MORPHOLOGY OF PSOAS MINOR AND PSOAS ACCESSORIUS S D Joshi, S S Joshi, U K Dandekar, S R Daini Deptt. of Anatomy, Rural Medical College, P.I.M.S. Loni Distt. Ahmednagar, Maharashtra

ABSTRACT

Psoas minor muscle is considered to be a vestigial muscle like palmaris longus and plantaris. It lies

anterior to the psoas major muscle. It has a small fleshy belly and a long flat tendon. This small muscle is the

representative in man of a relatively large muscle which in most mammals plays an important part of flexing the

lumbar part of vertebral column. In some lower mammals it is larger than psoas major and acts as a flexor of

pelvis; this being utilized in running by the rabbit and the apes in brachiating. It begins to disappear in those

primates which assume the erect posture. The presence of this muscle varies from person to person and on the

two sides. It is present in about 40 to 60 % of the population. In the majority it is inserted on the iliopubic

eminence but may also be attached to the arcuate line of ilium and the iliac fascia. While going through the

literature we did not come across any detailed study of the muscle in the population of this region. Hence, we

have conducted a detailed study of psoas minor muscle in 30 cadavers bilaterally. In the present series this

muscle was absent in 70%. Average length of fleshy belly was 8.23 em and that of the tendon 15.48 em. In some

cases an unusual interesting feature observed was the origin of some fibers of psoas major from the deep surface

of flattened tendon of psoas minor muscle, which we have named as Psoas accessorius.

Key Words-Morphology, Psoas minor, Psoas major, psoas accessorius.

INTRODUCTION

The Psoas Minor is also known as 'Psoas

parvous' (Bryce, 1923'; Keith, 1948l "Psoas" is a

Greekwordforthe loins -the muscle of the lower back

(Webster's comprehensive dictionary, 19863). This

muscle although well developed and constant in most

lower animals, sometimes exceeding the psoas major

in size, is not constant in man' and is considered by

many as a vestigial muscle. It begins to disappear in

those primates who assume the erect posture2.This

muscle lies anterior to psoas major (Hollinshead,

19854; Moore, 19855

; McMinn, 19906; Williams, 19957

;

Snell, 20088) but Wood Jones (1953)9 stated that it lies

anteromedial to the psoas major except at the pelvic

brim, where its expanded tendon turns to the medial

side of that muscle. Kendall (1993) 10 stated that it is

relatively unimportant and is only present in about

40% of the population; while Wood Jones (1953)9 has

described it to be present, on one or both sides, in

about 45% of the population. In a study of racial

discrepancies, psoas minor has been reported to be

absent in 50% in Orientals; 57% in whites and 67% in Correspondence Dr. S. D. Joshi Pro( & Head, Dept. of Anatomv Rural Medical College, PIMS, Loni, Dis//. Ahmednagar. Maharastra Ph . . 02422 • 272067 Email: sdjoshi _ 2003@hotmail.com

J. Anat. Soc. India 59(1) 31-34 (2010) 31

blacks (Hanson P et al, 1999 "). The short fleshy belly

soon gives way to a long, flat tendon which may be

attached to the pecten pubis, iliopectineal eminence

and/ or iliac fascia 1'7

'9

'12

'13

''4

· • Looking at the differences

regarding its presence, laterality, size and variability

of insertion and its evolutionary significance it was

decided to undertake a detailed study of this muscle;

although small and vestigial but not insignificant.

MATERIAL AND METHODS

This study was carried out on 30 embalmed

cadavers (males-24, females-6) available in this

department, over a period of two years. Clearance of

institutional ethical committee was obtained before

starting the work. These were the cadavers which

were utilized by medical students for routine

dissection. When all organs were removed from the

abdominal cavity then the dissection of posterior

abdominal wall was undertaken.· On the anterior

surface of Psoas major muscle, when present, Psoas

minor muscle was cleaned right from its origin (above

the medial arcuate ligament) up to its insertion. Any

variations of this muscle at its attachment (both at the

origin as well as insertion) or along its whole length

were recorded and, whenever deemed necessary,

photographs were taken. The length and width of

Morphology of Psoas ....... .. .. .. ... ... ........ .. . SD Joshi, S S Joshi, U K Dandekar, S R Daini

fleshy belly as well as long tendon were measured

with the help of divider, thread, scale and digital

vernier caliper. Careful dissection of distal attachment

was done to verify the insertion of tendon to the

neighbouring fascia and the bone.

RESULTS

In the present study psoas minor was found

to be present in 9 cases (30%). It was present

bilaterally in 23.3% and unilaterally only on the left

side in 6.6%. In all the cases the psoas minor was

present onthe anterior surface of psoas major and

tended to lie medial to the psoas major towards its

insertion (Fig. 1 A). The origin of psoas minor was from

the sides of 12'h thoracic and 1" lumbar vertebrae and

the intervening disc between them, except in two

cases, where the origin was from a lower level i.e.

from sides of 3'd and 4'h lumbar vertebrae (Fig. 3A) and

the intervening intervertebral disc bilaterally. In these

two. cases the muscle was quite bulky. In almost all

cases the tendon, when traced inferiorly, flattened out

and blended with the iliopsoas fascia (Fig. 1 8). Some

aponeurotic fibres were inserted on the iliopubic

eminence and the arcuate line. The average length of

fleshy belly was 8.23 em (Rt-7.71cm, Lt-8.75 em) and

that of the tendon was 15.52 em long (Rt-15.8 em, Lt-

15.24 em). The average maximum width of the fleshy

part of muscle was 1.31 em (Rt-1.45cm, Lt-1.18 em)

and that of the tendon was 0. 7cm (Rt-0.67 em, Lt-0. 73

em). In few specimens the fleshy fibres of the muscle

extended further lower down on the deep surface of

the tendon. In 4 cases (25 %) it was found that some

fibres of psoas major were arising from the deep

surface of the flattened tendon of psoas minor (

Fig.2A,28 & Fig.3A,38) and in this position these fibres

continued on the superficial surface of the muscle

right up to its insertion. In 7 cases (43.75%) there was

a zone of overlapping between fleshy belly and

tendon (Fig.2A).

Fig.1A: Showing Right Psoas minor muscle -'A' lying

superficial to Psoas major muscle- '8'.

Fig.1B: Showing left Psoas major (B) and Psoas minor (A).

When traced inferiorly tendon of Psoas minor merges with

the dence iliopsoas fascia (indicated by black dots). Also

seen is the genitofemoral nerve (black arrow).

Table 1: showing the Length and Width of Fleshy belly and the Tendon of Psoas minor muscle (In em):

FLESHY BELLY TENDON TOTAL LENGTH

Right Range Average Range Average Range Average

Length 3.10.5 7.71 " 14-18.5 15.8 18-29 23.51

Width 0 .8-2 1.45 0 .5-1 0.67

Left

Length 5-11 8.75 12.5-20 15.24 20-30 24

Width 0.5-2 1.18 0 .21.5 0.73

J. Anat. Soc. India 59(1) 31-34 (2010) 32

Morphology of Psoas ........... ... ... .... ......... SO Joshi, S S Joshi, U K Dandekar, S R Daini

Fig.2A: Showing left Psoas major (8), tendon of Psoas minor (A), the fleshy fibers of a muscle 'C' (Psoas accessorius) seen arising from the deep surface of Psoas minor and descending superficial to Psoas major. The tendon of Psoas minor has been turned to show the origin of this muscle from the deep surface of the tendon. Edge of Psoas minor has been indicated by red arrow.

Fig.28: Showing left Psoas major (B). The tendon of Psoas 'minor has been turned to expose its posterior surface wherein, the fleshy fibres of Psoas minor (A) are seen to join the tendon (red arrow). Belowthatthe origin of fleshy fibres of Psoas accessorius are seen-'C' as shown in fig.2A

DISCUSSION

The presence of Psoas minor muscle varies

from person to person and on the two sides. Many

workers have stated that it is present in about 60% of

the population5• s.

7' s. 12

' 13

' 15

• In the present study it was

present in 30% cases.

J. Anat. Soc. India 59(1) 31-34 (2010) 33

Fig.3A: Showing left Psoas major (B) and lying superficial

to it is seen a well developed Psoas minor. muscle (A)

vertebrae. A well defined tendon is present along the lateral

edge of this muscle. Also seen are the fibres of Psoas

accessorius (C) arising from the deep surface of Psoas

minor.

Fig.3B: Showing Left Psoas major (B), tendo~:~ of Psoas minor

(A) and the sheet of Psoas accessorius (C) arising from the

deep surface of Psoas minor tendon and spreading anterior

to Psoas major.

The study of racial differences described by Saib ( 1934) has been referred to by Hanson P et al ( 1999) 11

wherein it was reported that the psoas minor was absent in 50% in Orientals; 57% in whites and 67% in blacks. Hanson Petal (1999) 11 studied psoas minor in young blacks (21 cases) and white men (23 cases) during routine autopsies. They observed that the psoas minor was present in 2 subjects (9%) in blacks. In the whites, the psoas minor was present on both sides (87%). Further, on visual inspection the psoas minor in black subjects appeared as a slight thickening of the fascia surrounding the psoas major, rather than actual muscle. In contrast, in the white group the psoas minor was a well defined muscle positioned anterior to and separate from the psoas

Morphology of Psoas .............................. SD Joshi, S S Joshi, U K Dandekar, S R Daini

major fascia.

Anatomical texts have described the origin of psoas minor from the bodies of 12"' thoracic and 1 .. lumbar vertebrae and the disc between them'·5

•6

•7·'

0·'2.'6

In the present study the origin of the muscle was found to be similar except in 2 cases where psoas minor was arising from sides of L3 and L4 vertebrae. The short fleshy belly soon gives way to a long flat tendon, which is attached to the pecten pubis, iliopectineal eminence and iliac fascia'·9

·'2

•13

·'•· The psoas minor is a weak flexor of the lumbar spine. The muscle also flexes the pelvis on the trunk5·'3. As a flexor of the pelvis on the spine its value to the rabbit when running and to the apes while brachiating is quite apparent17

·

It has also been described that the tendon of psoas minor may be mistaken for the genitofemoral nerve 17

• It is active in cats when they arch their backs •s. It is also proposed that psoas minor could support the lumbar trunk during stance. While reviewing the literature it is observed that except for the incidence of presence or absence of the muscle no other morphological features have been described and as such the findings of the present work cannot be compared with that of any other workers.

CONCLUSION

This study has been conducted on 30 cadavers bilaterally. In the present series Psoas minor was found to be present in 30% cases. The absence of the muscle described in the literature ranges from 50-90%; also racial differences have been reported. During evolution and with the assumption of bipedal gait, the function and hence the incidence and size of psoas minor has been reduced. But, at times the muscle may be large. We have found a hitherto undescribed muscle mass in the literature which arises from deep surface of the tendon of psoas minor and spreads out as a substantial fleshy mass on the anterior surface of psoas major and continues in the same position till its insertion and have named it as -­Psoas Accessorius.

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J. Anat. Soc. India 59(1) 31-34 (2010) 34

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