MRI signs for intraneural ganglion cysts: a roadmap revealing the pathoanatomic and pathophysiologic principles underlying the unifying articular theory.

Intraneural ganglion cysts (IGCs) are benign, mucinous lesions that are often found within the epineurium of peripheral nerves, causing mononeuropathy due to nerve compression. These cysts arise from synovial fluid tracking from an adjacent synovial joint along an articular nerve branch to the parent nerve, as proposed by the unifying articular theory. This theory has transformed the understanding of IGCs from their being considered as isolated nerve lesions but as joint-related pathologies. The emphasis of identifying the cyst's origin and its connection to the neighboring joint has shifted the surgical strategy from evacuation of the cyst to addressing the origin of the cyst and its articular branch connection. The role of high resolution imaging in both detection and surgical planning is thereby critical, because in some cysts (i.e. affecting the peroneal and tibial nerves in the knee region), the cyst may be intraneural or extraneural and arise from different joints. In this article, we summarize the magnetic resonance imaging (MRI) signs for IGCs and introduce the concept of phase-based signs which are all explained by pathoanatomic and pathophysiologic principles underlying the unifying articular theory. These signs, in turn, thave expanded our understanding, changed our clinical practice, and advanced the field.