Spine. Volume 35(21), 1 October 2010, pp E1076-E1082

The Mechanical Effect of Commercially Pure Titanium and Polyetheretherketone Rods on Spinal Implants at the Operative and Adjacent Levels

Turner, Joseph L. MS*; Paller, David J. MS†; Murrell, Charles B. BS, MBA*. Author Information: From the *Medtronic's Spinal and Biologics Business, Memphis, TN; and †RIH Orthopaedic Foundation, Inc, The Warren Alpert Medical School of Brown University, Providence, RI. © 2010 Lippincott Williams & Wilkins, Inc.

Study Design. Single-level cadaveric lumbar constructs were instrumented with either polyetheretherketone (PEEK) or commercially pure (CP) titanium (Ti) rods and biomechanically evaluated. Strain from gauged bone screws and interbody (IB) spacers, kinematic motion, and caudal disc pressure measurements were recorded during testing.

Objective. The objective of this study was to determine the biomechanical differences in CP Ti rods and PEEK rods in conjunction with PEEK interbody spacers.

Summary of Background Data. Very little biomechanical data exist substantiating the performance of PEEK as a spinal rod material. This study is unique, because it combines strain, motion, and pressure measurement techniques to evaluate cadaveric constructs.

Methods. Twelve human cadaveric lumbar spine segments (T12-L3 and L4-S1) were tested in compression, flexion-extension, bilateral lateral bending, and bilateral axial torsion. Bending, axial, and shear strains were recorded from a gauged bone screw; axial and shear strains were also recorded from a gauged PEEK interbody spacer. Planar motion data and subadjacent disc pressure measurements were also collected.

Results. Highest screw strains were in bending; the lowest screw strains derived from the shear and axial gauges. Spacer strain was high to medium in some cases, especially in compression and flexion. PEEK constructs attained higher interbody strains than Ti constructs. Conversely, Ti construct screw strains were higher in most tests. Planar motion showed no differences at any level in almost every test. There was a trend toward decreased caudal intradiscal pressure for Ti constructs in compression.

Conclusion. Rigid CP Ti rods resulted in increased screw strain (bone-screw interface forces) and less interbody spacer compression (higher stress shielding). Furthermore, there was a trend toward decreased intradiscal pressure with Ti rods at the caudal segment. These trends suggest that segments instrumented with PEEK more closely mimicked intact physiologic loading in the subadjacent level, which may reduce the likelihood of adjacent level disease.