PurposeCurrently used classification systems and measurement methods are insufficient to assess fracture displacement. In this study, a novel 3D measure for fracture displacement is introduced and associated with risk on conversion to total knee arthroplasty (TKA).MethodsA multicenter cross-sectional study was performed including 997 patients treated for a tibial plateau fracture between 2003 and 2018. All patients were contacted for follow-up and 534 (54%) responded. For all patients, the 3D gap area was determined in order to quantify the degree of initial fracture displacement. A cut-off value was determined using ROC curves. Multivariate analysis was performed to assess the association of 3D gap area with conversion to TKA. Subgroups with increasing levels of 3D gap area were identified, and Kaplan–Meier survival curves were plotted to assess survivorship of the knee free from conversion to TKA.ResultsA total of 58 (11%) patients underwent conversation to TKA. An initial 3D gap area ≥ 550 mm2was independently associated with conversion to TKA (HR 8.4;p= 0.001). Four prognostic groups with different ranges of the 3D gap area were identified: excellent (0–150 mm2), good (151–550 mm2), moderate (551–1000 mm2), and poor (> 1000 mm2). Native knee survival at 10-years follow-up was 96%, 95%, 76%, and 59%, respectively, in the excellent, good, moderate, and poor group.ConclusionA novel 3D measurement method was developed to quantify initial fracture displacement of tibial plateau fractures. 3D fracture assessment adds to current classification methods, identifies patients at risk for conversion to TKA at follow-up, and could be used for patient counselling about prognosis.Level of evidencePrognostic Level III.