Multilevel anterior lumbar interbody cages: an in vitro biomechanical assessment

Abstract

Darrel S. Brodke, MD, R. Alexander Mohr, BS, Salt Lake City, UT, USA; Jeffrey C. Wang, MD, Los Angeles, CA, USA; Jim A. Youssef, MD, Durango, CO, USA; Bao-Khang N. Nguyen, BS, Kent N. Bachus, PhD, Salt Lake City, UT, USAIntroduction: Threaded titanium interbody cages have become widely popular for use in lumbar fusion surgery. They are commonly used as stand-alone devices for single-level fusions, occasionally for two-level fusions and rarely to fuse three or more levels. Despite initial enthusiasm about multilevel cage constructs, there has been an increase in reported pseudarthrosis rates and concern for stability. The published mechanical information on single-level cages cannot be directly applied to the multilevel construct. To date, no biomechanical studies have evaluated the stability of multilevel fusion constructs.Purpose: This study was designed to assess the stiffness of multilevel cage constructs in flexion-extension, lateral bending and axial torsion. A comparison is made between one-, two- and three-level stand-alone cage constructs, and each was compared with the intact spine and posterior pedicle screw instrumentation. Finally, a comparison was made between the three-level anterior cage construct, posterior pedicle screw instrumentation and combined anterior-posterior instrumentation. A better understanding of construct biomechanics will guide clinical decision making.Materials and methods: Twelve calf spines (L1–6) were used for this study. Each was tested on a custom pneumatic four-axis spine simulator. Moments of 5 Nm were applied in flexion-extension, lateral bending and torsion sequentially, while a 50-N axial compressive preload was maintained. In attempt to apply pure moments, one moment was independently applied while the remaining two were actively maintained at zero. After testing of the intact specimen, six of the spines were prepared with sequential cage constructs (single-level L3–4, two-level L3–5 and three-level L2–5), using BAK/L cages (Sulzer SpineTech, MN). Posterior instrumentation was then added at L2–5. Data were compared with six specimens that underwent posterior pedicle screw instrumentation, using Silhouette System (Sulzer SpineTech, MN). Statistical analysis was performed using analysis of variance and Fisher's post hoc test.Results: In the single-level cage construct, L3–4, stiffness was significantly increased in lateral bending 6 times over the intact spine, from 1.4 to 8.7 Nm (p<.01). Flexion-extension and torsional stiffness were increased 82% (7.9 to 14.4 Nm) and 47% (7.3 to 10.7 Nm), respectively, although these were not statistically significant. Single-level posterior pedicle screw instrumentation was stiffer than the anterior cage construct in flexion-extension (36.3 Nm) and lateral bending (25.7 Nm), similar in torsion (11.8 Nm), and significantly increased over the intact specimen in all three planes (p<.01).In the two-level cage construct, L3–4 and L4–5, lateral bending stiffness was again significantly increased over the intact specimen, from 0.9 to 4.1 Nm (p<.005). Neither the flexion-extension nor torsional stiffness of the cage construct were significantly different than the intact specimen. Posterior pedicle screw instrumentation significantly increased stiffness 4 times over the anterior cages in flexion-extension (14.4 to 39.5 Nm) and 8 times in lateral bending (4.1 to 31.9 Nm) (p<.001).The three-level cage construct, L2–3, L3–4 and L4–5, showed no significant differences from the intact specimen in flexion-extension, lateral bending or torsion. Posterior pedicle screw instrumentation was dramatically stiffer in all motion segments, as compared with both the intact spine and the anterior cage construct (p<.0001). Both flexion-extension and lateral bending stiffness were 8 times greater with the posterior instrumentation than the cages alone. However, when posterior nonsegmental pedicle screw instrumentation was added to the cage construct, the stiffness significantly increased (p<.0001).Conclusion: Multilevel anterior stand-alone cage constructs show progressively decreasing stiffness in all planes, flexion-extension, lateral bending and torsion, as fusion levels are added. Whereas the single-level cage construct is stiffer than the intact spine, comparable to posterior pedicle screw instrumentation, and clinically successful, the multilevel constructs are not. Two-level constructs are questionably able to maintain adequate stiffness. Three-level stand-alone cage constructs are no stiffer than the intact mobile spine. The addition of posterior pedicle screw instrumentation to the three-level anterior cage construct significantly increases stiffness in all planes. If a three-level interbody fusion is contemplated, this study supports the addition of posterior instrumentation.