(I-600) How does lumbosacral instrumentation strain vary with construct flexibility? Correlations between rod and screw strains and instrumented flexibilities across lumbar levels in long fusion constructs.

Introduction: Long construct PSR instrumentation results in greatly reduced flexibility across fixed levels. However, slight intervertebral mobility remains following fixation, with increased motion suggesting changes in stress and strain transfer. This study investigates the relationships between post fixation intervertebral flexibilities and rod and screw strains across lumbar spine constructs instrumented with PSR during sagittal plane loading.

Methods: Seven specimens were instrumented with PSR at L2-S. Rods and screws were instrumented with strain gauges to determine intervertebral rod strains at L2-3, L3-4, L4-5 and L5-S, and screw bending moments at L2, L3, L4, L5 and S. Specimens were tested to 7.5 Nm in flexion (FL) and extension (EX). Intervertebral motion was determined optically. Relationships between intervertebral ROM (after instrumentation), rod strains and screw bending moments were studied using Pearson Correlation Analysis (p < 0.05).

Results: Intervertebral ROM following PSR was the least at the center of the construct: 10% [L2-3], 3% [L3-4 and L4-5] and 11% [L5-S] of intact ROM during FL, and 22% [L2-3], 11% [L3-4], 7% [L4-5] and 26% [L5-S] during EX. In contrast to ROM, rod strains were the greatest at the center of the construct: (FL: -9uE [L2-3], 285uE [L3-4], 197uE [L4-5], 52uE [L5-S]; EX: -5uE [L2-3], -176uE [L3-4], -215uE [L4-5] -98uE [L5-S]), while screw bending moments were the greatest caudally: ([FL: 0.05Nm [L2], 0.03Nm [L3], 0.09Nm [L4], 0.35Nm [L5] and 0.33Nm [S]; EX: 0.05Nm [L2], 0.05Nm [L3], 0.10Nm [L4], 0.30Nm [L5] and 0.34Nm [S]).
Rod strains at L4-5 (level with greatest magnitude strains during both flexion and extension) increased significantly with increasing ROM at L5-S (level with greatest magnitude ROM during both flexion and extension): (FL) R= 0.906, p=0.013; (EX) R=0.767, p=0.044.
Screw bending moments at the most distal level, S (level with second to greatest bending moment during flexion and greatest bending moment during extension) increased significantly with increasing ROM at L5-S (level with greatest ROM during both flexion and extension): (FL) R= 0.82, p=0.024; (EX) R= -0.831, p=0.021.

Conclusion : There were significant relationships between post-fixation intervertebral flexibility and lumbosacral strains (rods and screws), especially caudally. Very small changes in instrumented mobility at L5-S can greatly influence stresses on lumbo-sacral instrumentation.

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