Radiographic Evaluation of Monocortical Versus Tricortical Purchase Approaches in Lumbosacral Fixation With Sacral Pedicle Screws: A Prospective Study of Ninety Consecutive Patients

Study Design. Prospective radiographic outcome analysis. Objective. Radiographic evaluation of the rigidity of monocortical and tricortical purchase in lumbosacral fixation. Summary of Background Data. The lumbosacral junction continues to be a difficult region to obtain a successful spinal arthrodesis and is one of the primary regions for construct failure. In inserting sacral screws, 3 types of purchase are known: monocortical, bicortical, and tricortical. Among them no clinical or basic studies have compared monocortical with tricortical purchase. Methods. Ninety consecutive patients diagnosed with spondylolisthesis who underwent lumbosacral fixation were evaluated. Fifty-three patients were treated with a monocortical approach and the other 37 patients were treated with a tricortical approach. Patients underwent surgery for posterolateral fusion or transforaminal lumbar interbody fusion (TLIF) according to their diagnosis. Their radiologic findings, radiolucent zones around sacral screws, and lumbosacral instability of more than 2° flexion just after the surgery and 1 year later were compared. The angulation and length of the inserted screw were also measured and evaluated. Results. There were no significances in the distributions of gender, average age, proportion of fixation method, and fusion range between the monocortical and tricortical groups (P > 0.05). Radiolucent zones and lumbosacral instability were more prevalent in the monocortical group than were in the tricortical group. Smaller angulation also affected the outcome in the monocortical group while screw length did not. Multivariate logistic regression analysis of possible risk factors revealed that female gender (odds ratio [OR]: 3.56, 95% confidence interval [CI]: 1.18–10.8), TLIF operative method (OR: 5.54, 95% CI: 1.08–8.2), number of fusion levels (OR: 1.84, 95% CI: 1.05–3.18), and monocortical purchase (OR: 7.11, 95% CI: 1.58–31.9) were statistically significant. Conclusion. More loosening of sacral screws was radiographically observed in the monocortical purchase group than in the tricortical group 1 year after the lumbosacral fixation surgery. A statistical analysis indicated that the tricortical approach should be used for patients undergoing multilevel fusion including lumbosacral junction by TLIF.