The influence of static axial torque in combined loading on intervertebral joint failure mechanics using a porcine model

The morphology of acute disc herniation: a clinically relevant model defining the role of flexion

STUDY DESIGN

Hydrostatically induced disruption of flexed lumbar intervertebral discs followed by microstructural investigation.

OBJECTIVE

To investigate how flexion affects the anulus' ability to resist rupture during hydrostatic loading, and determine how the characteristics of the resulting disc failures compare with those observed clinically.

SUMMARY OF BACKGROUND DATA

While compression of neutrally positioned motion segments consistently causes vertebral failure, compression of flexed segments can induce herniation. Why flexion has this effect remains unclear. A vast range of herniation characteristics have been documented clinically; whether flexion-related herniations are likely to possess a subset of these is unknown.

METHODS

Forty-two ovine lumbar motion segments, dissected from the same 3 levels of 14 spines, were each flexed 7 degrees or 10 degrees from the neutral position. While maintained at one of these angles, the nucleus of each segment was gradually injected with a viscous radio-opaque gel via an injection screw placed longitudinally within the inferior vertebra, until failure occurred. Each segment was then inspected using microcomputed tomography and oblique illumination microscopy in tandem. RESULTS.: Eighteen segments suffered disc failure; 14 of these were caused by direct radial rupture of the anular wall. All radial ruptures were located in the central posterior anulus. Nine radial ruptures contained nuclear material, which had breached the posterior longitudinal ligament in 1 disc, and reached it in 5 others forming transligamentous and subligamentous nuclear extrusions, respectively. The most common radial rupture route, occurring in 10 discs, involved a systematic anulus-endplate-anulus failure pattern.

CONCLUSION

Flexion places the anulus at risk by facilitating nuclear flow, limiting circumferential disruption while promoting radial rupture, and rendering the endplate/vertebra junction vulnerable to failure. Flexion may play a developmental role in those herniations possessing a central posterior radial rupture that incorporates a short span of endplate disruption along the apex of the vertebral rim.

Rat disc torsional mechanics: effect of lumbar and caudal levels and axial compression load

BACKGROUND CONTEXT

Rat models with altered loading are used to study disc degeneration and mechano-transduction. Given the prominent role of mechanics in disc function and degeneration, it is critical to measure mechanical behavior to evaluate changes after model interventions. Axial compression mechanics of the rat disc are representative of the human disc when normalized by geometry, and differences between the lumbar and caudal disc have been quantified in axial compression. No study has quantified rat disc torsional mechanics.

PURPOSE

Compare the torsional mechanical behavior of rat lumbar and caudal discs, determine the contribution of combined axial load on torsional mechanics, and compare the torsional properties of rat discs to human lumbar discs.

STUDY DESIGN

Cadaveric biomechanical study.

METHODS

Cyclic torsion without compressive load followed by cyclic torsion with a fixed compressive load was applied to rat lumbar and caudal disc levels.

RESULTS

The apparent torsional modulus was higher in the lumbar region than in the caudal region: 0.081+/-0.026 (MPa/degrees, mean+/-SD) for lumbar axially loaded; 0.066+/-0.028 for caudal axially loaded; 0.091+/-0.033 for lumbar in pure torsion; and 0.056+/-0.035 for caudal in pure torsion. These values were similar to human disc properties reported in the literature ranging from 0.024 to 0.21 MPa/degrees.

CONCLUSIONS

Use of the caudal disc as a model may be appropriate if the mechanical focus is within the linear region of the loading regime. These results provide support for use of this animal model in basic science studies with respect to torsional mechanics.

Disc prolapse: evidence of reversal with repeated extension

STUDY DESIGN

A basic science study that used a porcine cervical spine model to produce disc prolapse subsequently exposed to an extension protocol.

OBJECTIVE

This study investigated whether extension or combined extension and side flexion could move the displaced portion of nucleus from the anulus towards the nucleus.

SUMMARY OF BACKGROUND DATA

Previous research has established that repeated flexion can create disc prolapse, the question here is whether repeated extension can reverse the process.

METHODS

The C3/4 segments of 18 porcine cervical spines were dissected and potted in cups. Specimens were preloaded, then axially compressed (1472 N), and repeatedly rotated in either pure flexion or combined flexion and side flexion at a rate of 0.5 degrees /s. Specimens that prolapsed were axially compressed and repeatedly and rotated into extension.

RESULTS

Based on a blinded radiologist's review of the radiograph images, all 18 specimens contained healthy discs before testing, but after testing 2 of the 18 specimens had endplate fractures, whereas 11 of the 18 specimens had prolapsed. Prolapsed nucleus was reduced in 5 of the 11 prolapsed specimens after the reversal testing, whereas the remaining 6 did not change. Subclassification analysis revealed that the prolapsed discs that centralized had significantly less disc height loss (P < 0.01). Neither the classification of the herniation (circumferential or radial) nor the angle of lordosis of the specimens was linked to the behavior of the specimens.

CONCLUSION

This study showed that with repeated flexion, in porcine cervical spines, disc prolapse was initiated and that the displaced portion of nucleus can be directed back towards the center of the disc in response to particular active and passive movements/positions.

Evolving ergonomics?

The theme developed in this position paper follows the current evolution of injury prevention in the backs of workers. Job change or 'fitting the task to the person' has come far, but will probably not result in zero injury rates. This is because the cause of injury is heavily influenced by the way that a worker moves. A review of injury mechanisms reveals the need for the biomechanist/ergonomist to incorporate features in biomechanical models that recognise these injury mechanisms. The implication of one such model is that the next leap toward a zero injury rate may be approached with 'fitting the person to the task' or at least retraining the way that workers move. A few examples of movement-based back injury prevention strategies are provided. Finally, some thoughts on implementing such an approach are expressed. This is a review and position paper written in honour of Professor Don Chaffin's career.

Intervertebral neural foramina deformation due to two types of repetitive combined loading

BACKGROUND

Tissue compression and noxious stimuli are known to elicit pain from neural tissues in the spine. Compression of nerve roots due to decreases in the intervertebral foramina may be caused by posture, sustained loading and disc height loss, herniation, or altered mechanics. It has been established that non-neutral postures combined with repeated loading can cause disc herniations, however information regarding the effect of repetitive axial twist loading is limited. The objectives of this study were twofold; to measure the occlusion of the foramina due to two types of repetitive loading and to investigate whether repetitive combined axial twist loading can contribute to disc injury.

METHODS

Sixteen porcine cervical spine segments (C5/6) were subjected to 1500 N of compression combined with either repetitive flexion-extension motions or 16.4 degrees (Standard Deviation 2.1) of static flexion with repetitive axial twist motions. The foramina pressure was measured bilaterally using plastic tubing and a custom pressure monitoring system. Specimens were loaded until 10,000 cycles were reached or disc herniation occurred.

FINDINGS

Significantly larger pressure (pre-post difference) developed in the intervertebral foramina of specimens that were repetitively flexed-extended (P=0.028) compared to those that were repetitively twisted. All of the flexed-extended specimens herniated, whereas in the twisted specimens five (62.5%) had incomplete herniations, one (12.5%) sustained a facet fracture, and two (25%) had no damage. There was no difference between the loading groups for vertical height loss (P=0.994).

INTERPRETATION

Repetitive loading of flexion-extension motions are a viable pain generating pathway in absence of distinguishing height loss. This information may be useful to consider for the diagnosis and treatment of nerve root compression.

The influence of posture and loading on interfacet spacing: an investigation using magnetic resonance imaging on porcine spinal units

STUDY DESIGN

Basic scientific investigation using porcine spine segments and magnetic resonance imaging.

OBJECTIVE

To quantify the effects of flexion-extension postures and loading history on the distance between the facet articular surfaces.

SUMMARY OF BACKGROUND DATA

Increased axial twist motion is used clinically to indicate instability and has been implicated as a potential cause of low back pain. Recently, it has been demonstrated that larger twist angles can be achieved when coupled with forward flexion in vivo. These findings suggest a postural mechanism may be responsible for modulating how the facet joints articulate, thereby affecting the moment resisting capability of the facets and altering the load distribution between the facet joints and the disc.

METHODS

Four porcine cervical spine motion segments (C3-C4) were exposed to a compressive preload. Two of these specimens were also exposed to 5000 repeats of flexion-extension motions. The interfacet spacing was measured from magnetic resonance images of 6 postures: neutral, maximum flexed, maximum extended, neutral-twisted, maximum flexed-twisted, and maximum extended-twisted. The range of axial twist angle was quantified in the neutral, flexed, and extended postures.

RESULTS

Flexion-extension postures and loading history caused a difference in the interfacet spacing and twist angle measured. Repetitive loading and flexed postures independently increased the spacing and twist angle, whereas the preload condition and extended postures independently decreased the measures. The 2 specimens that underwent the preload only condition suffered no damage to the disc or vertebrae. Of the repetitively loaded specimens, 1 had a vertebral fracture with initiation of herniation, and the second had a complete herniation.

CONCLUSION

The findings support a posture-dependent injury mechanism and may account for the previously reported in vivo posture-dependent passive rotational differences quantified for combined postures. The changes in spine mechanics and resulting load distribution due to coupled postures may be a key to understanding the formation of low back injuries and eventually clinical spine instability.

Do flexion/extension postures affect the in vivo passive lumbar spine response to applied axial twist moments?

BACKGROUND

The injury potential and mechanical effects of combining axial rotation with non-neutral flexion/extension postures in vivo remains poorly understood, despite being identified as a risk factor in epidemiological and in vitro studies. The purpose of this experiment was to quantify the passive axial twist motion of the lumbar spine in various postures, and to assess whether non-neutral flexion/extension postures cause a detectable change in the range of twist motion and/or spine rotational stiffness.

METHODS

Ten healthy male participants were passively rotated three times from a neutral and six flexed/extended starting postures (maximum-, mid-, mild-), while the moment-angle relationships were measured. The upper body was fixed to an adjustable rigid harness and the lower body was fixed to a cradle that rested on a frictionless table, thereby isolating the lumbar spine.

FINDINGS

The lumbar spine stiffness and rotational range of motion were modulated by the different flexion/extension postures. The average maximum rotational stiffness values were smallest in maximum-flexion (81.0%, SD 16.6), and largest in maximum-/mid-extension postures at 125.4% (SD 24.4, P<0.0001) of the neutral stiffness magnitude. The axial twist angle was significantly different for each posture (P<0.0001), with 13.8% (SD 8.9) greater rotation in the maximum-flexion and 23.8% (SD 7.8) less rotation in the maximum-extension posture. The lateral bend coupled motion with axial twist was significantly different (P<0.0001) between the maximum-flexion (11.4 degrees , SD 6.3), mid-flexion/maximum-extension/mid-extension (6.5 degrees , SD 4.5), and mid-extension/mild-flexion/mild-extension postures (4.4 degrees , SD 3.8).

INTERPRETATION

The lumbar spine stiffness and rotational range were modified by flexed-extended postures. The postural mechanism observed may be due to a change in the initial distance separating the facets prior to rotation. This information will be useful in determining spine rotational injury mechanisms through comparison with in vitro literature and for patient positioning during diagnostic tests.

Chiropractic: a critical evaluation

Chiropractic was defined by D.D. Palmer as "a science of healing without drugs." About 60,000 chiropractors currently practice in North America, and, worldwide, billions are spent each year for their services. This article attempts to critically evaluate chiropractic. The specific topics include the history of chiropractic; the internal conflicts within the profession; the concepts of chiropractic, particularly those of subluxation and spinal manipulation; chiropractic practice and research; and the efficacy, safety, and cost of chiropractic. A narrative review of selected articles from the published chiropractic literature was performed. For the assessment of efficacy, safety, and cost, the evaluation relied on previously published systematic reviews. Chiropractic is rooted in mystical concepts. This led to an internal conflict within the chiropractic profession, which continues today. Currently, there are two types of chiropractors: those religiously adhering to the gospel of its founding fathers and those open to change. The core concepts of chiropractic, subluxation and spinal manipulation, are not based on sound science. Back and neck pain are the domains of chiropractic but many chiropractors treat conditions other than musculoskeletal problems. With the possible exception of back pain, chiropractic spinal manipulation has not been shown to be effective for any medical condition. Manipulation is associated with frequent mild adverse effects and with serious complications of unknown incidence. Its cost-effectiveness has not been demonstrated beyond reasonable doubt. The concepts of chiropractic are not based on solid science and its therapeutic value has not been demonstrated beyond reasonable doubt.