The powerful back-rower had only just returned from a long-term hamstring injury that kept him on the sidelines for six months.
But Elsom suffered the ankle damage in his comeback for the Brumbies against Australian rivals Western Force in Perth on Saturday night.
The Brumbies have not yet confirmed whether Elsom will miss the remainder of the season. But the struggling two-time Super Rugby Champions are out of the race to reach the knock-out stages.
A syndesmosis injury usually requires four to six weeks to heal and the Brumbies only have four matches of the current campaign left.
The latest injury is a bitter end to a frustrating season for Elsom but he will hope to be fully fit for the Tri-Nations and World Cup later this year.
Elsom will also be hoping to be injury-free come 2013 when the Lions venture 'down under' to face to Wallabies.