What is an ankle accessory ossicle?

Ankle Accessory Ossicles

Ankle accessory ossicles are small, extra bones that develop from unfused secondary ossification centers during skeletal maturation and are typically found adjacent to normal bones of the ankle and foot. These ossicles represent normal anatomical variants that are usually asymptomatic but can become a source of pain and dysfunction when subjected to trauma, stress, or degenerative changes 1.

Characteristics of Ankle Accessory Ossicles

• Formation: Develop from secondary ossification centers that fail to fuse with the main bone during skeletal development 2
• Appearance: Well-corticated bony structures found close to bones or joints 2
• Prevalence: May be unilateral or bilateral, with at least 24 different types identified in the foot and ankle region 1
• Detection: Often discovered incidentally on imaging studies performed for other reasons 2

Common Types of Accessory Ossicles in the Ankle Region

1. Os Tibiale Externum (Accessory Navicular)

   • Located within the tibialis posterior tendon near its insertion on the navicular bone 2
   • Also known as os naviculare secundarium or accessory tarsal scaphoid
   • May cause medial foot pain when symptomatic

2. Os Peroneum

   • Small sesamoid bone within the peroneus longus tendon adjacent to the cuboid 2
   • Can become painful following trauma or tendon pathology

3. Os Trigonum

   • Located posterior to the talus
   • Can cause posterior ankle impingement syndrome, especially in ballet dancers and soccer players 3

4. Os Sustentaculi

   • Rare accessory bone in the ankle and foot region
   • Can become symptomatic and cause pain 4

5. Calcaneus Secundarius

   • Found at the anterior calcaneal facet in up to 5% of the population
   • May limit subtalar joint motion and mimic calcaneonavicular coalition 5

Clinical Significance

Most accessory ossicles remain asymptomatic throughout life. However, they can become clinically significant in several scenarios:

• Trauma: Fractures or dislocations of the ossicle
• Degenerative changes: Osteoarthritis at the synchondrosis (junction between ossicle and main bone)
• Tendon pathology: When the ossicle is embedded within a tendon (e.g., os peroneum in peroneus longus)
• Impingement: Mechanical irritation of adjacent structures
• Misdiagnosis: May be mistaken for fractures, especially following trauma 2

Diagnostic Approach

When accessory ossicles are suspected as a source of ankle pain:

1. Initial imaging: Radiography is the appropriate first-line imaging study for chronic ankle or foot pain 6

   • Standard views include anteroposterior, lateral, and mortise views

2. Advanced imaging (when radiographs are negative or equivocal):

   • MRI without IV contrast: Best for evaluating symptomatic accessory ossicles, allowing visualization of bone marrow edema, synchondrosis, and associated soft tissue pathology 6
   • CT without IV contrast: Useful for detailed osseous characterization, confirming presence of accessory ossicles, and evaluating for fragmentation or fracture 6

3. Other modalities:

   • Bone scan: Can exclude symptomatic accessory ossicles (negative scan is helpful) but positive findings lack specificity 6
   • Ultrasound: Allows dynamic assessment and direct clinical correlation but has limitations in characterizing the synchondrosis 6

Management of Symptomatic Ossicles

Most symptomatic accessory ossicles can be managed conservatively:

• Rest and activity modification
• Analgesics and anti-inflammatory medications
• Appropriate footwear modifications or orthotic devices (e.g., medial arch support for symptomatic os tibiale externum) 2

Surgical intervention may be considered for persistent symptoms despite conservative management, particularly for:

• Painful os peroneum syndrome
• Symptomatic os naviculare
• Os trigonum syndrome causing posterior impingement
• Bulky ossicles limiting joint motion (e.g., calcaneus secundarius) 5

Clinical Pitfalls

• Misdiagnosis as fracture: Accessory ossicles may be mistaken for acute fractures, leading to unnecessary immobilization 2
• Overlooking as cause of pain: Failure to recognize symptomatic ossicles can lead to missed diagnoses and ineffective treatments 1
• Incidental findings: Most accessory ossicles are asymptomatic and incidental findings; correlation with clinical symptoms is essential before attributing pain to their presence 2

Awareness of these normal anatomical variants and their potential to become symptomatic is crucial for accurate diagnosis and appropriate management of ankle and foot pain.