Version Changes
Revised. Amendments from Version 2
As suggested by Reviewer’s following changes are made in our manuscript. Changes in title Morphometry of pyramidalis muscle and its role in reconstructive surgeries: A cadaveric study is modified to Morphometry of pyramidalis muscle and its role in reconstructive surgeries: A cadaveric study in South Indian population. Changes in Text In the Introduction: Embryogenesis and evolution of pyramidalis muscle is added. In the Discussion: Clinical importance of some of the measured parameters was introduced. In the Conclusions: Abbreviated words are replaced by elongated words. Changes in figures and legends Hand drawn diagrams are inserted and Legends modified accordingly. Figure number is retained and subsections like 1A, 1B removed since now we have only hand drawn image, no cadaveric image.
Abstract
Background
The pyramidalis muscle is a tiny triangular-shaped muscle that is located in the anterior abdominal wall, which originates from the body of the pubis and pubic symphysis and is inserted into linea alba. This study aimed to measure the different parameters of the pyramidalis muscle in adult cadavers.
Methods
This study was carried out on 31 adults (26 males and five females) 10% formalin embalmed cadavers of both sexes from the Department of Anatomy at Kasturba Medical College, Manipal.
Results
The mean length of the right pyramidalis muscle was 64.44 ± 12.52 mm and the left pyramidalis muscle was 64.73 ± 12.81 mm. The mean width of the muscle was 15 ± 4.18 mm and 15.03 ± 3.52 mm on the right and left sides, respectively. The mean thickness of the muscle was 1.32 ± 0.55 mm and 1.4 ± 0.80 mm on the right and left sides, respectively. The distance between the umbilicus to pubic symphysis ranged from 130–192 mm and their mean was calculated to be 159.77 ± 15.36. The distance between the umbilicus and the apex of the muscle ranged from 72–123 mm.
Conclusions
The measured parameters like length, width and thickness may help the surgeons to locate the muscle during infra umbilical surgeries.
Keywords: Abdominal muscles, Anterior abdominal wall, Pyramidalis, Rectus sheath
Introduction
The pyramidalis muscle (PM) is a tiny triangular-shaped muscle that is located in the anterior abdominal wall in humans. 1 Muscle is present in the lower anterior part of the rectus abdominis muscle enclosed by the rectus sheath. The muscle originates from the anterior surface of the body of the pubis and pubic symphysis. Through the tendinous fibers, PM is fixed in the anterior part of the pubis and through the ligamentous fibers; it is attached to the pubic symphysis. Inferiorly PM has a broad base, as it ascends, the muscle becomes slender and gets inserted medially into linea alba. Insertion of muscle takes place halfway between the umbilicus and pubis.
It is believed that the action of the PM contracts the linea alba and strengthens the abdominal wall. 2 , 3 On the other hand, its absence does not tend to result in a functional loss. 4 As a result, authors believe it is a vestigial muscle left behind from the pouches of marsupials and monotremes. 5 Myotomes of the lower thoracic region migrate to the ventral part of the abdomen forming a pair of mesodermal primordium which further differentiate into fleshy bellies of the anterior abdominal wall in the prenatal period. 6 The literature regarding the embryogenesis of the pyramidalis muscle is scarce however, pyramidalis muscle is linked phylogenetically to the pouch inside monotremes like - hedgehog and the platypus and marsupials like - the koala or kangaroo. 7 With the evolution of the human, the pouch is responsible for the variation in the morphology and prevelance of the PM. 7 In primates and prosimians, the contraction of the pyramidalis muscle help in the expression of milk from the mammary gland. However due to the regression of supernumerary nipples in humans, pyramidalis muscle lost its importance and considered vestigial. 7
In humans, the PM has been linked to the assumption of upright posture. 8 The PM flap can be used to treat tiny intractable lesions in the foot and ankle, such as chemical wounds and prolonged osteomyelitis. 4 The PM, when cryopreserved, can be used as a source of stem cells that can then be employed to treat post-prostatectomy stress urinary incontinence. 9 When performing a longitudinal incision for a standard caesarean section, the PM was used to locate the linea alba and midline. 10
The morphometric study on PM in the south Indian adult human population is less studied. So this study aimed to explore the various parameters of PM in adult cadavers.
Methods
Ethical approval
The protocol was approved by the Institutional Ethics Committee (IEC-617/2021) Kasturba Medical College and Kasturba Hospitals, Manipal on September 15, 2021. Written informed consent was given by the body donors for teaching and research when they were alive.
Study design and data collection
In this descriptive cross-sectional study, 31 adult cadavers of both sexes (26 males and five females) were utilized from the Department of Anatomy at Kasturba Medical College (KMC), Manipal for morphometric measurements. Data were collected between October 1 st, 2021 to April 30 th, 2022, for a period of six months. Available cadavers in the department were used for the study purpose with the sample size 31. It was a convenient sampling approach. Specimens with damage to the anterior abdominal wall were not considered for the study. With the surgical instruments, cadavers were dissected and muscles were exposed. The following parameters were measured using a vernier caliper:
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Length of muscle from the pubic symphysis to its attachment to linea alba (apex) along its medial border ( Figure 1).
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Width at the base/origin of the muscle ( Figure 1).
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The thickness of the muscle at the midpoint of the measured length.
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A small gap was found in few cadavers between right and left PM. The length and breadth of this gap were measured ( Figure 2).
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Distance between the umbilicus and pubic symphysis (DUP) and distance between the umbilicus and apex of the muscle (DUA) ( Figure 3).
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Distance between the anterior superior iliac spine (ASIS) and lateral part of the base of the muscle (LBM) ( Figure 4).
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Distance between ASIS to apex of the muscle ( Figure 4).
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Distance between ASIS and the pubic symphysis (ASIS-PS) ( Figure 4).
Figure 1. Showing the measurement – length of the pyramidalis muscle (LM); and width of the pyramidalis muscle (WM) at the origin; PS-pubic symphysis; ASIS-Anterior superior iliac spine; U-umbilicus.
Figure 2. Showing the measurement of length (LGM) and width (WGM) of gap between the two PM. PM-pyramidalis muscle; PS-pubic symphysis; ASIS-Anterior superior iliac spine; U-umbilicus.
Figure 3. Showing the distance between the umbilicus to the pubic symphysis (DUP); distance between umbilicus to apex of the muscle (DUA); PS-pubic symphysis; ASIS-Anterior superior iliac spine; U-umbilicus.
Figure 4. Showing: Distance between the anterior superior iliac spine to the lateral part of the base of the muscle (ASIS-LBM); Distance between ASIS to tip of the muscle (ASIS-TIP); Distance between ASIS and Pubic symphysis (ASIS-PS); ASIS-anterior superior iliac spine; LBM-lateral part of the base of the muscle; PS-pubic symphysis; U-umbilicus.
All the dimensions were completed by the same researcher to avoid inter observer variation.
Data analysis
Descriptive statistics (mean, standard deviation, and range) were done for all measured parameters. To compare the right and left of each continuous variable, paired T-test was performed. P-value ≤ 0.05 was considered significant. The normally distributed data were correlated using Pearson's correlation coefficient analysis under the following combinations: i) Right and left PM length; ii) Right and left PM width; iii) Right PM length and right PM width; iv) Left PM length and left PM width; v) DUP and PM length (R and L); vi) DUP and PM width (R and L); vii) ASIS-PS (R) and PM width (R); viii) ASIS-PS (L) and PM width (L); ix) ASIS-PS (R) and PM length (R); and x) ASIS-PS (L) and PM length (L). SPSS (RRID:SCR_002865) version 16.0 (Chicago, SPSS Inc.) was utilized for data analysis.
Results
Morphometric measurements
Descriptive statistics
The length of the right PM ranged from 40–95 mm and the left PM ranged from 38–100 mm. The mean length of the right PM was 64.44 ± 12.52 mm and the left PM was 64.73 ± 12.81 mm. There was a 1–2 mm difference between the length of the right and left PM.
The width of the right PM and left PM ranged from 8–24 mm and 9–22 mm, respectively. The mean width of the right PM and left PM was 15 ± 4.18 mm and 15.03 ± 3.52 mm, respectively.
The PM thickness ranged from 0.5–2 mm on both sides. The mean thickness of muscle on the right side was 1.32 ± 0.55 mm and 1.4 ± 0.80 mm on the left side.
In 18 cadavers, a triangular gap was observed between right and left PM. Its length ranged from 8–26 mm and its width ranged from 5–17 mm.
Descriptive statistics of the remaining parameters are tabulated in Table 1. 16
Table 1. Overview of descriptive statistics calculated in adult cadavers (mm).
| Variables | Mean ± Standard deviation | Range |
|---|---|---|
| Length (R) | 64.44 ± 12.52 | 40–95 |
| Length (L) | 64.73 ± 12.81 | 38–100 |
| Width (R) | 15 ± 4.18 | 8–24 |
| Width (L) | 15.03 ± 3.52 | 9–22 |
| Thickness (R) | 1.32 ± 0.55 | 0.5–2 |
| Thickness (L) | 1.4 ± 0.80 | 0.5–4 |
| Length of the triangular gap * | 17.33 ± 5.30 | 8–26 |
| Breadth of the triangular gap * | 10.05 ± 3.60 | 5–17 |
| ASIS - LBM (R) | 122.5 ± 10.41 | 104–149 |
| ASIS - LBM (L) | 120.75 ± 8.00 | 105–138 |
| ASIS - Apex (R) | 118.25 ± 10.75 | 100–148 |
| ASIS - Apex (L) | 114.55 ± 12.16 | 94–144 |
| ASIS - PS (R) | 139.81 ± 7.74 | 122–158 |
| ASIS - PS (L) | 140.81 ± 7.10 | 128–152 |
| DUA | 97.48 ± 14.33 | 72–123 |
| DUP | 159.77 ± 15.36 | 130–192 |
R, right; L, left; ASIS, anterior superior iliac spine; LBM, lateral part of base of muscle; PS, pubic symphysis; DUA, distance between umbilicus to apex of muscle; DUP, distance between umbilicus to pubic symphysis.
Paired T-test
A paired T-test was used to compare all the right and left dependent variables. ASIS-Apex (R) and ASIS-Apex (L) were significantly different from each other since the p-value was 0.0014. There was no significant difference between the two respective groups of other dependent variables.
Pearson's correlation coefficient analysis
Pearson's correlation coefficient analysis was done since the data were distributed normally. A significant positive correlation was observed between right and left PM length and right and left PM width. The above result suggests that the increase in length or width of the right PM was directly proportional to the length/width of left PM.
Correlation analysis between right PM length and width and left PM length and width showed a weak but positive correlation. From this, we can note that as the length of the muscle increases, the width of the muscle may or may not increase.
A weak correlation was observed when DUP and ASIS-PS was compared with the right and left PM length and width. From this, we can conclude that the DUP and ASIS-PS have negligible effects on the length and width of the PM. The correlation coefficient and its level, analyzed between the parameters are tabulated in Table 2.
Table 2. Correlation coefficient and level of correlation between the measured parameters in adults.
| Correlated variables | Correlation coefficient | Level of correlation |
|---|---|---|
| Muscle length R & L | 0.8824 | Very strong positive |
| Muscle width R & L | 0.7149 | Strong positive |
| Right side muscle length and width | 0.3411 | Weak positive |
| Left side muscle length and width | 0.2525 | Weak positive |
| DUP & Muscle length (R) | 0.3036 | Weak positive |
| DUP & Muscle length (L) | 0.2602 | Weak positive |
| DUP & Muscle width (R) | 0.2719 | Weak positive |
| DUP & Muscle width (L) | 0.1586 | Very weak positive |
| ASIS-PS (R) & width (R) | 0.0908 | Very weak positive |
| ASIS-PS (L) & Width (L) | 0.1841 | Very weak positive |
| ASIS-PS (R) & length (R) | 0.2315 | Weak positive |
| ASIS-PS (L) & length (L) | 0.2609 | Weak positive |
R, right; L, left; ASIS, anterior superior iliac spine; DUP, distance between umbilicus to pubic symphysis; PS, pubic symphysis.
Discussion
Length and width of PM varies in different populations and it is tabulated in Table 3. It was observed in the present study, the length of PM in the female population was almost equal to the length of the PM reported by Natsis et al., 11 Hojaij et al., 12 and Kipkorir et al. 13 The width of the PM noted in the present study is almost equal to the study by Natsis et al. 14 A study by Das et al., 7 reported the mean thickness of 32 ± 0.55 mm in the right PM and 1.4 ± 0.80 mm in the left PM. In our present study, the mean thickness of muscle on the right side was 1.32 ± 0.55 mm and 1.4 ± 0.80 mm on the left side, which was relatively less than the thickness reported in the study by Das et al. 7
Table 3. Morphometric measurements of pyramidalis muscle done in other studies.
| Authors (ref.) | Year | Population | Pyramidalis measurements (mm) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Male | Female | |||||||||||||
| Mean length | Mean width | Mean length | Mean width | |||||||||||
| R | L | R+L | R | L | R+L | R | L | R+L | R | L | R+L | |||
| Anson (8) | 1938 | American | - | - | 68.2 | - | - | 19.8 | - | - | - | - | - | - |
| Ashley-Montagu (1) | 1939 | White Americans | - | - | 83.5 | - | - | 21.8 | - | - | 72.3 | - | - | 59.2 |
| African Americans | - | - | 76.3 | - | - | 19.2 | - | - | 59.2 | - | - | 17.7 | ||
| Kipkorir V (11) | 2021 | Kenyan | - | - | 72 | - | - | 71.5 | - | - | 62.2 | - | - | 64.7 |
| Kaur H (6) | 2016 | North Indian | 49.7 | 49.7 | - | 17.5 | 17.2 | - | 48.7 | 47.8 | - | 12.5 | 14.5 | - |
| Natsis K (9) | 2016 | Greek | 83.7 | 75.0 | - | 16.1 | 15.6 | - | 61.8 | 65.6 | - | 15.0 | 15.5 | - |
| Das SS (12) | 2017 | Indian | 52.2 | 53.9 | - | 18.3 | 17.0 | - | 50.1 | 51.2 | - | 17.8 | 16.2 | - |
| Hojaij FC (10) | 2020 | Brazilian | 70.6 | 68.0 | - | 18.5 | 18.3 | - | 64.2 | 63.8 | - | 19.1 | 19.4 | - |
| Present study | 2023 | South Indian (Male & female) | 64.4 | 64.7 | - | 15.0 | 15.0 | - | - | - | - | - | - | - |
R, right; L, left.
In the present study, strong positive correlations were found between the lengths and widths of the PM on both sides, which is in line with the results of the study by Natsis et al. 11
The present study also measured ASIS-PS, ASIS-LBM, ASIS-Apex, DUP and DUA. The mean, standard deviation and range values of the aforementioned parameters are tabulated in Table 1. These parameters were not recorded in the previous studies; therefore we were not able to compare it with the other studies.
The present study also showed correlations between the DUP and PM length (R and L), DUP and PM width (R and L), ASIS-PS (R) and PM width (R), ASIS-PS (L) and PM width (L), ASIS-PS (R) and PM length (R), ASIS-PS (L) and PM length (L). The correlation analysis showed a weak but positive correlation between them. This suggests that increases or decreases in ASIS-PS and DUP have negligible effects on the length and width of PM.
Plastic surgeons and general surgeons must be aware of potential regional differences, to reduce the chance of consequences and implement the most effective therapeutic strategy. 14 A prostate-cystopexy employing PM for prostate stabilization can treat individuals with functional dysuria caused by the prostate's retrograde inclination and those with bladder obstruction caused by motor neuron injuries. 15
Pyramidalis Pubo-Umbilical Index can be calculated by using length of PM and distance between umbilicus to pubic symphysis (DUP). Further the knowledge of PPUI could be useful for a variety of clinical and surgical procedure s as it encloses about 40.78% of lower abdominal wall. 3
There is no literature regarding the importance of each parameters or distances measured in this study.
However, it is essential for the surgeons to be aware of the morphometry of the pyramidalis muscle prior to making the different incisions needed for procedures involving the anterior abdominal wall.
Implications
PM act as a landmark for surgeons during infra-umbilical and suprapubic incisions while performing caesarean sections. The PM may be collected through a reasonably unnoticeable Pfannenstiel (pubic) opening with minimal donor site complications making it a desirable donor muscle for microsurgical transfer.
Limitations
The limitation of the present study is the smaller female cadaver sample size, with respect to the male cadaver sample size. Therefore a sex-based comparison was not performed in this study.
Recommendations for further research
This study can be done to explore the foetus morphometric data and its clinical implications.
Conclusions
The parameters like length, width, and thickness and new parameters such as Anterior superior iliac spine Pubic symphysis, Anterior superior iliac spine - Lateral part of the base of the muscle, Anterior superior iliac spine - Apex, Distance between umbilicus to pubic symphysis and Distance between umbilicus to apex of the muscle may help the surgeons to locate the muscle in the suprapubic region while performing surgical procedures to remove the flap of donor pyramidalis muscle.
Acknowledgements
Authors are thankful to all the teaching and non-teaching staffs at department of Anatomy, KMC Manipal for their support in completing this study.
Funding Statement
The author(s) declared that no grants were involved in supporting this work.
[version 3; peer review: 3 approved]
Data availability
Underlying data
Figshare: Morphometry of pyramidalis muscle and its role in reconstructive surgeries: A cadaveric study. https://doi.org/10.6084/m9.figshare.22200181. 16
This project contains the following underlying data:
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EX sheet - Morphometric study of pyramidalis muscle.xlsx
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STROBE-checklst.doc
Data are available under the terms of the Creative Commons Zero “No rights reserved” data waiver (CC0 1.0 Public domain dedication).
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