Posterior lumbar interbody fusion. A biomechanical comparison, including a new threaded cage.

STUDY
DESIGN: In vitro biomechanical testing was performed on eight lumbar calf spines.
OBJECTIVES: To compare the initial stiffness of a standard method of posterior lumbar interbody fusion using structural autograft with the same procedure using additional posterior instrumentation. These constructs also were compared to a new titanium implant. SUMMARY OF BACKGROUND DATA: Posterior lumbar interbody fusion is gaining wide acceptance for the treatment of segmental spinal instability, spondylolisthesis, and discogenic pain. Many methods have been described, including use of autograft or allograft bone, in either structural or nonstructural form, with or without additional fixation. A new threaded titanium interbody implant has been designed to increase initial stability while allowing bony ingrowth for fusion.
METHODS: Eight lumbar calf spines were subjected to axial compression, sagittal moments (flexion-extension), and axial torque while displacement was measured. Stiffness was calculated from the load displacement curves for each construct under each load pattern.
RESULTS: The posterior lumbar interbody fusion by bone graft alone was the least stiff construct of all modes tested. In two of eight specimens the bone graft dislodged posteriorly into the canal during torsional testing. The titanium interbody implant was similar in stiffness to the bone graft posterior lumbar interbody fusion with posterior instrumentation group in all three modes. They were both significantly stiffer than the normal spine, the destabilized spine, and the posterior lumbar interbody fusion by bone graft alone (P < 0.05).
CONCLUSIONS: In this model, the posterior lumbar interbody fusion with bone graft alone had less initial stiffness than that of the intact spine. The addition of posterior instrumentation or interbody implants can increase initial stiffness significantly.