QUIZ: No. 36, 23/11/2025

Xray

CT

MRI

ANS – Stieda process / Os trigonum pseudoarthrosis with posterior ankle impingement syndrome

Findings: Well-corticated ossicle posterior to the talus , consistent with an os trigonum and an elongated lateral tubercule of talus consistent with the Stieda process. CT shows a narrow pseudoarthrosis at the synchondrosis, associated with subtle subchondral sclerosis and irregularity, indicating chronic stress. MRI demonstrates marrow edema in the os trigonum, a fluid-filled pseudoarthrosis cleft and posterior capsular thickening with synovitis—all features supportive of posterior ankle impingement syndrome.

The Stieda process represents an elongated lateral tubercle of the posterior talar process, which can itself cause posterior ankle impingement when prominent.

The os trigonum is an accessory ossicle resulting from failure of fusion of this lateral tubercle; both the Stieda process and os trigonum can coexist, as in this case.

Posterior capsular thickening, synovitis, and narrowing of the posterior tibiotalar/subtalar recesses contribute to the symptomatic posterior ankle impingement syndrome (PAIS).

Imaging correlation with patient symptoms helps differentiate symptomatic impingement from an incidental os trigonum or Stieda process.

Take home points

Os trigonum or Stieda process is the most common structural cause of posterior ankle impingement.

CT is best for assessing cortication, pseudoarthrosis, sclerosis, and chronic bony changes at the synchondrosis.