Abstract
Background: Continuous bone “creep” under constant load can cause measurable deformity in cadaveric vertebrae, but the phenomenon is extremely variable.
Purpose: We test the hypothesis that vertebral micro-damage accelerates creep deformity.
Methods: Twenty-six thoracolumbar “motion segments” were tested from cadavers aged 42–92 yrs. Bone mineral density (BMD) of each vertebral body was measured using DXA. A 1.0 kN compressive force was applied for 30 mins, while the height of each vertebral body was measured using a MacReflex optical tracking system. After 30 mins recovery, one vertebral body from each specimen was subjected to controlled micro-damage (< 5mm height loss) by compressive overload, and the creep test was repeated. Load-sharing between the vertebral body and neural arch was evaluated from stress measurements made by pulling a pressure transducer through the intervertebral disc.
Results: Creep was inversely proportional to BMD (P=0.041) and did not recover substantially after unloading. Creep was greater in the anterior vertebral body cortex compared to the posterior (p=0.002). Vertebral micro-damage usually affected a single endplate, causing creep of that vertebra to increase in proportion to the severity of damage. Anterior wedging of the vertebral bodies during creep increased by 0.10o (STD 0.20o) for intact vertebrae, and by 0.68o (STD 1.34o) for damaged vertebrae.
Conclusion: Creep is substantial in elderly vertebrae with low BMD, and is accelerated by micro-damage. Preferential loss of trabeculae from the anterior vertebral body could explain why creep is greater there, and so causes wedging deformity, even in the absence of fracture.
Conflicts of Interest: none
Source of Funding: Action Medical Research
Correspondence should be addressed to: SBPR at the Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE, England.
