI Investigated tissue
Gender, age, BMI
Direction of load
N
Newton /mm2
SD
Significance P value
Epigastric midline
Male Female
Horizontal
30 36
9.6 8.3
3.1 2.4
0.14
Hypogastric midline
Male Female
Horizontal
29 35
8.1 6.6
2.1 2.4
0.10
Epigastric midline
Age < 80 Age P 80
Horizontal
28 38
9.7 8.7
2.7 2.8
0.26
Hypogastric midline
Age < 80 Age P 80
Horizontal
28 36
8.4 7.1
3.0 2.8
0.18
Epigastric midline
BMI < 23 BMI P 23
Horizontal
30 36
9.2 9.0
2.9 2.9
0.94
Hypogastric midline
BMI < 23 BMI P 23
Horizontal
29 35
7.8 7.4
2.2 2.5
0.59
C. Hollinsky, S. Sandberg / Clinical Biomechanics 22 (2007) 88–92
91
12
10
Newton
8
6
4
2
0
0
5
10
15
20
25 years
epigastric scar
30
35
40
45
50
hypogastric scar
epigastric linea alba
hypogastric linea alba
Fig. 3. Tensile strength of the epigastric and hypogastric median laparotomy scar in horizontal direction 1–40 years after the operation in comparison with the epigastric and hypogastric linea alba, shown in N/mm2. (– - – - –) Tensile strength of the epigastric linea alba (............) Tensile strength of the hypogastric linea alba.
fibers are oriented horizontally while others are oriented cross-wise in the linea alba. Due to the absence of vertically oriented fiber bundles, the strength of the linea alba in vertical direction is reduced by 50%; this difference in the tensile strength of the midline should also be taken into account when designing meshes. Currently we have meshes that possess nearly equal stability in all directions. Foreign tissue could be reduced to 50% without increasing the risk of recurrence, purely on the basis of static and dynamic features. Based on our results, it would be meaningful to devise new types of meshes in linear form that neutralize horizontal forces, while avoiding the potential complications of a large scar, restriction of abdominal wall motility (Junge et al., 2001) and chronic pain (Machairas et al., 2004). 4.3. Anterior and posterior rectus sheath Our results concerning the stability of the two rectus sheaths showed significantly greater stability for the anterior rectus sheath in horizontal and vertical direction compared to the posterior rectus sheath. Thus, the results obtained by Rath et al. (1997) were confirmed in the present study. Such greater resistance capacity was seen in the epigastrium as well as in the hypogastrium, with all measurements performed above the arcuate line. Below this limit, adequate stable tissue from the posterior rectus sheath was not available for performing tensile load tests. Thus, it may be concluded that an increase in intraabdominal pressure (such as that resulting from laparoscopic procedures) is of secondary importance for the structures of the abdominal wall. Otherwise the anatomy of the abdominal wall would be different, with the stronger structures oriented towards the inner side. These concepts are
confirmed by the structure of the linea alba and the posterior rectus sheath: practically no force-absorbing structures are present below the arcuate line in the posterior rectus sheath. Notably, these findings show that the selection of the position of a mesh when using the onlay technique or the sublay technique cannot be based on the argument proposed by Schumpelick et al. (2002), namely, that intraabdominal pressure requires reinforcement of the abdominal wall from within. Rather, one is inclined to believe that tensile forces in the abdominal wall are caused by abdominal muscles, which play a very important role in the emergence of incisional hernias. Tension in the oblique muscles of the abdominal wall (e.g. when coughing, laughing or during defecation) causes very high tensile loads in horizontal direction on the Linea alba because the forces are passed on to the anterior and posterior rectus sheath. Due to the orientation of its fibres the Linea alba is able to withstand these loads. However, the scar tissue is unable to resist the load and this may lead to hernias. Therefore, when the abdominal wall is reinforced, the position of the mesh on the anterior or posterior rectus sheath is of secondary importance, because recurrences following mesh-reinforced abdominal walls may be caused by the mesh shearing off or being torn from the fixation (Hollinsky and Hollinsky, 1999). It is much more important to achieve better fixation of the mesh in the abdominal wall, since nearly all recurrent hernias occur at the point of transition between the mesh and tissue. 5. Conclusion It may be concluded that scar tissue is unstable in terms of its stress resistance and therefore poses a high risk of
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hernia for patients with median laparotomies. The fact that the horizontal load on the linea alba is twice as high as the vertical load should be taken into account during primary abdominal wall closure and also during mesh construction. This field calls for extensive research in the future. References Axer, H., Graf v Keyserlingk, D., Prescher, A., 2001. Collagen fibers in linea alba and rectus sheaths. J. Surg. Res. 96, 127–134. Cassar, K., Munro, A., 2002. Surgical treatment of incisional hernia. Br. J. Surg. 89, 534–545. Flum, D.R., Horvath, K., Koepsell, T., 2003. Have outcomes of incisional hernia improved with time? A population-based analysis. Ann. Surg. 237, 129–135. Gutierrez de la Pena, C., Medina Achirica, C., Dominguez-Adame, E., Medina Diez, J., 2003. Primary closure of laparotomies with high risk of incisional hernia using prosthetic material: analysis of usefulness. Hernia 7, 134–136. Hoer, J., Lawong, G., Klinge, U., Schumpelick, V., 2002. Factors influencing the development of incisional hernia. A retrospective study of 2983 laparotomy patients over a period of 10 years. Chirurg 73, 474–480. Hollinsky, C., Hollinsky, K.H., 1999. Static calculations for mesh fixation by intraabdominal pressure in laparoscopic extraperitoneal herniorrhaphy. Surg. Laparosc. Endosc. Percutan. Tech. 9, 106–109. Junge, K., Klinge, U., Prescher, A., Giboni, P., Niewiera, M., Schumpelick, V., 2001. Elasticity of the anterior abdominal wall and impact for reparation of incisional hernias using mesh implants. Hernia 5, 113–118. Korenkov, M., Beckers, A., Koebke, J., Lefering, R., Tiling, T., Troidl, H., 2001. Biomechanical and morphological types of the linea alba and
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