ENHANCED LUMBO-SACRAL FIXATION USING BUTTRESS SUPPORT: BIOMECHANICAL COMPARISON OF INTRA-ILIAC AND INTRA-SACRAL TECHNIQUES

Participants: M.S. Ominsky, M.J. Hernandez, E. Yian, D. Demetropoulos, R. Banglmaier, M. Stock, D. Kayner, J. Pinto, G. Graziano, S. A. Goldstein

Keywords: Spine, sacral fixation, scoliosis, cadaver testing

Introduction

Long instrumentation constructs, as treatments for scoliosis, have a commonly weak lumbo-sacral junction. Enhanced fixation across the lumbosacral junction to the iliac wings has been popularized -- structurally, the ilia provide buttress support. This type of buttress is used in the Galveston intra-iliac technique (Fig. 1a), which employs L-shaped rods which pass above the sacral notch and insert obliquely into the ilia. Recent attempts at increasing L-S fixation without fixation to the pelvis involve the use of additional screw attachments. The Jackson intrasacral fixation technique (Fig. 1b) utilizes such an approach, with pedicle screws in S1, and rods driven into the sacral masses on both sides.

The purpose of this study was to compare the mechanical properties and kinematics of uninstrumented lumbosacral spines with those of instrumented spines employing these intrasacral and intra-iliac techniques, and to quantify the contribution of the buttresses to the stability of the instrumented spines.

Fig. 1: (a) Galveston Intra-iliac & (b) Jackson Intrasacral spinal instrumentation constructs

Materials and Methods

Six fresh human female cadaveric lumbar spine/sacrum/pelvis specimens (age: 43-90) were harvested, dissected down to the ligament attachments, and L2 was potted in an aluminum test block with PMMA. Uninstrumented spines were tested in torsion and flexion to determine control stiffnesses and rotations. For both tests, the pelvis was fixed in a loading frame and the rotations of L3, L4, L5, and S1 were measured using electronic clinometers (Lucas AccuStar® system) attached to the anterior surface of each vertebral body. The constructs were tested in torsion using a servohydraulic torsional MTS machine which rotated the L2 base through a range from -3° to 3°. The constructs were tested in flexion using a servohydraulic MTS machine with the specimen fixed with its superior-inferior axis parallel and offset from the loading axis of the MTS actuator. The offset acted as a moment arm, and the spine was displaced to 1". The moment at each vertebra was calculated from the spatial orientation of each clinometer.

Each specimen was instrumented and tested with both techniques in order to reduce inter-specimen variablity. Three specimens were instrumented initially with the intrasacral technique while the three specimens were instrumented with the intra-iliac technique to eliminate any bias. The sacral screws used in the intrasacral construct and the L5 screws in both techniques were instrumented with strain gages in order to assess the pull-out forces. After instrumentation, the specimens were tested again in torsion and flexion. Finally, in order to evaluate the contribution of buttressing to the stability of the construct, the rods were cut below L5, and tested pre-yield in torsion and flexion, and to yield in flexion.

Results /Progress

The original data based on six specimens tested in only one instrumentation construct was presented as a poster at the 44th Annual Orthopaedic Research Society meeting [1]. Briefly, the intra-iliac construct appeared to be stiffer in torsion and less stiff in flexion than the intrasacral technique. The sacrum itself was more mobile in the intrasacral construct, but the relative L5-S1 rotation was reduced. All data was normalized to uninstrumented values, which created a wide range of variablity due to differences in normal human spine kinematics. Therefore, each subsequent spine was tested with both sets of instrumentation to allow a direct comparison between techniques. Four specimens have been tested using both techniques, and the trends from the original ORS data seem to be consistent. A full analysis of the completed study is needed before conclusions on comparative stability can be drawn.

Discussion

The Galveston intra-iliac technique has long been used to improve stability in surgeries involving lumbo-sacral fixation and/or pelvic obliquities. Its L-shaped rods undergo excessive stresses at the lumbo-sacral junction upon hip flexion, a problem which has resulted in failure in at least one patient [2], and can be painful due to lack of fusion at the S1 joint [3]. The intrasacral technique was introduced to reduce these problems and simplify the surgical technique, while maintaining proper spine stability.

As an overall construct, the intra-iliac instrumented spine was stiffer in torsion and less stiff in flexion than the intrasacral spine, which may be a result of the proximity of the buttress axes to the axes of rotation. Both techniques provided stability well above that of the uninstrumented spine.

Bypassing the sacrum, the intra-iliac technique deflects the loads into the ilia, making the sacrum stiffer and less mobile, an idea which both our flexion and torsion results support. Whether this stiffness decreases the difference in rotation between L5 and S1, and thus improves fixation at the lumbosacral joint is a question that needs to be answered with a completed study.

Ultimately the question of which instrumentation construct to use may depend on factors other than mechanics, such as ease of implementation, quality of sacral bone, and degree of obliquity.

References:

[1] Ominsky et al. Trans 44th Ann ORS 1998; 23: 639

[2] Allen BL and Ferguson RL. Spine 1984; 9: 388-393.

[3] Camp JF et al. Spine 1990; 15: 932-941.