Introduction Vertebral deformities often occur in patients who recall no trauma, and display no evident fracture on radiographs. We hypothesise that vertebral deformity can occur by a gradual creep mechanism which is accelerated following minor damage. ''Creep'' is continuous deformation under constant load. Materials and methods Forty-five thoracolumbar spine motion segments were tested from cadavers aged 42-92 years. Vertebral body areal BMD was measured using DXA. Specimens were compressed at 1 kN for 30 min, while creep in each vertebral body was measured using an optical MacReflex system. After 30 min recovery, each specimen was subjected to a controlled overload event which caused minor damage to one of its vertebrae. The creep test was then repeated. Results Vertebral body creep was measurable in specimens with BMD \0.5 g/cm 2 . Creep was greater anteriorly than posteriorly (p \ 0.001), so that vertebrae gradually developed a wedge deformity. Compressive overload reduced specimen height by 2.24 mm (STD 0.77 mm), and increased vertebral body creep by 800 % (anteriorly), 1,000 % (centrally) and 600 % (posteriorly). In 34 vertebrae with complete before-and-after data, anterior wedging occurring during the 1st creep test averaged 0.07°(STD 0.17°), and in the 2nd test (after minor damage) it averaged 0.79°(STD 1.03°). The increase was highly significant (P \ 0.001). Vertebral body wedging during the 2nd creep test was proportional to the severity of damage, as quantified by specimen height loss during the overload event (r 2 = 0.51, p \ 0.001, n = 34). Conclusions Minor damage to an old vertebral body, even if it is barely discernible on radiographs, can accelerate creep to such an extent that it makes a substantial contribution to vertebral deformity.

The audit demonstrated an acceptable level of quality and accuracy of primary grading in the Bristol and Weston diabetic retinopathy screening programme and provided a standard against which future interobserver agreement can be measured for quality assurance within a screening programme. Diabet. Med. 26, 820-823 (2009).