Influence of Pedicle Screw Insertion Depth on Posterior Lumbar Interbody Fusion: Radiological Significance of Deeper Screw Placement

Study Design: Retrospective case series. Objectives: To investigate the influence of screw size on achieving bone fusion in posterior lumbar interbody fusion (PLIF). Methods: In total, 137 consecutive patients with L4 degenerative spondylolisthesis who underwent single-level PLIF at L4-L5 were evaluated. Factors investigated for their contribution to bone fusion included: 1) age, 2) sex, 3) body mass index, 4) bone mineral density, 5) intervertebral mobility, 6) screw diameter, 7) screw length, 8) screw fitness in the pedicle (%fill), 9) screw depth in the vertebra (%depth), 10) screw angle, 11) facetectomy, 12) crosslink connector, and 13) cage material. Results: Bone fusion was confirmed in 88.2% of patients. The comparison between fusion (+) and fusion (-) groups showed no significant differences in screw size. The %fill and %length were significantly greater in the fusion (+) group than in the fusion (-) group (%fill: 58.5% +/- 7.5% vs 52.3% +/- 7.3%, respectively, P = .005; %depth: 59.8% +/- 9.7% vs 50.3% +/- 13.8%, respectively, P = .025). Multivariate logistic regression analysis revealed that %fill (odds ratio [OR]= 1.11, P = .025) and %depth (OR = 1.09, P = .003) were significant independent factors affecting bone fusion. Receiver operating characteristic curve analyses identified a %fill of 60.0% and a %depth of 54.2% as optimal cutoff values for achieving bone fusion Conclusions: Screw size should be determined based on the screw fitness in the pedicle (%fill > 60%) and screw insertion depth in the vertebral body (%depth > 54.2%) according to individual vertebral anatomy in L4-L5 PLIF.

Automated summary

This retrospective case series aimed to investigate the influence of screw size on bone fusion in posterior lumbar interbody fusion (PLIF). The study found that screw fitness in the pedicle and screw insertion depth in the vertebra were significant factors affecting bone fusion. The results suggest that screw size should be determined based on individual vertebral anatomy, considering the screw fitness in the pedicle and screw insertion depth in the vertebra for L4-L5 PLIF.