Persistent postoperative step-off of the posterior malleolus leads to higher incidence of post-traumatic osteoarthritis in trimalleolar fractures.

Background: Traditionally, size of the posterior fragment is considered the most important indicator for fixation in trimalleolar fractures. It remains unclear which factors contribute to worse functional and radiological outcome. This study was designed to determine predictors for the development of posttraumatic osteoarthritis and worse functional outcome in trimalleolar fractures.

Methods: This retrospective cohort study evaluated outcomes of 169 patients with a trimalleolar fracture treated between 1996 and 2013 in a level-1 trauma hospital in the Netherlands after a mean follow-up of 6.3 (range 2.4 to 15.9) years. The average fragment size was 17%. Twenty patients had a posterior fragment smaller than 5% of the intra-articular surface, 119 patients a fragment of 5-25% and 30 patients a posterior fragment larger than 25%. In total, 39 patients (23%) underwent fixation of the posterior fragment.

Results: Clinical union was achieved in all 169 patients. The median AOFAS score after follow-up was 93 (interquartile range 76-100) and the median AAOS score was 92 (interquartile range 81-98). A persistent postoperative step-off larger than 1 mm was found in 65 patients (39%) and osteoarthritis was present in 49 patients (30%). Higher age and postoperative step-off > 1 mm were independent, significant risk factors for the development of osteoarthritis. Osteoarthritis and BMI were independent, significant risk factors for worse functional outcome.

Conclusion: It is advisable to correct intra-articular step-off of intraarticular posterior malleolar fragments to reduce the risk of developing osteoarthritis and, consequently, the risk of worse functional outcome after long-term follow-up.

Level Of Evidence: Level IIB.