What forms the rectus sheath?

Anatomy of the Rectus Sheath

The rectus sheath is formed by the aponeuroses of the three lateral abdominal wall muscles: the external oblique, internal oblique, and transversus abdominis muscles, which split to enclose the rectus abdominis muscle.

Composition and Arrangement

The arrangement of these aponeuroses varies at different levels of the abdomen:

Above the Arcuate Line (Superior Two-Thirds)

• Anterior layer: Formed by the external oblique aponeurosis and the anterior lamina of the internal oblique aponeurosis
• Posterior layer: Formed by the posterior lamina of the internal oblique aponeurosis and the transversus abdominis aponeurosis

Below the Arcuate Line (Inferior One-Third)

• Anterior layer: All three aponeuroses (external oblique, internal oblique, and transversus abdominis) pass in front of the rectus abdominis muscle
• Posterior layer: Absent - only the transversalis fascia separates the rectus muscle from the peritoneum 1

Three-Dimensional Architecture

The rectus sheath has a complex three-dimensional architecture:

• Ventral rectus sheath: Consists primarily of intermingling oblique fibril bundles
• Dorsal rectus sheath: Composed mainly of transverse fibril bundles 2

Linea Alba

The linea alba is the midline fibrous structure formed by the fusion of the aponeuroses from both sides and has three distinct layers from ventral to dorsal:

1. Lamina fibrae obliquae: Intermingling oblique fibers
2. Lamina fibrae transversae: Mainly transverse fibril bundles
3. Lamina fibrae irregularium: An inconstant, small layer composed of oblique fibers 2

Transversus Abdominis Relationship

Recent anatomical studies have challenged traditional understanding of the rectus sheath, particularly regarding the transversus abdominis muscle:

• There is significant presence of the transversus abdominis within the rectus sheath at the costal margin (4.2 cm overlap)
• This presence diminishes caudally: 99% at L1-L2 (3.2 cm overlap), 86% at L2-L3 (1.4 cm overlap), 36% at the umbilicus, and only 2% at L5-S1 3

Clinical Significance

1. Surgical Implications:

   • Understanding this anatomy is crucial for abdominal wall reconstruction and hernia repair techniques 3
   • The arcuate line is a key anatomical landmark for surgeons performing abdominal procedures 1

2. Regional Anesthesia:

   • The rectus sheath block is an effective technique for providing analgesia in abdominal surgeries
   • It's particularly useful in both adults and children undergoing umbilical and paraumbilical hernia repairs 4, 5

3. Pathological Considerations:

   • Rectus sheath hematomas occur most frequently in the lower abdomen due to the absence of the posterior rectus sheath below the arcuate line
   • In this region, perforating branches of the inferior epigastric artery can rupture, causing hematomas that spread widely in the loose preperitoneal space 1

Understanding this complex anatomy is essential for surgeons performing abdominal wall procedures and for anesthesiologists administering regional blocks for pain management.