Os Trigonum Syndrome (OTS) is a cause of posterior ankle pain that results from impingement between the talus and the calcaneus due to the presence of an accessory bone called the os trigonum. This small, extra bone develops when a secondary ossification center at the back of the talus fails to fuse during childhood. While many people with an os trigonum remain asymptomatic, repetitive plantar flexion (downward pointing of the toes) or trauma can cause pain, swelling, and stiffness at the back of the ankle, especially in athletes and dancers.
How Common It Is and Who Gets It? (Epidemiology)
The os trigonum is found in approximately 7–13% of the general population, though only a small fraction develop symptoms. OTS occurs most commonly in individuals who repeatedly perform plantar flexion movements such as ballet dancers, soccer players, swimmers, and runners. Both men and women are affected, but it is slightly more prevalent in athletes between ages 15 and 30, when the accessory bone becomes ossified and exposed to mechanical stress.
Why It Happens – Causes (Etiology and Pathophysiology)
During development, the posterior talus forms from two ossification centers that usually fuse between ages 7 and 13. Failure of fusion leads to the formation of the os trigonum, a small accessory bone attached to the talus by fibrous tissue.
Pain and inflammation arise when the os trigonum impinges between the tibia and calcaneus during forceful plantar flexion, known as a “nutcracker” effect.
Common causes include:
- Repetitive trauma from sports or ballet movements (en pointe or relevé).
- Acute injury, such as an ankle sprain, that stretches or tears the synchondrosis between the talus and os trigonum.
- Associated tendon pathology, particularly flexor hallucis longus (FHL) tendinitis, as the tendon passes close to the os trigonum and may become inflamed from friction.
- Posterior ankle impingement syndrome (PAIS), where soft tissues and accessory bones limit ankle motion and cause posterior pain.
How the Body Part Normally Works? (Relevant Anatomy)
The talus forms the lower part of the ankle joint, articulating with the tibia, fibula, and calcaneus to allow dorsiflexion and plantar flexion. The os trigonum lies posterior to the talus within the tendon sheath of the flexor hallucis longus. When the ankle plantar flexes, the os trigonum can be compressed between the talus and calcaneus, particularly during activities that involve repetitive pointing of the foot.
What You Might Feel – Symptoms (Clinical Presentation)
- Deep, aching pain in the back of the ankle, worse with plantar flexion (pointing the toes) or push-off movements.
- Swelling or tenderness over the posterior ankle, especially along the lateral side.
- Stiffness or limited range of motion in plantar flexion.
- Pain during activities like ballet, kicking a soccer ball, or sprinting.
- Occasionally, numbness or tingling if nearby nerves are irritated.
Symptoms may develop gradually from repetitive motion or acutely following injury.
How Doctors Find the Problem? (Diagnosis and Imaging)
Diagnosis relies on history, examination, and imaging studies.
- Physical examination:
- Pain reproduced by forced plantar flexion (the posterior impingement test).
- Tenderness to palpation over the back of the ankle between the Achilles tendon and peroneal tendons.
- Assessment for associated FHL tendon pain with resisted big toe flexion.
- X-rays: Reveal the os trigonum as a small round or triangular bone behind the talus.
- MRI: Confirms inflammation, bone marrow edema, or soft tissue involvement (tendinitis, synovitis, or joint effusion).
- CT scans: Useful for detailed bony anatomy or differentiating a large Stieda process (elongated talar process) from a true os trigonum.
Classification
OTS is classified as a subtype of posterior ankle impingement syndrome (PAIS), with severity ranging from mild inflammation to chronic bone and tendon impingement.
- Type I: Inflammation without impingement.
- Type II: Mechanical impingement with restricted motion.
- Type III: Associated with additional pathologies, such as FHL tendinopathy or posterior capsule fibrosis.
Other Problems That Can Feel Similar (Differential Diagnosis)
- Achilles tendinitis or retrocalcaneal bursitis.
- Posterior talar process fracture.
- FHL tendinopathy.
- Posterior tibial tendonitis.
- Tarsal tunnel syndrome.
- Ankle sprain or stress fracture.
Treatment Options
Non-Surgical (Conservative) Management
Most patients improve with conservative treatment aimed at reducing inflammation and mechanical irritation:
- Rest and activity modification: Avoid sports or movements requiring plantar flexion (e.g., ballet en pointe).
- RICE protocol: Rest, Ice, Compression, and Elevation to reduce pain and swelling.
- NSAIDs: Anti-inflammatory medications such as ibuprofen for pain relief.
- Immobilization: A walking boot or brace may be used for 2–4 weeks to offload the joint.
- Physical therapy:
- Stretching of calf and Achilles to reduce posterior ankle pressure.
- Strengthening exercises for the ankle stabilizers.
- Ultrasound and soft-tissue mobilization to relieve tendon irritation.
- Corticosteroid injections: Provide short-term relief for persistent inflammation around the os trigonum or FHL tendon.
Surgical Care
Surgery is indicated for patients who fail to improve after 3–6 months of conservative therapy or athletes requiring early return to activity. The goal is to remove the os trigonum and decompress the posterior ankle.
Common techniques:
- Endoscopic Excision (Posterior Hindfoot Endoscopy):
- Minimally invasive; two small incisions behind the ankle.
- Allows removal of the os trigonum and FHL tendon release if needed.
- Advantages: faster recovery, minimal scarring, lower risk of complications.
- Arthroscopic Excision:
- Camera-guided removal through small portals; allows treatment of coexisting joint pathology.
- Open Excision:
- Used for large os trigonum or when multiple structures are involved.
- Provides full exposure but has a longer recovery period.
Recovery and What to Expect After Treatment
- After surgery, a bulky dressing or walking boot is applied for 1–2 weeks.
- Partial weight-bearing with crutches is recommended initially.
- Physical therapy begins after 2–3 weeks to restore mobility and strength.
- Return to sport or full activity typically occurs within 3–6 months depending on the procedure.
Athletes often experience complete relief of symptoms and return to previous performance levels.
Possible Risks or Side Effects (Complications)
- Persistent pain or stiffness.
- Scar tenderness or nerve irritation (especially sural nerve).
- Infection (rare).
- FHL tendon scarring or re-impingement.
- Prolonged recovery in cases of chronic inflammation.
Long-Term Outlook (Prognosis)
The prognosis is excellent for most patients, especially with early diagnosis and appropriate management. Conservative treatment succeeds in most mild cases, and surgical excision provides durable relief for athletes and dancers. Studies show >90% of patients return to full activity with minimal recurrence.
Out-of-Pocket Costs
Medicare
CPT Code 29891 – Arthroscopic/Endoscopic Excision of Os Trigonum: $158.80
CPT Code 27635 – Open Excision of Os Trigonum: $136.51
Medicare Part B typically covers 80% of the approved cost for these procedures once your annual deductible has been met, leaving you responsible for the remaining 20%. Supplemental Insurance plans such as Medigap, AARP, or Blue Cross Blue Shield usually cover the remaining 20%, minimizing or eliminating out-of-pocket expenses for Medicare-approved surgeries. These plans work together with Medicare to fill the coverage gap and reduce patient responsibility.
If you have Secondary Insurance such as TRICARE, an Employer-Based Plan, or Veterans Health Administration coverage, it will act as a secondary payer. These plans generally cover any remaining coinsurance or deductibles, which typically range from $100 to $300, depending on your plan and provider network.
Workers’ Compensation
If your Os Trigonum Syndrome developed or was aggravated by a work-related activity, Workers’ Compensation will cover all medical expenses, including the procedure, postoperative care, and rehabilitation. You will not have any out-of-pocket costs, as the employer’s insurance carrier pays directly for all approved treatments.
No-Fault Insurance
If your Os Trigonum Syndrome is related to an automobile accident, No-Fault Insurance will typically cover the total cost of treatment, including arthroscopic or open excision and postoperative care. The only possible out-of-pocket cost may be a small deductible or co-payment depending on your policy.
Example
Rachel Kim suffered from chronic posterior ankle pain due to Os Trigonum Syndrome and underwent arthroscopic excision (CPT 29891). Her estimated Medicare out-of-pocket cost was $158.80. Since Rachel had supplemental insurance through AARP Medigap, her remaining balance was fully covered, leaving her with no out-of-pocket expenses for the procedure.
Frequently Asked Questions (FAQ)
Q. What is Os Trigonum Syndrome?
A. Os Trigonum Syndrome is a condition where an extra bone at the back of the ankle becomes irritated, often due to repetitive stress or trauma.
Q. What causes Os Trigonum Syndrome?
A. It is commonly caused by repeated downward pointing of the toes, especially in ballet dancers, soccer players, and other athletes.
Q. Who is most likely to develop Os Trigonum Syndrome?
A. Athletes involved in activities requiring frequent plantarflexion, such as ballet dancers and soccer players, are more likely to develop the condition.
Q. What are the symptoms of Os Trigonum Syndrome?
A. Symptoms include pain at the back of the ankle, swelling, tenderness, and difficulty pointing the toes.
Q. How is Os Trigonum Syndrome diagnosed?
A. Diagnosis is made through clinical examination and confirmed with imaging studies like X-rays, MRI, or CT scans.
Q. What imaging technique is most useful for diagnosing soft tissue involvement in Os Trigonum Syndrome?
A. MRI is most useful for assessing soft tissue inflammation around the os trigonum.
Q. What non-surgical treatments are available for Os Trigonum Syndrome?
A. Non-surgical treatments include rest, ice, anti-inflammatory medications, physical therapy, and immobilization with a walking boot or brace.
Q. When is surgery considered for Os Trigonum Syndrome?
A. Surgery is considered when conservative treatments fail and symptoms persist, typically involving removal of the os trigonum.
Q. What surgical options are available for Os Trigonum Syndrome?
A. The os trigonum can be removed via open surgery or arthroscopically, depending on the surgeon’s preference and the specific case.
Q. What is the recovery time after surgery for Os Trigonum Syndrome?
A. Recovery typically involves a few weeks of immobilization followed by physical therapy, with most patients returning to activity within a few months.
Q. Can Os Trigonum Syndrome be prevented?
A. Prevention involves avoiding repetitive forceful plantarflexion and addressing any biomechanical issues that contribute to the condition.
Summary and Takeaway
Os Trigonum Syndrome is a common cause of posterior ankle pain due to impingement from an accessory bone behind the talus. It often affects athletes and dancers who perform repetitive plantar flexion movements. Diagnosis is made clinically and confirmed with imaging. Most cases respond to conservative measures, but surgery is highly effective when needed. With proper treatment, patients regain full mobility and can return to their usual activities without limitation.
Clinical Insight & Recent Findings
A case series from Guy’s & St Thomas’ Hospital, London found that some patients thought to have Os Trigonum Syndrome actually had small unhealed fractures of the inner ankle bone (posteromedial talus).
These fractures can mimic os trigonum symptoms and may occur alongside a true os trigonum. Because these fragments lie close to major nerves and vessels, the authors recommend open surgical removal rather than endoscopic techniques. Patients treated with open excision had full symptom relief and returned to activity.
The study underscores the importance of advanced imaging (MRI or CT) to distinguish between these conditions for proper treatment. (“A study of ankle pain labeled as Os Trigonum Syndrome found it may stem from a small hidden fracture. Imaging and open surgery can effectively relieve it — see PubMed.“)
Who Performs This Treatment? (Specialists and Team Involved)
Treatment is performed by orthopedic foot and ankle surgeons or sports medicine specialists with expertise in ankle arthroscopy. Physical therapists aid in rehabilitation, and radiologists assist with diagnostic imaging.
When to See a Specialist?
If you experience persistent posterior ankle pain that worsens with pointing your toes, or if conservative measures fail after several weeks, consult a foot and ankle specialist.
When to Go to the Emergency Room?
Seek immediate care only if severe trauma, sudden swelling, or inability to bear weight occurs following an injury.
What Recovery Really Looks Like?
Early rest and immobilization typically resolve mild cases. After surgery, patients transition gradually from a boot to full weight-bearing, followed by physical therapy. Most return to normal activity pain-free within 3–6 months.
What Happens If You Ignore It?
Chronic untreated OTS can lead to persistent pain, stiffness, and FHL tendon irritation, potentially limiting athletic performance.
How to Prevent It?
- Avoid repetitive forced plantar flexion without proper conditioning.
- Strengthen and stretch calf and ankle muscles regularly.
- Use supportive footwear and orthotics for high-impact activities.
Nutrition and Bone or Joint Health
A diet rich in calcium, vitamin D, and anti-inflammatory foods supports tendon and bone healing. Staying hydrated and maintaining a healthy body weight reduce stress on the ankle joint.
Activity and Lifestyle Modifications
Gradually return to sport, avoid overtraining, and incorporate ankle flexibility and strengthening exercises into daily routines. Consistent conditioning and awareness of early symptoms help prevent recurrence.
Dr. Mo Athar
