Segmental Stability and Compressive Strength of Posterior Lumbar Interbody Fusion Implants

Seth M. Zeidman, MD
Anthony Tsantrizos MSc
Hani G. Baramki, MD, PhD
Thomas Steffen, MD, PhD (Montreal QB)

Introduction:

Multiple devices augmenting interbody fusion have been developed. However, currently available devices are suboptimal in either design or application. To address cage shortcomings, the machined allograft spacer with anchoring teeth was developed. In this study, the initial stability and compressive strength of a machined allograft spacer was compared to that of threaded and non–threaded metal cages in posterior lumbar interbody fusion.

Methods:

Multisegmental specimens were tested intact, with implants inserted into the L4/5 interbody space and with supplemental posterior fixation. Three PLIF implant constructs (Ray Threaded Fusion Cage, TFC; Contact Fusion Cage, CFC and; PLIF Spacer, PLIF–S) were tested in axial rotation, flexion/extension and lateral bending. The implant–specimen constructs were then isolated and compressed to failure. Changes in neutral zone (NZ), range of motion (ROM), yield strength (YS) and ultimate strength (US) were analyzed.

Results:

None of the stand–alone implant constructs reduced the NZ. Supplemental posterior fixation decreased NZ in flexion/extension and lateral bending. Stand–alone implant constructs decreased ROM in flexion and lateral bending. Differences in ROM between stand–alone cage constructs were found in flexion and extension (marginally significant). Supplemental posterior fixation further decreased ROM in all loading directions with no differences between implant constructs. The CFC and PLIF–S constructs had a higher US than the TFC.

Discussion / Conclusion:

The PLIF–S is biomechanically equivalent to the other tested implant but is devoid of the deficiencies associated with other interbody cage technologies. A PLIF–S is a valid alternative and may be preferable for posterior lumbar interbody fusion.