Introduction <p>We compared long term clinical and radiological outcomes of patients with trimalleolar ankle fractures including a posterior malleolar fragment (PMF) between those who underwent PMF fixation and those who did not. We also compared complication rates and identified independent risk factors associated with worse outcome.</p> Material and Methods <p>We included 69 consecutive patients undergoing operation for a trimalleolar fracture between 2008 and 2013. Mean follow-up was 11.3 years. Patients completed the SF-12 and EFAS scores. Radiological osteoarthritis (OA) was assessed using the Kellgren-Lawrence classification. Postoperative complications were classified according to Sink. PMF size was measured on preoperative x-ray and CT and classified according to the Haraguchi classification.</p> Results <p>The non-fixated group (<i>n</i> = 48) had a mean PCS score of 47.9, a mean MCS score of 54.1, a mean EFAS score of 17.1, and a mean EFAS-Sport score of 4.35. The fixated group (<i>n</i> = 21) had a mean PCS score of 49.2, a mean MCS score of 56.5, a mean EFAS score of 17.5, and a mean EFAS-Sport score of 6.05. There was no statistical difference between the two groups in the long term clinical PROMS. Patients in the fixated group developed more advanced OA (<i>p</i> = 0.013).</p> Conclusion <p>Patients who underwent PMF fixation had a larger PMF size, more complications and more advanced OA, but with similar long term clinical outcomes than the non-fixated group.</p> Level of Clinical Evidence <p>III, Retrospective comparative study.</p>