P23. Variations in stresses and strains along the rod during in-vitro loading of long fusion constructs

Abstract

BACKGROUND CONTEXT Rod strains at one or two levels of spinal constructs are measured during in vitro studies to help evaluate the local biomechanical effects of different pedicle screw and rod (PSR) configurations. However, little is known about patterns of rod strain distribution particularly in longer PSR constructs that span the entire lumbar spine. PURPOSE The purpose of this study was to analyze intervertebral rod strains at all six instrumented levels during multi-directional pure moment loading of lumbar spine constructs instrumented with PSR across T12-S. STUDY DESIGN/SETTING In vitro biomechanical study using human cadaveric specimens. PATIENT SAMPLE A total of 7 (T11-S) cadaveric spines (2F/5M, 50.8±9.7 yrs, DEXA 0.985 g/cm2) were studied. OUTCOME MEASURES T12-L1, L1-2, L2-3, L3-4, L4-5 and L5-S rod strains. METHODS There were 7 cadaveric specimens potted at T11 and S were instrumented with bilateral T12-S PSR including contoured CoCr rods. Uni-axial strain gauges were placed midway between pedicle screws on the posterior surface of the right rod. Longitudinal (cranial-caudal) rod strains were recorded during applications of continuous dynamic loads in a 6DOF robot to 7.5 Nm in flexion (FL), extension (EX), right lateral bending (RLB), left lateral bending (LLB), right axial rotation (RAR) and left axial rotation (LAR). Intervertebral rod strains (RS) were compared using One-way ANOVA followed by Holms-Sidak paired analysis (p RESULTS During FL and EX, the mean rod strain increased in magnitude towards the center of the construct and decreased towards the end levels (ie, T12-L1 and L5-S) where the mean strain was significantly less than at any of the intermediate levels (p 0.2), but with opposite signs. The mean strains (±SD) in uE during FL were: [T12-L1] -50(±83); [L1-2] 157(±82); [L2-3] 205(±84); [L3-4] 278(±63); [L4-5] 217(±84); and [L5-S] 18(±88); and during EX: [T12-L1] 45(±77); [L1-2] -190(±134); [L2-3] -226(±56); [L3-4] -294(±68); [L4-5] -269(±78); and [L5-S] -75(±95)]. The mean posterior rod strain during LB increased towards the proximal and distal ends of the construct where the strain was significantly greater than at any of the intermediate levels (p 0.1) at all levels, but with opposite sign. The greatest variability in posterior rod strain among specimens within a given level occurred during AR. Mean strains during RAR were: [T12-L1] -12(±58); [L1-2] 88(±119); [L2-3] 103(±140); [L3-4] 16(±100); [L4-5] 51(±115); and [L5-S] 30(±56); and during LAR: [T12-L1] -20(±42); [L1-2] -106(±113); [L2-3] -110(±116); [L3-4] -3(±80); [L4-5] -79(±111); and [L5-S] -78(±69)]. Mean posterior rod strains during RAR and LAR were the smallest at the proximal and more central level (L3-L4). There were no significant differences in strain between levels (p>0.19). CONCLUSIONS There are clear trends in the distribution of posterior rod strain along a T12-S PSR construct during pure moment loading, which are highly dependent on the direction of bending. Understanding of these distributions can help to guide clinical decision making towards optimizing construct strength and longevity. Additional strain gauges and rosette gauge configurations are needed to characterize the strain distributions on the rods during LB and AR in more detail. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.