Despite the widely held belief that residual incongruities from intra-articular fractures subject the joint to contact stresses that predispose to post-traumatic osteoarthritis (PTOA), objective evidence has been lacking. This study tested the hypothesis that a metric of elevated contact stress exposure would predict the onset of PTOA. The ankles of 10 tibial plafond fracture patients were treated initially using a spanning fixator, with subsequent screw fixation of the articular surface. Following up on an earlier report of finite element computed post-operative contact stress distributions in these patients' ankles, Kellgren–Lawrence (KL) scores were assessed from minimum 2-year follow-up radiographs to characterize the presence/severity of PTOA. At that time point, seven patients had developed PTOA (KL ≥ 2). Five different metrics of contact stress exposure were calculated, all of which exhibited excellent concordance with KL scores, ranging from 88% to 95%. When time of stress exposure was included, one metric was able to predict PTOA development (KL ≥ 2) with 100% reliability, and all metrics exhibited >94% prediction reliability. These findings, albeit in a small population, support the existence of a contact stress exposure threshold above which incongruously reduced tibial plafond fractures are highly likely to develop PTOA.