Interpedicular kinematics in an in vitro biomechanical assessment of a bilateral lumbar spondylolytic defect

•We performed a biomechanical in vitro assessment of a bilateral pars defect at L4.•Interpedicular metrics were used to quantify intervertebral translatory motions.•Bending-plane and out-of-plane translatory motions did not change after the defect.•Interpedicular displacement increased in flex-ex and bilateral bending motions.•A bilateral pars defect in lumbar spine does not cause spondylolisthesis per se.

BackgroundA spondylolytic defect in lumbar vertebra is a common condition during early childhood and adolescence, and is considered a precursor to spondylolisthesis. This study examined whether a bilateral spondylolytic defect in lumbar spine intrinsically results in increased intervertebral translations during different bending motions.MethodsSeven fresh frozen cadaveric kangaroo lumbar (L1-L6) spine specimens were tested in a kinematic spine simulator; first in their intact state, followed by creating a bilateral spondylolytic defect at L4 and retesting. In addition to recording global and segmental range of motions, the pedicles at L3, L4, and L5 vertebrae were digitized bilaterally and virtually tracked throughout testing. Interpedicular kinematic metrics were employed to capture any changes in translatory motions during flexion-extension, bilateral bending, and axial torsion testing modes.FindingsFollowing the defect, range of motion at the defect level (L4-L5) increased significantly in all the three motion planes. At L4-L5, normalized interpedicular displacement increased significantly in flexion-extension (median change + 156%) and bilateral bending (median change + 58%) motions, but changes in bending-plane and out-of-plane intervertebral translations were not significant in any of the testing modes.InterpretationIn the absence of any significant changes in bending-plane and out-of-plane intervertebral translations at L4-L5, changes in interpedicular displacement would directly correspond with the stretching of posterior annulus of the L4-L5 intervertebral disc. A bilateral spondylolytic defect at L4 may result in significant overstretching of the posterior annulus of the L4-L5 disc during flexion-extension and bilateral bending motions.