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 lumbopelvic segments, including mobility of the sacrum. The loads were applied at T9 using pure couples up to 5 Nm. The displacements were measured by an optoelectronic method (VICON 140). Stress-displacement curves were obtained for the three angular components of the vertebra studied according to the plane 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 of 1.28 degrees (0.5 to 2.8 degrees); 3 sacrums showed a mobility of the order of one degree for torsional stresses. There was no sacral mobility during stresses in lateral flexion. The use of this experimental protocol with low mechanical stresses should allow the evaluation of long osteosyntheses extending to the sacrum.