Abstract
Introduction: Painful anterior vertebral wedge “fractures” can occur without any remembered trauma, suggesting that vertebral deformity could accumulate gradually through sustained loading by the process of “creep”. If the adjacent intervertebral discs are degenerated, they press unevenly on the vertebral body in a posture- dependent manner, producing differential creep of the vertebra. We hypothesise that differential creep due to sustained asymmetrical loading of a vertebral body can cause anterior vertebral wedge deformity.
Materials And Methods: Eleven thoracolumbar motion segments aged 64–88 yrs were subjected to a 1.5 kN compressive force for 2 hrs, applied via plaster moulded to its outer surfaces. Specimens were positioned in 2° flexion to simulate a stooped posture. Reflective markers attached to pins inserted into the lateral cortex of each vertebral body enabled anterior, middle and posterior vertebral body heights to be measured at 1Hz using an optical tracking device. Compressive ‘stress’ acting vertically on the vertebral body was quantified by pulling a miniature pressure transducer along the midsagittal diameter of adjacent discs.
Results: Elastic deformation (strain) was higher anteriorly (−2018 ± 2983 μ strain) than posteriorly (−1675 ± 1305 μ strain). Creep strain (−2867 ± 2527 μ strain) was significantly higher anteriorly (p< 0.05) than posteriorly (−1164 ± 1026 μ strain), and was associated with a higher compressive stress in the anterior annulus of the adjacent disc. Non-recoverable creep deformations were significantly higher anteriorly (p< 0.05), and were equivalent to a wedging angle of 0.01–0.3°.
Conclusion: Creep can cause anterior wedge deformity of the vertebral body. In the long term, accumulating creep could cause more severe (and painful?) deformity.
Correspondence should be addressed to: Mr John O’ Dowd, SBPR, c/o BOA, The Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
