A proposed novel action of psoas minor

Psoas minor is a curious muscle in humans, lacking a clear and universally agreed function and reported to be present in only 33–65% of individuals ( , , , ). This commentary proposes a novel functional role for psoas minor, with the aim of stimulating interest in this muscle, both anatomically and clinically.

Psoas minor is located retroperitoneally deep in the abdomen, running nearly parallel with and immediately anterior to the much larger psoas major ( Fig. 9.2.1 ). Although structural variation is not uncommon, it typically originates proximally on the vertebral bodies of T12–L1 and the intervening intervertebral disc ( ; see Ch. 77 ). The relatively long tendon of psoas minor typically has a distal bony attachment to the iliopubic ramus (see Fig. 9.2.1 ): this attachment is usually medial to the anterior edge of the acetabulum and medial and slightly superior to the iliopubic eminence ( ). Therefore, unlike psoas major, psoas minor typically has a bony distal attachment to the pelvis, not to the femur, that likely accounts for its described functions as a weak flexor of the trunk or of the lumbar spine ( , ). However, given its small muscle belly and the presence of other larger surrounding muscles that are capable of these actions, it seems unlikely that these are either the sole or dominant functions of psoas minor.

Anatomical connections between the tendon of psoas minor and the iliac fascia

In addition to the distal attachment of psoas minor to the pelvis, anatomy textbooks usually report a secondary attachment into adjacent fascia. In a cadaveric study, the tendon of psoas minor was found to be consistently fused with the structural ‘backbone’ of the iliac fascia, and the entire musculofascial entity was referred to as the psoas minor–iliac fascia complex ( ). Detailed measurements showed that the iliac fascia component of this complex draped directly over the distal, pre-tendinous portion of iliopsoas and the femoral nerve ( Fig. 9.2.2A ). The variably curved superior edge of the iliac fascia has been referred to as the iliopectineal arch ( ), and this is the term that will be used throughout this commentary.

Fig. 9.2.2, Anterior views of the iliac fascia component of the left psoas minor–iliac fascia complex. A , Psoas minor tendon blending with the iliac fascia (see tip of probe). Note that the tendon of psoas minor has been manually pulled medially. Normally, in situ, the tendon of psoas minor lies nearly parallel and in contact with the anterior belly of psoas major. Parallel black lines show the approximate location of the inguinal ligament. B , Pictorial summary of the distal attachments of the psoas minor–iliac fascia complex. The dashed black lines represent the location of the iliac fascia. The locations of the attachment of the tendon of psoas minor into the iliac fascia are indicated by variably spaced, red horizontal areas; and into the pelvic bone by overlapping red, vertical areas. For perspective, the estimated location of psoas major and iliacus, which for the most part lie deep to the iliac fascia, are depicted using a translucent outline.

A composite tracing of the location of the iliac fascia obtained through study of cadaveric specimens is shown in Fig. 9.2.2B ( ). Of the 21 hips in which psoas minor was present, the muscle was attached distally to bone and to the iliac fascia in 19 (90.5%) (see vertical and horizontal red areas in Fig. 9.2.2B , respectively). In the remaining two hips (from the same cadaver) psoas minor was attached distally only to the iliac fascia (which was fully formed). Thus, in all cases, the tendon of psoas minor was firmly blended with the iliac fascia. The location of this tendon–fascial junction was on average 69.9 ± 9.0 mm medial and 29.8 ± 16.1 mm superior to the anterior–superior iliac spine (see Fig. 9.2.2B ), placing the tendon–fascial junction immediately lateral and anterior to the sacroiliac joint.

As is apparent in the composite perspective in Fig. 9.2.2B , the arched superior edge of the iliac fascia (i.e. the iliopectineal arch) was generally concave, faced superiorly and could be pulled up and away from the underlying iliopsoas. The superior–medial aspect of the iliac fascia was attached to bone and crossed the sacroiliac joint in 11 hips (52.4%). Inferior to the sacroiliac joint, the fascia was attached to the inner pelvic brim and eventually became continuous with the pelvic attachment of the tendon of psoas minor (see Fig. 9.2.2B ). The iliac fascia continued and was attached inferiorly to a point generally medial to the iliopubic junction, i.e. the junction between the anterior ilium and superior pubic ramus. En route, some fibres flowed posteromedially to interlace with the external fascia covering the superior part of obturator internus.

The lateral extent of the iliopectineal arch was attached to bone in a region immediately superior and inferior to the anterior superior iliac spine. However, the posterior side of the iliac fascia lacked a pelvic attachment. The anterior surface of the iliac fascia blended with the deep part of the inguinal ligament and the proximal fascia lata of the thigh. A near continuous sheet of iliac fascia therefore existed between the iliopectineal arch superiorly and fascia lata inferiorly, providing an open vertical aponeurotic sleeve that allowed unrestricted passage of the underlying iliopsoas and femoral nerve into the thigh (see and Fig. 9.2.3 ). Although iliopsoas and the femoral nerve passed posterior to the iliac fascia, the femoral vein and artery (embedded within the femoral sheath) passed superficial to the fascia. The anterior surface of iliopsoas and the femoral nerve were in direct contact with the posterior surface of the iliac fascia, whereas the posterior surface of iliopsoas was in firm contact with a segment of the anterior pelvic brim.

Fig. 9.2.3, Anterior view showing the left pelvic region of a cadaver, with intestines and inguinal ligament removed (orientation seen in Fig. 9.2.2A has been reversed). The probe is shown lifting the iliopectineal arch (and iliac fascia) away from the underlying left iliacus, psoas major and femoral nerve. The tendon of psoas minor has been manually pulled superiorly and medially, adding to the tension in the iliac fascia. Caudally, the iliac fascia blends with the fascia lata of the proximal thigh.

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