Does the interfacing angle between pedicle screws and support rods affect clinical outcomes after posterior thoracolumbar fusion? A retrospective clinical study.
Noriega DC, Eastlack RK, Hernández GL, Kafchitsas K, Ryang YM, Spitz SM, Lite IS.
Spine J. 2023 Sep 17:S1529-9430(23)03382-X.
doi: 10.1016/j.spinee.2023.09.008. Epub ahead of print. PMID: 37726089.
Highlights
- Achieving proper alignment and tightening of pedicle screw/rod assembly is crucial
- Pedicle screw/rod mismatch is a relevant occurrence after fusion surgery
- Mismatches indicate that the construct was assembled under mechanical stress
- Mismatches can affect clinical outcomes and be associated with higher risk of revision
- All preventable mechanical stresses should be avoided as much as possible
Abstract
Background context
Proper alignment and tightening of the pedicle screw/rod assembly after instrumented posterior fusion of the lower spine is known to be crucial in order to achieve satisfactory clinical results. Such interfacing angle mismatches indicate stress overloading of the implant system.
Purpose
The objective of this study is to investigate the incidence of postoperative screw/rod interfacing angle mismatch and to analyze the impact of mismatches on clinical outcome in terms of (1) revision surgery, (2) adjacent segment degeneration (ASD), and (3) pain.
Study design
This is a monocentric retrospective observational study.
Patient sample
Patients underwent fusion surgery with pedicle screw/rod systems for predominantly degenerative pathologies.
Outcome measures
Pedicle screw/rod interfacing angle mismatch (mismatch is the angular deviation from 90° formed by the rod axis and the pedicle screw head axis as an indicator for missing form-fit) revision rate, ASD at the immediately adjacent cranial segment and VAS pain.
Methods
Revision refers to subsequent procedures in which all or part of the original implant configuration is changed or removed. Radiographic parameters are evaluated using a/p and lateral radiographs at final follow-up. The interfacing angle mismatch between pedicle screw and rod is measured as the angle between two parallel lines on either side of each pedicle screw head and a line laterally along the associated rod. Multiple comparisons are counteracted by Bonferroni correction, adjusted significance level is at *p<.01.
Results
Pedicle screw and rod interfacing angle mismatch was found in 171/406 (42.1%) of patients undergoing fusion surgery, affecting 613/3016 (20.3%) screws. The overall revision incidence was 11.8% (48/406), and a new ASD occurred in 12.1% of all patients (49/406) with an average follow-up of 5 years. Mean VAS pain score at final follow-up was 2.0. Comparison of the two groups with and without mismatches revealed statistically significantly higher (1) numbers of revision procedures performed (26.9% vs. 0.9%), (2) numbers of new ASD developed (27.5% vs. 3.8%), and (3) higher VAS pain scores (2.8/10 vs. 1.4/10) for cases with mismatch. When comparing patients who underwent intraoperative correction and/or reduction with those who did not, statistically significant more screw mismatches (63.4% vs. 39.7%) and revision surgeries (29.3% vs. 9.9%) were noted in patients who had these forceful maneuvers.
Conclusion
Pedicle screw/rod interfacing angle mismatch is a frequent occurrence after fusion surgery. Mismatches indicate that the construct was assembled under mechanical stress. All preventable mechanical stresses, for example, unintentional uncontrolled forces on the instrumentation, should be avoided as much as possible, as they can negatively influence the clinical outcome.