Influence of deviated centers of rotation on kinematics and kinetics of a lumbar functional spinal unit: An in vitro study

Center of rotation (COR) has been used for assessing spinal motion quality. However, the biomechanical influence of COR deviation towards different directions during flexion-extension (FE) remains largely unknown. This study aimed to investigate the alteration in the range of motion (ROM), compressive force, shear force, and neutral zone size (NZ) in a lumbar functional spinal unit (FSU), caused by the deviated COR in different directions during FE. Twelve human cadaveric lumbar FSUs (6 for L2–L3, 6 for L4–L5) were tested in a 6-degree-of-freedom servo-hydraulic load frame. These FSUs were firstly applied a 7.5 Nm pure moment to perform FE to obtain their natural COR during FE. Subsequently, they were subjected to FE around 9 established deviated CORs with 6 Nm cyclical loading. It was found that the ROM and NZ increased significantly when the COR moved from the superior plane to the inferior plane for the L2–L3 unit and when the COR located in the superior plane compared with the inferior plane for the L4–L5 unit. The compressive forces for both FSUs demonstrated significant changes caused by COR shift in the same horizontal plane, while the shear forces demonstrated significant changes caused by COR shift in the same vertical plane. The ROM, NZ, and shear force of FSU are sensitive to the vertical COR shift, while the compressive force of FSU is highly sensitive to the horizontal COR shift. Additionally, the kinematics and kinetics of the L2–L3 unit are more sensitive to COR location than those of the L4–L5 unit.