Predictive equations to estimate spinal loads in symmetric lifting tasks

Lumbopelvic rhythm during forward and backward sagittal trunk rotations: combined in vivo measurement with inertial tracking device and biomechanical modeling

BACKGROUND

The ratio of total lumbar rotation over pelvic rotation (lumbopelvic rhythm) during trunk sagittal movement is essential to evaluate spinal loads and discriminate between low back pain and asymptomatic population.

METHODS

Angular rotations of the pelvis and lumbar spine as well as their sagittal rhythm during forward flexion and backward extension in upright standing of eight asymptomatic males are measured using an inertial tracking device. The effect of variations in the lumbopelvic ratio during trunk flexion on spinal loads is quantified using a detailed musculoskeletal model.

FINDINGS

The mean of peak voluntary flexion rotations of the thorax, pelvis, and lumbar was 121° (SD 9.9), 53.0° (SD 5.2), and 60.2° (SD 8.6), respectively. The mean lumbopelvic ratios decreased from 2.51 in 0-30° of trunk flexion to 1.34 in 90-120° range during forward bending while it increased from 1.23 in 90-120° range to 2.86 in 0-30° range during backward extension. Variations in the lumbopelvic ratio from 0.5 to 3 (with an interval of 0.25) at any trunk flexion angle generally reduced the L5-S1 compression and shear forces by up to 21 and 45%, respectively. The measured lumbopelvic ratios resulted overall in near-optimal (minimal) L5-S1 compression forces.

INTERPRETATION

A simultaneous rhythm between the lumbar and pelvis movements was found during both forward and backward trunk movements. While the lumbar spine contributed more to the trunk rotation during early and final stages of forward flexion and backward extension, respectively, the pelvis contributed more during final and early stages of forward flexion and backward extension, respectively. Our healthy subjects adapted a lumbopelvic coordination that diminished L5-S1 compression force.

An in vitro model of degenerative lumbar spondylolisthesis

STUDY DESIGN

A biomechanical human cadaveric study.

OBJECTIVE

To create a biomechanical model of low-grade degenerative lumbar spondylolisthesis (DLS), defined by anterior listhesis, for future testing of spinal instrumentation.

SUMMARY OF BACKGROUND DATA

Current spinal implants are used to treat a multitude of conditions that range from herniated discs to degenerative diseases. The optimal stiffness of these instrumentation systems for each specific spinal condition is unknown. Ex vivo models representing degenerative spinal conditions are scarce in the literature. A model of DLS for implant testing will enhance our understanding of implant-spine behavior for specific populations of patients.

METHODS

Four incremental surgical destabilizations were performed on 8 lumbar functional spinal units. The facet complex and intervertebral disc were targeted to represent the tissue changes associated with DLS. After each destabilization, the specimen was tested with: (1) applied shear force (-50 to 250 N) with a constant axial compression force (300 N) and (2) applied pure moments in flexion-extension, lateral bending and axial rotation (±5 Nm). Relative motion between the 2 vertebrae was tracked with a motion capture system. The effect of specimen condition on intervertebral motion was assessed for shear and flexibility testing.

RESULTS

Shear translation increased, specimen stiffness decreased and range of motion increased with specimen destabilization (P < 0.0002). A mean anterior translation of 3.1 mm (SD 1.1 mm) was achieved only after destabilization of both the facet complex and disc. Of the 5 specimen conditions, 3 were required to achieve grade 1 DLS: (1) intact, (3) a 4-mm facet gap, and (5) a combined nucleus and annulus injury.

CONCLUSION

Destabilization of both the facet complex and disc was required to achieve anterior listhesis of 3.1 mm consistent with a grade 1 DLS under an applied shear force of 250 N. Sufficient listhesis was measured without radical specimen resection. Important anatomical structures for supporting spinal instrumentation were preserved such that this model can be used in future to characterize behavior of novel instrumentation prior to clinical trials.

A meta-model analysis of a finite element simulation for defining poroelastic properties of intervertebral discs

Finite element analysis is an effective tool to evaluate the material properties of living tissue. For an interactive optimization procedure, the finite element analysis usually needs many simulations to reach a reasonable solution. The meta-model analysis of finite element simulation can be used to reduce the computation of a structure with complex geometry or a material with composite constitutive equations. The intervertebral disc is a complex, heterogeneous, and hydrated porous structure. A poroelastic finite element model can be used to observe the fluid transferring, pressure deviation, and other properties within the disc. Defining reasonable poroelastic material properties of the anulus fibrosus and nucleus pulposus is critical for the quality of the simulation. We developed a material property updating protocol, which is basically a fitting algorithm consisted of finite element simulations and a quadratic response surface regression. This protocol was used to find the material properties, such as the hydraulic permeability, elastic modulus, and Poisson's ratio, of intact and degenerated porcine discs. The results showed that the in vitro disc experimental deformations were well fitted with limited finite element simulations and a quadratic response surface regression. The comparison of material properties of intact and degenerated discs showed that the hydraulic permeability significantly decreased but Poisson's ratio significantly increased for the degenerated discs. This study shows that the developed protocol is efficient and effective in defining material properties of a complex structure such as the intervertebral disc.

The complex spine: the multidimensional system of causal pathways for low-back disorders

OBJECTIVE

The aim of this study was to examine the logic behind the knowledge of low-back problem causal pathways.

BACKGROUND

Low-back pain and low-back disorders (LBDs) continue to represent the major musculoskeletal risk problem in the workplace,with the prevalence and costs of such disorders increasing over time. In recent years, there has been much criticism of the ability of ergonomics methods to control the risk of LBDs.

METHOD

Logical assessment of the systems logic associated with our understanding and prevention of LBDs.

RESULTS

Current spine loading as well as spine tolerance research efforts are bringing the field to the point where there is a better systems understanding of the inextricable link between the musculoskeletal system and the cognitive system. Loading is influenced by both the physical environment factors as well as mental demands, whereas tolerances are defined by both physical tissue tolerance and biochemically based tissue sensitivities to pain. However, the logic used in many low-back risk assessment tools may be overly simplistic, given what is understood about causal pathways. Current tools typically assess only load or position in a very cursory manner.

CONCLUSION

Efforts must work toward satisfying both the physical environment and the cognitive environment for the worker if one is to reliably lower the risk of low-back problems.

APPLICATION

This systems representation of LBD development may serve as a guide to identify gaps in our understanding of LBDs.

Predictive equations for lumbar spine loads in load-dependent asymmetric one- and two-handed lifting activities

BACKGROUND

Asymmetric lifting activities are associated with low back pain.

METHODS

A finite element biomechanical model is used to estimate spinal loads during one- and two-handed asymmetric static lifting activities. Model input variables are thorax flexion angle, load magnitude as well as load sagittal and lateral positions while response variables are L4-L5 and L5-S1 disc compression and shear forces. A number of levels are considered for each input variable and all their possible combinations are introduced into the model. Robust yet user-friendly predictive equations that relate model responses to its inputs are established.

FINDINGS

Predictive equations with adequate goodness-of-fit (R(2) ranged from ~94% to 99%, P≤0.001) that relate spinal loads to task (input) variables are established. Contour plots are used to identify combinations of task variable levels that yield spine loads beyond the recommended limits. The effect of uncertainties in the measurements of asymmetry-related inputs on spinal loads is studied.

INTERPRETATION

A number of issues regarding the NIOSH asymmetry multiplier are discussed and it is concluded that this multiplier should depend on the trunk posture and be defined in terms of the load vertical and horizontal positions. Due to an imprecise adjustment of the handled load magnitude this multiplier inadequately controls the biomechanical loading of the spine. Ergonomists and bioengineers, faced with the dilemma of using either complex but more accurate models on one hand or less accurate but simple models on the other hand, have hereby easy-to-use predictive equations that quantify spinal loads under various occupational tasks.

A combined numerical and experimental technique for estimation of the forces and moments in the lumbar intervertebral disc

Evaluation of the loads on lumbar intervertebral discs (IVD) is critically important since it is closely related to spine biomechanics, pathology and prosthesis design. Non-invasive estimation of the loads in the discs remains a challenge. In this study, we proposed a new technique to estimate in vivo loads in the IVD using a subject-specific finite element (FE) model of the disc and the kinematics of the disc endplates as input boundary conditions. The technique was validated by comparing the forces and moments in the discs calculated from the FE analyses to the in vitro experiment measurements of three corresponding lumbar discs. The results showed that the forces and moments could be estimated within an average error of 20%. Therefore, this technique can be a promising tool for non-invasive estimation of the loads in the discs and may be extended to be used on living subjects.