Biomechanical rationale for using polyetheretherketone (PEEK) spacers for lumbar interbody fusion-A finite element study.
METHODS
To determine the effect of cage/spacer stiffness on the stresses in the bone graft and cage subsidence.


OBJECTIVE
To investigate the effect of cage stiffness on the biomechanics of the fused segment in the lumbar region using finite element analysis.


BACKGROUND
There are a wide variety of cage/spacer designs available for lumbar interbody fusion surgery. These range from circular, tapered, rectangular with and without curvature, and were initially manufactured using titanium alloy. Recent advances in the medical implant industry have resulted in using medical grade polyetheretherketone (PEEK). The biomechanical advantages of using different cage material in terms of stability, subsidence, and stresses in bone graft are not fully understood.


METHODS
A previously validated 3-dimensional, nonlinear finite element model of an intact L3-L5 segment was modified to simulate posterior interbody fusion spacers made of PEEK ("E" = 3.6 GPa) and titanium ("E" = 110 GPa) at the L4/5 disc with posterior instrumentation. Bone graft ("E" = 12 GPa) packed between the spacers in the intervertebral space was also simulated. The posterior lumbar interbody fusion spacer with instrumentation and graft represent a simulation of the condition present immediately after surgery.


RESULTS
The peak centroidal Von Mises stresses in the graft bone increased by at least 9-fold with PEEK spacers as compared to titanium spacer. The peak centroidal Von Mises stresses in the endplates increased by at least 2.4-fold with titanium spacers over the PEEK spacers. These stresses were concentrated at places where the spacer interfaced with the endplate. The stiffness of the spacer did not affect the relative motion (stability) across the instrumented (L4/5) segment.


CONCLUSIONS
Spacers less stiff than the graft will: (1) provide stability similar to titanium cages in the presence of posterior instrumentation, (2) reduce the stresses in endplates adjacent to the spacers, and (3) increase the load transfer through the graft, as evident from the increase in stresses in graft.
Vadapalli
Sasidhar
S
Engineering Center for Orthopaedic Research Excellence (E-CORE), Departments of Bioengineering and Orthopaedic Surgery, Colleges of Engineering and Medicine, University of Toledo, Toledo, OH 43606, USA.
Sairyo
Koichi
K
Goel
Vijay K
VK
Robon
Matt
M
Biyani
Ashok
A
Khandha
Ashutosh
A
Ebraheim
Nabil A
NA
D001577:Benzophenones; D001672:Biocompatible Materials; D001696:Biomechanical Phenomena; D016025:Bone Transplantation / Q000295:instrumentation; D020342:Finite Element Analysis; D006801:Humans; D007659:Ketones; D008159:Lumbar Vertebrae; D008962:Models, Theoretical; D011092:Polyethylene Glycols; D011108:Polymers; D019736:Prostheses and Implants; D013123:Spinal Fusion / Q000295:instrumentation*; D014025:Titanium
D001577:Benzophenones; D001672:Biocompatible Materials; D007659:Ketones; D011108:Polymers; C063834:polyetheretherketone; D011092:Polyethylene Glycols; D014025:Titanium