POSTURE AND THE COMPRESSIVE STRENGTH OF THE LUMBAR SPINE

The effect of posture on spinal compressive strength was examined in a series of three experiments on cadaveric material. Lumbar 'motion segments', consisting of two vertebrae and the intervening disc and ligaments, were compressed while positioned in various angles of flexion and extension. In the first experiment load sharing between the disc, the apophyseal joint surfaces, and the intervertebral ligaments was inferred from measurements of intradiscal pressure (IDP). Results showed that extension caused the apophyseal joints to become load-bearing, and damage could occur at compressive loads as low as 500 N. Flexion angles greater than about 75% of the full range of flexion (as defined by the posterior ligaments) generated high tensile forces in these ligaments, and caused substantial increases in IDP. The optimum range for resisting compression therefore appeared to be 0-75% flexion. The second experiment compared the distribution of compressive stress within the disc at the endpoints of this range, and showed that at O% flexion high stress concentrations occur in the posterior annulus of many discs, whereas an even distribution of stress was usually found at 75% flexion. However, the third experiment showed that there was no significant difference in the compressive strength of motion segments positioned in 0% and 75% flexion. A comparison of the range of flexion/extension movements in vivo and in vitro led us to conclude that in life a position of moderate flexion is to be preferred when the lumbar spine is subjected to high compressive forces.