Abstract
Osteoporotic vertebral deformities are conventionally attributed to fracture, although deformity is often insidious, and bone is known to “creep” under constant load. We hypothesise that deformity can arise from creep that is accelerated by minor injury.
Thirty-nine thoracolumbar “motion segments” were tested from cadavers aged 42-92 yrs. Vertebral body BMD 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.
Creep was inversely proportional to BMD below a threshold BMD of 0.5 g/cm2 (R2=0.30, P<0.01) and did not recover substantially after unloading. Creep was greater in the anterior cortex compared to the posterior (p=0.01) so that anterior wedge deformity occurred. 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 vertebral bodies during creep increased by 0.10o (STD 0.20o) for intact vertebrae, and by 0.68o (STD 1.34o) for damaged vertebrae.
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 greater anterior creep and vertebral wedging.
