POSTERIOR LUMBAR INTERBODY FUSION - A BIOMECHANICAL COMPARISON, INCLUDING A NEW THREADED CAGE

Study Design. In vitro biomechanical testing was performed on eight lu mbar calf spines. Objectives. To compare the initial stiffness of a st andard method of posterior lumbar interbody fusion using structural au tograft with the same procedure using additional posterior instrumenta tion. These constructs also were compared to a new titanium implant. S ummary of Background Data. Posterior lumbar interbody fusion is gainin g wide acceptance for the treatment of segmental spinal instability, s pondylolisthesis, and discogenic pain. Many methods have been describe d, including use of autograft or allograft bone, in either structural or nonstructural form, with or without additional fixation. A new thre aded titanium interbody implant has been designed to increase initial stability while allowing bony ingrowth for fusion. Methods. Eight lumb ar 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 co nstruct under each load pattern. Results. The posterior lumbar interbo dy fusion by bone graft alone was the least stiff construct of all mod es tested. In two of eight specimens the bone graft dislodged posterio rly into the canal during torsional testing. The titanium interbody im plant was similar in stiffness to the bone graft posterior lumbar inte rbody fusion with posterior instrumentation group in all three modes. They were both significantly stiffer than the normal spine, the destab ilized spine, and the posterior lumbar interbody fusion by bone graft alone (P < 0.05). Conclusions. In this model, the posterior lumbar int erbody fusion with bone graft alone had less initial stiffness than th at of the intact spine. The addition of posterior instrumentation or i nterbody implants can increase initial stiffness significantly.