Oblique fissure of left lung

The oblique fissure of the left lung (Left major fissure) is the deep cleft in the left lung that separates its two lobes the upper (superior) lobe from the lower (inferior) lobe. It is the only major fissure of the left lung (unlike the right lung, which also has a horizontal fissure).

Course and surface anatomy:

The left oblique fissure begins posteriorly at a high level, most commonly at the fourth rib (45% of individuals), courses inferolaterally across the lung surface crossing the sixth rib in the midaxillary line (52%) and extends anteroinferiorly to reach the anterior third of the hemidiaphragm (60%). Its upper portion faces laterally and is typically concave, while its lower portion usually faces medially and is convex a configuration that distinguishes it from the right oblique fissure.

Morphology:

The left oblique fissure most commonly follows a linear (straight) configuration (78%), in contrast to the right oblique fissure, which more often has an S-shaped or variable course. The uppermost extent of the left oblique fissure is almost always at or above the level of the right oblique fissure; a higher right-sided fissure compared to the left should raise suspicion for a pathological process.

Completeness and variations:

Although standard anatomy textbooks describe the fissure as complete, studies show it is complete in approximately 72-82% of individuals, incomplete in 16-26%, and absent in about 2%. Incompleteness is most often found in the parahilar region, with the area of discontinuity typically spanning 21-40% of the fissure's length. An incomplete oblique fissure allows for collateral air drift and potential disease spread between lobes, which has important implications for thoracic surgery and radiologic interpretation.