Neutral zone

Abstract: Study design This study investigated the influence of five different muscle groups on the monosegmental motion (L4-L5) during pure flexion/extension, lateral bending, and axial rotation moments. Objectives The results showed and compared the effect of different muscle groups acting in different directions on the stability of a single motion segment to find loading conditions for in vitro experiments that simulate more physiologically reasonable loads. Summary of background data In spine biomechanics research, most in vitro experiments have been carried out without applying muscle forces, even though these forces stabilize the spinal column in vivo. Methods Seven human lumbosacral spines were tested in a spine tester that allows simulation of up to five symmetrical muscle forces. Changing pure flexion/extension, lateral bending, and axial rotation moments up to +/- 3.75 Nm were applied without muscle forces, with different muscle groups and combinations. The three-dimensional monosegmental motion was determined using an instrumented spatial linkage system. Results Simulated muscle forces were found to strongly influence load-deformation characteristics. Muscle action generally decreased the range of motion and the neutral zone of the motion segments. This was most evident for flexion and extension. After five pairs of symmetrical, constant muscle forces were applied (80 N per pair), the range of motion decreased about 93% in flexion and 85% in extension. The total neutral zone for flexion and extension was decreased by 83% muscle action. The multifidus muscle group had the strongest influence. Conclusion This experiment showed the importance of including at least some of the most important muscle groups in in vitro experiments on lumbar spine specimens.

Abstract: The human spinal column, devoid of musculature, is incapable of carrying normal physiologic loads. In an in vitro experiment, the effect of simulated intersegmental muscle forces on spinal instability was investigated. Intact and sequentially injured fresh lumbar functional spinal units were subjected to three-dimensional biomechanical tests with increasing muscle forces. With the application of muscle forces, range of motion (ROM) increased and neutral zone (NZ) decreased in flexion loading, while both ROM and NZ decreased in extension loading. In lateral bending, ROM and NZ were unaffected by the application of the muscle forces. In axial rotation, ROM decreased significantly, while NZ decrease was statistically insignificant. It was concluded that the action of the intersegmental muscle forces is to maintain or decrease intervertebral motions after injury, with the exception of the flexion ROM, which increased with the application of muscle forces. In addition, the study suggested that Neutral Zone is a better indicator of spinal instability than Range of Motion.

Abstract: Knowledge of the normal movements of the occipito-atlanto-axial joint complex is important for evaluating clinical cases that may be potentially unstable. The purpose of this in vitro study was to quantitatively determine three dimensional movements of the occiput-C1 and C1-C2 joints. Ten fresh cadaveric whole cervical spine specimens (occiput to C7) were studied, using well-established techniques to document the movements in flexion, extension, left and right lateral bending, and left and right axial rotation. Pure moments of a maximum of 1.5 N-m were applied incrementally, and three-dimensional movements of the bones were recorded using stereophotogrammetry. Each moment was applied individually and in three load/unload cycles. The motion measurements were made on the third load cycle. Parameters of neutral zone, elastic zone, and range of motion were computed. Neutral zones for flexion/extension, right/left lateral bending, and right/left axial rotation were, respectively: 1.1, 1.5, and 1.6 (occiput-C1); and 3.2, 1.2, and 29.6 degrees (C1-C2). Ranges of motion for flexion, extension, lateral bending (one side), and axial rotation (one side) were, respectively: 3.5, 21.0, 5.5, and 7.2 degrees (occiput-C1 joint) and 11.5, 10.9, 6.7, and 38.9 degrees (C1-C2 joint). The greatest intervertebral motion in the spine was axial rotation at the C1-C2 joint, with the neutral zone constituting 75% of this motion.