Screw density in the talar transverse plane is associated with early bone union following arthroscopic ankle arthrodesis: a retrospective comparison of three- versus four-screw fixation

Although arthroscopic ankle arthrodesis (AAA) is a well-established procedure with lower complication rates than open arthrodesis, the optimal screw configuration for stable fixation and reliable bone union remains unclear. We compared outcomes between three- and four-screw fixation in AAA and evaluated whether screw density at the fusion interface, quantified using the screw-to-talus (S/T) area ratio, could predict bone union. A retrospective review was performed on patients who underwent AAA between 2018 and 2023 using a standardized fixation construct comprising a home run screw and medial parallel screws. After excluding two ankles, 40 ankles in 38 patients were categorized into three-screw (14 ankles) and four-screw (26 ankles) groups. Primary outcomes included time to bone union and delayed union or nonunion incidence. The S/T area ratio was measured using postoperative computed tomography and the sagittal inter-screw distance/tibial width (ISD/TW) ratio was evaluated from radiographs. Receiver operating characteristic (ROC) analysis was performed to evaluate predictors of bone union. The delayed union or nonunion rate was significantly lower in the four-screw group than in the three-screw group (7.7 vs. 42.9%, P = 0.014). Among ankles that ultimately achieved bone union, time to union was significantly shorter in the four-screw group (7.3 ± 1.6 vs. 12.4 ± 4.5 weeks, P < 0.001). The union group demonstrated significantly higher S/T area ratios ( P = 0.003) and sagittal ISD/TW ratios ( P = 0.034). ROC analysis showed that the S/T area ratio had the highest predictive accuracy for bone union (AUC = 0.846), with an exploratory cutoff value of approximately 10%. Four-screw fixation was associated with fewer delayed unions or nonunions than three-screw fixation. Screw density across the fusion interface, quantified by the S/T area ratio, may serve as a more informative indicator of fixation quality than screw number alone. Retrospective comparative study.