Comparison of stiffness and failure load of two cervical spine fixation techniques in an in vitro human model

Objective: Recently, an unpaired threaded cage has been introduced as a fusion device for the cervical spine. No biomechanical comparison of a stand-alone single interbody threaded cage to a standard plated Smith-Robinson construct has been reported. Accordingly, an in vitro biomechanical comparison of a single threaded cylindrical interbody fusion cage versus a plated Smith-Robinson cervical discectomy and fusion construct was conducted to establish whether a single cylindrical interbody cage in the cervical spine would perform mechanically as well as a plated structural interbody graft. Methods: Six fresh-frozen human cadaveric cervical spines were used for biomechanical testing. Flexion-extension and load-to-failure testing were performed on two single-level discectomy and interbody fusion constructs from each specimen. Results: Initial range of motion (ROM) was significantly greater for the specimens implanted with a cage than specimens implanted with a structural graft and plate (9.1degrees +/- 3.7degrees vs 5.8degrees +/- 2.4degrees; P = 0.040). Initial stiffness in flexion in caged specimens was significantly less than in plated specimens (0.7 +/- 0.3 vs 0.9 +/- 0.3 Nm/degrees; P = 0.028). Cage specimens also failed at a significantly lower load than plated specimens (9.8 +/- 3.5 vs 15.8 +/- 4.1 Nm; P = 0.0104). Conclusions: This study demonstrates that a plated SmithRobinson cervical discectomy and fusion construct provides greater stiffness and failure load and reduced ROM across operated levels than a single interbody cage construct. Although clinical success may not directly correlate with biomechanical data, these results raise concern regarding the use of a single threaded interbody cage as a stand-alone device for cervical interbody fusion.