Mechanical properties of osteopenic vertebral bodies monitored by acoustic emission

A microdamage event, either bone microfracture or microcrack propagation, releases energy. Some of this energy is in the form of acoustic waves. We measured acoustic emission (AE) in normal and osteopenic vertebral bodies during compression loading to confirm the microdamage accumulation. The 2nd to 4th lumbar vertebrae taken from the embalmed cadavers of a 37-year-old male and a 75-year-old female were used. Bone mineral densities (BMDs) were measured by dual energy X-ray absorptiometer (DEXA). The male vertebrae had normal BMDs, while the female vertebrae had lower BMDs than the normal range and were graded osteopenic. Mechanical parameters (maximum load, maximum stress, stiffness, strain at maximum load, and apparent elastic modulus) and AE event count rates in the load-deformation curve were measured during quasi-static compression loading (deformation rate 0.1 mm/min). For all mechanical parameters, the normal vertebrae had higher values than the osteopenic vertebrae. Cumulative AE event counts until maximum load of the osteopenic vertebrae were much greater than that of normal vertebrae. The vertebrae which were well compressed to the plastic range in the load-deformation curve displayed dome-shaped fracture lines just above the end plates. These results are consistent with the hypotheses that microcracks of osteopenic vertebral bodies are generated and accumulate at lower strains than those of normal vertebrae at a specific site.