Variables associated with loss of fixation of retrograde rami screws in minimally displaced lateral compression type 1 pelvic ring injuries

Abstract

OBJECTIVES: To evaluate variables associated with loss of fixation of retrograde rami screws in the treatment of stress-positive minimally displaced lateral compression type 1 (LC1) injuries. METHODS: Design: Retrospective comparative study Setting: Level one trauma center Patient Selection Criteria: Stress-positive minimally displaced (<1 cm) LC1 pelvic ring injuries treated with retrograde rami screws Outcome Measures and Comparisons : Loss of fixation, defined as >5 mm of pelvic fracture displacement based on the radiographic tear-drop distance, or >5 mm of implant displacement on follow-up radiographs; Revision surgery for loss of fixation RESULTS: 38 patients with 40 retrograde rami screws were analyzed. Median patient age was 64 years (IQR 42.5 to 73.3 years), 71.1% (n=27/38) were female, 52.6% (n=20/38) of injuries were secondary to low-energy mechanisms. Loss of fixation occurred in 17.5% (n=7/40) of screws with 10% (n=4/40) requiring revision surgery. Unicortical rami screws failed at a significantly higher rate than bicortical screws (62.5% (5/8) vs. 3.1% (1/32); observed difference (OD) 59.4%, 95% confidence interval (CI) 22.7% to 85.5%; p=0.0005). Other variables associated with loss of fixation included dynamic displacement on stress radiographs (22.1 vs. 15.4 mm; OD 6.6 mm, CI 2.0 to 11.3; p=0.02), unicortical screws (83.3% vs. 8.8%; OD -74.5%, CI -95.6% to -32.3%; p=0.0005), short screws ending medial to the acetabular sourcil (50.0% vs. 8.8%; OD 41.1%, CI 2.7% to 75.0%; p=0.03), and partially-threaded screws (66.7% vs. 23.5%; OD 43.1%, CI 1.1% to 73.9%; p=0.04). On univariate analysis, patients who had a loss of fixation were more likely to have greater dynamic displacement on stress radiographs (22.0 vs. 15.2 mm; median difference (MD) 5.6 mm, CI -19.2 to 10.3; p=0.04), unicortical rami screws (71.4% vs. 9.1%; PD 62.3%, CI 8.8% to 22.6%; p=0.001), and partially-threaded rami screws (71.4% vs. 21.2%; PD 50.2%, CI 10.0% to 77.6%; p=0.01). The remaining variables had no observed association (p≥0.05) with loss of fixation, including age, gender, BMI, energy of injury mechanism, tobacco use, ASA score, sacral fracture type, distal rami fractures, rami comminution, number of sacral screws, fully-threaded sacral screws, transsacral screws, or rami screw diameter. On multivariate analysis, only unicortical rami screws (p=0.01) remained associated with loss of fixation. CONCLUSIONS: Retrograde rami screws had a high rate of loss of fixation in minimally displaced LC1 pelvic ring injuries and this was associated with unicortical screws. These screws should be avoided when possible. LEVEL OF EVIDENCE: Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.