S1 Dual Outer Diameter Screws Accelerate Early Fusion Compared with Conventional Pedicle Screws in Posterior Lumbosacral Interbody Fusion.

INTRODUCTION

Achieving successful lumbosacral interbody fusion is challenging due to the unique anatomical characteristics of the sacrum. The predominantly cancellous bone and short pedicles limit screw purchase, which frequently leads to S1 screw loosening and pseudarthrosis, resulting in persistent pain and poor outcomes. Therefore, the S1 Dual Outer Diameter (DOD) screw, featuring a dual-zone thread design with greater thread depth and sharper edges proximally and reduced thread depth with blunter edges distally, was developed to maximize screw purchase in cancellous and cortical bones. This study aimed to compare the clinical and radiological outcomes of DOD and conventional pedicle screws in posterior lumbosacral interbody fusion.

METHODS

A total of 90 patients (control group, n=51; DOD group, n=39) who underwent lumbosacral fusion (≤2 levels) between 2017 and 2023 were retrospectively reviewed. The outcomes included the Japanese Orthopaedic Association (JOA) score and computed tomography-assessed interbody fusion and S1 screw loosening rates over 2 years.

RESULTS

The DOD group demonstrated a significantly higher early fusion rate at six months than the control group (33.3% vs. 9.8%; p=0.006). This early fusion was associated with a significantly higher 6-month JOA recovery rate in the DOD group (59.2%±26.9% vs. 47.8%±22.6%; p=0.029). No significant difference in overall loosening rates was observed between the two groups (20.5% vs. 35.3%; p=0.12). However, patients with one-level fusions in the DOD group showed a significantly lower loosening rate than those in the control group (5.3% vs. 25.8%; p=0.048). A higher preoperative Oswestry Disability Index score, lower lumbar lordosis, and multi-level fusion were risk factors for loosening, even with DOD screws.

CONCLUSIONS

S1 DOD screws were associated with significantly improved early fusion and functional recovery, as assessed by the JOA score, and with a significantly lower loosening rate in one-level fusion. These findings indicate that DOD screws are an effective option for short-segment lumbosacral fusion.