An incomplete fissure on a CT scan of the lungs refers to the partial or absent separation between adjacent lobes of the lung due to incomplete development or fusion of the pleura during embryogenesis. Normally, the lungs are divided into lobes by fissures that consist of visceral pleura; the major (oblique) and minor (horizontal) fissures are typically well-defined on imaging. When a fissure is incomplete, portions of the lung parenchyma that should belong to separate lobes are instead continuous.

Characteristics of Incomplete Fissures on CT:
Blunted or absent fissure line: Instead of a sharp line delineating the lobes, the fissure may appear as a hazy, poorly defined or incomplete boundary.
Lobar connection: Adjacent lobes may appear directly contiguous due to the lack of full pleural separation.
Variability: The degree of incompleteness can vary; some fissures are partially formed, while others are nearly absent.
Displacement of fissure anatomy: In conditions like atelectasis or lung collapse, the incomplete fissure may not conform to expected anatomical positions.
Clinical Significance:
Normal Variation:
Incomplete fissures are a common anatomical variant and often have no clinical significance.
Impact on Disease Spread:
Infections, tumors, or fluid collections (e.g., pleural effusions) may cross incomplete fissures more easily compared to fully formed fissures, affecting disease localization.
Surgical Considerations:
Incomplete fissures can complicate lung resections (e.g., lobectomies) by obscuring anatomical planes and increasing the risk of complications such as air leaks.
Imaging Artifacts:
An incomplete fissure can mimic pathology like a mass or consolidation on chest radiographs if not correctly identified.
Common Causes for Misinterpretation:
Artifacts on imaging (motion, partial volume effects).
Adjacent pathology (e.g., scarring or fibrosis affecting the fissure).
In summary, an incomplete fissure is an anatomical variant where the separation between lung lobes is not fully developed, appearing on CT as a poorly defined or absent fissure line, with potential implications for disease spread and surgical planning.