Abstract
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”.
The abstracts were prepared by Editorial Secretary, Dr Charles Pither. Correspondence should be addressed to SBPR at the Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PN
