Introduction: Low back pain (LBP) is an ailment affecting a large portion of the population and may result from degeneration of the intervertebral discs. Degeneration of the discs may be characterized by a loss of hydration, a more granular texture in the disc components and the presence of anular lesions which are tears in the anulus fibrosus. Research to date has been lacking in defining a relationship between LBP and anular lesions. In this study a materially and geometrically accurate finite element model (FEM) of an L4/5 intervertebral disc was developed in order to study the effects of anular lesions on the disc mechanics.

Methods: An anatomically accurate transverse profile for the disc FEM was derived from transversely sectioned human cadaveric discs. The anulus fibrosus ground substance was represented as an incompressible material using an Ogden hyperelastic strain energy equation. Material parameters were derived from experimentation on sheep discs. In order to separately assess the effects of degeneration of the nucleus and of the entire disc, four models were analysed. A healthy disc was modelled as reference and the three degenerate models comprised a degenerate nucleus (no hydrostatic nucleus pressure) with either a healthy anulus, or with a radial or rim anular lesion. Loading conditions to simulate the extreme range of physiological motions about the 6 axes of rotation were applied to the models and the peak rotation moments compared.

Results: The reduction in peak moment between the Healthy Disc FEM and the Healthy Anulus FEM ranged from 24% under flexion to 86% under right lateral bending. When the lesions were simulated, the rim and radial lesion resulted in variations in peak moment from the Healthy Anulus FEM of 1–10% and 0–4%, respectively.

Conclusions: The analysis suggested that loss of the nucleus pulposus pressure had a much greater effect on the disc mechanics than the presence of anular lesions. This indicated that the development of anular lesions prior to the degeneration of the nucleus would have minimal effect on the disc mechanics. But the response of an entirely degenerate disc would show significantly different mechanics compared to a healthy disc. With the degeneration of the nucleus, the disc stiffness will reduce and the outer innervated anulus may become overloaded and painful.

Introduction: A computer model of the L4/5 human intervertebral disc is currently under development. An integral aspect of this model is the material properties assigned to its components. Detailed data on the material properties of the anulus fibrosus ground matrix are not available in the existing literature. To determine these properties, mechanical tests were carried out on specimens of anulus fibrosus harvested from sheep spines. The tests included unconfined uniaxial compression, simple shear and biaxial compression. Data on the strain required to cause permanent damage in the anulus ground matrix and data on the mechanical response of the anulus to repeated loading were obtained.

Methods: Intervertebral discs were isolated from the lumbar spines of recently sacrificed sheep. These discs were sectioned into test specimens ensuring there were no continuous collagen fibres bearing load. The edge dimensions of the cubic specimens were 3 ± 0.2mm. To ascertain the strain to initiate tissue damage, the specimens underwent successive loadings, which were carried out 1 hour apart to allow recovery. The maximum strain in each test was increased incrementally by 5% until a reduction in stiffness was observed in the following test. Separate tests were carried out to quantify and characterise the response of the anulus ground matrix in the three modes of loading and to strains greater than that which initiates damage.

Results: The strains at which permanent tissue damage occurred were between 20 and 27% in uniaxial compression and between 25 and 35% in simple shear. Testing the specimen beyond these strains showed an obvious reduction in stiffness. The biaxial compression tests showed similar changes but did not result in such pronounced losses in stiffness. The material characteristics were reproducible up to 20% strain. Following deformation to higher strains the altered mechanics were also shown to be reproducible indicating that the matrix had been deranged but not failed.

Discussion: Average physiological strains in the L4/5 intervertebral disc are in the order of 10–50% based on maximum deformations observed in vivo. The current results demonstrate that this strain will cause some permanent damage to the anulus ground matrix, however, the matrix will still be capable of carrying stress upon repeated loading. Thompson et. al.¹ found that people over the age of 35 all exhibited signs of disc degeneration. We hypothesise that the regenerative ability of the anulus ceases to function effectively as we age and the continual damage caused to the anulus tissue by daily activities may lead to the degenerative changes seen in the anulus.

Knowledge of the material characteristics up to 20% strain and following exposure to higher strains will enable a more realistic model of the intervertebral disc and the effects of degeneration to be studied.

Introduction: The complexity of the spine has made a complete understanding of its mechanical function difficult. As a consequence, biomechanical models have been used to describe the behaviour of the spine and its various components. A comprehensive mathematical model of the muscles of the lumbar spine and trunk is presented to enable computation of the forces and moments experienced by the lumbar intervertebral joints during physiological activities.

Methods: The model includes the nine major muscles crossing the region and concentrates on improving the estimated line of action for the muscles. The muscles are considered to consist of numerous fascicles, each with its own force producing potential based on size and line of action. The model respects the physical constraints imposed by the skeletal structure by ensuring that muscles maintain their anatomical position in various spinal postures. Validation was performed by comparing model predictions of maximum moments to published data from maximum isometric exertions in male volunteers. To highlight the potential novel uses of the model, three examples of muscle injury caused by surgical procedures were investigated; posterior lumbar surgery, impairment of abdominal muscles from anterior surgery and removal of the psoas major unilaterally during total hip replacement.

Results: The validation indicated that the model predicted forces similar to those measured in normal volunteers. The biomechanical changes resulting from the muscle injuries during the surgical procedures share several common features: decreased spinal compression and production of asymmetric moments during symmetric tasks.

Discussion: The results suggest that interference with muscles crossing or attaching to the lumbar spine can have a significant impact on its function.

INTRODUCTION: A computer model of the L4/5 human intervertebral disc is currently under development. An integral aspect of this model is the material properties assigned to its components. Detailed data on the material properties of the anulus fibrosus ground matrix are not available in the existing literature. To determine these properties, mechanical tests were carried out on specimens of anulus fibrosus harvested from sheep spines. The tests included unconfined uniaxial compression, simple shear and biaxial compression. Data on the strain required to cause permanent damage in the anulus ground matrix and data on the mechanical response of the anulus to repeated loading were obtained.

METHODS: Intervertebral discs were isolated from the lumbar spines of recently sacrificed sheep. These discs were sectioned into test specimens ensuring there were no continuous collagen fibres bearing load. The edge dimensions of the cubic specimens were 3 ± 0.2 mm. To ascertain the strain to initiate tissue damage, the specimens underwent successive loadings, which were carried out one hour apart to allow recovery. The maximum strain in each test was increased incrementally by 5% until a reduction in stiffness was observed in the following test. Separate tests were carried out to quantify and characterise the response of the anulus ground matrix in the three modes of loading and to strains greater than that which initiates damage.

RESULTS: The strains at which permanent tissue damage occurred were between 20 and 27% in uniaxial compression and between 25 and 35% in simple shear. Testing the specimen beyond these strains showed an obvious reduction in stiffness. The biaxial compression tests showed similar changes but did not result in such pronounced losses in stiffness. The material characteristics were reproducible up to 20% strain. Following deformation to higher strains the altered mechanics were also shown to be reproducible indicating that the matrix had been deranged but not failed.

DISCUSSION: Average physiological strains in the L4/5 intervertebral disc are in the order of 10–50% based on maximum deformations observed in vivo. The current results demonstrate that this strain will cause some permanent damage to the anulus ground matrix, however, the matrix will still be capable of carrying stress upon repeated loading. Thompson et. al1 found that people over the age of 35 all exhibited signs of disc degeneration. We hypothesise that the regenerative ability of the anulus ceases to function effectively as we age and the continual damage caused to the anulus tissue by daily activities may lead to the degenerative changes seen in the anulus.

Knowledge of the material characteristics up to 20% strain and following exposure to higher strains will enable a more realistic model of the intervertebral disc and the effects of degeneration to be studied.

INTRODUCTION: The complexity of the spine has made a complete understanding of its mechanical function difficult. As a consequence, biomechanical models have been used to describe the behaviour of the spine and its various components. A comprehensive mathematical model of the muscles of the lumbar spine and trunk is presented to enable computation of the forces and moments experienced by the lumbar intervertebral joints during physiological activities.

METHODS: The model includes the nine major muscles crossing the region and concentrates on improving the estimated line of action for the muscles. The muscles are considered to consist of numerous fascicles, each with its own force producing potential based on size and line of action. The model respects the physical constraints imposed by the skeletal structure by ensuring that muscles maintain their anatomical position in various spinal postures. Validation was performed by comparing model predictions of maximum moments to published data from maximum isometric exertions in male volunteers. To highlight the potential novel uses of the model, three examples of muscle injury caused by surgical procedures were investigated; posterior lumbar surgery, impairment of abdominal muscles from anterior surgery and removal of the psoas major unilaterally during total hip replacement.

RESULTS: The validation indicated that the model predicted forces similar to those measured in normal volunteers. The biomechanical changes resulting from the muscle injuries during the surgical procedures share several common features: decreased spinal compression and production of asymmetric moments during symmetric tasks.

DISCUSSION: The results suggest that interference with muscles crossing or attaching to the lumbar spine can have a significant impact on its function.