Abstract
Introduction: Vertebral bodies and intervertebral discs resist most of the compressive force acting on the spine. However, experiments on lumbar spines have shown that apophyseal joints can resist more than 50% of applied compression, and that the proportion varies with spinal level, disc narrowing, and posture. In the cervical spine, the situation is likely to be complicated by the presence of uncovertebral joints on the lateral margins of the disc. Load-sharing is important because it influences injury risk, and predisposition to degenerative changes. The present study aims to characterise compressive load-sharing in the cervical spine.
Methods: Sixteen cervical motion segments, consisting of two vertebrae and the intervening disc and ligaments, were dissected from nine cadaveric spines, aged 48-77 yrs (mean 63 yrs) which had been stored at -17degC. Specimens were subjected to 200N of compression while the distribution of compressive ‘stress’ was measured along the mid-sagittal diameter of the disc, using a pressure transducer side-mounted in a 0.9mm-diameter needle. ‘Stress profiles’ effectively were integrated over area to calculate the total compressive force acting on the disc. Experiments were performed with each specimen in flexion, extension and neutral posture. They were repeated after creep compressive loading (2 hrs at 150N) to simulate diurnal loading in life, and again following removal of the apophyseal joints. Eight specimens were re-tested following bi-lateral removal of the uncovertebral joints.
Results: Creep loading reduced disc height by an average 0.64mm (approximately 12%). Creep reduced overall computed disc loading by 14% and 25% in neutral and extended postures respectively (P< 0.005). Apophyseal joint removal increased disc loading in extension (only) by 14% (P< 0.05). Uncovertebral joint removal further increased disc loading in flexed, neutral and extended postures by 28%, 33% and 21% respectively (P< 0.05).
Conclusion: Creep loading of the cervical spine transfers loading to the apophyseal joints and uncus. The former effect is small, and significant only in extended postures. The latter effect is larger, and is greatest in flexed and neutral postures.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
