The Filament-Union Sign: Prevalence and Clinical Outcomes of Incomplete Bone Remodeling Following Percutaneous Hallux Valgus Surgery

Research Type: Level 3 - Retrospective cohort study, Case-control study, Meta-analysis of Level 3 studies Introduction/Purpose: Minimally invasive or percutaneous hallux valgus surgery can occasionally result in a distinct radiological finding termed the "Filament-Union sign," characterized by a thin, filamentous bone bridge at the osteotomy site < 25% of the metatarsal width and associated with minimal medial/lateral/central remodelling. This study aimed to determine its prevalence and identify potential contributing factors. Methods: A retrospective radiographic cohort study analyzed 727 feet that underwent percutaneous transverse osteotomy for hallux valgus correction between November 2017 and January 2023. The primary outcome was the presence of the filament-union sign. Secondary outcomes included clinical patient reported outcome measure and radiographic deformity analysis. Results: The filament-union sign was identified in 24 feet (3.3%, 95% CI: 2.0-4.6%). Mean radiographic follow-up was significantly longer in the non-filament-union group (2.6±1.3 vs 1.8±0.6 years, p< 0.001), with no significant difference in mean PROM follow-up between groups (1.7±0.8 vs 1.9±0.9 years, p=0.145). While both groups showed similar baseline characteristics, the filament-union group demonstrated slightly worse functional outcomes at final follow-up, with significantly higher MOXFQ Index scores (20.6±16.9 vs 13.0±15.7, p=0.040). Preoperative hallux valgus angle was significantly associated with filament-union (OR 1.08, 95% CI 1.02-1.15, p=0.006). All cases in the filament-union group achieved bony union, with one case of non-union observed in the non-filament group. There was no remodelling on long term radiographs. No instances of fracture or metalwork failure were observed in the filament-union group. Conclusion: The filament-union sign is an uncommon radiographic finding following percutaneous hallux valgus surgery, occurring in 3% of cases. While its presence was associated with slightly worse functional outcomes, it did not lead to serious complications or require revision surgery. Further research is needed to fully understand the mechanical and biological factors contributing to this pattern of bone healing.