Motion Segment Units Research Articles

The Effect of Lumbar Disc Herniation on Spine Loading Characteristics during Trunk Flexion and Two Types of Picking Up Activities.

The main purpose of this study was to investigate the compensatory response of the muscle activities of seventeen major muscle groups in the spinal region, intradiscal forces of the five lumbar motion segment units (MSUs), and facet forces acting on the ten lumbar facet joints in patients with lumbar disc herniation (LDH). Twenty-six healthy adults and seven LDH patients performed trunk flexion, ipsilateral picking up, and contralateral picking up in sequence. Eight optical markers were placed on the landmarks of the pelvis and spinal process. The coordinates of these markers were captured to drive a musculoskeletal model to calculate the muscle activities, intradiscal forces, and facet forces. The muscle activities of the majority of the seventeen major muscle groups were found increases in LDH patients. In addition, the LDH patients displayed larger compressive forces and anteroposterior forces on all the five lumbar MSUs and more lumbar facet inventions on most facet joints. These findings suggest that the LDH patients demonstrate compensatory increases in the most trunk muscle activities and all spinal loads. These negative compensatory responses increase the risk of the aggravation of disc herniation. Therefore, treatment should intervene as earlier as possible for the severe LDH patients.

Robotic application of a dynamic resultant force vector using real-time load-control: Simulation of an ideal follower load on Cadaveric L4–L5 segments

Standard in-vitro spine testing methods have focused on application of isolated and/or constant load components while the in-vivo spine is subject to multiple components that can be resolved into resultant dynamic load vectors. To advance towards more in-vivo like simulations the objective of the current study was to develop a methodology to apply robotically-controlled, non-zero, real-time dynamic resultant forces during flexion–extension on human lumbar motion segment units (MSU) with initial application towards simulation of an ideal follower load (FL) force vector.A proportional-integral-derivative (PID) controller with custom algorithms coordinated the motion of a Cartesian serial manipulator comprised of six axes each capable of position- or load-control. Six lumbar MSUs (L4–L5) were tested with continuously increasing sagittal plane bending to 8Nm while force components were dynamically programmed to deliver a resultant 400N FL that remained normal to the moving midline of the intervertebral disc. Mean absolute load-control tracking errors between commanded and experimental loads were computed. Global spinal ranges of motion and sagittal plane inter-body translations were compared to previously published values for non-robotic applications.Mean TEs for zero-commanded force and moment axes were 0.7±0.4N and 0.03±0.02Nm, respectively. For non-zero force axes mean TEs were 0.8±0.8N, 1.3±1.6Nm, and 1.3±1.6N for Fx, Fz, and the resolved ideal follower load vector FLR, respectively. Mean extension and flexion ranges of motion were 2.6°±1.2° and 5.0°±1.7°, respectively. Relative vertebral body translations and rotations were very comparable to data collected with non-robotic systems in the literature.The robotically coordinated Cartesian load controlled testing system demonstrated robust real-time load-control that permitted application of a real-time dynamic non-zero load vector during flexion–extension. For single MSU investigations the methodology has potential to overcome conventional follower load limitations, most notably via application outside the sagittal plane. This methodology holds promise for future work aimed at reducing the gap between current in-vitro testing and in-vivo circumstances.

Design and validation of a novel Cartesian biomechanical testing system with coordinated 6DOF real-time load control: application to the lumbar spine (L1–S, L4–L5)

Robotic methods applied to in-vitro biomechanical testing potentially offer more comprehensive evaluations however, standard position control algorithms make real-time load control problematic. This paper describes and evaluates a novel custom developed Cartesian force controlled biomechanical testing system with coordinated 6 degree of freedom (DOF) real-time load control.A custom developed 6-DOF serial manipulator with cascaded force over position control algorithms was designed, assembled, and programmed. Dial gauge tests assessed accuracy of custom linear axes. Standard test input and tuning procedures refined control performance. Two single motion segment units (L4–L5) and lumbar (L1–S) spine segments were tested under continuous pure moment application in flexion–extension, left–right lateral bending and axial rotation to 8Nm under full 6-DOF load control. Mean load control tracking errors between commanded and experimental loads were computed. Global spinal ranges of motion were compared to previously published values for standard non-robotic protocols.Individual linear and rotational axis position control accuracies were equal to or less than 6.35μm and 0.0167° respectively. Pilot pure bending tests demonstrated stable load control performance, as well as load rates, rotational velocities, and ranges of motion comparable to those for standard non-robotic in-vitro tests. Tracking errors for zero commanded forces and all moment controlled axes were less than 0.81±0.68N and 0.18±0.19Nm over all tests, respectively.The Cartesian based system simplified control application and demonstrated robust position and load control that was not limited to single axis or zero commanded loads. In addition to emulating standard biomechanical tests, the novel Cartesian force controlled testing system developed is a promising tool for biomechanical assessments with coordinated dynamic load application and coupled motion response in 6DOF.

The influence of fixed sagittal plane centers of rotation on motion segment mechanics and range of motion in the cervical spine

The center of rotation (CoR) has become an increasingly used metric for biomechanical evaluation of spinal joints however traditional methods of determination remain prone to high degrees of uncertainty. The objective was to use a novel robotic testing protocol to investigate the effects of placement of fixed CoRs in the cervical spine.Human cadaveric C4–C5 (n=3) and C6–C7 (n=5) motion segment units (MSU) were rotated in flexion–extension to limits of 2.5Nm bending or 225N resultant force about three points in a disc plane (A1, C1, P1) located at 25%, 50% and 75% along the length of the midline of the intervertebral disc respectively in the sagittal view, and three points (A2, C2, P2) in a sub-adjacent plane 5mm below the disc plane.Significant differences in range of rotation occurred between CoRs within the same plane but not between the same points in different planes (e.g. A1–A2). In flexion and extension axial forces at posterior points of rotation (P1, P2) were significantly different from those at anterior and central points. Shear forces were significantly different between points within the same plane except for the disc plane in extension, and between the same points in different planes in flexion and extension.The results indicate that the native cervical MSU is highly sensitive to the CoR location in terms of mechanics and range of motion, and that the CoR location likely varies between flexion and extension. The methodology developed has potential for application towards investigation of optimal CoR locations and in-vitro evaluations of the effects of implantable instrumentation.

INFLUENCE OF THE CONSTITUTIVE MATERIAL BEHAVIOR MODEL ASSIGNED TO THE ANNULUS FIBROSUS AND THE NUCLEUS PULPOSUS ON THE BIOMECHANICAL PERFORMANCE OF A MODEL OF THE CERVICAL SPINE: A FINITE ELEMENT ANALYSIS STUDY

One feature of the literature on finite element analysis of models of cervical spine segment(s) is that an assortment of constitutive models has been used for the elastic behavior of the annulus fibrosus (AF) and the nucleus pulposus (NF). The extent to which the model assigned to each of these tissues affects the values of the biomechanical parameters of interest of the model is lacking. This issue was the subject of the present study. We used a three-dimensional solid model of the C4–C6 motion segment units (which comprised the vertebral bodies, the bony posterior elements (transverse processes, pedicles, laminae, spinous processes, and facet joints), the intervertebral discs (IVDs), the endplates, and the five major ligaments) and eight combinations of constitutive models. It was found that (1) the influence of the constitutive material models used depended on the tissue considered, with some, such as the posterior endplate of C5 and the cancellous bone of C6, showing marked sensitivity, while others, such as the cancellous bone of C4 and the cortical bone of C5, were moderately affected; and (2) the biomechanical performance of the spine model is more sensitive to the material behavior model used for the AF than it is to that used for the NF. These results suggest that experimental and computational efforts expended in obtaining the most appropriate constitutive model for the elastic behavior of the two parts of the IVD, in particular the AF, are justified.

Influence of loading cycle profile and frequency on a biomechanical parameter of a model of a balloon kyphoplasty-augmented lumbar spine segment: A finite element analysis study

For patients who are suffering debilitating and persistent pain due to vertebral compression fracture(s) and for whom conservative therapies have not provided relief, balloon kyphoplasty (BKP) is used as a surgical option. There are only a very few literature reports on the use of the finite element analysis (FEA) method to obtain biomechanical parameters of models of spine segments that include BKP augmentation at a given level. In each of these studies, the applied loading used was quasi-static. During normal activities of daily living, the patient's spine would be subject to dynamically-applied loading. Thus, the question of the influence of the characteristics of a dynamically-applied loading cycle on biomechanical parameters of a spine that includes BKP-augmented segment(s) is germane; however, a study of this issue is lacking. We investigated this issue in the present FEA work, with the spine segment model being the L1-L3 motion segment units (MSUs) (a segment that is commonly augmented using BKP) and prophylactic BKP simulated at L2. The dynamic load was the compressive load-versus-time cycle to which the L3-L4 MSU is subjected during gait. Four cases of the cycle were considered, corresponding to slow-, normal-, fast- and very fast-paced gait. The loading cycle was applied to the superior surface of L1 while the inferior surface of L3 was fully constrained. It was found that (1) the global mean von Mises stress during the loading cycle (σVMG), in each tissue in the model increased in going from a slow-paced gait cycle to a very fast-paced gait cycle; and (2) for the slow-paced gait cycle, with increase in frequency of the cycle, f (1 ≤ f ≤ 3 Hz), σVMG in each of these tissues increased. Potential uses of the present findings are identified.

In vitro biomechanics of cervical disc arthroplasty with the ProDisc-C total disc implant

An in vitro biomechanical study was conducted to compare the effects of disc arthroplasty and anterior cervical fusion on cervical spine biomechanics in a multilevel human cadaveric model. Three spine conditions were studied: harvested, single-level cervical disc arthroplasty, and single-level fusion. A programmable testing apparatus was used that replicated physiological flexion/extension, lateral bending, and axial rotation. Measurements included vertebral motion, applied load, and bending moments. Relative rotations at the superior, treated, and inferior motion segment units (MSUs) were normalized with respect to the overall rotation of those three MSUs and compared using a one-way analysis of variance with Student-Newman-Keuls test (p < 0.05). Simulated fusion decreased motion across the treated site relative to the harvested and disc arthroplasty conditions. The reduced motion at the treated site was compensated at the adjacent segments by an increase in motion. For all modes of testing, use of an artificial disc prosthesis did not alter the motion patterns at either the instrumented level or adjacent segments compared with the harvested condition, except in extension.

Biomechanical Testing of an Artificial Cervical Joint and an Anterior Cervical Plate

An in vitro biomechanical study was conducted to determine the effects of fusion and nonfusion anterior cervical instrumentation on cervical spine biomechanics in a multilevel human cadaveric model. Three spine conditions were studied: harvested, single-level artificial cervical joint, and single-level graft with anterior cervical plate. A programmable testing apparatus was used that replicated physiologic flexion/extension and lateral bending. Measurements included vertebral motion, applied load, and bending moments. Relative rotations at the superior, implanted, and inferior motion segment units (MSUs) were normalized with respect to the overall rotation of those three MSUs and compared using a one-way analysis of variance (P < 0.05). Application of an anterior cervical plate decreased motion across the fusion site relative to the harvested and artificial joint spine conditions. The reduced motion was compensated for by an increase in motion at the adjacent segments. Use of an artificial cervical joint did not alter the motion patterns at either the instrumented level or the adjacent segments compared with the harvested condition for all modes of testing.