A CINERADIOGRAPHIC STUDY ON THE LUMBAR DISC DEFORMATION DURING FLEXION AND EXTENSION OF THE TRUNK

To reveal deformation behaviour of normal lumbar discs (L(3/4r) L(4/5) and L(5)/S-1), the lumbar spines of eight asymptomatic volunteers wer e examined cineradiographically during flexion and extension of the tr unk. Disc deformation could be evaluated by displacement of the superi or corners of the disc, which were measured with respect to the upper surface of the adjacent lower vertebra. Furthermore the in-vivo strain distribution of each lumbar disc was analysed by the finite element m ethod and in-vivo measurement of the disc deformation. During flexion, deformation of the lumbar disc increased rapidly after a certain dela y from the start of trunk motion and reached maximum value before the finish of trunk motion. It was also confirmed that time lags were pres ent between the onsets of disc deformation. Namely, each disc deformed not simultaneously but stepwise from the upper to the lower level wit h time lags during flexion. When the lower disc deformation started, t he strain at the adjacent upper discs had already reached more than ha lf of the value at full flexion.