Symptomatic ganglion cyst of ligamentum flavum as a late complication of lumbar fixation

We report the case of a 72-year-old man who underwent surgery for a mobile spondylolisthesis L4-L5. Six months later, magnetic resonance imaging revealed an extradural cyst of the ligamentum flavum at L5-S1, which was then removed. Histological examination revealed a ganglion cyst of the ligamentum flavum. Cyst formation could be explained primarily according to the natural history of chronic degenerative disease of spine elements. Nevertheless, we could also consider the cyst formation as demonstrative of an adjacent segment syndrome: hypermobility of the L5-S1 segment just below three fixed vertebral segments would have triggered the mechanical stress necessary for L5-S1 ligamentum flavum degeneration.