Psoas Major
Muscular anatomy of the Psoas Major (Saiko & Stuber, 2009)
The psoas has fibrous attachments to the anterior aspect of L1-5 transverse processes and to the anteromedial aspect of L1-5 discs (except L5/S1) and vertebral bodies.
Desouki et al (2016) found the psoas composed of two sections. The whole of the lumbar plexus runs between these two sections:
Anterior section: comprises of fleshy slips that arise from the anterolateral part of the vertebral bodies of T12-L5 and their corresponding intervertebral discs (except L5/S1). The fascicles are oriented inferolaterally and make up the main part of the psoas.
Posterior section: comprises of strips that originate from the front and lower border of the transverse processes of T12-L5. This makes the accessory part of the psoas.
The L1-5 spinal nerves roots emerge in front of the lumbar transverse processes and enter into the part of the psoas which is in-between the muscle fibres originating from (i) the vertebral bodies and intervertebral discs and (ii) the muscle fibres originating from the transverse processes.
An anchoring retinaculum at the level of the lumbosacral junction keeps the psoas in place alongside the lumbar spine, making its action on the increase of the lumbar lordosis independent of the lumbar curvature (Siccardi et al 2021).
The fascicles come together as a common tendon which descends over the pelvic brim. It shares a common insertion with the iliacus muscle on the lesser trochanter of the femur.
The psoas major is the largest muscle in cross section at the lower levels of the lumbar spine and sacroiliac joint.
Fascial relations of the Psoas Major (Saiko & Stuber (2009)
The fascial relations of the psoas are:
Medial arcuate ligament: this is a tendonous continuation of the superior psoas fascia. The medial arcuate ligament continues superiorly to the diaphragm to attach on to the transverse processes of L1 and L2 (Cai et al 2013) and in some cases L3 (Deviri et al 1988).
Right and left crus of the diaphragm: the crura constitute the spinal attachments of the diaphragm. They attach to the anterolateral component of the L1-3 vertebral bodies and anterior longitudinal ligament. The crura and their fascia overlap the psoas major and appear to be continuous with this muscle until they come more anterior and blend with the anterior longitudinal ligament.
Pelvic floor fascia, conjoint tendon, transverse abdominis and internal oblique: as the psoas descends, its inferomedial fascia becomes thick at its inferior portion and is continuous with the pelvic floor fascia. This forms a link with the conjoint tendon, transverse abdominis, and the internal oblique. As the psoas major courses over the pelvic brim, the fascia of the fascicles from the transverse processes attach firmly to the pelvic brim.
Neurological relations of the Psoas Major
The nerves travelling through the psoas are:
The L1-5 spinal nerves roots emerge in front of the lumbar transverse processes and enter into the part of the psoas major which is in-between the muscle fibres originating from (i) the vertebral bodies and intervertebral discs and (ii) the muscle fibres originating from the transverse processes. The lumbosacral plexus lies in the psoas major between the transverse process and vertebral body exiting it distally along the medial edge of the muscle (Benglis et al 2009).
Femoral nerve.
Lateral femoral cutaneous nerve.
Obturator nerve. This was refuted by Iwanaga et al (2020) who found the obturator nerve descends posterior to the psoas and never through it.
Superior cluneal nerve (Tubbs et al 2010).
Function of the Psoas Major
Hip
Skyryme et al (1999) found the main function of the psoas as a hip flexor with other movements dependent upon the position of the subject:
Anatomical position: hip flexion with no rotation.
Hip abduction: hip flexion, adduction, and external rotation of the hip.
Hip adduction: hip flexion with no rotation.
Maximal hip flexion: hip adduction.
Yoshio et al (2002) found the psoas works phasically at different degrees of hip flexion producing predominately:
0-15 degs of hip flexion: stabiliser of the femoral head in the acetabulum. Erects the lumbar spine.
15-45 degs of hip flexion: still as an erector of the Lumbar spine!
45 to 60 degs of hip flexion: hip flexor.
Lumbar spine
Ipsilaterally sidebends the lumbar spine (Kim et al 2013).
Lordoses and compresses the lumbar spine: L1/2 and L2/3: extension (in midrange lordosis). L3-5 pulls the lumbar spine downwards into compression. L5/S1: flexion (in midrange lordosis) (Penning 2000). Whilst the extension and flexion movement are only very small at maximum contraction the psoas exerts severe compression and large shear forces (Bogduk et al 1992). An anchoring retinaculum at the level of the lumbosacral junction keeps the psoas in place alongside the lumbar spine, making its action on the increase of the lumbar lordosis independent of the lumbar curvature (Siccardi et al 2021).
This flexion-extension movement was also generated by discrete fascicles in the muscle. In midrange lordosis the posterior fascicles of arising from the lumbar transverse process are biased towards producing trunk extension with a limited role effect on hip flexion, functioning just to maintain hip flexion in a lordotic posture; in midrange lordosis the anterior fascicles arising from the vertebral body are biased towards producing trunk and hip flexion (Park et al 2013).
Ipsilateral rotation: Jeon et al (2016) found that during SLRT there is an ipsilateral rotation of the lumbar spine e.g left SLRT produces rotation of the Lumbar spine to the left. This rotation was counterbalanced by the contralateral psoas producing ipsilateral rotation in the opposite direction of the SLRT e.g. the right psoas would produce rotation of the Lumbar spine to the right. Andersson et al (2002) also found the psoas as an ipsilateral rotator.
References
Psoas Major: a case report and review of its anatomy, biomechanics, and clinical implications (2009). Sandy Sajko & Kent Stuber.
Anatomy and landmarks for the superior and middle cluneal nerves: application to posterior iliac crest harvest and entrapment syndromes (2010). Tubbs RS, Levin MR, Loukas M, Potts EA, Cohen-Gadol AA.
Psoas muscle and lumbar spine stability: a concept uniting existing controversies Critical review and hypothesis (2000). L Penning
An anatomical study of the lumbosacral plexus as related to the minimally invasive transpsoas approach to the lumbar spine (2009). Benglis DM, Vanni S, Levi AD.
Psoas major and its controversial rotational action (1999). Skyrme AD, Cahill DJ, Marsh HP, Ellis H.
The function of the psoas major muscle: passive kinetics and morphological studies using donated cadavers. (2002). Yoshio M, Murakami G, Sato T, Sato S, Noriyasu S.
Asymmetry of the cross-sectional area of paravertebral and psoas muscle in patients with degenerative scoliosis (2013). Hyoungmin Kim, Choon-Ki Lee, Jin S. Yeom, Jae Hyup Lee, Jae Hwan Cho, Sang Ik Shin, Hui-Jong Lee, andBong-Soon Chang
Diverging intramuscular activity patterns in back and abdominal muscles during trunk rotation. (2002). Andersson EA, Grundström H, Thorstensson A.
Comparison of psoas major muscle thickness measured by sonography during active straight leg raising in subjects with and without uncontrolled lumbopelvic rotation (2016) In-cheol Jeon, Oh-yun Kwon, Jong-hyuck Weon, Sung-dae Choung, Ui-jae Hwang
Medial arcuate ligament: a new anatomic landmark facilitates the location of the renal artery in retroperitoneal laparoscopic renal surgery (2013). Cai W, Li HZ, Zhang X, Song Y, Ma X, Dong J, Chen W, Chen GF, Xu Y, Lu JS, Wang BJ, Shi TP.
Medial and lateral arcuate ligaments of the diaphragm: attachment to the transverse process. Deviri E, Nathan H, Luchansky E.
Relationship of the Obturator Nerve and Psoas Major: Anatomic Study with Application to Avoiding Iatrogenic Injuries. (2020). Iwanaga J, Warner T, Scullen TA, von Glinski A, Ishak B, Bui CJ, Dumont AS, Tubbs RS.
N Bogduk N, M Pearcy M, G Hadfield G (1992). Anatomy and biomechanics of psoas major
Siccardi M, Tariq M, Valle C (2021). Anatomy, Bony Pelvis and Lower Limb, Psoas Major
PARK R, TSAO H, CLAUS A, CRESSWELL A, HODGES P (2013). Changes in Regional Activity of the Psoas Major and Quadratus Lumborum With Voluntary Trunk and Hip Tasks and Different Spinal Curvatures in Sitting
