Post-Implantation Subsidence Characteristics of Lateral Interbody Constructs During Cyclic Testing

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:
Even with the advantages of the lateral approach, with placement of a spacer on the lateral cortex of cortical bone, there remains a potential subsidence risk. Implant design may be one factor that affects that risk.

Aims/Objectives:
To investigate the role of lateral implant design at influencing the potential of endplate subsidence (measured through implant displacement), in a post-fatigue cadaver model. The implant characteristics considered were 1) Static/expandable interbody devices, and 2) Supplemental fixation. Additionally a transforaminal lumbar interbody fusion (TLIF) construct was included as a well-established comparison to lateral technique.

Methods:
Twenty lumbar motion segments (L2-3 or L4-5) were scanned and distributed into the following groups: 1) 22mm wide static lateral spacer (22mm LS), 2) 22mm wide expandable lateral spacer (22mm ELS); 3) 26mm wide static lateral spacer (26mm LS), 4) 22mm wide static lateral spacer + bilateral pedicle screws (22mm LS+BPS); and 5) TLIF + bilateral pedicle screws (TLIF+BPS). Instrumentation was performed under 50lb and 30lb axial loads for lateral and TLIF constructs, respectively. Specimens were then placed on a MTS machine and a compressive axial load of 50-300N was applied for 6hours at 1Hz. Displacement vs. time graphs were averaged for the groups.

Results:
The T-scores for the 22mm LS, 22mm ELS, 26mm LS, 22mm LS + BPS and TLIF + BPS groups are -2.74 ± 0.84, -2.66 ± 1.24, -2.21 ± 0.80, -2.86 ± 1.01, and -2.46 ± 1.40, respectively, with no significant difference between the groups (p=0.92). initial displacement of the interbody devices (22mm LS, 22mm ELS, and 26mm LS), were 0.74 ± 0.19mm, 0.76 ± 0.17mm, and 0.72 ± 0.13mm, respectively. Initial displacement of interbody with pedicle screws (22mm LS + BPS and TLIF + BPS) were -0.54 ± 0.22mm and 0.66 ± 0.12mm respectively. During cyclic testing, 22mm LS + BPS had had the least amount subsidence (change in displacement). All other interbody constructs performed similarly during cyclic testing.

Conclusions:
Lateral interbody with pedicle screws had the highest resistance to subsidence, which was greater than TLIF with pedicle screws. The width of the lateral spacer did make a marginal, though non-statistical, improvement in subsidence resistance. The expandable spacer seemed to reduce initial subsidence, but was not statistically different than a 22mm or 26mm lateral spacer after cycling. An expandable spacer may help reduce the risk of immediate iatrogenic subsidence but is not significantly better than a static spacer in preventing postoperative subsidence.