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: 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”.