Spondylolysis. Bending of the inferior articular processes of lumbar vertebrae during simulated spinal movements.

STUDY DESIGN: This was a cadaveric lumbar motion segment experiment.

OBJECTIVES: To measure bending about the pars interarticularis during simulated flexion and extension movements, with and without high compressive forces. This was designed to indicate which activities are most likely to cause spondylolysis.

SUMMARY OF BACKGROUND DATA: The mechanical etiology of spondylolysis is controversial.

METHODS: Motion segments were loaded to simulate: A, full flexion; B, full extension; C, high compression in flexed posture, and D, high compression in lordotic posture. Bending of the inferior articular processes about the pars was measured in the sagittal plane, using a two-dimensional "MacReflex" infrared motion analysis system.

RESULTS: In full flexion, tension in the intervertebral ligaments bent the processes forward and down (by 3.25 degrees +/- 3.07 degrees), whereas in the loading conditions B, C, and D, inter-facet compressive forces caused the processes to bend backward and upward, by 2.24 +/- 2.48 degrees, 0.40 +/- 0.23 degrees, and 0.74 +/- 0.56 degrees respectively.

CONCLUSIONS: Activities that involve alternating flexion and extension movements cause large stress reversals in the pars and pose the greatest threat of spondylolysis. Compressive loading has little effect.