Revision: Lumbosacral Fusion

Revision
According to Weiner and Fraser, ⁹⁸ interbody cage devices have 4 goals, correct the existing mechanical deformation; provide stability to the segment until arthrodesis is obtained; provide the best possible environment for successful arthrodesis; and achieve this with limited morbidity associated with their use. As previously discussed, clinical failures and morbidity are usually due to factors such as surgical technique or patient selection, rather than an actual material failure of the threaded cylindrical interbody device, be it titanium or cortical bone dowel. Clinical failure has been associated with undersized cages, which fail to achieve adequate distraction of the annulus fibrosis. This is less problematic in ALIFs than PLIFs, where size is constrained by neurologic structures and the facet joints. Failure is also higher with the use of local bone graft instead of iliac crest cancellous bone, and excessive lateral placement of the threaded interbody fusion device (as a result of failing to correctly identify the midline). ^{60, 65}

Postoperatively, approximately 20% of the initially distracted disc space height is lost, largely due to bony subsidence. ⁸⁷ In a biomechanical study examining the effect of cyclical loading on threaded interbody cages (simulating patient activity); one group found a gradual loss of stability due to local trabecular bone failure and subsequent loss of annular tension. ⁷⁶Ahrens et al1 reported that anterior interbody cage subsidence occurred throughout the first 12 months after surgery, but found no correlation with vertebral bone mineral density on DEXA evaluation. Conversely, others have reported that vertebral body bone mineral density is a significant factor in threaded interbody device stability and is correlated with failure rates. ^{51, 63}

ALIF pseudarthrosis can lead to continued lumbar axial back pain and, rarely, interbody device migration. In this setting, one theoretical disadvantage of titanium devices (versus threaded cortical bone dowels) is the potential for the generation of titanium debris. Although not well studied, significant concentrations have been identified in the paraspinous soft tissues of pseudarthrosis patients with more traditional titanium spinal implants. ⁹⁶Titanium debris has been shown to stimulate a macrophage cellular response^{96, 98} and cytokine release, which could possibly have a deleterious effect on spinal tissues. The most compelling theoretical advantage of threaded bone dowels over titanium cages, however, is the fact that they are much easier to revise.