Degeneration and mechanics of the segment adjacent to a lumbar spine fusion : a biomechanical analysis

Introduction: The development of adjacent segment degeneration (ASD) as a common complication of spinal fusion is believed by some clinicians and researchers to have roots in kinematic changes and altered loading at the intervertebral levels beside the fusion (i.e. adjacent). Dynamic spinal implants and minimally invasive surgeries were introduced to minimize such kinematic changes and load alterations in attempts to prevent ASD. However, little is known whether the kinematic changes at the adjacent level to a fusion are common in vivo occurrences. Further, the role of iatrogenic muscle damage on loading at the adjacent levels has not been investigated previously. Objectives: (1) To assess the current clinical evidence of in vivo kinematic changes at the levels adjacent to a lumbar spinal fusion. (2) To investigate the role of iatrogenic muscle damage on loading at the adjacent levels. Methods: (1) A systematic search in the PubMed database was performed for studies that addressed kinematics of the segment adjacent to a lumbar spine fusion or any other spinal implant. (2) A musculoskeletal model of the lumbar spine with 210 muscles was developed. Muscle damage was simulated by detaching the muscles from the posterior elements of the operated vertebrae and its effect on spinal loads at the adjacent levels was assessed during upright standing. Results: (1) The search identified 39 articles, among which 29 studied fusion. None of the studies observed any increase in range of motion (ROM) of the caudal adjacent segment, while for the rostral adjacent level the ROM was reported to increase in 10-30% of the patients. (2) The axial forces at the adjacent levels increased with muscle damage, with the largest increases being at the rostral adjacent level (73%) in comparison to the caudal level (32%). Discussion: The results of both studies imply higher susceptibility of the rostral adjacent level to disc degeneration, which is in harmony with the clinical prevalence of ASD occurring in 70 to 100% of the cases at the rostral level. The findings suggest that muscle damage secondary to spine surgery may play a key role in adjacent segment changes, independent of the spinal instrumentation.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat