Psoas Quartus and Femoral Nerve Branching: A Case Report and Potential Clinical Implications

Introduction

The iliopsoas muscle complex is comprised of psoas major, psoas minor, and iliacus muscles. Together, these muscles induce flexion at the hip-coxal joint. The fusiform psoas major descends the vertebral column toward the femur where it fuses with the triangular iliacus muscle at the level of the inguinal ligament to form the conjoined iliopsoas muscle prior to attaching on the lesser trochanter of the femur [1,2].

Variations in psoas minor are well documented, with 40–66% of individuals having complete agenesis of the muscle [3,4]. Associated with psoas muscle variations are lumbar plexus variations, ranging from 8.8% to 47.1%, including femoral nerve variations such as 2 to 3 branches separated by psoas major muscle fibers [5]. In contrast, variations in psoas major muscles are rare. Among the few known variants for this muscle are the psoas tertius and psoas quartus. These variants were first noted by Clarkson and Rainy in 1889, who initially homologized these anomalous muscle bellies with the psoas muscle complex of seals [6]. Psoas quartus is a rare, anomalous muscle belly found posterior to the psoas major and anterior to the quadratus lumborum and iliacus. Only 4 reported cases of psoas quartus are recorded in the literature [6–9]. This case report details a fifth observation of psoas quartus, including a description of its anatomical attachments, relationship to the femoral nerve, and a review of our current knowledge of the muscle variant.

Case Report

ANATOMY:

Our psoas quartus muscle belly originated from the transverse process of L5 and the anteromedial aspect of quadratus lumborum. The iliacus muscle originated from the iliac crest and traveled inferiorly past the inguinal ligament. The iliacus remained posterior to the psoas quartus throughout its extent. The main belly of the psoas major originated from the T12-L5 vertebral bodies and their transverse processes. The muscle traveled inferiorly past the inguinal ligament and anterior to the psoas quartus muscle belly. Inferior to the inguinal ligament, the psoas quartus fused with the posterior aspect of the psoas major prior to forming the iliopsoas tendon. The iliopsoas tendon inserted on the lesser trochanter of the femur. We did not observe psoas quartus on the contralateral side of the body. However, we did observe that the distal attachments of both psoas minor muscles expanded into a large aponeurosis that contributed to the iliac fascia, as has recently been reported [10].

REVIEW OF THE LITERATURE:

Clarkson and Rainy reported 2 muscle variants of the psoas major, named the psoas quartus and psoas tertius based on their proximal attachments to the corresponding lumbar transverse processes [6]. These muscles were observed bilaterally on a dissected male subject [6]. Their reported psoas tertius arose from the proximal half of the 12th rib near the quadratus lumborum and the tips of the L1–L5 transverse processes on the right side, and the anterior surface of the L4–L5 transverse processes on the left side. Their psoas quartus arose proximally deep to the medial tendon of quadratus lumborum and the transverse process of L5 on the right side, whereas it arose from the anterior surface of the L4–L5 transverse processes on the left side. Both the psoas tertius and psoas quartus fused with the psoas major just prior to the inguinal ligament [6]. In contrast, Tubbs et al reported psoas quartus as originating from the transverse process of L3 rather than L5 (Figure 2A), and along the medial aspect of quadratus lumborum [7].

Parker et al described a femoral nerve trifurcation in relation to the psoas quartus [8]. In that report, the authors noted that the psoas quartus originated from the anteromedial aspect of quadratus lumborum and the transverse process of L5 (Figure 2B). The 2 lateral-most branches of the femoral nerve traveled around the muscle belly, whereas the medial branch penetrated the psoas quartus, splitting the muscle belly into 2 parts. Both parts of the psoas quartus rejoined distal to the split femoral nerve and prior to fusion with the iliacus. Wong et al reported an L5 level femoral nerve split into 4 separate branches [9]. The medial-most branch traveled posterior to the psoas quartus, whereas the lateral-most branch traveled anterior to the muscle (Figure 2). The 2 midline branches split the psoas quartus muscle belly, leading to a similar distal coalescence prior to iliacus fusion. Table 1 provides a summary of these variations. There was no mention of abnormal gait or additional neuropathies by any of the previous authors.

Vázquez et al performed a statistical comparison of the occurrences of the psoas and iliacus muscle group impinging on a nerve and found that 19 out of 242 (7.9%) specimens contained a muscular slip from the iliacus and psoas major that pierced or covered the femoral nerve [11]. A distinction should be drawn between a muscle slip of the psoas major or iliacus and the distinct muscle belly of the psoas quartus. A muscle slip is a variant in muscle branching and trajectory. The psoas quartus constitutes a separate muscle, containing a proximal attachment, distal attachment, and a distinct muscle belly [12].

Reports of psoas quartus are rare. Our PubMed search recovered only 4 cases of this muscle variant. In addition to our report, 4 of the 5 observations were found unilaterally on the left side of male cadavers [6–9].

Discussion

Aberrant branching of the femoral nerve in relation to the psoas quartus can have significant clinical implications related to compression neuropathy. Orthopedic surgeons should be aware of anomalous branching patterns of the femoral nerve and their potential for iatrogenic nerve injuries [3]. Variations in anatomical structures complicate diagnoses of nerve-related pathologies and can affect the efficacy of femoral nerve blocks for procedures such as quadriceps muscle tears, femoral fractures, hip replacement surgery, and postoperative pain relief in hip surgery [13–15]. In addition, radiological interpretations require more careful assessment of this unique nerve–muscle relationship, such as when performing ultrasound-guided nerve blocks to the femoral nerve [16].

Compression neuropathy is particularly relevant to our case as anomalous psoas quartus muscle bellies occupy the anatomical space where the femoral nerve typically traverses (Figure 1B–1D). The position of the femoral nerve between the psoas quartus and psoas major muscle bellies creates an opportunity for partial or complete nerve compression. This compression may present as an abnormal gait or episodic chronic lower limb pain. As noted by Yi et al, even partial, continuous femoral nerve compression can lead to weakness in the iliacus, sartorius, pectineus and quadriceps muscles [17]. Our donor was reported to have an abnormal gait which may have been caused by this psoas quartus–femoral nerve relationship. However, we cannot directly attribute his gait abnormality to the presence of an additional psoas muscle belly nor any potential femoral nerve compression due to the donor’s co-morbidities, including Huntington’s disease [18].

Conclusions

We reported on the presence of a unilateral psoas quartus muscle during routine anatomical dissection. This muscle belly traversed through the anatomical space normally attributed to the femoral nerve, resulting in abnormal branching of the nerve within the abdomen. There are limited reports of this anomalous muscle in the literature. Our survey of these reports indicates that this is a rare anatomical variant that appears to be predominantly unilateral. This aberrant muscle placement may precipitate other complications and indicate potential clinical implications for diagnostics imaging and surgical procedures. The potential for the psoas quartus to partially or completely compress the femoral nerve may have contributed to the gait abnormality noted for our donor.