Radiological and clinical evaluation of cage subsidence after transforaminal lumbar interbody fusion with unilateral pedicle screw fixation

English Keywords:Transforaminal lumbar interbody fusion  Unilateral pedicle screw fixation  Cage  Subsidence

English Abstract:

  【Abstract】 Objectives: To assess the radiographic and clinical characteristics of cage subsidence in transforaminal lumbar interbody fusion(TLIF) with unilateral pedicle screw fixation. Methods: A total of 43 patients who underwent unilateral instrumented TLIF from January 2014 to January 2015 was included in this retrospective study. The total number of fusion segment was 51. The duration of follow-up was 23.8±3.7 months(19-28 months). Intervertebral height of fusion segment was measured by comtupter tomography. The time points of measurement were preoperation, immediate, half a year, 1 year and the final follow-up after operation. Subsidence was classified by using the following scale: Grade 0, 0-24% loss of postoperative disc height; Grade Ⅰ, 25%-49%; Grade Ⅱ, 50%-74%; and Grade Ⅲ, 75%-100%. Changes of overall cage subsidence were analyzed at different follow-up time point(6 months, 1 year and the final follow-up after operation). According to the cage anteroposterior diameter and fusion length, patients were divided into 32mm(n=15) and 36mm group(n=28), 1-level(n=35) and 2-level(n=8) group, respectively. Statistical difference of cage subsidence degree between groups at different follow-up time point was analyzed. Visual analogue scale(VAS) and the Oswestry disability index(ODI) were recorded, changes of VAS and ODI were analyzed at different follow-up time point(6 months, 1 year and the final follow-up after operation). The correlation between cage subsidence degree and VAS and ODI at the final follow-up was tested by Pearson correlation coefficient. Results: At the final follow-up after operation, most fusion levels were classified as Grade 0(45/51, 88.2%), and 6 fusion levels with Grade Ⅰ subsidence (6/51, 11.8%). No Grade Ⅱ or Grade Ⅲ subsidence was found. The subsidence degree showed significant difference between 6 months follow-up[(9.6±3.4)%] and 1 year follow-up[(14.2±5.6)%](P<0.05), while no significant difference between 1 year and the final follow-up(P>0.05). Cage subsidence degree in 32mm group was higher than that in 36mm group at the final follow-up(P<0.05), while there was no difference between 1-level and 2-level group at the final follow-up(P>0.05). Both VAS and ODI improved after operation(6 months, 1 year and the final follow-up)(P<0.05), and it showed low correlation between VAS of low back pain and cage subsidence(Pearson coefficient: 0.334, P＜0.05), while no significant relationship between ODI, VAS of leg pain and cage subsidence(P<0.05). Conclusions: Cage subsidence degree in TLIF with unilateral pedicle screw fixation is relatively low, and most of the cage subsidence occurrs within 1 year after operation. The anteroposterior diameter of the cage will affect the cage subsidence. If conditions permit, the cage with large anteroposterior diameter should be selected as far as possible to reduce the cage subsidence degree, while no relationship between fusion length and cage subsidence. The VAS score of lumbar pain is weakly correlated with the cage subsidence.