Biomechanical analysis and modeling of the in vivo lumbar spine
Low back pain is the most prevalent musculoskeletal disorder in the United States and worldwide. To better understand the mechanical antecedents which exacerbate low back pain, further investigation of lumbar mechanics during functional activity is required. Advancements in medical imaging techniques have paved the way to address current knowledge gaps regarding in vivo lumbar mechanics, providing the capability of capturing motion of the lumbar spine with high accuracy during dynamic activities. The current work comprises three aims. The first aim was to accurately quantify in vivo deformation of the lumbar intervertebral discs in healthy subjects during dynamic lifting tasks. The second aim was to evaluate lumbar facet joint kinematics during the same lifting tasks. Utilizing directly measured subject-specific lumbar vertebral kinematics, the third aim was to investigate the potential for obtaining more accurate joint reaction and muscle force estimates. To accomplish this, in vivo data were incorporated within subject-specific musculoskeletal models, whereby the joint reaction and muscle force patterns of the lumbar spine during the lifting motion were estimated. The current study found uniquely different intervertebral disc morphometry, disc deformation, and facet join translational kinematics at the L5S1 disc during the lifting tasks. The incorporation of accurately measured lumbar vertebral kinematics within musculoskeletal models led to decreased joint reaction forces compared to those with generic, rhythm-based lumbar kinematic inputs. Lumbar kinematic input also displayed significant interaction with passive stiffness properties and the neutral state configuration defined at the lumbar joints of the musculoskeletal models. The results suggest that the mechanical behavior of the L5S1 is distinctly different from the rest of the lumbar segments, and that approaches to restore normal, functional motion at the segment should differ from other joint levels. Furthermore, results indicate that inclusion of the accurate vertebral kinematics - including rotational as well as translational kinematics - within musculoskeletal models may lead to improved estimates of lumbar loading patterns. Such input datasets can also provide a better insight into the stabilizing role of deep intrinsic muscles such as the multifidus. On the other hand, it may also heighten the demand for accuracy of accompanying parameters.