Clinical course and significance of the clear zone around the pedicle screws in the lumbar degenerative disease

Study Design. A retrospective study. Objective. To report the clinical course and significance of postoperative radiolucent zones around pedicle screws. Summary of Background Data. Radiolucent zones around pedicle screws have been considered to indicate loss of fixation, delayed union, or pseudarthrosis. Few reports have described the course and significance of them. Methods. Plain radiographs were evaluated from 190 patients with pedicle screw fixation. When 1 mm or greater circumferential lucency was confirmed around a screw on plain radiographs from 2 or more directions, the patient was judged as clear-zone positive. The course of clear-zone positivity was investigated and the relationships between clear-zone positivity and the following items were investigated: (1) number of levels fused, (2) bone union, (3) posterolateral lumbar fusion versus posterior lumbar interbody fusion, (4) clinical results, (5) bone mineral density, and (6) screw types. Results. Although clear zones were observed in 78 patients 6 months after surgery, the clear zone-positive rate decreased over time and the progression of bone union and the number of patients decreased to 28 at the final follow-up 3 years or more after surgery. Interobserver and intraobserver coefficient of variability in clear-zone judgment was over 0.86, and agreement was over 96%. At final follow-up, pseudarthrosis was found in 15 patients. Although the predictive value of the positive test for the presence of clear zones for pseudarthrosis at final follow-up was 0.50, sensitivity was 0.93 and specificity was 0.92. There were significant differences in the clear zone-positive rate in the number of levels fused and bone mineral density among the investigated items. Conclusion. As approximately two thirds of the clear zones disappeared over time, the presence of clear zones did not necessarily mean pseudarthrosis. However, clear zones persisting for 2 years or longer after surgery are a great risk of pseudarthrosis. Therefore, careful observation of clear zones around pedicle screws is of great significance as an evaluation of bone union.