Fig. 1.

The direction and magnitude of the mechanical load are indicated by arrows. a In the normal case without contralateral spondylolysis, the mechanical stress from the posterior is evenly distributed to the left and right vertebral arches. b When a bone defect occurs due to terminal stage spondylolysis, the mechanical stress from the posterior is concentrated at the site of acute spondylolysis where bone continuity remains. Therefore, stability is not attained and the union rate is low