Factors associated with intervertebral cage subsidence in posterior lumbar fusion
Authors: liu yan , LI nianhu 
Published: November 2023

Abstract
Background The interbody fusion apparatus is a key component of the operation and plays a key role in the postoperative efficacy. Fusion sink is one of the common complications after lumbar fusion and internal fixation.Clinical studies on the risk factors of fusion sink are incomplete and inaccurate, especially paravertebral muscle atrophy and intervertebral bone fusion time.
Methods The data of 60 patients with L4/5 posterior lumbar fusion surgery admitted to the orthopedic ward of Shandong Hospital of Traditional Chinese Medicine from January 2016 to January 2019 were retrospectively analyzed. All patients were divided into a sedimentation group (30 cases) and a non-sedimentation group (30 cases) according to whether the fusion device was settled or not at the last follow-up. Operative time, intraoperative blood loss, postoperative time to the ground, interbody fusion time, preoperative intraoperative space endlaminitis, and the improvement of VAS score and Oswestry index after treatment were analyzed and compared between the two groups. Preoperative paravertebral muscle CSA, FI, paravertebral muscle FCS, psoas major muscle CSA, and vertebral body CSA were measured and compared by CT and MRI. Intraoperative space height and Angle of immediate SL correction were measured and compared, as well as postoperative SS, LL, PT, and PI-LL. Paravertebral rCSA, psoas major rCSA and paravertebral rFCSA were calculated. logistic regression analysis was used to determine the high risk factors affecting the fusion instrument subsidence after posterior lumbar fusion.
Results Bone mineral density (BMD) in the sedimentation group was lower than that in the non-sedimentation group, the difference was statistically significant (P=0.018). There were 4 patients with endplate injury in the sedimentation group (P=0.038). rCSA of psoas major muscle, CSA of paravertebral muscle, rCSA of paravertebral muscle and rFCSA of paravertebral muscle in sedimentation group were significantly lower than those in sedimentation group, and the differences were statistically significant (P=0.043, P=0.047, P<0.001, P<0.001). The vertebral body area, the height of immediately corrected intervertebral space, the Angle of immediate SL correction and the time of intervertebral fusion in the sedimentation group were significantly higher than those in the non-sedimentation group (P=0.004, P=0.020, P=0.009, P=0.023). Binary multivariate logistic regression analysis showed that the time of intervertebral fusion (OR=1.158, P=0.045), the height of immediate intervertebral space correction (OR=1.438, P=0.038), and the Angle of immediate SL correction (OR=1.101, P=0.019) were the risk factors for fusion fusion. Bone mineral density (OR=0.544, P=0.016) and paravertebral muscle rFCSA (OR=0.525, P=0.048) were protective factors.
Conclusion Long intervertebral fusion time, correctable intervertebral space height and immediate SL correction Angle are independent high risk factors for fusion sink after posterior lumbar fusion. The greater the BMD and the paravertebral muscle rFCSA, the less likely the fusion subsidence was to occur.