Factors associated with intervertebral cage subsidence in posterior lumbar fusion

The interbody fusion apparatus is a key component of the operation and plays a key role in the postoperative efficacy. Cage subsidence is one of the common complications after lumbar fusion and internal fixation. Clinical studies on the risk factors of cage subsidence are incomplete and inaccurate, especially paravertebral muscle atrophy and intervertebral bone fusion time.

Among the patients who underwent PLIF surgery in our hospital from January 2016 to January 2019, 30 patients with cage subsidence and 30 patients without cage subsidence were randomly selected to be included in this study. The differences between the two groups were compared, and the relevant factors of cage subsidence were explored by single factor comparison and multiple logistic regression analysis.

Bone mineral density (T) of the subsidence group [(- 1.84 ± 1.81) g/cm² vs (- 0.87 ± 1.63) g/cm², P = 0.018] was significantly lower than that of the normal group. There were 4 patients with end plate injury in the subsidence group (P = 0.038). Preoperative end plate Modic changes [I/II/III, (7/2/2) vs (2/5/8), P = 0.043] were significantly different between the two groups. In the subsidence group, preoperative rCSA of psoas major muscle [(1.43 ± 0.40) vs (1.64 ± 0.41), P = 0.043], CSA of paravertebral muscle [(4530.25 ± 776.55) mm2 vs (4964.75 ± 888.48) mm², P = 0.047], paravertebral muscle rCSA [(3.03 ± 0.72) vs (3.84 ± 0.73), P < 0.001] and paravertebral muscle rFCSA [(2.29 ± 0.60) vs (2.89 ± 0.66), P < 0.001] were significantly lower than those in normal group. In the subsidence group, the vertebral body area [(1547.81 ± 309.89) mm2 vs (1326.48 ± 297.21) mm2, P = 0.004], the height of the immediately corrected vertebral space [(2.86 ± 1.10) mm vs (1.65 ± 1.02) mm, P = 0.020], immediately SL corrective Angle [(5.81 + 4.71)° vs (3.24 + 3.57) °, P = 0.009), postoperative PI-LL [(11.69 + 6.99)° vs (6.66 + 9.62) °, P = 0.029] and intervertebral fusion time [(5.38 ± 1.85) months vs (4.30 ± 1.49) months, P = 0.023] were significantly higher than those in the normal group. 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 cage subsidence. Bone mineral density (OR = 0.544, P = 0.016) and preoperative paravertebral muscle rFCSA (OR = 0.525, P = 0.048) were protective factors.

Intervertebral fusion time, correctable height of intervertebral space, excessive Angle of immediate SL correction, bone mineral density and preoperative paravertebral muscle rFCSA are risk factors for cage subsidence after PLIF.