Muscular anatomy (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.
The fascicles that attach to the vertebrae and discs are oriented inferolaterally. They 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
Fascial relations of the Psoas (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.
The nerves travelling through the Psoas are:
- 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).
- 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)
Function of the Psoas
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 Major works phasically at different degrees of hip flexion producing predominately:
- 0-15 degs of hip flexion: stabilizer 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.
- Ipsilaterally sidebends the lumbar spine (Kim et al 2013).
- Lordoses and compresses the Lumbar spine: L1/2 and L2/3: extension. L3-5 pulls the lumbar spine downwards into compression. L5/S1: flexion (Penning 2000).
- 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.
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.