Optimizing Biomechanics of Anterior Column Realignment (ACR) Constructs: A Cadaveric Study of Rod Strain and Construct Stability

Presented at SMISS Annual Forum 2018
By Jay Turner MD, PhD
With Jakub Godzik MD, Bernardo De Andrada Pereira MD, Anna Newcomb MS, Jennifer Lehrman MS, Gregory Mundis MD, Randy Hlubek MD, Juan Uribe MD, FACS, Brian Kelly PhD,

Disclosures: Jay Turner MD, PhD A; NuVasive. B; NuVasive, SeaSpine. Jakub Godzik MD None, Bernardo De Andrada Pereira MD None, Anna Newcomb MS None, Jennifer Lehrman MS None, Gregory Mundis MD A; ISSGF, Nuvasive. B; K2M, Medicrea, Misonix, Nuvasive. C; K2M, Nuvasive. F; Nuvasive., Randy Hlubek MD None, Juan Uribe MD, FACS A; Nuvasive. B; Nuvasive. C; Nuvasive. D; Nuvasive. F; Nuvasive, Brian Kelly PhD None,

Introduction:

Anterior column realignment (ACR) is capable of achieving large segmental lordotic correction through a minimally invasive lateral approach. However, the technique, which typically involves both ALL release and posterior column osteotomies, is highly destabilizing to the spine. Construct optimization is critical for preventing instrumentation failure and maximizing fusion rates.

Aims/Objectives:

The goal of this study was to evaluate the impact of ACR construct design on rod strain and stability in a cadaveric model.

Methods:

Standard nondestructive flexibility tests (7.5 Nm) were performed on 6 cadaveric specimens (T12-Sacrum) to assess range of motion stability (ROM) in flexion (F), extension (E), and compression, and rod strain (RS) across L3/4 ACR with single screw anterior fixation into one (+1XLP) versus two (+2XLP) vertebral bodies through an integrated plate, and two-rod (+2R) versus four-rod (+4R) conditions. Conditions: 1) intact, 2) intact disc + pedicle screw fixation (+2R), 3) ACR+1XLP+2R, 4) ACR+2XLP+2R, 5) ACR+1XLP+4R, 6) ACR+2XLP+4R. Order of testing was randomized. Data were analyzed using RM-ANOVA or ANOVA (p<0.05).

Results:

ACR+1XLP+2R demonstrated significantly greater ROM than intact+2R in F, E(p<0.006); this difference was significantly reduced both by using 4-rods (4R) or 2 screws (2XLP), such that both ACR+1XLP+4R and ACR+2XLP+2R were similar to intact+2R (p>0.99) and more stable than ACR+1XLP+2R (p<0.04). Similarly, ACR+1XLP+2R had the greatest RS compared to other conditions (p<0.013); addition of 4R to +1XLP significantly reduced strain compared to 1XLP+2R in F, E, C (p<0.029) and were comparable with intact+2R (p>0.68). However, use of +2XLP did not significantly provide any significant reduction in RS across testing conditions. Ultimately, there was no difference in rod strain between ACR+1XLP+4R and ACR+2XLP+4R (p>0.85).

Conclusions:

In order to achieve equivalent or better stability as standard pedicle screw-rod construct with intact disc space, ACR constructs should either contain anterior fixation into both vertebral bodies (+2XLP) or accessory rods (+4R) when fixating into only one vertebral body. Use of 4 rods provides greater posterior rod strain reduction compared to anterior fixation, however clinical benefit of this remains to be seen. Combing these two techniques (+2XLP +4R) did not provide any additional benefit.