Biomechanical behaviour in vitro of the spine and lumbosacral junction

Six fresh human specimens extending from the 9th thoracic vertebra (T9) to the pelvis were used to study the biomechanical behaviour of the long lumbo pelvic segments, including mobility of the sacrum. The loads were applied a t T9 using pure couples up to 5Nm. The displacements were measured by an op toelectronic method (VICON 140). Stress-displacement curves were obtained f or the three angular components of the vertebra studied according to the pl ane of the principal stresses and of the two other planes corresponding to the coupled mobilities. Mobility decreased from T9 to the sacrum. There was mobility of the sacrum in relation to the pelvis in flexion, with a mean o f 1.28 degrees (0.5 to 2.8 degrees); 3 sacrums showed a mobility of the ord er of one degree for torsional stresses. There was no sacral mobility durin g stresses in lateral flexion. The use of this experimental protocol with l ow mechanical stresses should allow the evaluation of long osteosyntheses e xtending to the sacrum.