A 20-year-old male patient presented to our emergency clinic with an ankle sprain after falling down the stairs. In the examination performed, the ankle posterior was swollen, oedematous and painful. Also achilles tendon was palpated intact. The pain was more intense when plantarflexing the ankle.
Radiographs of the ankle were performed, which demonstrated the existence of an os trigonum, with the suspicion of a lucent fracture (arrow) line through its base (Fig 1). CT was performed and this demonstrated the fracture line in the os trigonum of the left ankle in axial, sagittal, and coronal imaging (Figure 2, 3, 4). There is heterogeneity in the surrounding soft tissues (blue arrow) adjacent to the os trigonum and fluid values at the joint distance (red arrow head) (Fig 5).
Os trigonum is an accessory bone observed in 7-13% of society and in some cases can be fragmented or bipartite [1, 2]. It appears between the ages of 8 and 11 years as a secondary centre of ossification and usually fuses with the talus within one year of its appearence [3, 4]. When the ossification centre remains separate from the talus it is referred to as the os trigonum, but it is still connected to the lateral tubercle of the posterior process of the talus by a fibrocartilaginous synchondrosis [3, 5]. When fusion does occur and a large posterolateral process forms, it is referred to as a ‘fused os trigonum’, a ‘Stieda’s process’, or a ‘trigonal process’. [5, 6] This bone is often asymptomatic but can become symptomatic during, or after, strenuous physical activities or an acute injury to the ankle. We report an unusual case in which a fracture of the os trigonum was observed. In the literature, the fracture of this accessory bone is extremely rare [7]. Hyperplantarflexion of the ankle is the mechanism of injury, compressing the os trigonum between the posterior malleolus of the tibia and the tuber calcaneus [8]. Clinical symptoms include pain during forced plantarflexion of the ankle [8]. Os trigonom fractures can be skipped due to radiologically difficult detection. It may be radiographically confused with fractures of the posterior process of the talus [4, 6]. Axial computed tomographic (CT) scanning is helpful for differentiating between true talar fractures and an os trigonum. CT scanning is the imaging choice in cases where posterior talus structure fractures, including an os trigonum fracture, are suspected. Fracture of the os trigonum is a rare entity, thus a high index of suspicion is necessary to diagnose an os trigonum fracture after severe plantarflexion to the ankle. A missed diagnosis can make the pathologies more complex and successful treatment more difficult. Conservative treatment is usually successful although nonunion has been reported [3]. İn our case, direct lateral radiograph showed the os trigonum with the suspicion of a lucent fracture (Fig 11). CT axial, sagittal and coronal examinations of the ankle showed a fracture of the os trigonum (Fig 2, 3, 4). There is heterogeneity in the surrounding soft tissues (blue arrow) adjacent to the os trigonum and fluid values at the joint distance (red arrow) (Fig 5). It was distinguishable from an os trigonum bipartite by having remarkably sharp edges and discontinuity of the cortical lining, thus a fracture of the os trigonum was diagnosed.
Os trigonum fracture
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Based on the provided X-ray and CT images, there is a roughly round or slightly irregular bony structure (“accessory bone”) behind the talus, adjacent to the posterior talar process. This structure has the following characteristics:
• The small bony fragment has sharply defined edges, and there is an interruption of the cortical continuity, suggesting the presence of a fracture line.
• Mild swelling and fluid-density shadows are noted in the surrounding soft tissue (indicated by blue arrows in the images), suggesting local inflammation or edema.
• A small amount of effusion is also visible near the joint space (indicated by red arrows in the images), consistent with post-traumatic joint effusion.
In the routine lateral X-ray of the ankle, one can see this accessory bone shadow; the small fragment appears separated from the adjacent portion of the talus, raising suspicion of a fracture line. Further CT scans in the axial, sagittal, and coronal planes clearly reveal this independent bony fragment with sharp edges and discontinuity from the surrounding bone, which is consistent with a fracture.
Based on the imaging findings and the patient’s history of trauma, the main considerations are:
1) Fracture of the posterior talar process (Stieda’s process) or fused accessory talus:
- Severe plantar flexion or acute injury can cause a fracture in the previously present accessory bone (os trigonum) located behind the talus.
2) Primary fracture of the posterior talar process:
- In the absence of an independent accessory bone, the fracture might involve the posterior talar process itself.
3) Congenital bipartite os trigonum:
- Relatively rare. A bipartite os trigonum typically has smooth edges without a distinct fracture line, and clinical pain may not present acutely.
Given the clear fracture line, surrounding soft tissue edema, and the patient’s significant pain during plantar flexion after trauma, the most likely diagnosis is a fracture rather than a simple congenital variant.
Taking into account the patient’s history of falling, severe pain on ankle plantar flexion, imaging findings (sharp fracture fragment, soft tissue swelling, joint effusion), and physical examination (marked tenderness in the posterior ankle region), the most probable diagnosis is:
“Fracture of the Accessory Talus (Os Trigonum)”.
If any doubt remains, MRI can be used to further evaluate the soft tissue and bone marrow edema. However, the current imaging findings are sufficient to support the diagnosis of an os trigonum fracture.
(1) Overview of Treatment Strategies
• Conservative Treatment: For stable os trigonum fractures with minimal displacement, consider using an ankle brace or cast immobilization, combined with NSAIDs to relieve pain, and local cold compresses to reduce soft tissue swelling.
• Surgical Treatment: If the fracture fragment is significantly displaced, if joint function is impaired, or if conservative treatment fails, surgical removal of the fragment or fracture fixation may be considered.
(2) Rehabilitation Exercises and Exercise Prescription (FITT-VP Principle)
1) Early Stage (Acute Phase and Immobilization):
- F (Frequency): Perform 2-3 sessions per day of gentle exercises around the ankle joint or isometric muscle contractions.
- I (Intensity): Maintain a small range of motion that does not provoke severe pain.
- T (Time): Each session lasts about 5-10 minutes, divided into segments to avoid fatigue.
- T (Type): Gentle ankle movements in non-weight-bearing or partial weight-bearing positions (such as toe flexion-extension, ankle pump exercises) with protective measures.
- V (Volume): Gradually increase total exercise time and number of repetitions over time.
- P (Progression): As pain and swelling subside, gradually increase ankle range of motion and repetition count.
2) Intermediate Stage (Functional Recovery):
- F (Frequency): 3-4 targeted training sessions per week.
- I (Intensity): Progressively introduce light resistance (e.g., elastic bands) or partial weight-bearing exercises in standing posture, without causing further injury.
- T (Time): 15-20 minutes per session, mainly focusing on restoring joint function and muscle strength.
- T (Type): Gradually include proprioception exercises such as single-leg stance balance training and gentle ankle inversion-eversion drills to enhance ankle stability.
- V (Volume): Increase gradually based on the degree of inflammation and pain.
- P (Progression): If there is no marked increase in pain after activities, further increase exercise intensity and complexity.
3) Late Stage (Return-to-Sport):
- F (Frequency): 3-5 sessions per week, depending on the patient’s desired activity level.
- I (Intensity): Incorporate running, jumping, and other dynamic exercises to improve ankle strength and agility.
- T (Time): 20-30 minutes or more per session, combined with comprehensive functional training.
- T (Type): In addition to routine strength and balance exercises, add quick directional changes, deceleration drills, and agility training.
- V (Volume): Adjust training volume according to individual recovery status and monitor for recurrent pain or swelling.
- P (Progression): Based on performance and pain levels, evaluate and progressively increase training load.
(3) Safety Precautions
• At any stage, if severe pain, increased swelling, or a sudden decrease in range of motion occurs, re-evaluation is required and adjustments to the rehabilitation plan may be necessary.
• For patients with poor bone quality or other comorbidities, weight-bearing and exercise intensity should be reduced accordingly, and protective measures may need to be extended.
• Even after pain subsides, continuous ankle strengthening and balance exercises are important to prevent re-injury.
Disclaimer: This report is based on the currently provided medical history and imaging information and is for clinical reference only. Specific diagnosis and treatment should be guided by in-person evaluation and professional medical advice.
Os trigonum fracture
