James A A Miller

Influence of some biomechanical factors on low-back pain in pregnancy

Several biomechanical factors were recorded intermittently in 855 pregnant women from the 12th to the 36th week of gestation and were related to back pain occurrence during pregnancy. The three factors related to the development of back pain were abdominal sagittal diameter, which correlated with back pain, with a coefficient of 0.15 [P< 0.01); transverse diameter (r=0.13, P< 0.01); and depth of the lumbar lordosis, which correlated with a coefficient of 0.11 (P< 0.01). In the group of women who were pregnant for their first time, there was a significantly lower peripheral joint laxity in the 12th week in those women who, later in pregnancy, developed back pain. These correlations suggest that back pain in pregnancy can not be explained primarily by biomechanical factors.

Anatomy and significance of fixation of the lumbosacral nerve roots in sciatica.

The anatomy of 54 pairs of lumbosacral nerve roots was described in nine fresh adult cadaver specimens, with particular attention given to the fixation of the nerve roots to surrounding skeletal and ligamentous structures in the lumbar spine. Dural ligaments were identified fixing the dura and nerve roots at their exit from the main dural sac to the posterior longitudinal ligament and vertebral body periosteum proximal to the intervertebral disc. Distal fixation occurs at the intervertebral foramen where the epineural sheath of the spinal nerve is attached. The overall arrangement is one which tends to hold the exiting nerve root anteriorly in the spinal canal. Mechanical analysis of this anatomical arrangement explains how pressure can be applied to the extrathecal nerve root by a disc protrusion without compression of the nerve root against the posterior elements. The possible role of the dural ligaments in the pathogenesis of the sciatica syndrome is discussed.

Mechanical properties of lumbar spine motion segments under large loads

The mechanical behavior of fourteen fresh human lumbar motion segments taken at autopsy from males with an average age of 29 yr was studied. Forces up to 1029 N were applied in anterior, posterior and lateral shear; and moments up to 95 Nm were applied in flexion, extension, lateral bending and torsion. In response to these loads endplate displacements up to 9 mm and rotations up to 18 degrees were measured. Stiffness values ranged from 53 to 140 N mm-1 in response to the shear forces and 6-11 Nm degree-1 in response to the moments. Lumbar motion segments can develop significant passive resistances to loads in situations where they are allowed to undergo substantial deformations.

Posterior element loads in lumbar motion segments

This report concerns the manner in which the intervertebral disc and the posterior elements share loads placed on a lumbar motion segment. A two-dimensional biomechanical model was used to examine this. The model incorporated two rigid bodies to represent the vertebrae and six elastic springs to represent the tissues of the intervertebral disc and the posterior elements. Compression loads were resisted almost totally by the model intervertebral disc, but both the intervertebral disc and the posterior elements contributed substantially to resisting anteroposterior shear and flexion-extension loads. Motion segment morphology was a major determinant of load-sharing in model response to anteroposterior shear.

The effect of intervertebral disc space narrowing on the contact force between the nerve root and a simulated disc protrusion.

An instrumented probe mounted on the anterior surface of the lumbar spine over an excised lumbar Intervertebral disc was used to simulate a disc protrusion in 12 fresh cadavers. The contact force between probe and nerve root was measured as a function of two independent variables: probe protrusion depth and disc space height. The contact force on the nerve root was found to increase with increasing probe depth. Disc space widening increased the contact force while narrowing the disc space decreased it. A simple mechanical model analysis confirmed that the force exerted on the nerve root by the probe is the result of tension produced in the nerve root as it is deformed by the probe. The mechanical principle that disc narrowing can reduce the pressure on a nerve root produced by a disc protrusion may be an explanation of how chemonycleolysis relieves sciatic pain

The effects of chemonucleolysis on the mechanical properties of the canine lumbar disc

One hundred and twenty-two lumbar intervertebral discs from 43 mongrel dogs were used to study the effect of chemonucleolysis on the flexion, torsion, and lateral bending flexibilities of the disc. The dogs were killed 2, 4, 12, 26, and 52 weeks following injection with 0.1-0.15 ml of either crude collagenase, semipurified collagenase, or chymopapain. Controls consisted of saline-injected and uninjected discs. The bending and torsional properties of each disc were determined by applying incremental moments up to 0.8 Nm and measuring the resultant rotations 60 seconds after each load increment was applied. The discs were then sectioned for morphologic evaluation. Increases in disc flexibilities ranging from 1.4 to 5.8-fold were found 2 weeks after injection with all three enzymes. The largest increase was noted in flexion in discs injected with chymopapain. By 3 months, all lateral bending flexibilities had returned to control values. In general, however, flexion and torsion flexibilities did not return to control values 6 months following chemonucleolysis. The extent of the gross morphologic changes produced by each of the three enzyme preparations did not correlate with the acute increases in disc flexibilities. Chymopapain and semipurified collagenase had similar morphologic and mechanical effects. The temporary increases in flexibility appeared to be due to decreases in the overall compression, tensile and shear stiffness of the annulus caused by the enzymes.

Stiffness properties and geometry of lumbar spine posterior elements

This paper reports measurements made in five fresh cadaver lumbar spine motion segments of the load-deformation properties of the posterior element soft tissues. These properties were measured and the stiffness corresponding to them were calculated for loading in anterior, posterior and lateral shear; in longitudinal tension and compression; and in flexion, extension, lateral bending and axial torsion. In addition, measurements were made in six motion segments of the positions of the inferior facet joint centers relative to the vertebral body centers, and of the orientations of the facet joint surfaces.

The mechanism of sciatic pain relief by chemonucleolysis.

Chemonucleolysis must relieve sciatic pain by some means other than the removal of the disc protrusion from the neural canal, because the protrusion characteristically persists despite the resolution of sciatic pain. This paper analyzes the mechanism of symptom production in sciatica and identifies how chemonucleolysis relieves these symptoms.Pressure can be exerted on a nerve root by a disc protrusion without compression against the posterior wall of the neural canal. This pressure is the result of tension produced in the nerve root when it is deformed by the disc protrusion. A previously unrecognized principle of nerve root-disc protrusion mechanics is identified. For a given disc protrusion, narrowing the disc reduces the tension in the nerve root and therefore the pressure on the nerve root; conversely, widening the disc increases the tension and pressure on the nerve root. Disc narrowing is in itself a mechanism by which chemonucleolysis relieves sciatic pain.