Maximizing screw length in expandable lateral lumbar interbody spacers with integrated fixation may obviate the need for supplemental pedicle screws.

BACKGROUND CONTEXT

Lateral lumbar interbody fusion (LLIF) is a minimally invasive surgical technique that provides a wide footprint interbody cage for correction of lumbar coronal and sagittal deformity. Traditional spinal interbody fusion procedures utilize pedicle screws and rods for additional stability. An expandable lateral titanium interbody cage with an integrated lateral fixation (eLLIFp) device provides a stand-alone LLIF that is intended to function autonomously. This may reduce the complexity of the surgery and the potential risks associated with supplemental posterior instrumentation. The minimum-acceptable screw length to promote adequate biomechanical fixation and stability for a stand-alone eLLIFp has not been determined.

PURPOSE

To investigate the effective ratio (of screw length/cage length) of a stand-alone eLLIFp construct that provides adequate biomechanical fixation and stability as compared to the eLLIFp with supplemental bilateral pedicle screw-rod fixation.

STUDY DESIGN/SETTING: In vitro cadaveric biomechanical testing and finite element modeling.

PATIENT SAMPLE

Eight fresh-frozen human cadaveric lumbar spine specimens (L2-5) were used.

OUTCOME MEASURES

Range-of-motion (ROM) measurements of intact and treated specimens with simulated stresses within the construct and surrounding bone during flexion-extension (FE), lateral bending (LB), and axial rotation (AR).

METHODS

Specimens with similar age and DEXA scores were selected. ROM of intact specimens was measured before treatment with LLIF at L3-4. Specimens were treated with expandable lateral cages with integrated fixation (stand-alone eLLIFp) or eLLIFp with supplemental posterior fixation using bilateral pedicle screws and rods (eLLIFp + BPS). ROM was measured using a custom-built 6°-of-freedom motion simulator (±7.5Nm) and normalized as a percentage of intact. Four patient-specific lumbar functional spinal unit finite element models (FEMs) were developed, validated, and then instrumented with eLLIFp stand-alone devices. The integrated screw lengths were varied to achieve screw-to-cage length ratios of 0.6, 0.75 and 0.9. Stresses were compared among the constructs under a 7.5Nm pure moment load in FE, LB, and AR.

RESULTS

The stand-alone and posteriorly supplemented eLLIFp constructs were not sensitive to the ratio during FE and LB (with only a 4%-9% change in motion trends from low-to-high ratios, relative to intact). Independent of ratio, these constructs had minimal differences in FE and LB motion. However, during AR both constructs were sensitive to the ratio showing greater stability and less variability in performance with higher ratios (≥0.65). Regression analysis revealed that posteriorly supplemented eLLIFp constructs had a linear 13% reduction in AR motion as the ratio increased from low-to-high (p<.05). AR also imposed the highest stresses on the eLLIFp and these stresses increased with higher ratios (maximum stress 259MPa for ratio 0.9 during AR), yet implant failure was improbable because of the material properties of the titanium alloy used. Similarly, surrounding bone stresses were higher during AR and longer screws reduced these stresses (63MPa with a 0.6 ratio compared to 38MPa with a 0.9 ratio).

CONCLUSIONS

Independent of screw-to-cage length ratio, eLLIFp had comparable reduction of FE and LB motion with or without posterior fixation. For eLLIFp with and without posterior fixation, torsional performance and repeatability increased with screw-to-cage length ratio. Torsional stability was comparable between stand-alone eLLIFp with high ratios (≥0.65) and posteriorly supplemented eLLIFp with low ratios (0.55). However, a threshold screw-to-cage length ratio for optimizing the clinical performance of eLLIFp cannot be prescribed. Implant stress findings reinforced torsion as the critical loading condition. Surrounding bone stress decreased as the screw length increased, indicating the benefit of using longer screws. Surgeons using eLLIFp should consider longer screw lengths based on anatomical considerations.

CLINICAL SIGNIFICANCE

eLLIFp cages can be used as a stand-alone device in appropriately selected patients. This avoids the morbidity and cost associated with futher supplemental posterior fixation. Surgeons using eLLIFp should consider using longer screws to optimize fixation.