Biomechancial Quantification of Pathologic Manipulable Spinal Lesions: An In Vivo Ovine Model of Spondylolysis and Intervertebral Disc Degeneration

Abstract

Objective: The purposes of this study were to quantify the biomechanical and pathologic consequences of surgically induced spinal lesions and to determine their response to spinal manipulation (SMT) in an in vivo ovine model. Methods: Of 24 Merino sheep, 6 received L5 spondylolytic defects, 6 received L1 annular lesions, and 12 served as respective controls. Dorsoventral (DV) stiffness was assessed using oscillatory loads (2-12 Hz). Two SMT force-time profiles were administered in each of the groups using a randomized and repeated-measures design. Stiffness and the effect of SMT on the DV motions and multifidus needle electromyographic responses were assessed using a repeated-measures analysis of variance (α = .05). Postmortem histologic analysis and computed tomography validated the presence of lesions. Results: L5 DV stiffness was significantly increased (40.2%) in the spondylolysis (6.28 N/mm) compared with the L5 control group (4.48 N/mm) (P < 03). Spinal manipulations delivered to the spondylolysis group resulted in less DV vertebral displacement (P < .01) compared with controls. Dorsoventral stiffness of the disc degeneration group was 5.66 N/mm, 94.5% greater than in the L1 control group (2.91 N/mm) (P < .01). One hundred–millisecond SMTs resulted in significantly reduced DV displacements in the disc degeneration group compared with the L1 control group (P < .01). Animals in the disc degeneration group showed a consistent 25% to 30% reduction in needle electromyographic responses to all SMTs. Conclusions: Quantifiable objective evidence of spinal lesions and their response to SMT were confirmed in this study. Neuromechanical alterations provide novel insights into quantifying manipulable spinal lesions and a means to biomechanically assess SMT outcomes.