P20. Evaluation of interbody design parameters on graft subsidence in the 3D-printed era: separating fact from fiction

Abstract

<h3>BACKGROUND CONTEXT</h3> Cage subsidence is a known complication of spinal interbody fusion and is associated with poor clinical outcomes. Three-dimensional-printed titanium interbodies are becoming increasingly popular and their unique manufacturing allows for alteration of several features including stiffness and porosity. Though there are many claims made regarding the influence of these features on interbody subsidence, there is a lack of rigorous scientific evaluation. <h3>PURPOSE</h3> The purpose of this study is to determine how changes in implant parameters biomechanically affect subsidence using an in-vitro sawbone model. <h3>STUDY DESIGN/SETTING</h3> In vtiro biomechanical saw bone model. <h3>OUTCOME MEASURES</h3> Subsidence measured as relative displacement of the interbody and foam block using digital imaging correlation. Stiffness measured using thethe linear portion of the load displacement curve. <h3>METHODS</h3> Eight groups were tested (n = 8/group) including seven 3D-printed titanium (Seaspine, Inc.) varying in surface area (aperture: standard, small and none), porosity (high vs low) and surface topography (nonporous vs porous) as well as one standard PEEK and one solid titanium. Subsidence testing was performed in a standardized foam block model compressed to 150 N, cycled between 50-250N for 5000 cycles at a rate of 1 Hz. Digital imaging correlation was used to determine relative displacement of the interbody and foam block. Stiffness testing was performed by compressing implants between parallel steel blocks by 500 N at a rate of 50N/sec for three cycles and measuring the linear portion of the load displacement curve. <h3>RESULTS</h3> Subsidence was decreased as the surface area of contact with the sawbone was increased (all p 0.353) or based on material property (all p>.192). Apparent stiffness of the implant was affected by porosity (p 0.146). The PEEK cage had similar stiffness to porous Ti (p= 0.959). A nonporous surface led to greater stiffness compared to the standard porous interbody (p= 0.010). Surface area was negatively correlated with subsidence (r= -0.808, p=0.0284), but was not correlated with stiffness (r= 0.568, p=0.183). <h3>CONCLUSIONS</h3> Implant surface area and surface topography have the greatest influence on interbody subsidence. Apparent stiffness, implant porosity, as well as material property do not affect subsidence in an in vitro model. Biologic response in the in vivo setting likely also influences clinical subsidence, which is a topic of future study. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.