A retrospective, single-center clinical study with follow-up of more than 24 months. To evaluate the union rates and relevant risk factors for non-union after posterior lumbar interbody fusion (PLIF) using porous-coated closed-box titanium spacers. Although the use of a closed-box interbody spacer for PLIF could avoid potential complications associated with the harvesting of autologous bone, few studies have reported detailed follow-up of fusion progression and risk factors for non-union in the early postoperative period. PLIF using closed-box spacers without filling the autologous bone was performed in 78 (88 levels) consecutive patients. Surgical procedures included PLIF using traditional pedicle screw fixation (PLIF, n=37), PLIF using cortical bone trajectory screw fixation (CBT-PLIF, n=30), and transforaminal lumbar interbody fusion with traditional pedicle screw fixation (TLIF, n=11). Lateral dynamic radiography and computed tomography findings were investigated, and the relationship between the union status and variables that may be related to the risk of non-union was tested statistically. The overall bone union rates at 12 and 24 months were 68.0% and 88.5%, respectively. Incidences of bone cyst formation, subsidence, and retropulsion of spacers were 33.3%, 47.4%, and 14.1%, respectively. Union rates at 24 months were 94.6% in PLIF, 80.0% in CBT-PLIF, and 90.9% in TLIF. Multivariate logistic regression analyses showed that at 12 months postoperatively, the risk factor for non-union was age >75 years (p =0.02). In contrast, no significant risk factor was observed at 24 months. These findings demonstrated the efficacy of interbody closed-box spacers for PLIF without the need to fill the spacer with autologous bone. However, the risk of non-union should be considered in elderly patients, especially intra-operatively and during the early postoperative stage.

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.