Endplate volumetric bone mineral density measured by quantitative computed tomography as a novel predictive measure of severe cage subsidence after standalone lateral lumbar fusion

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Abstract
Purpose Quantitative computed tomography (QCT) is an alternate imaging method to dual X-ray absorptiometry to measure bone mineral density (BMD). One advantage of QCT is that it allows site-specific volumetric BMD (vBMD) measurements in a small region. In this study, we utilized site-specific, endplate vBMD (EP-vBMD) as a potential predictive marker of severe cage subsidence in standalone lateral lumbar interbody fusion (SA-LLIF) patients and conducted a retrospective comparative study between EP-vBMD and trabecular vBMDs (Tb-vBMD) in the vertebrae. Methods Patients undergoing SA-LLIF from 2007 to 2016 were retrospectively reviewed. EP-vBMD was defined as the average of the upper and lower endplate volumetric BMDs measured in cortical and trabecular bone included in a 5-mm area of interest beneath the cage contact surfaces. We compared Tb-vBMDs and EP-vBMDs between disk levels that had severe cage subsidence and levels with no severe subsidence. Results Both EP-vBMD and Tb-vBMD could be measured in 210 levels of 96 patients. Severe cage subsidence was observed in 58 levels in 38 patients. Median (IQR) Tb-vBMD was 120.5 mg/cm3 (100.8–153.7) in the non-severe subsidence group and 117.9 mg/cm3 (90.6–149.5) in the severe subsidence group (p = 0.393), whereas EP-vBMD was significantly lower in the severe subsidence group than the non-severe subsidence group (non-severe subsidence 257.4 mg/cm3 (216.3–299.4), severe subsidence 233.5 mg/cm3 (193.4–273.3), p = 0.026). Conclusion We introduced a novel site-specific vBMD measurement for cage subsidence risk assessment. Our results showed that EP-vBMD was a reproducible measurement and appeared more predictive for severe cage subsidence after SA-LLIF than Tb-vBMD. Graphic abstract These slides can be retrieved under Electronic Supplementary Material.

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