Oblique Talus

Oblique Talus (also known as Supranavicular Subluxation) occupies the middle ground in the spectrum of congenital flatfoot deformities. It is more severe than a flexible flatfoot but less rigid than a Congenital Vertical Talus (CVT).

The topic repeatedly names the spring ligament - it is "attenuated/stretched" in the pathoanatomy, "spring ligament failure" is listed as an irreversible sequela, and the surgical technique "reefs the talonavicular capsule/spring ligament" - yet what it is and why its failure is central is never developed. It is the key static stabiliser, so it deserves to be understood.

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

• the superomedial band - broad, contains fibrocartilage where the talar head rests, and blends with the tibialis posterior tendon sheath and the deltoid ligament; and
• the inferior (plantar/short) band.

Function - static vs dynamic support of the arch:

Why it is the linchpin in oblique talus: as the talar head sags plantar-medially, the spring ligament is progressively stretched and attenuated. Once the sling fails, the static support of the talar head is lost, the head drops further, and the dorsal talonavicular subluxation deepens - tipping a positional sag towards fixed deformity. Because the superomedial band blends with the tibialis posterior, spring-ligament incompetence and tibialis posterior failure go together - the dynamic and static stabilisers fail as a unit, which is the substrate for progressive collapse here and for adult acquired flatfoot later (developed in the adult-flatfoot topic). This is exactly why reconstructive surgery does not just realign bone but reefs/imbricates the spring ligament and talonavicular capsule to restore the medial sling.

The oblique talus is a dynamic failure of the medial column in which the talonavicular (TN) joint is the weak link of the medial longitudinal arch.

Deforming sequence:

1. Lax or imbalanced TN joint — generalised ligamentous laxity (hypotonia, syndromic collagen disease) or a neuromuscular muscle imbalance (spastic peroneals/gastrocnemius overpowering tibialis posterior) destabilises the joint.
2. Repetitive stance-phase loading drives the talar head plantar-medially while ground reaction force pushes the navicular dorsally and laterally — a progressive dorsal subluxation rather than a fixed dislocation.
3. Spring (plantar calcaneonavicular) ligament attenuation removes the static sling that supports the talar head, deepening the sag.
4. Dorsal soft-tissue contracture (extensor tendons, dorsal TN capsule) plus a tight triceps surae forces the midfoot to "break" dorsally for ground clearance, perpetuating the deformity.
5. Conversion along the spectrum — if the dorsal contractures become fixed, the reducible oblique talus migrates towards the rigid, irreducible congenital vertical talus.

The crucial distinction (Harris, Clin Podiatr Med Surg 2000) is that in oblique talus the TN joint is pushed to, or just past, its end range but is not truly dislocated — which is why it remains reducible and why prognosis is better than in CVT.

Gait Analysis:

• Stance Phase: Increased medial foot contact. The "arch" is absent.
• Heel Rise: Watch for the calcaneus to invert (varus). In Oblique Talus/Flexible Flatfoot, the heel should invert due to the windlass mechanism (though often delayed or weak). In Vertical Talus, the heel remains in valgus.
• Propulsion: Disorganized. Push-off occurs from the midfoot rather than the metatarsal heads due to the midfoot break (sag).
• Fatigue: Use of accessory muscles to stabilize the foot leads to early tiring ("carry me" sign).

Radiographs (Standard + Stress Views):

• Weight-Bearing AP:
  • Kite's Angle (Talocalcaneal): Increased (greater than 35-40 degrees). Hindfoot Valgus.
  • Talar Head Coverage: The navicular is shifted laterally, uncovering the talar head.
• Weight-Bearing Lateral:
  • Meary's Angle: The talus axis points plantar to the 1st MT axis (Angle greater than 20 degrees).
  • Calcaneal Pitch: Decreased.
• Forced Plantarflexion Lateral: The Money View.
  • Oblique Talus: The axis of the talus lines up with the 1st metatarsal. The deformity reduces.
  • Vertical Talus: The axis remains broken. The deformity is rigid.
• Forced Dorsiflexion Lateral:
  • Assesses ankle vs midfoot stiffness.

The investigations use Meary's and Kite's angles, and the Hamanishi evidence card refers to TAMBA / CAMBA with a flexible-to-rigid threshold - but the dedicated measurement system for this exact spectrum is never explained. It is the tool examiners expect for quantifying the deformity and, crucially, for proving reducibility on the stress view (Hamanishi, J Pediatr Orthop 1984).

The two angles (measured on the lateral radiograph):

• TAMBA - Talar-Axis–Metatarsal-Base Angle: the angle between the long axis of the talus and the base of the first metatarsal. It captures talar obliquity and the severity of the dorsal talonavicular displacement.
• CAMBA - Calcaneal-Axis–Metatarsal-Base Angle: the angle between the long axis of the calcaneus and the base of the first metatarsal. It captures calcaneal (hindfoot) obliquity and reflects the tendo Achilles contracture / equinus.

Together they describe the obliquity of both the talus and the calcaneus, the severity of the talonavicular dislocation, and the heel-cord contracture - more completely than Meary's angle alone.

The key application here: the angles are measured on the forced plantarflexion stress view (the "money view"), and the change in TAMBA is the quantitative proof of reducibility that defines oblique talus. If TAMBA falls toward normal on plantarflexion, the navicular is reducing (oblique talus); if it stays high and fixed, the joint is dislocated (CVT). Hamanishi placed the transition from flexible oblique talus to rigid CVT at a TAMBA of roughly 60 degrees and a CAMBA of roughly 20 degrees, and recommended open reduction for a typical CVT if three months of casting fails to bring the TAMBA down to about 50 degrees. (The full CVT measurement and classification detail belongs to the congenital-vertical-talus topic; here the point is using TAMBA on the stress view to confirm the oblique talus is genuinely reducible.)

• Casting: Long leg cast (knee flexed) is essential to control the gastrocnemius pull and prevent the pin from migrating.
• Duration: 6-8 weeks non-weight bearing.
• Pin Removal: At 6-8 weeks.
• Orthotics: Use of AFO (Ankle Foot Orthosis) or UCBL for 6-12 months post-correction to prevent recurrence.
• Monitoring: Radiographs every 6 months to check for recurrence of the "sag".

• Natural History: Some debate exists. Some authors believe Oblique Talus is simply a severe flatfoot and benign. Others believe it leads to inevitable degenerative changes. The truth is likely in the middle – symptomatic cases do poorly without treatment.
• Conservative: Orthotics provide symptom relief but do not change the radiographic angles.
• Surgical: Joint-sparing reconstructive surgery (osteotomies + soft tissue) yields good results in children. Arthrodesis is reserved for salvage in adults.
• Neuromuscular Cases: Have a higher recurrence rate and may require bony stabilization (arthrodesis) earlier.

Global epidemiology

• Flexible flatfoot is near-universal in toddlers and falls to roughly 15-25 percent prevalence by adolescence as the arch develops; oblique talus is a rare subset within this and is over-represented in children with hypotonia, ligamentous laxity syndromes and neuromuscular disease.
• There is no dedicated registry for oblique talus; evidence is drawn from single-centre paediatric series across North America, Europe and Asia.

Side-by-side guidance

The consistent global message: treat the symptomatic or rigid foot, not the radiograph; investigation and intervention are triggered by pain, rigidity or documented progression, not by appearance alone.

High- versus limited-resource practice

• Well-resourced settings: stress radiographs, gait analysis, custom UCBL/AFO orthoses, and image-guided minimally invasive casting/pinning are routinely available.
• Limited-resource settings: diagnosis rests on clinical reducibility and a single lateral radiograph; serial casting (reverse Ponseti) is favoured as a low-cost, effective first line, with reconstructive surgery reserved for the disabling or rigid foot.

• Is oblique talus a real entity or just severe flatfoot? Some authors regard it as the severe end of flexible flatfoot rather than a true diagnosis. Harris (Clin Podiatr Med Surg 2000) argues for keeping it distinct because it carries a better prognosis than CVT and prevents over-diagnosis of CVT.
• Definition of "subluxation": Harris notes that in many feet the TN joint is merely pushed to its maximum range and does not even meet a strict definition of subluxation, blurring the boundary with flexible flatfoot.
• Natural history is unknown: There are no large prospective trials defining which children progress to symptomatic disease, so the threshold for any intervention is judgement-based (Carr, Pediatrics 2016).
• Orthotics: Widely prescribed, but there is no high-level evidence that they alter bony architecture or natural history — their role is symptom relief only.
• Subtalar arthroereisis: Remains controversial — variable evidence, high hardware-removal and sinus-tarsi pain rates; not a substitute for correcting fixed valgus or equinus.
• Timing and choice of surgery: When casting is attempted versus straight to reconstruction, and joint-sparing osteotomy versus arthrodesis in neuromuscular feet, are not standardised.