The present study was conducted on 25 (17 males and 8 females) embalmed cadavers in the age group 35-60 years in the Department of Anatomy, Maulana Azad Medical College, New Delhi, India from August 2014 to August 2016. The cadavers with injury scars or surgery marks on abdominal wall were excluded from the study. During dissection of anterior abdominal wall, anterior wall of rectus sheath was reflected laterally to expose the pyramidalis muscle on the lower part of rectus abdominis muscle. Following variables were studied: Presence of pyramidalis muscle; total muscle length; width and thickness of the muscle; presence of any morphological variation of the origin and insertion of the muscle. The width and thickness was measured at the mid-point of the muscle. Measuring tape and digital vernier caliper were used for measurements. Pyramidalis- puboumbilical index which is the percentage of distance between pubic symphysis and umbilicus into which the pyramidalis extends (length along the medial border) was also determined.

Statistical analysis was performed using the Chi-Square test for nominal categorical data and Student’s t-test for normally distributed continuous variables in SPSS version 23 to assess the relationship between the examined variables. A level of significance of p < 0.05 was used. All the observations and results were tabulated and compared with previous studies.

The results of various biometric data of pyramidalis muscle is summarised in [Table/Fig-1,2,3 and 4]. Pyramidalis muscle was present in 23 (92%) cases. Out of which in 18 cases (72%) (13 males (76.47%), 5 females (62.5%)) it was bilaterally present, in 2 cases (8%) {1 male (5.88%), 1 female (12.5%)} it was bilaterally absent, in 3 cases (12%) {2 males (11.76%), 1 female (12.5%)} it was unilaterally present on right side and in 2 cases (8%) {1 male (5.88%), 1 female (12.5%)} it was unilaterally present on left side [Table/Fig-1,5]. No anatomical variations related to origin or insertion of muscle was seen. The average value of total muscle length, width and thickness of muscle are depicted in [Table/Fig-2,3 and 4] respectively. The mean pyramidalis-puboumbilical index was 35.15±4.38%, 36.01±4.97% in males and females respectively [Table/Fig-2]. No anatomical variations with regard to origin and insertion were seen.

[Table/Fig-5]:

Lower part of anterior abdominal wall showing- A) Bilateral presence of pyramidalis muscle, B) Bilateral absence of pyramidalis muscle, C) Unilateral left side pyramidalis muscle, D) Unilateral right side pyramidalis muscle. P- pyramidalis muscle, RA – rectus abdominis muscle, RS- rectus sheath, U- umbilicus.

Vestigial muscles in human beings are those muscles which are tendinous in larger part or reduced in size compared to the homologous muscles in other species or is frequently absent within or between populations. Pyramidalis muscle is also such muscle which is known to exhibit variations.

Phylogenetically, pyramidalis is linked to the pouch inside monotremes such as hedgehog and the platypus and marsupials such as the koala or kangaroo. In humans, standard anatomical textbooks describe the incidence of the pyramidalis muscle to be 83–90% [1]. Different authors around the globe had reported an incidence of pyramidalis in the range 82- 100% in various ethnic groups [Table/Fig-6] [3,6–9]. In our study in Indian population, an incidence of 92% (males-94.11%, females-87.5%) was reported. In Japanese [6] population, the incidence of pyramidalis was greater (94.5%) as compared to our study population. Incidence in the present study was greater than that reported in American blacks (82%) [3]. Kaur et al., and Jit et al., reported incidence of 87% and 100% in north Indian population [3,7]. Present study incidence fall between these data as our population included various subjects of different regions of India. The frequency of pyramidalis muscle in humans is probably influenced by racial and ethnic characteristics. Highest incidence of unilateral pyramidalis was reported in both the genders (male-17.64%, females-25%) of our study population as compared to the rest of populations of the world. Bilateral presence of pyramidalis muscle (72%) was the most common variety. Males showed a greater incidence of bilaterality (76.47%) than females (62.5%). Unilateral pyramidalis on right side (12%) was more common than unilateral left side (8%). Unilateral left pyramidalis was more common in females (12.5%) than males (5.88%). These differences were not statistically significant.

The morphometry of pyramidalis also varies with its frequency. Side asymmetry in length, width and thickness was noticed which was statistically insignificant [Table/Fig-3,4,5]. The biometric parameters of right side pyramidalis muscle were greater than the left side except the muscle length which was greater on left. The average muscle lengths in both the genders were lesser than the corresponding Greek ethnicity [8] and almost equal to the north Indian population [3,7]. The difference may be due to racial variations i.e., difference of stature in different populations. In contrast the average muscle width was greater than the Greek population [8] and roughly equal to the north Indians [3,7]. No statistically significant gender dimorphism was found on the basis of muscle length, width and thickness [Table/Fig-2,3 and 4]. No anatomical variations related to origin or insertion of muscle was seen.

Since, pyramidalis muscle serves as a landmark for surgeon for making a midline infraumbilical incision [10], pyramidalis- puboumbilical index [3] was calculated which could be clinically significant. The average pyramidalis- puboumbilical index was 35.15±4.38% in males and 36.01±4.97% in females [Table/Fig-2]. These values may be useful for surgeons as pyramidalis muscle covers the lower 35-36% of distance between umbilicus and pubic symphysis.

Though literature on the functions of pyramidalis has been vague yet it is unquestionably said to tense the linea alba. Some authors consider it to be immaterial and vestigial, but it is frequently encountered by gynaecologists during caesarean sections [11]. Lovering and Anderson studied the architecture and the fibre type of pyramidalis muscle and predicted that, this muscle generates less than 1% of the anticipated force generated by the rectus abdominis in normal sized adult [12]. This led them to the conclusion that the importance of this less amount of force on linea alba is not understandable. But, yet on a different note, it is frequently harvested to conduct electrophysiological experiments on muscles [13]. Apart from this, clinically the pyramidalis muscle has been used as a source of skeletal muscle stem cells for the treatment of post-prostatectomy stress urinary incontinence [5] and also been used for microsurgical transfer to treat small recalcitrant wounds in the foot/ankle region because of reduced donor site morbidity as compared to traditional free flaps [14]. Therefore, the biometric measurements of pyramidalis could help surgeons in proper sizing of pyramidalis muscle flaps. Pyramidalis muscle has been used successfully to treat severe dysuria due to insufficiency of the perineal floor muscles caused by upper and lower motor neuron lesions, by fixing the prostate with the pyramidalis muscle, associated with a bladder-pexy [15]. Considering the above facts, the role of the pyramidalis muscle cannot be overlooked.

The limitation of current study was that the sample size was less (25 cadavers). Also fetal specimens were not included in this study, which could have given appropriate idea about the development of this muscle.

This cadaveric study shows that, the pyramidalis muscle is nearly a constant feature in Indian population. The morphometry of pyramidalis also varies along with its incidence. The muscle morphometric unevenness and its proven applications will help surgeons during lower abdominal wall surgeries.