Risk of vertebral insufficiency fractures in relation to compressive strength predicted by quantitative computed tomography

Vertebral insufficiency fractures may result from excessive loading of normal and routine loading of osteoporotic spines. Fracture occurs when the mechanical load exceeds the vertebral compressive strength, i.e., the maximum load a vertebra can tolerate. Vertebral compressive strength is determined by trabecular bone density and the size of endplate area. Both parameters can be measured non-invasively by quantitative computed tomography (QCT). In 75 patients compressive strength (i.e., trabecular bone density and endplate area) of the vertebra L3 was determined using QCT. In addition, conventional radiographs of the spines were analysed for the prevalence of insufficiency fractures in each case. By relating fracture prevalence to strength, three fracture risk groups were found: a high-risk group with strength values of L3 5 kN and a fracture risk near 0%. Biomechanical measurements and model calculations indicate that spinal loads of 3 to 4 kN at L34 will be common in everyday activities. These data and the results described above suggest that spines with strength values of L3 < 3 kN are at an extremely high risk of insufficiency fractures in daily life. Advantages of fracture risk assessment by strength determination over risk estimation based on clinically used trabecular bone density measurements are discussed.