P4. Independent expansion of the anterior and posterior height of a lateral expandable cage can improve the contact area and load distribution at the cage-endplate interface

Abstract

<h3>BACKGROUND CONTEXT</h3> Compared to their traditional static counterparts, expandable cages offer clinical advantages by allowing for the use of smaller surgical access corridors to achieve similar lordosis correction without endplate damage from large impaction forces. However, as with static cages, the risk of implant subsidence due to stresses on the vertebral endplate remains an area of concern. Although early clinical outcomes have been reported for individual expandable cage designs, how the endplate contact area and load distribution are affected by specific cage expansion trajectories remains unclear. <h3>PURPOSE</h3> To assess and quantify differences in contact area and load distribution from different lateral expandable interbody cage designs. <h3>STUDY DESIGN/SETTING</h3> Comparative biomechanics analysis using established synthetic spine units. <h3>OUTCOME MEASURES</h3> Cage-endplate contact area, anterior-posterior load distribution, segmental lordosis angle. <h3>METHODS</h3> Discectomies and cage placement were performed on synthetic functional lumbar spine units (n = 3). Three different expandable cage designs were evaluated, each with their own unique expansion trajectory: cranio-caudal, arc-based, and independent anterior and posterior height expansion. For the third cage, a differential expander was used to increase the two heights, anterior and posterior, according to the path of least resistance. A Tekscan calibrated pressure mapping sensor was placed between the cage and the synthetic endplate. The functional spine units were mounted on a servo-hydraulic testing frame and a constant 250N compressive axial preload was applied with unconstrained rotations and translations. The cages were then fully expanded and segmental lordosis angle along with pressure measurements were collected. To allow for a consistent comparison among cages, the surface contact area was normalized by the footprint of each respective cage. To determine the uniformity in the contact load profile, the local load distribution was demarcated in half along the anterior-posterior (A-P) aspect of the cage. The symmetry of the loading profile was characterized by dividing the cumulative anterior force magnitude by the overall force (50% indicates perfectly balanced A-P loading) quantified using the pressure mapping sensor. Friedman's test was performed (α = 0.05) to compare the effect of expansion trajectory on the outcome measures (values reported as median [range]). <h3>RESULTS</h3> Expansion trajectory had a significant effect on cage-endplate contact area and the resultant A-P loading distribution (p = 0.050), but not segmental lordosis angle (p = 0.097). Independent expansion resulted in the highest cage-endplate contact area (93% [92 to 98%]) compared to cranio-caudal (84% [81 to 90%]) and arc-based (62% [56 to 63%]) expansion. Furthermore, independent expansion resulted in a near-even A-P loading distribution (48% [42 to 51%]), compared to cranio-caudal (34% [22 to 36%]), and arc-based (97% [94 to 99%]) expansion. <h3>CONCLUSIONS</h3> Of the cage designs evaluated, the expandable cage with independent anterior and posterior height expansion resulted in the highest cage-endplate contact area and most uniform load distribution, both important considerations fundamental for a successful fusion procedure and the mitigation of implant subsidence risk. With a larger sample size, our continuing studies will investigate the interaction of various cage placement options and expansion mechanisms. <h3>FDA DEVICE/DRUG STATUS</h3> MOD-EX XLIF Interbody System (Approved for this indication), RISE-L Spacer (Approved for this indication)