Lightning Podiums: Spinal Gumbo - 803A

Presented by: P. Nunley

Author(s):

L. Ferrara(1), P. Nunley(2)

(1) OrthoKinetic Technologies, Southport, NC, United States
(2) Spine Institute of Louisiana, Orthopaedics, Shreveport, LA, United States

Abstract

Introduction: A successful intervertebral fusion requires biomechanical stability created by the structural support of the interbody device and loading of the bone graft material to accelerate mechanotransduction and bone remodeling. However, the rigidity and architecture of an interbody implant can influence the fusion process.

Objective: The objective of this study was to generate a quantitative map of the contact area and load profile for two implant designs; a rigid monolithic PEEK lateral cage (Rampart-L) and a unique hybrid interbody design which includes PEEK terminal supports surrounding an expandable porous mesh that contains bone graft (Duo).

Materials and Methods: The construct for each test consisted of a device sandwiched between two Grade 15 foam blocks. Pressure sensitive film and thin film sensors were placed between the device and each of the foam blocks. A series of each implant type was compressed at a rate 0.1mm/second for three loads (1,100N, 2,000N, 3,000N). The implants were tested without bone graft initially and placed between two polyurethane foam blocks (G15pcf) that simulate vertebral bone, followed by nominal cage filling with morselized bone graft with testing repeated in the same fashion. Two phases of tests were conducted on flat and shaped bone blocks to mimic the curvature of the vertebral endplates. Areas were measured using ImageJ analysis software. Images of each pressure profile were calibrated, cropped to the specified region of interest, and a threshold applied using a uniform value across all samples to isolate the contact area region of the device on medium range pressure sensitive film. The resultant area was used to determine the bone graft area for each test condition and corresponding load profiles were quantified and mapped with thin film sensors.

Results: The Duo demonstrated 34% greater graft volume than the Rampart L resulting in a 28% larger area for bone exchange when filled. The load profiles for all applied loading paradigms for the Duo demonstrated significant direct loading on the bone graft contained within the mesh, resulting in at least 170% greater loaded area than the areas measured for the standard monolithic lateral cage. Furthermore, the Duo demonstrated load sharing with the terminal PEEK supports as shown by the pressure profiles. The monolithic lateral cage for all loading conditions demonstrated minimal bone graft loading. Post-analysis demonstrated that the bone graft material within the lateral cage was compressed under loading to just below the edge of the open pores of the implant, resulting in minimal direct bone graft loading for all loads applied.

Conclusions: The mesh component of the Duo allows for an optimized contact area for bone exchange and graft incorporation. The compliant mesh stiffens with bone graft fill, but is able to conform to the unique contours of the individual patient vertebral endplates, thus contributing to the overall structural support. The load profiles confirmed that the filled mesh does not stress shield terminal PEEK supports and will load share. The expandable, compliant, porous mesh of the Duo provides a greater multiplanar area for bone exchange and allows for direct contact with the viscoelastic vertebral endplates, improving the endplate and graft interface mechanics.