Os Trigonum Syndrome

Os trigonum is an accessory ossicle resulting from failure of fusion of the lateral tubercle of the posterior talar process. It is the most common accessory ossicle of the foot, present in roughly 2 to 14% of the population (around 9.8% in large radiographic series) and bilateral in about 50% of those with the ossicle. Most are asymptomatic; symptomatic disease clusters in athletes who load the ankle in plantarflexion.

Anatomy

The posterior process of the talus has two tubercles: medial and lateral. The lateral tubercle may develop from a secondary ossification center that appears at 8-13 years and usually fuses within 1 year. When it fails to fuse, the os trigonum remains as a separate bone connected to the talus by a synchondrosis. A Stieda process is the same lateral tubercle when it is elongated but fused to the talus.

The flexor hallucis longus (FHL) tendon runs in a groove between the medial and lateral tubercles. This close relationship means FHL tenosynovitis commonly accompanies os trigonum syndrome.

Forced or repetitive plantarflexion (as in ballet dancing en pointe or downward kicking in soccer) compresses the os trigonum in a "nutcracker" fashion between the posterior tibia and calcaneus. This microtrauma leads to bone marrow edema, synchondrosis injury, and inflammation. Chronic impingement can enlarge the ossicle and narrow the FHL fibro-osseous tunnel, producing secondary FHL stenosing tenosynovitis and triggering of the great toe. Acute hyperplantarflexion can instead fracture the synchondrosis or the posterolateral talar process.

History

Posterior ankle pain in a dancer or athlete requiring plantarflexion. Pain worse with en pointe position or downward kicking. May describe deep posterior pain. May have triggering or clicking of the great toe if FHL involved.

Examination

Posterior Impingement Test: Passive forced plantarflexion reproduces posterior ankle pain. Highly suggestive.

Palpation: Deep posterior tenderness between Achilles and peroneal tendons.

FHL Assessment: Passive flexion/extension of hallux with ankle in different positions may reveal tenosynovitis or triggering.

Differential Diagnosis of Posterior Ankle Pain

The os trigonum is found in roughly 2 to 14% of feet (most asymptomatic), so its presence on radiograph never proves it is the pain source. Work systematically through the differential before attributing symptoms to the ossicle.

Algorithm

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Imaging

Lateral Radiograph: Shows os trigonum as a separate ossicle posterior to the talus. May see enlarged Stieda process if the tubercle is fused but prominent.

MRI: Shows bone marrow edema in os trigonum and adjacent talus. Shows FHL tenosynovitis. Soft tissue inflammation.

CT: Useful for showing degenerative changes at the synchondrosis and surgical planning.

Nuclear Medicine (SPECT-CT): Can confirm symptomatic os trigonum with increased uptake localizing to the ossicle.

Diagnostic Injection: Local anesthetic to the posterior ankle. Greater than 70% pain relief confirms diagnosis.

The topic leans on the posterior-ankle injection as both the confirmatory test (greater than 70% relief confirms the source) and a therapeutic option, and the controversies flag an FHL-rupture caveat, but never describes how the injection is actually done or interpreted.

• How it is done. An image-guided (ultrasound or fluoroscopic) injection of a small volume of local anaesthetic (with or without corticosteroid) into the posterior ankle around the os trigonum / posterior recess. Image guidance matters because the target is deep and the posterior tibial neurovascular bundle and the FHL are close.
• How it is interpreted (the diagnostic half). Because an os trigonum is a common incidental finding, the injection is used to prove it is the pain generator: repeat the posterior impingement test and the patient's aggravating activity immediately after the local anaesthetic - greater than roughly 70% pain relief confirms the ossicle (or posterior impingement) as the source and predicts a good response to excision. Poor relief should send you back to the differential (Achilles, peroneal, primary FHL, or other pathology).
• The therapeutic half and its ceiling. Adding corticosteroid can give weeks to months of relief and may let an athlete finish a season or a dancer bridge to off-season surgery, but it is symptom control, not cure - the mechanical impingement persists.
• The FHL-rupture caveat. Corticosteroid deposited close to the FHL tendon carries a theoretical risk of tendon weakening or rupture - catastrophic in a dancer who depends on the FHL for en pointe - so keep the injection in the posterior recess rather than into the tendon, limit repetition, and counsel accordingly.

Posterior hindfoot surgery is generally low-morbidity, but the candidate must be able to list the specific risks.

• Neurovascular injury - sural nerve and lesser saphenous vein (posterolateral portal); posterior tibial neurovascular bundle and medial calcaneal/plantar branches (posteromedial). Kept safe by staying lateral to the FHL, which marks the medial safe boundary.
• Incomplete excision - a retained ossicle remnant or under-resected Stieda process leaves persistent impingement and pain.
• Persistent FHL symptoms - failure to release the FHL sheath when tenosynovitis coexists; triggering and posteromedial pain continue.
• FHL weakness - over-aggressive tendon debridement reduces hallux plantarflexion power - functionally catastrophic for a dancer.
• Posterior ankle stiffness / scarring - fibrosis can paradoxically limit the plantarflexion the surgery was meant to restore; mitigated by early mobilisation.
• Wound complications and infection - low overall; minor wound issues more frequent with open approaches (roughly 10% minor in open dancer series, mostly self-limiting).
• Approach-specific complication rate - systematic review data: open lateral approximately 12.7%, open medial approximately 3.9%, arthro-endoscopic approximately 4.8%.

The van Dijk EvidenceCard, the SAFE-FHL mnemonic and two separate viva follow-ups all demand the two-portal hindfoot endoscopy technique - "where do you place the portals and what are the key steps?" - yet the Management section only names "posterolateral and posteromedial portals" without the landmarks that make the operation safe.

• Positioning. The patient is prone with the foot over the end of the table so the ankle can be dorsi- and plantarflexed; thigh tourniquet, with a support under the lower leg.
• The two portals (at the level of the fibular tip).
  • Posterolateral portal: just lateral to the Achilles tendon at or just above the tip of the lateral malleolus, made first and directed toward the first interdigital web space. This is the viewing portal - the sural nerve and short saphenous vein lie lateral, so hug the Achilles.
  • Posteromedial portal: just medial to the Achilles at the same level, directed onto the shaft of the arthroscope and then slid down to bone - the instrument portal.
• The FHL is the medial safety line. The posterior tibial neurovascular bundle lies medial to the FHL, so all dissection is kept lateral to (or directly on) the FHL tendon. The FHL is identified by moving the hallux and watching its excursion, and is used as the landmark not to cross - working medial to it risks the tibial nerve and vessels.
• The steps. Debride the posterior capsule and scar to expose the ossicle and its synchondrosis; release the synchondrosis and remove the entire os trigonum (a retained remnant leaves persistent impingement); open the FHL sheath and release any stenosis if tenosynovitis coexists; confirm free hallux gliding at the end. The same two portals also access the posterior subtalar and tibiotalar joints.

Global Epidemiology

• Os trigonum is the most common accessory ossicle of the foot, reported in roughly 2 to 14% of feet across radiographic studies (9.8% in a 1651-foot series), with wide variation by population and imaging method; bilateral in about half.
• The vast majority are asymptomatic incidental findings - symptomatic posterior ankle impingement is concentrated in plantarflexion-loading athletes: ballet dancers (the dominant group in the literature, around 62% of reported surgical cases) and footballers/downhill runners.

Society Guidance (Side by Side)

There is no high-level guideline for this niche condition; recommendations are derived from specialist society consensus and reviews rather than RCT-based protocols.

All converge on the same pathway: confirm the ossicle is symptomatic, exhaust conservative care, then minimally invasive excision (releasing FHL when involved).

Registry Note

This is a soft-tissue/bony excision rather than an implant procedure, so it is not captured by arthroplasty registries (NJR, AJRR, AOANJRR). The evidence base is therefore limited to single-centre series and systematic reviews; there is no registry-level survival or revision data.

High- vs Limited-Resource Practice Variation

• Well-resourced settings: MRI and SPECT-CT for confirmation, image-guided diagnostic/therapeutic injection, and posterior endoscopic excision with dancer-specific rehabilitation.
• Limited-resource settings: diagnosis rests on history, posterior impingement test and plain lateral radiograph; treatment is conservative or open excision, which remains safe and effective where endoscopic equipment or expertise is unavailable.

The entire evidence base is Level III to V (case series and one Level II prospective comparison) - there are no randomised trials, so most recommendations are expert consensus.

• Endoscopic vs open vs arthroscopic excision. Ahn et al found no functional difference between subtalar arthroscopic and posterior endoscopic excision, and open posteromedial series (Heyer & Rose) report comparable results. The posterior 2-portal endoscopic technique dominates practice for faster return and lower morbidity, but no approach has proven superior long-term outcomes. Open access is still favoured for very large ossicles or extensive FHL work.
• How aggressively to treat the FHL. Posterior impingement and FHL tenosynovitis frequently coexist. There is no consensus on when simple sheath release suffices versus when tendon debridement is needed - and over-debridement risks catastrophic hallux flexion weakness, especially in dancers.
• Confirming the ossicle is symptomatic. Because os trigonum is a common incidental finding, opinion varies on whether MRI bone marrow oedema, a diagnostic injection, or SPECT-CT should be mandatory before surgery. Most authors require at least one positive correlating test.
• Conservative ceiling and injection use. The optimal length of conservative treatment (commonly quoted as 3 to 6 months) and the role/repetition of corticosteroid injection are not standardised; injection near the FHL also carries a theoretical rupture risk.
• Staged vs simultaneous bilateral surgery. No comparative data exist; the staged approach is preferred pragmatically in high-demand athletes to avoid bilateral non-weight-bearing and bilateral complication risk.