The medial clearspace is a risk factor for secondary dislocation following cast immobilization after closed reduction in closed ankle fracture dislocations.

Purpose Ankle fractures represent about 10% of all adult fractures, with increasing incidence. Dislocated ankle fractures often require delayed open reduction and internal fixation due to swelling, necessitating temporary stabilization using a cast or an external fixator. This study aims to assess risk factors for insufficient preliminary reduction immobilized by a cast, focusing on medial clearspace and posterior malleolus fragment size, to identify fractures that would benefit from initial stabilization with an external fixator. Methods Patients treated for dislocated ankle fractures at our level-1 trauma center from 2011 to 2023 were retrospectively reviewed. The primary outcome was the rate of insufficient reduction during immobilization in a cast. Secondary outcomes included time to definitive surgery, length of surgery and hospital stay. Results 134 patients met the inclusion criteria. The most common fracture type was AO 44B3, with 71.6%. Sufficient reduction was achieved in 53.7% of patients. Multiple regression analyses revealed the initial medial clearspace at the time of dislocation as an independent risk factor for insufficient reduction after reduction. ROC-analysis revealed that a initial medial clearspace at the time of dislocation of 9 mm is a predictor for insufficient reduction with a sensitivity of 88% and a specificity of 55%. Conclusion Initial medial clearspace was an important predictor for insufficient reduction in a cast, with 9 mm being identified as the cutoff for critical initial medial clearspace. Therefore, we recommend primary external fixation or acute internal fixation, if the soft tissue allows it for those patients with initial medial clearspace of > 9 mm. This approach may prevent secondary dislocation, reduce swelling, and expedite definitive surgery.