291. Influence of lumbar lordosis on posterior rod strain in long-segment pedicle screw and rod instrumentation with anterior column realignment: cadaveric study

Abstract

BACKGROUND CONTEXT Lumbar lordosis (LL) has a close geometric relationship with pelvic parameters and sagittal profile. The association between the alignment of the pelvis (sacral slope) and the lumbar spine is an important determinant of sagittal balance, with the lower arc of lordosis being the most important determinant of the global lordosis. Posterior rod strain (RS) monitoring during biomechanics tests is an effective method to infer the stress on spinal implants. Yet it remains unknown whether lumbar lordosis and final construct geometry have a significant impact on instrumentation loading. An improved understanding of key factors that influence stress and strain in spinal instrumentation may serve to maximize construct longevity and avoid implant failure. PURPOSE Investigate the relationship between lordosis angle in different levels and different conditions with rod strain during loading. METHODS Seven fresh-frozen specimens underwent standard dynamic nondestructive bending tests in flexion (FL), extension (EX) (7.5 Nm) as well as compression (C) (400N) in a 6DOF robotic apparatus. Conditions tested were: 1) intact; 2) pedicle screws and rods only (PSR); 3) Anterior column realignment at L3-L4 (ACR). The posterior right rod at L3-L4 and at L5-S1 was instrumented with strain gauges. Lateral x-rays of all conditions were taken before testing. Lumbar lordosis spanning different levels were measured in all different conditions (unloaded) using the Cobb method (L5-S1, L4-S1, L3-S1, L2-S1 and L1-S1) and correlated with rod strain (RS) (during loading) using a Pearson correlation analysis (p RESULTS L3-S1 intact (P=0.028) and PSR (p=0.007) lordotic angle demonstrated strong correlation with L3-4 RS in PSR condition during FL. The L3-S1 intact angle also correlated with RS with PSR in EX (p=0.018). L3-S1 intact angle demonstrated a relationship with L5-S1 RS with ACR during C (p=0.008). L2-S1 intact angle also demonstrated a correlation with L3-4 RS with PSR during FL (p=0.007) and EX (p=0.002). The L2-S1 intact lordosis was linked with L5-S RS with PSR during FL (p=0.03) and with ACR during compression (p=0.03). All correlations were positive (i.e. strains increased in magnitude with increasing Cobb angles). CONCLUSIONS Different lumbar lordotic measures taken in intact and PSR conditions demonstrated strong correlations with in vitro posterior RS. Both native lumbar curvature and after placement of pedicle screw and rod instrumentation have a significant influence on in vitro posterior hardware strain changes during loading. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.