Intra-articular gas on knee MRI: a meniscal tear mimic and description of a novel MRI sign.

Intra-articular gas is a recognized mimic of meniscal tears on knee MRI, typically resulting from a vacuum phenomenon caused by joint distension and negative intra-articular pressure. This study presents three cases demonstrating transient intra-articular gas that simulated meniscal pathology but was ultimately identified as a benign vacuum phenomenon. We describe a novel diagnostic clue, the "rising tide sign," which refers to the progressive disappearance of intra-articular gas and the posterior-to-anterior migration of the fluid interface in a gravity-dependent manner, resembling rising tidal water. Recognition of this sign enables differentiation between true meniscal tears and gas artifacts, thereby avoiding false-positive diagnoses and potential unnecessary surgical intervention.