Oblique Talus

Congenital oblique talus (COT) is a rocker-bottom deformity with a dorsally dislocated navicular that is less severe and more flexible than congenital vertical talus (CVT). Current objective work rejects the old assumption that the diagnosis is simply a talonavicular subluxation that returns to normal on plantarflexion.

Definition. COT is persistent dorsal navicular dislocation on plantarflexion stress imaging, with less severe angular deformity and greater flexibility than CVT. Plantarflexion imaging therefore does not simply divide a reducible COT from an irreducible CVT: both retain dorsal navicular dislocation. If the talonavicular relationship becomes anatomically aligned on plantarflexion, consider flexible flatfoot with a short tendo-Achilles (FFF-STA), not COT.

Associations. COT is associated with neuromuscular disorders, genetic syndromes of ligamentous laxity and developmental delay.

• Cerebral palsy, especially spastic diplegia: the muscle imbalance (spastic peroneals and gastrocnemius) pulls the foot into valgus
• Spina bifida: muscle weakness leading to instability
• Ehlers-Danlos syndrome: a collagen defect causing hypermobility
• Marfan syndrome: a similar mechanism; arachnodactyly may be noted
• Down syndrome (trisomy 21): hypotonia and ligamentous laxity are hallmark features
• Larsen syndrome: multiple joint dislocations
• Developmental delay: hypotonia contributes to the "flat" appearance

The defining lesion. COT is best understood as a less severe, more flexible dorsal navicular dislocation within the rocker-bottom group. The defining abnormality is at the talonavicular joint; talar verticality is associated but not determinative.

• Navicular: dorsally dislocated and proximally translated. On infant radiographs the first metatarsal axis is used as its proxy
• Talus: generally less vertical and more flexible than in CVT, although talar position alone does not define the diagnosis
• Calcaneus: valgus and everted within the rocker-bottom configuration
• Soft tissues: the dorsal talonavicular structures and the tendo-Achilles contribute to rigidity, and their relative contracture must be assessed separately

Biomechanics. The medial longitudinal arch is obliterated. During weight-bearing the ground reaction force pushes the talar head further plantarward and the navicular further dorsal. In a flexible flatfoot the locking mechanism of the midfoot is intact during push-off; the oblique talus foot remains unlocked and unstable, leading to an inefficient gait and fatigue.

Anatomy. The spring (plantar calcaneonavicular) ligament complex runs from the sustentaculum tali of the calcaneus to the plantar-medial navicular, forming a hammock or sling beneath the head of the talus. It has two principal components:

• the superomedial band, which is broad, contains fibrocartilage where the talar head rests, and blends with the tibialis posterior tendon sheath and the deltoid ligament
• the inferior (plantar or short) band

Static and dynamic support. The spring ligament is the primary static support of the talar head, and it works alongside the other stabilisers of the medial longitudinal arch.

Its role in COT. The plantar calcaneonavicular complex supports the talar head, but its precise causal role in congenital oblique talus has not been established. Do not import the adult progressive-collapsing-foot model wholesale into this congenital deformity.

Why the anatomy still matters. The sling and its blend with tibialis posterior matter during talonavicular reconstruction. If reconstruction is considered, define the actual talonavicular, subtalar and tendon pathology rather than assuming universal spring-ligament failure. The diagnosis itself rests on stress-radiographic navicular position and rigidity, not on an assumed spring-ligament tear.

The objective framework (Mosca, Ganjwala and Shah, 2024). Maximum plantarflexion and dorsiflexion lateral radiographs separate the normal foot, FFF-STA, COT and CVT.

Angle and CORA together. The plantarflexion talus-first metatarsal angle measures navicular angulation, while the foot-CORA location detects proximal translation. Use both: the angle alone exists on a continuum, whereas CORA location separates aligned from dislocated feet.

The dorsiflexion view. The dorsiflexion tibiotalar angle describes talar verticality and tendo-Achilles rigidity. In FFF-STA it shows the short tendo-Achilles; in COT the talus is generally oblique and more flexible, and in CVT generally vertical and more rigid.

Categories, not stages. These categories should not be presented as a proven temporal progression from flexible flatfoot through COT to CVT.

Hamanishi (historical). Hamanishi classified CVT by aetiology into neural-tube, neuromuscular, malformation-syndrome, chromosomal and idiopathic groups, and the same paper introduced TAMBA and CAMBA. It did not create a three-group sequence of flexible flatfoot, oblique talus and vertical talus by age.

History. Parents describe a foot posture that "fails to thrive" and ankles "rolling in". The older child reports pain or fatigue with sport, and shoes show medial wear.

Standing. A severe planovalgus foot with a prominent medial malleolus. The talar head bulges on the medial border, the "double malleolus" sign.

Tiptoe and heel rise. On tiptoe the heel may go into some varus, indicating some power, but the arch often does not reconstitute fully. In oblique talus and flexible flatfoot the heel should invert at heel rise, though often delayed or weak; in vertical talus the heel remains in valgus.

Jack's test. Extending the big toe fails to elevate the arch: windlass failure.

Manual flexibility. Plantarflex and dorsiflex the foot while palpating the talar head and midfoot. Record the range and rigidity, but do not claim anatomical talonavicular reduction from palpation alone.

Neurological examination. Essential. Check tone, reflexes and gait for mild cerebral palsy or spina bifida, and assess the Beighton score for hyperlaxity.

Gait. Stance shows increased medial foot contact and an absent arch. Propulsion is disorganised: push-off occurs from the midfoot rather than the metatarsal heads because of the midfoot break (sag). Using accessory muscles to stabilise the foot leads to early tiring, the "carry me" sign.

Paired stress laterals. The diagnosis needs a maximum dorsiflexion lateral and a maximum plantarflexion lateral. Maximum plantarflexion tests the severity and flexibility of dorsal navicular angulation and proximal translation. Maximum dorsiflexion tests the severity and rigidity of talar plantarflexion and the tendo-Achilles contracture.

Measuring them. The infant navicular is unossified, so the first metatarsal is its proxy.

1. On the maximum plantarflexion lateral, draw the central axes of the talus and first metatarsal and measure their angle. A negative (dorsiflexed) relationship means talonavicular malalignment persists.
2. Locate the foot-CORA, the intersection of those axes, along the talar axis. A CORA at the talar head supports normal alignment; proximal migration supports navicular dislocation.
3. On the maximum dorsiflexion lateral, measure the tibiotalar angle to quantify talar plantarflexion (verticality) and tendo-Achilles rigidity.
4. Interpret the measurements together, against the categories in the classification table.

AP foot. Documents forefoot abduction and talocalcaneal divergence but does not replace the paired lateral stress views.

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Historical measurements. Hamanishi's TAMBA and CAMBA relate the talar or calcaneal axis to a point at the first-metatarsal base. They are historically important and still appear in CVT outcome series, but a point cannot represent the rotational axis of the first metatarsal. Different first-metatarsal rotations can therefore produce the same TAMBA or CAMBA.

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The diagnosis must be settled before treatment, because the evidence cannot support one shared pathway for FFF-STA, COT and CVT.

1. Confirm the category with paired stress laterals before predicting prognosis, and screen any congenital rocker-bottom foot for neurological, spinal and syndromic associations.
2. FFF-STA: treat symptoms and heel-cord contracture; orthoses may improve comfort but do not remodel alignment.
3. COT: refer an objectively dislocated talonavicular joint to a paediatric foot specialist. Current objective definitions are newer than the treatment literature, so natural history, casting thresholds and operative indications remain insufficiently established. Follow symptoms, function, rigidity and radiographic alignment rather than appearance alone.
4. CVT: use the established vertical-talus pathway, serial reverse-Ponseti manipulation and casting followed by talonavicular reduction and temporary pin fixation, with Achilles tenotomy when indicated.
5. Do not route by age or one angle alone. Symptoms, rigidity, residual dislocation, associated diagnosis and prior treatment determine escalation.

Postoperative care is procedure-specific. For CVT reduction and pinning, follow the selected casting, pin-surveillance and brace protocol, with serial skin, neurovascular and radiographic checks.

For any COT intervention, document the corrected talonavicular relationship and follow rigidity, recurrence, pain and function. There is no validated universal six-to-eight-week or six-month radiograph schedule for COT.

Natural history. The natural history of objectively defined COT is not yet established. Older reports combined COT with severe flexible flatfoot or mild CVT, so claims about spontaneous resolution, inevitable arthritis or a fixed treatment sequence are not reliable.

Non-operative treatment. Orthoses may relieve symptoms in flexible flatfoot, but no high-level evidence establishes their effect on COT dislocation.

Casting and fixation. The outcome evidence is predominantly CVT evidence and must not be presented as COT-specific.

Reconstruction. Procedure selection depends on persistent joint dislocation, rigidity, associated deformity and underlying diagnosis. COT-specific comparative outcomes are unavailable.

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Global evidence

• COT is rare and lacks a dedicated registry; the best current diagnostic evidence is a single Level II radiographic study.
• No AAOS, POSNA, BOA/BSCOS, NICE or EFORT guideline provides a COT-specific treatment algorithm.
• Flexible-flatfoot guidance cannot be applied to a persistently dislocated talonavicular joint, and CVT treatment series cannot automatically be relabelled as COT evidence.

Practice principles

1. Obtain paired stress laterals when a congenital rocker-bottom deformity is suspected.
2. Use the same objective measurements in all settings; a single comfortable-position lateral is insufficient.
3. Screen for neurological, spinal, chromosomal and syndromic associations.
4. Refer persistent navicular dislocation to a paediatric orthopaedic foot service.
5. Record uncertainty honestly and use shared decision-making when considering treatment without COT-specific comparative evidence.

• Objective definition is recent: the 2024 axis-and-foot-CORA framework supersedes the traditional reducing-subluxation description, but external validation is still needed.
• Threshold direction and nomenclature: published tables and figure conventions can be confusing; inspect the radiographs and use severity plus CORA location rather than memorising an isolated sign.
• Natural history is unknown: older series mixed COT with FFF-STA and mild CVT, so prognosis cannot be inferred reliably.
• Treatment evidence lags diagnosis: there are no comparative trials defining observation, casting, pinning or reconstruction thresholds for objectively defined COT.
• Associated disease matters: neuromuscular and syndromic feet may behave differently, but diagnosis still requires the same stress-radiographic discipline.