P15. Biomechanical analysis of bone density and depth of subsidence after expandable interbody cage placement

Abstract

BACKGROUND CONTEXT Expandable interbody fusion cages have been developed to improve the ability to restore disc height and segmental lordosis. However, expandable cages have been shown to be associated with increased rates of postoperative subsidence. To optimize patient outcomes, it is critical to evaluate all variables which may influence a patient's risk of subsidence following the placement of an expandable interbody cage. PURPOSE To evaluate the association between bone density and depth of subsidence due to expandable interbody cages. STUDY DESIGN/SETTING Biomechanical study. PATIENT SAMPLE No patients were included. OUTCOME MEASURES Depth of subsidence (mm). METHODS In the first stage of the study, the depth of subsidence was measured following 150 N output force exerted by an expandable interbody cage. In the second stage, subsidence depth was measured with 1 Nm of input expansion torque. Within each stage, different bone foam densities were analyzed, including 5, 10, 15, and 20 pounds per cubic foot (PCF). Five experimental trials were performed for each PCF material, and the mean subsidence depths were calculated. Trials which failed to reach 150 N output force were considered outliers and were excluded from the analysis. The Mojave PL 3D Expandable Interbody System (Stryker, Leesburg, VA, USA) was utilized for all trials. RESULTS There was an overall decrease in subsidence depth with increasing bone foam density. The subsidence depths (mean ± standard deviation) at 150 N output force were 2.0±0.3 mm for 5 PCF, 1.8±0.2 mm for 10 PCF, 1.1±0.2 mm for 15 PCF, and 1.1±0.2 mm for 20 PCF bone foam. The mean subsidence depths at 1 Nm of input torque were 2.3±0.5 mm for 5 PCF, 2.3±0.5 mm for 10 PCF, 1.2±0.2 mm for 15 PCF, and 1.1±0.1 mm for 20 PCF bone foam. CONCLUSIONS The current study demonstrates the significant correlation between bone density and depth of subsidence due to an expandable interbody cage. At the same expansion torque, an expandable interbody cage implant will subside further into lower density bone. To apply the same distraction force, an implant on lower density bone will lose more expansion height to subsidence. These findings highlight the concern for catastrophic subsidence and resultant construct failure. Notably, because tactile feedback of cage expansion into subsiding bone cannot be reliably distinguished from true expansion of disc space height, surgeons should take bone quality into account when deploying expandable cages. FDA DEVICE/DRUG STATUS Mojave PL 3D Expandable Interbody System (Stryker, Leesburg, VA, USA) (Approved for this indication) Expandable interbody fusion cages have been developed to improve the ability to restore disc height and segmental lordosis. However, expandable cages have been shown to be associated with increased rates of postoperative subsidence. To optimize patient outcomes, it is critical to evaluate all variables which may influence a patient's risk of subsidence following the placement of an expandable interbody cage. To evaluate the association between bone density and depth of subsidence due to expandable interbody cages. Biomechanical study. No patients were included. Depth of subsidence (mm). In the first stage of the study, the depth of subsidence was measured following 150 N output force exerted by an expandable interbody cage. In the second stage, subsidence depth was measured with 1 Nm of input expansion torque. Within each stage, different bone foam densities were analyzed, including 5, 10, 15, and 20 pounds per cubic foot (PCF). Five experimental trials were performed for each PCF material, and the mean subsidence depths were calculated. Trials which failed to reach 150 N output force were considered outliers and were excluded from the analysis. The Mojave PL 3D Expandable Interbody System (Stryker, Leesburg, VA, USA) was utilized for all trials. There was an overall decrease in subsidence depth with increasing bone foam density. The subsidence depths (mean ± standard deviation) at 150 N output force were 2.0±0.3 mm for 5 PCF, 1.8±0.2 mm for 10 PCF, 1.1±0.2 mm for 15 PCF, and 1.1±0.2 mm for 20 PCF bone foam. The mean subsidence depths at 1 Nm of input torque were 2.3±0.5 mm for 5 PCF, 2.3±0.5 mm for 10 PCF, 1.2±0.2 mm for 15 PCF, and 1.1±0.1 mm for 20 PCF bone foam. The current study demonstrates the significant correlation between bone density and depth of subsidence due to an expandable interbody cage. At the same expansion torque, an expandable interbody cage implant will subside further into lower density bone. To apply the same distraction force, an implant on lower density bone will lose more expansion height to subsidence. These findings highlight the concern for catastrophic subsidence and resultant construct failure. Notably, because tactile feedback of cage expansion into subsiding bone cannot be reliably distinguished from true expansion of disc space height, surgeons should take bone quality into account when deploying expandable cages.