This short contribution aims to explain how intervertebral disc ‘degeneration’ differs from normal ageing, and to suggest how mechanical loading and constitutional factors interact to cause disc degeneration and prolapse. We suggest that disagreement on these matters in medico-legal practice often arises from a misunderstanding of the nature of ‘soft-tissue injuries’.

The belief that an intervertebral disc must degenerate before it can herniate has clinical and medicolegal significance, but lacks scientific validity. We hypothesised that tissue changes in herniated discs differ from those in discs that degenerate without herniation. Tissues were obtained at surgery from 21 herniated discs and 11 non-herniated discs of similar degeneration as assessed by the Pfirrmann grade. Thin sections were graded histologically, and certain features were quantified using immunofluorescence combined with confocal microscopy and image analysis. Herniated and degenerated tissues were compared separately for each tissue type: nucleus, inner annulus and outer annulus.

Herniated tissues showed significantly greater proteoglycan loss (outer annulus), neovascularisation (annulus), innervation (annulus), cellularity/inflammation (annulus) and expression of matrix-degrading enzymes (inner annulus) than degenerated discs. No significant differences were seen in the nucleus tissue from herniated and degenerated discs. Degenerative changes start in the nucleus, so it seems unlikely that advanced degeneration caused herniation in 21 of these 32 discs. On the contrary, specific changes in the annulus can be interpreted as the consequences of herniation, when disruption allows local swelling, proteoglycan loss, and the ingrowth of blood vessels, nerves and inflammatory cells.

In conclusion, it should not be assumed that degenerative changes always precede disc herniation.

Cite this article: Bone Joint J 2013;95-B:1127–33.

Introduction

The feature of disc degeneration most closely associated with pain is a large fissure in the annulus fibrosus. Nerves and blood vessels are excluded from normal discs by high matrix stresses and by high proteoglycan (PG) content. However, they appear to grow into annulus fissures in surgically-removed degenerated discs. We hypothesize that anulus fissures provide a micro-environment that is mechanically and chemically conducive to the in-growth of nerves and blood vessels.

Introduction: There are extensive differences in structure and composition between cervical and thoracolumbar discs, yet practically nothing is known about the time-dependent “creep” behaviour of cervical discs.

Methods: 41 cadaveric cervical motion segments aged 48–89 yrs were subjected to a static compressive load of 150N for 2 hrs. Specimen height was recorded by the displacement of the actuator of the testing machine. Digitized radiographs were analysed to obtain dimensions of the vertebrae and discs. A three-parameter solid viscoelastic model was fitted to experimental data using nonlinear regression. Model parameters represent compressive stiffness of the wet tissue (E₂) and of the drained solid matrix (E₁), and tissue viscosity (η₁).

Results:Model and experimental data were in good agreement (r²> 0.98) and the average absolute error was always < 2%. E₁ was 11% and 39% lower than published values for thoracic and lumbar discs, respectively, whereas E₂ was 43% and 53% higher. The ratio E₂/E₁ for cervical discs (1.63) was greater than for thoracic (1.01) and lumbar (0.66) discs. η₁ for cervical discs was 108% and 21% higher than in thoracic and lumbar discs, resulting in a creep rate (E₁/η₁) which was lower by 51% and 43% respectively. Comparisons between younger (mean age 58 yrs) and older (79 yrs) cervical discs showed that in the latter, η₁ was reduced by 32% (p=0.01), E2 reduced by 18% (p=0.06), whereas E₁/η₁ was increased by 47% (p=0.02).

Discussion: Cervical discs appear to resist water loss more than thoracolumbar discs, but this resistance falls in old age.

Introduction: Neck pain often arises without any evident trauma suggesting that everyday loading may cause fatigue damage to spinal tissues. However, little is known about the forces acting on the cervical spine in everyday life. The purpose of this study was to determine spinal compressive forces using an electromyo-graphic (EMG) technique.

Methods: Eight subjects performed a number of tasks while cervical flexion/extension and surface EMG activity of upper trapezius and sternocleidomastoid were measured. Dynamic EMG signals were corrected for contraction speed, using a correction factor obtained from lumbar muscles, and were then compared with isometric calibrations in order to predict moment generation. Calibrations were performed in different amounts of cervical flexion/extension by each subject to account for changes in the EMG-moment relationship with muscle length. Compressive force on the C7-T1 intervertebral disc was determined by dividing the generated moments by the resultant lever arm of flexor or extensor muscles obtained from MRI scans on the same subjects.

Results: Peak values (mean ± SD) of extensor and flexor moments increased from 1.9±1.6Nm and 1.4±1.0Nm respectively in standing to 52.7±32.2Nm and 4.2±1.8Nm when lifting above the head. Resultant muscle lever arms ranged between 3.0–5.2cm and 1.6–3.5cm for extensor and flexor muscles respectively. Therefore, peak compressive forces on the C7–T1 disc were 110±74N in standing and 1570±940N during overhead lifting.

Conclusion: Neck muscles generate high forces in activities such as overhead lifting. If applied on a repetitive basis, such forces could lead to the accumulation of fatigue damage in life.

Introduction: Intradiscal electrothermal therapy (IDET) is a novel minimally invasive treatment for discogenic back pain. It involves inserting a catheter into discs which are suspected of being symptomatic in order to heat certain regions of the disc matrix and thereby influence the pain process. The clinical efficacy of IDET appears to be variable, and the scientific evidence suggests that the heating effect on disc tissues is very local to the catheter. We test the hypothesis that IDET can affect the internal mechanical functioning of lumbar intervertebral discs.

Methods: Eighteen cadaveric lumbar “motion segments” (aged 64–97 yrs) were used, 16 of which had degenerated intervertebral discs. Following incubation at 37°C, a miniature pressure transducer, side mounted in a 1.3mm diameter needle, was used to measure the distribution of compressive “stress” along the mid-sagittal diameter of each disc while it was compressed at 1.5 kN. Measurements were repeated in three simulated postures. IDET was then performed, using biplanar radiography to confirm placement of the heating element, and an independent thermocouple to measure temperature in the inner lateral annulus. Stress profilometry was repeated immediately after IDET.

Results: Peak temperatures in the inner lateral annulus during IDET averaged 40°C (STD 2.3°). Differences between stress measurements repeated before IDET never exceeded 8% (NS), and a sham IDET procedure produced no consistent changes. After IDET, pressure in the nucleus fell significantly by 6–13%, and stress peaks in the annulus were reduced (P< 0.008). In 12/18 specimens, annulus stress peaks were reduced by more than 8%, and in these “responders”, the mean reduction was 78%. Stress concentrations were increased by more than 8% in two specimens.

Conclusion: IDET has a significant but inconsistent affect on compressive stresses within intervertebral discs. These results may partly explain the variable clinical success of IDET.

Introduction: Peripheral rim tears in the annulus fibrosus are a common finding in autopsy specimens, and animal experiments suggest that they lead eventually to degenerative changes throughout the disc. We test the hypothesis that injury to the outer annulus decompresses the nucleus, thereby providing a progressive stimulus for disc degeneration.

Methods: Seven human cadaveric lumbar “motion segments” aged 49–70 yrs were compressed at 2 kN while the distribution of compressive stress was measured in each disc by pulling a 1.3 mm-diameter pressure transducer along its mid-sagittal diameter. Measurements were repeated after “rim tears” were simulated by 10 mm-deep scalpel cuts into the anterior annulus, as follows. 1st cut: horizontal, 15 mm right lateral; 2nd cut: vertical, 15 mm left lateral; 3rd cut: horizontal, midline (through the transducer needle track). Stress measurements were repeated a final time following compressive overload sufficient to fracture the endplate.

Results: “Rim tears” had negligible effect on compressive stress distributions more than 15mm from the scalpel cut, and nucleus pressure fell by only 1.0% (STD 1.3%, NS). However, compressive stresses in the outer annulus adjacent to the cut were greatly reduced, and a steep stress gradient appeared in the middle annulus. The effective decrease in the A-P diameter of the disc was 7.1% (STD 1.7%, P< 0.01). Endplate fracture reduced nucleus pressure by 36.1% (STD 16.7%, P< 0.001).

Discussion: Stress gradients generated in the middle annulus could cause the “rim tear” to progress inwards until it reached the nucleus, at which point it might decompress it. However, the present results suggest that injuries to the outer annulus are unlikely to have any direct effect on the pressure in, or metabolism of, the nucleus pulposus. This is in contrast to injuries to the vertebral endplate, which do affect the nucleus directly.

Introduction: Little is known about how the cervical spine resists the high complex loading to which it is often subjected in life. In this study, such loading was applied to cadaveric cervical motion segments in order to a) measure their strength in forward and backwards bending, b) indicate which structures resist bending most strongly, and c) indicate how compressive injury influences the bending properties.

Methods: Ten human cervical spines aged 65–88yrs were obtained post-mortem, dissected into 14 motion segments, and stored at −20°C. Subsequently, motion segments were defrosted and secured in dental plaster for testing on a hydraulic materials testing machine. An optical motion capture system recorded specimen movement simultaneously. Specimens were loaded in 2.5sec in combined bending and compression to reach their elastic limit in flexion, and then extension. Experiments were repeated following creep loading, removal of spinous processes, removal of apophyseal joints, and vertebral body compressive damage.

Results: On average, full flexion was reached at an angle of 7.2° and a bending moment of 6.8Nm. Full extension occurred at 9.2° and 9.0Nm. Creep loading reduced specimen height by 0.37mm, increased flexion by 1.5° (P< 0.01) but had little effect on extension. After creep, resistance to flexion came from the spinous processes and related ligaments (46%), apophyseal joints (30%), and disc (24%). Resistance to extension came from spinous processes (23%), apophyseal joints (45%), and disc (32%). The compressive strength of discvertebral body specimens was 1.87kN (STD 0.63kN). Compressive damage reduced specimen height by 0.83mm (STD 0.29mm). This reduced the disc’s resistance to flexion by 44% and extension by 18%.

Conclusion: Cervical motion segments have approximately 20% of the bending strength, and 45% of the compressive strength, of lumbar specimens of similar age. The relative weakness of the cervical spine in bending may influence the patterns of injury seen in “whiplash”.

Introduction. : Measurements of overall vertebral bone mineral density (BMD_v) do not adequately explain the observed patterns of osteoporotic vertebral fracture. Perhaps bone loss from specific regions of the vertebra has a more important effect on vertebral strength, and risk of fracture, than overall bone loss? We hypothesise that ‘stress shielding’ of the anterior vertebral body by the neural arch in erect standing postures can reduce BMD_v in the anterior vertebral body and thereby reduce vertebral compressive strength.

Materials and Methods: A compressive force of 1.5kN was applied to lumbar ‘motion segments’. positioned to simulate erect standing posture. Compressive stresses within the intervertebral disc were measured by pulling a miniature pressure transducer through it. ‘Stress profiles’ were integrated over area to calculate the total compressive force on the disc¹. This was subtracted from the 1.5kN to calculate the force resisted by the neural arch. Motion segments were then compressed to failure in moderate flexion (to simulate heavy lifting) and their compressive strength obtained. After disarticulation, the BMD_v, of the whole and the anterior half of each vertebral body was measured by dual energy x-ray absorptiometry (DXA). We report preliminary results from 9 specimens, aged 72–92 yrs.

Results: Vertebral strength (in flexion) was inversely related to load-bearing by the neural arch in erect posture (r²=0.42, p=0.05). Strength was directly related to the BMD_v of the whole (r²=0.65, p=0.06) and the anterior (r²=0.8, p=0.005) vertebral body.

Conclusions: These results suggest that habitual load-bearing by the neural arch in erect postures can lead to stress shielding of the anterior vertebral body so that the latter losesBMD_v, and the vertebra is weakened in the anterior vertebral body appears to be a BMD_v better predictor of vertebral strength than BMD_v, of the whole vertebra.

Introduction: Intradiscal electrothermal therapy (IDET) is a novel treatment for discogenic back pain. A heating element is inserted percutaneously into a disc in order to denature the collagen of the posterior annulus. Clinical success is claimed, although laboratory studies indicate that temperature increases may be insufficient to cause widespread collagen denaturation, or denervation, and that IDET has little effect on gross mechanical properties. We report on changes in internal disc mechanics following IDET.

Methods: Ten cadaveric lumbar ‘motion segments’ (aged 72–79 yrs) were stored at −17°C. Subsequently, each was equilibrated at 37°C. A miniature pressure transducer was used to measure the distribution of compressive stress along the mid-sagittal diameter of each disc while it was compressed at 1.5kN. IDET was performed, using bi-planar radiography to confirm placement of the heating element, and an independent thermocouple to measure temperature in the inner lateral annulus. Stress profilometry was repeated irnmediately after IDET.

Results: Before IDET, all discs exhibited stress concentrations typical of mild degeneration. Accurate placement of the element was confirmed in all discs. Temperatures in the inner lateral annulus during IDET reached only 40.9°C (STD 2.3°C). Differences between stress measurements repeated before IDET never exceeded 8% (NS). After IDET, peak stresses (above nucleus pressure) were reduced by more than 8% in 6/10 specimens (mean reduction 55%), increased in 2/10, and were unchanged in 2/10. Nucleus pressure fell by 13% (n=10 0, P=0.05).

Discussion: IDET had a variable effect on these 10 degenerated discs. In six of them, stress concentrations in the annulus were reduced, suggesting that IDET can cause disc material to resist compression in a more coherent fashion, possibly by ‘bonding’ fragmented tissue together, and thereby distributing load more evenly across the endplate. Reduction in nucleus pressure following IDET suggests load transfer to the neural arch, although this could not be confirmed. Reducing annulus stress concentrations could conceivably reduce pain in some individuals.

Introduction: Pathological changes in the elderly spine include intervertebral disc degeneration, apophyseal joint arthritis and osteoporotic fracture of the vertebral body. Such changes are likely to be inter-related through alterations in the sharing of load between the apophyseal joints and the intervertebral disc unit. We describe an accurate, non-destructive method for calculating the load sharing based on measurements of the distribution of stress within the intervertebral disc.

Materials and Methods: Twenty three motion segments, consisting of two vertebrae and the intervening disc and ligaments, were dissected from 17 human lumbar spines. A preliminary “creep” test was used to reduce disc height and water content by an amount equivalent to the diurnal variation seen in vivo. Then, a constant load was applied to each motion segment, using a computer-controlled hydraulic materials testing machine, for a period of 20s while a pressure-transducer, sensitive to spatial variations in compressive stress, was pulled through the disc along its mid-sagittal diameter. Profiles of vertically-acting compressive stress were obtained in each disc positioned in 2° of extension (appropriate for an erect standing posture). The total compressive force acting on the intervertebral disc was calculated by modelling the disc using approximately 20 elliptical rings of known cross-sectional area. The force acting on each ring was given by the product of area and the average compressive stress acting on it, which was obtained from the appropriate region of the stress profile. The total force acting through the disc was obtained by summing up the force contribution from each ring. The force acting on the apophyseal joints was calculated from the difference between applied (known) load and the calculated load acting on the disc. A correction factor was obtained separately for each disc to account for deviations in the cross-section from the elliptical, and variations in the sensitivity of the transducer in disc tissues of different ages. The correction factor was obtained by comparing the applied force with the force calculated from a stress profile measured before creep loading while the disc was in a neutral position, when the load passing through the apophyseal joints is negligible.

Results: The proportion of load passing through the apophyseal joints increased significantly with age (r²=0.48, p< 0.01), from 7% at age 27 yrs to 42% at 82yrs. Similarly, the proportion of load passing through the apophyseal joints increased with degree of disc degeneration (r²=0.5, p< 0.05 Pearson, Chi-square) from 8% in “grade 1” discs to 40% in “grade 4” discs.

Discussion: The compressive load passing through the apophyseal joints is higher than that predicted by previous, inaccurate, methods, or by experiments which failed to reduce the height and water content of the intervertebral disc. Increased load-bearing may be a contributing factor in apophyseal joint degeneration. Also, in lordotic postures, “stress shielding” by the apophyseal joints could contribute to bone loss in the vertebral body, leaving it vulnerable to osteoporotic fracture when the spine is loaded in flexion.

We investigated the distribution of compressive ‘stress’ within cadaver intervertebral discs, using a pressure transducer mounted in a 1.3 mm diameter needle. The needle was pulled along the midsagittal diameter of a lumbar disc with the face of the transducer either vertical or horizontal while the disc was subjected to a constant compressive force. The resulting ‘stress profiles’ were analysed in order to characterise the distribution of vertical and horizontal compressive stress within each disc. A total of 87 discs from subjects aged between 16 and 87 years was examined.

Our results showed that age-related degenerative changes reduced the diameter of the central hydrostatic region of each disc (the ‘functional nucleus’) by approximately 50%, and the pressure within this region fell by 30%. The width of the functional annulus increased by 80% and the height of compressive ‘stress peaks’ within it by 160%. The effects of age and degeneration were greater at L4/L5 than at L2/L3, and the posterior annulus was affected more than the anterior. Age and degeneration were themselves closely related, but the stage of degeneration had the greater effect on stress distributions.

We suggest that structural changes within the annulus and endplate lead to a transfer of load from the nucleus to the posterior annulus. High ‘stress’ concentrations within the annulus may cause pain, and lead to further disruption.