BIOMECHANICAL TESTING OF THE LUMBOSACRAL SPINE

Objective. To assess various forms of anterior and posterior sacral fi xation and to study the influence of anterior lumbosacral fixation and posterior pedicle fixation at L5 in conjunction with lumbosacral fixa tion. Summary of Background Data. Moments at the lumbosacral junction are high in the long constructs requiring lumbosacral fixation. The pu rpose of this study was to assess bending moments in flexion-extension and lateral bending and rotational forces at the lumbosacral junction involving a variety of long constructs to the lumbosacral junction. T he incidence of pseudarthrosis in such constructs in the adult spine l iterature ranges from 7% to 40%. Methods. An alignment jig was designe d to display three-dimensional motion in the three orthogonal planes. Nine constructs of five specimens each were tested. These consisted of fixation at T12-L5-S1 (construct 1), T12-L5-S1 with anterior L5-S1 fi xation and grafting (construct 2), T12-L5-S1, S2 with and without L5-S 1 fixation and grafting anterior (constructs 3 acid 4, respectfully), T12-S1, S2 with and without L5-S1 anterior grafting and fixation (cons tructs 5 and 6, respectfully), T12 Jackson intrasacral fixation with o r without L5-S1 grafting anteriorly at the anterior fixation (construc ts 7 acid 8, respectfully), and T12-L5-S1, S2 fixation with anterior g rafting only (construct 9). Results. The use of anterior fixation stat istically increased stiffness in extension. There was a trend toward i ncreasing stiffness in constructs with anterior fixation (two anterior anterior-oblique L5-S1 screws) and in other loading modes as well, Fa ilure to use L5 screw fixation significantly decreased torsional rigid ity in long constructs without anterior fixation. Conclusions. In long constructs, particularly in scoliosis surgery requiring lumbosacral f ixation, the addition of anterior fixation at L5-S1 is recommended. Th e addition of L5 fixation in addition to sacral fixation significantly decreases rotational stresses acid is recommended as well.