Safe Zone for Placement of Talar Screws When Fusing the Ankle With an Anterior Plating System

Background: Ankle fusions fixed with anterior plates use fluoroscopic guidance to direct screws toward the subtalar joint. Special imaging views that visualize the subtalar joint are difficult to use and can be unreliable. This study evaluated whether a single lateral ankle view would provide adequate information to judge whether a screw penetrated the subtalar joint and identified strategies that would improve this technique. Methods: In 5 cadaveric ankles fixed with anterior plates, talar screws were placed up to the subtalar joint without penetration using lateral fluoroscopy to guide screw length. After dissection, the true distance from the screw tip to subchondral surface was measured. In addition, 4 readers measured the perceived distance from screw tip to subchondral surface using direct lateral, 10 degrees cephalad tilt lateral, and 10 degrees caudal tilt lateral fluoroscopic images on 2 separate occasions. Results: Nineteen (63%) of 30 screws penetrated the subchondral bone, and screw length determined using fluoroscopy was significantly longer than screw length measured directly (29.4 ± 5.5 mm vs 27.3 ± 8.5 mm, P = .014). Measurement of screw tip to bone distance demonstrated a high level of within-reader (kappa = .871, P < .001) and between-reader agreement (kappa = .807, P < .001), but poor specificity of determining screw penetration (0.50, χ2 = 22.1, P < .001) and poor correlation between radiographically measured and actual distances between screw tip and bone margin ( r = .35, P < .001). Tilting the c-arm 10 degrees cephalad and directing screws toward the posterior facet improved the ability to detect screw penetration and directing screws toward the middle facet diminished it ( P < .05). A safe zone of screw placement was defined by region. Conclusion: Use of a lateral fluoroscopic image to guide talar screw placement may lead to an unacceptably high rate of subtalar joint penetration. Clinical Relevance: Understanding the limitations of lateral fluoroscopy when using anterior ankle fusion plates may minimize screw penetration into the subtalar joint and diminish development of subtalar arthropathy.