Learning Objectives
Identify the five primary muscles of the posterior abdominal wall and their spatial relationships. Master the attachments and innervations of the Psoas major and Iliacus (Iliopsoas). Analyze the three layers of the thoracolumbar fascia and the muscles they enclose. Understand the clinical significance of the Psoas Sign in diagnosing appendicitis.
Muscles of the Posterior Abdominal Wall
The posterior wall is a complex musculoskeletal region formed by the lumbar vertebrae, pelvic girdle, and specialized muscle groups.
| Muscle | Key Attachments | Primary Action | Innervation |
|---|---|---|---|
| Psoas Major | T12–L5 vertebrae to the lesser trochanter | Thigh flexion; Lateral vertebral flexion | L1–L3 (Anterior rami) |
| Iliacus | Iliac fossa to Lesser Trochanter | Thigh flexion and lateral rotation | Femoral Nerve (L2–L4) |
| Quadratus Lumborum | Iliac crest to 12th rib & L1–L4 transverse processes | Lateral flexion: Fixes the 12th rib for the diaphragm | T12–L4 (Anterior rami) |
| Psoas Minor | T12/L1 to Pectineal line (Pelvis) | Weak flexion of the vertebral column | L1 spinal nerve |
Note: The Iliopsoas (Iliacus + Psoas Major) is the most powerful flexor of the thigh. The Psoas Minor is absent in approximately 40% of people.
Fascial Layers
The posterior wall fascia is continuous with the transversalis fascia. It is divided based on the muscles it covers.
Thoracolumbar Fascia
- Anterior Layer: Covers the anterior aspect of the quadratus lumborum. Thickens superiorly to form the lateral arcuate ligament.
- Middle Layer: Sits between the Quadratus Lumborum and the deep back muscles.
- Posterior Layer: Covers the deep back muscles (Erector spinae). Laterally, it attaches to the Internal Oblique and Transversus Abdominis.
Clinical Relevance: The Psoas Sign
The Psoas Sign is an essential physical exam finding for retrocecal irritation.
- Mechanism: Passive extension or active flexion of the thigh causes the iliopsoas to contract and rub against an inflamed appendix.
- Positive Result: Pain in the lower right quadrant.
Activity
You must be logged in to post a comment.