Isthmic spondylolisthesis at L4–L5 in a 44–year–old male. No clinical data were available in this case. This sagittal section runs through the medial border of the pedicle. At this level the "beak" of the pars interarticularis is displayed as it "plunges" into the interlaminar ligamentum flavum, causing it to mushroom anteriorly and into the lateral recess. The body of L4 is markedly rotated posteriorly which appears to be facilitated by the severe internal disruption of the L4 disc. Its posterior annulus is partially detached superiorly from the apophyseal rim and allows the posterior annulus fibrosus to project posteriorly, virtually closing the lateral recess inferiorly. The extension–rotation of L4 causes the sharp lower rim of the pedicle to "fall" down on the ganglion like a guillotine. Sub–chondral endplate sclerosis appears to designate areas of particular axial stress exposition.