General Session: Innovative Technologies II - Hall F

Presented by: G.M. Williams

Author(s):

W.R. Walsh(1), M.H. Pelletier(1), T. Wang(1), C. Christou(1), E.R. Walsh(1), J. Malik(2), R.M. Donahoe(2), S.S. Bannigan(2), G.M. Williams(2)

(1) University of New South Wales, Prince of Wales Hospital, Surgical and Orthopaedic Research Laboratories, Sydney, NSW, Australia
(2) NuVasive, San Diego, CA, United States

Abstract

Introduction: Recent advances in material science and manufacturing, including methods of creating coatings, composites, surface textures and 3D porous structures, are increasingly being applied to spinal interbody cages with the goals of improving osseointegration and creating a more favorable environment for bony union. This study evaluated and compared the functional osseointegration of implants created from seven "advanced" materials in an established ovine bone implantation model used for nearly two decades.

Methods: 13 skeletally mature wethers were assigned to receive 3 bi-cortical tibial implants and 2 cancellous implants (distal femur and proximal tibia) in each hind limb following ethical approval. Dowel-shaped implants (6x25mm) were fabricated using commercially available processes and characterized prior to implantation by stereomicroscopy and SEM. Implant groups were: 1) Ti (cp) Plasma Spray (TPS) on PEEK; 2) Nano-Hydroxyapatite (HA) on PEEK; 3) TiO2 Nanotexture on Ti Alloy; 4) Ti (cp) Vapor Deposition on PEEK; 5) Grit Blasted Ti Alloy; 6) 3D Printed Porous Ti Alloy; and 7) HA-Impregnated PEEK. Cortical implants were randomly allocated for a sample size of n=8 at 4 and 12 weeks for mechanical push out testing and histology while all remaining implants were processed solely for histology. Data were analyzed by ANOVA.

Results: The topography of the implants varied on nano- to macro-scales depending on the material and its method of manufacture. Bone on-growth was noted histologically on all implant materials, but only the 3D printed porous Ti alloy implants supported bony in-growth with marked quantities of bone within the interconnected pores. Functional differences in osseointegration were noted between groups during mechanical push out testing. 3D printed porous Ti alloy implants had the highest push out strength at 12 weeks, followed by TPS on PEEK. These two materials were also the only two groups that demonstrated continued improvement in strength from 4 to 12 weeks (p< 0.001).

Discussion: This study demonstrated differences in the osseointegration potential of various materials and manufacturing methods used in the design of spinal interbody cages. Non-porous materials were limited to bone on-growth which improved with time but did not manifest in a marked change in mechanical properties due to the lack of surface features large enough to interlock with bone. The porous 3D printed Ti alloy also supported bone in-growth which resulted in an increase in mechanical properties with time. Functional differences in shear (push-out) strength were measured and may have implications for the ability of these materials to resist intervertebral forces and micromotion. The comparative performance data of the osseointegration potential of the materials tested in this study may guide the development of improved interbody cage designs for use in spinal fusion. These findings may also be instructive to surgeons choosing to adopt new implant designs into their clinical practice in advance of any comparative data of clinical outcomes becoming available.

Figure 1: Integration strength and histology