Strain in Posterior Instrumentation Resulted by Different Combinations of Posterior and Anterior Devices for Long Spine Fusion Constructs

Study Design Clinically related experimental study. Objective Evaluation of strain in posterior low lumbar and spinopelvic instrumentation for multilevel fusion resulting from the impact of such mechanical factors as physiologic motion, different combinations of posterior and anterior instrumentation, and different techniques of interbody device implantation. Summary of Background Data Currently different combinations of posterior and anterior instrumentation as well as surgical techniques are used for multilevel lumbar fusion. Their impact on risk of device failure has not been well studied. Strain is a well-known predictor of metal fatigue and breakage measurable in experimental conditions. Methods Twelve human lumbar spine cadaveric specimens were tested. Following surgical methods of lumbar pedicle screw fixation (L2–S1) with and without spinopelvic fixation by iliac bolt (SFIB) were experimentally modeled: posterior (PLF); transforaminal (TLIF); and a combination of posterior and anterior interbody instrumentation (ALIF+PLF) with and without anterior supplemental fixation by anterior plate or diverging screws through an integrated plate. Strain was defined at the S1 screws, L5–S1 segment of posterior rods, and iliac bolt connectors; measurement was performed during flexion, extension, and axial rotation in physiological range of motion and applied force. Results The highest strain was observed in the S1 screws and iliac bolt connectors specifically during rotation. The S1 screw strain was lower in ALIF+PLF during sagittal motion but not rotation. Supplemental anterior fixation in ALIF+PLF diminished the S1 strain during extension. Strain in the posterior rods was higher after TLIF and PLF and was increased by SFIB; this strain was lowest after ALIF+PLF, as supplemental anterior fixation diminished the strain during extension, in particular, cages with anterior screws more than anterior plate. Strain in the iliac bolt connectors was mainly determined by direction of motion. Conclusions Different devices modify strain in low posterior instrumentation, which is higher after transforaminal and posterior techniques, specifically with spinopelvic fixation. Level of Evidence N/A.