In Vitro Biomechanics of Large-Sized Lateral Interbody Footprints and Expandable Lateral Spacers

Presented at SMISS Annual Forum 2014
By Stefan Mindea MD
With Brandon Bucklen PhD, Mark Moldavsky MS, Kanaan Salloum BS,

Disclosures: Stefan Mindea MD B; Depuy, Globus Medical Inc. Brandon Bucklen PhD E; Globus Medical Inc., Mark Moldavsky MS E; Globus Medical Inc., Kanaan Salloum BS E; Globus Medical Inc.,

Introduction:
The lateral transpsoas approach is used for treating patients with lumbar disc degeneration. It is advantageous over other techniques because it allows maintenance of the anterior and posterior longitudinal ligaments. The optimal width has yet to be determined, but it is hypothesized that higher widths may have biomechanical advantages.

Aims/Objectives:
The effect of lateral interbody device width on range of motion (ROM) was evaluated and compared to an expandable spacer. It is unknown whether interbody width or expansion of the disc space plays a more major role in the stability of the spine.

Methods:
Six L3-sacrum human cadaver specimens were tested. A pure moment of 8N-m was applied at a rate of 1o/sec in flexion-extension, lateral bending, and axial rotation. The testing order was: 1) 22mm wide static lateral spacer (22mm LS); 2) 22mm wide expandable lateral spacer (22mm ELS); 3) 26mm wide static lateral spacer (26mm LS); and 4) 22mm wide static lateral spacer + bilateral pedicle screws (22mm LS+BPS). All surgical constructs were performed at L4-L5. Data was normalized to intact and significant differences (p≤0.05) were determined using a repeated measures ANOVA followed by a Tukeys post hoc analysis.

Results:
In flexion, ROM for 22mm LS, 22mm ELS, 26mm LS, and 22mm LS+BPS was 30±10%, 27±7%, 25±8%, and 16±8%, respectively. All constructs reduced motion with significance compared to intact. During extension, ROM for 22mm LS, 22mm ELS, and 26mm LS was 91±30%, 84±30%, and 90±31%, respectively. ROM for 22mm LS+BPS was 32±11% and significantly less compared to intact and all stand-alone constructs. ROM for 22mm LS, 22mm ELS, 26mm LS, and 22mm LS+BPS in lateral bending was 38±22%, 28±10%, 41±21%, and 14±8%, respectively. All constructs significantly reduced motion compared to intact. The 22mm LS+BPS significantly reduced motion compared to 26mm LS. In axial rotation, ROM for 22mm LS, 22mm ELS, 26mm LS, and 22mm LS+BPS was 64±27%, 59±23%, 61±20%, and 33±12%, respectively. The 22mm LS, 22mm ELS, and 22mm LS+BPS constructs significantly reduced motion. The 22mm LS+BPS construct significantly reduced motion compared to 26mm LS.

Conclusions:
During flexion-extension, interbody spacers act as flexion blockers. Increasing the width of the spacer did not correlate with increased stability. Pedicle screws significantly increased the stability of the construct and was more effective in reducing motion than increasing implant width. Interestingly, in most modes, the smaller-width expandable spacer provided marginally more stability than a larger-width static spacer.