A repeatable ex vivo model of spondylolysis and spondylolisthesis.

STUDY
DESIGN: An ex vivo biomechanical study using porcine spinal segments.
OBJECTIVE: To produce a biomechanical model of both spondylolysis and spondylolisthesis using an accelerated cyclic loading model with intermittent impulse loads. SUMMARY OF BACKGROUND DATA: Only a few models of spondylolisthesis appropriate for biomechanical testing have been presented previously. Past modeling attempts have largely required nonphysiologic gross fracture of the pars before testing and have resulted in nonphysiologic endplate fracture. In these tests no clinically relevant spondylolisthesis was seen at the end of testing. A reproducible, clinically relevant model of both spondylolysis and spondylolisthesis would allow study of these disease processes, and facilitate the development and evaluation of advanced spinal implants optimized specifically for these pathologies.
METHODS: Five porcine lumbar functional spinal units were tested (2 L4-L5, 3 L6-S1) after small notches had been created in the pars and after the disc had specific collagen fibers in the anterior anulus sectioned. Specimens were loaded with a constant cranial-caudal compressive force of 300 N and the application of cyclic anterior shear loads between 300 and 600 N with intermittent impulse loads to 1500 N until pars fracture occurred. Elevated cyclic loading then continued between 500 and 800 N.
RESULTS: All specimens displayed bilateral pars fracture with the fractures passing through the points of notching and no damage to endplates or facet joints. Clinically-relevant Grade II spondylolisthesis was achieved in all 5 specimens. The mean slip at the conclusion of testing was 33%.
CONCLUSION: Cyclic shear loading with intermittent impulse loads can reliably create fracture in the pars interarticularis in ex vivo porcine spine segments. Subsequent cyclic anterior motion of the superior vertebra results in clinically-relevant spondylolysis and spondylolisthesis.