Biomechanical testing of the lumbosacral spine

To assess various forms of anterior and posterior sacral fixation and to study the influence of anterior lumbosacral fixation and posterior pedicle fixation at L5 in conjunction with lumbosacral fixation.

Moments at the lumbosacral junction are high in the long constructs requiring lumbosacral fixation. The purpose 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. The incidence of pseudarthrosis in such constructs in the adult spine literature ranges from 7% to 40%.

An alignment jig was designed 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 fixation and grafting (construct 2), T12-L5-S1, S2 with and without L5-S1 fixation and grafting anterior (constructs 3 and 4, respectfully), T12-S1, S2 with and without L5-S1 anterior grafting and fixation (constructs 5 and 6, respectfully), T12 Jackson intrasacral fixation with or without L5-S1 grafting anteriorly at the anterior fixation (constructs 7 and 8, respectfully), and T12-L5-S1, S2 fixation with anterior grafting only (construct 9).

The use of anterior fixation statistically increased stiffness in extension. There was a trend toward increasing stiffness in constructs with anterior fixation (two anterior anterior-oblique L5-S1 screws) and in other loading modes as well. Failure to use L5 screw fixation significantly decreased torsional rigidity in long constructs without anterior fixation.

In long constructs, particularly in scoliosis surgery requiring lumbosacral fixation, the addition of anterior fixation at L5-S1 is recommended. The addition of L5 fixation in addition to sacral fixation significantly decreases rotational stresses and is recommended as well.