Correlations between intervertebral rod and screw strains and instrumented flexibilities across lumbar levels in long fusion constructs; an in-vitro human cadaveric study.

BACKGROUND CONTEXT
Long construct posterior instrumentation results in greatly reduced flexibility across instrumented levels. However, slight intervertebral mobility remains following fixation, with increased motion suggesting changes in transfer of stresses and strains. Little is currently known regarding the relationships between intervertebral flexibilities following posterior fixation and instrumentation strains during loading of longer construct models.
PURPOSE
The purpose of this study was to study potential relationships between post fixation intervertebral flexibilities, and the distribution of rod strains and screw bending moments across L2-S spine constructs instrumented with pedicle screws and rods during flexibility testing in-vitro in the sagittal plane.

DESIGN/SETTING
In vitro biomechanical

METHODS
Seven L1-S specimens were instrumented at L2-S with bilateral pedicle screws and rods. Strain gages were placed on the right side rod midway between screw heads. Screws were instrumented with four strain gages circumferentially (90° apart) around the screw neck. Each screw was calibrated to determine resultant bending moments. Specimens were dynamically tested to 7.5 Nm in flexion (FL) and extension (EX). Intervertebral motion was determined optically (intact and instrumented). 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.
CONCLUSIONS
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.