Oblique Talus
The 'Grey Zone' Deformity
Severity Classification
Critical Must-Knows
- Reducibility: TN joint reduces on max plantarflexion (unlike CVT)
- Radiographic Sign: Talar axis passes plantar to 1st MT on lateral view (Meary's broken)
- Talonavicular Relationship: Dorsolateral subluxation
- Differentiation: Distinguish from Rigid CVT and Flexible Flatfoot
- Treatment: Often requires treatment if painful or progressing
Examiner's Pearls
- "In the exam, demonstrate RECUCTION of the deformity on plantarflexion lateral X-ray
- "Always screw the hips and spine (neuromuscular check)
- "If it doesn't reduce, it is a Vertical Talus
- "Look for the 'sliding' navicular
The Critical Differentiator
The single most important feature to distinguish Oblique Talus from Congenital Vertical Talus (CVT) is the reducibility of the talonavicular joint. In Oblique Talus, the navicular slides back onto the talar head when the foot is plantarflexed. In CVT, the navicular is dislocated dorsally and rigidly locked, and will NOT reduce. This must be demonstrated on a forced plantarflexion lateral radiograph.
The Spectrum of Valgus Foot
| Feature | Flexible Flatfoot | Oblique Talus | Vertical Talus |
|---|---|---|---|
| Congruent | Subluxed Dorsally | Dislocated Dorsally | |
| Fully Reducible | Reduces with PF | Irreducible ( Rigid) | |
| Normal (NWB) / Sag (WB) | Broken (Extension) | Severely Broken | |
| Parallel to 1st MT | Plantar to 1st MT | Vertical orientation | |
| Valgus (Corrects with toe rise) | Fixed Valgus | Fixed Valgus (Equinus) |
Oblique Features
Memory Hook:The navicular SLIDES back into place in Oblique talus.
Radiographic Views
Memory Hook:The Dynamic Stress Views are mandatory.
Differentiation: RED
Memory Hook:Oblique Talus is REDucible.
Overview/Epidemiology
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).
Definition: A condition where the talus is plantarflexed and the navicular is subluxed dorsally upon it. The key definition is that the relationship is reducible; the navicular can be brought back into alignment with the talus, usually by plantarflexion.
Associations:
- Neuromuscular Disorders:
- Cerebral Palsy: Especially spastic diplegia. The muscle imbalance (spastic peroneals/gastroc) pulls the foot into valgus.
- Spina Bifida: Muscle weakness leading to instability.
- Genetic Syndromes (Ligamentous Laxity):
- Ehlers-Danlos Syndrome: Collagen defect causing hypermobility. The spring ligament fails.
- Marfan Syndrome: 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.
Pathophysiology of Deformity Progression: In these syndromic cases, the talonavicular joint is the "weak link" in the medial longitudinal arch. The repetitive loading of a lax joint during the stance phase of gait causes progressive dorsal subluxation. Over time, the soft tissues on the dorsal aspect contract, and the plantar structures (spring ligament) become attenuated and incompetent. This converts a flexible deformity into a rigid one (Oblique Talus to Vertical Talus spectrum).
Clinical Course: Many are asymptomatic in childhood. However, because the joint is subluxed (incongruent), it is prone to progressive deformity and degenerative changes. Pain typically develops in adolescence along the medial border (talar head prominence) or sinus tarsi (impingement).
Long Term Sequelae (Untreated):
- Medial Column Collapse: Complete loss of the medial arch.
- Spring Ligament Failure: Irreversible elongation.
- Talonavicular Arthritis: Due to the incongruent joint surfaces wearing unevenly.
- Forefoot Abduction: Leading to hallux valgus (bunions).
- Functional Impairment: Inability to run or play sports without pain.
Anatomy/Biomechanics
Pathoanatomy:
- Talus: Plantarflexed attitude (more vertical than normal).
- Navicular: Subluxed dorsally and laterally. It articulates with the dorsal aspect of the talar head.
- Calcaneus: In valgus and eversion.
- Spring Ligament: Attenuated (stretched), failing to support the talar head.
- Triceps Surae: Often tight (Achilles contracture), which forces the midfoot to breach/break dorsally to achieve clearance during gait.
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. Unlike a flexible flatfoot where 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.
Classification Systems
Hamanishi Classification (1984)
This system classifies congenital vertical talus and its variants based on the reducibility of the talonavicular joint.
- Group 1 (Flexible Flatfoot):
- Resting position: Normal or Valgus.
- TN Joint: Congruent/Reduced.
- Stress Views: Stable.
- Group 2 (Oblique Talus):
- Resting position: Valgus/Plantarflexed Talus.
- TN Joint: Subluxed Dorsally.
- Stress Views: Reduces on Plantarflexion.
- Group 3 (Vertical Talus):
- Resting position: Rocker Bottom.
- TN Joint: Dislocated Dorsally.
- Stress Views: Irreducible.
- Subgroups: 3a (Less than 3 months), 3b (Older than 3 months).
Clinical Assessment
History:
- "Fail to thrive" of foot posture.
- Parents report "ankles rolling in".
- Pain/Fatigue with sport (older child).
- Shoe wear issues (medial wear).
Physical Examination:
- Standing: Severe planovalgus foot. The medial malleolus is prominent. The talar head bulges on the medial border ("double malleolus" sign).
- Tiptoe Test: The heel may go into some varus (indicating some power), but the arch often does not reconstitute fully.
- Jack's Test: Extending the big toe fails to elevate the arch (Windlass failure).
- Reduction Test: With the child seated, plantarflex the ankle and foot. Palpate the TN joint. The "hollow" dorsal to the talus disappears as the navicular slides back into place.
- Neurological Exam: Essential. Check tone, reflexes, and gait for mild CP or spina bifida. Assess Beighton score for hyperlaxity.
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).
Investigations
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.
Management Algorithm
Surgical Technique
Minimal Invasive Pinning (Dobbs Technique modified)
Indication: After successful serial casting where stability is uncertain.
Technique:
- Percutaneous stabilization.
- Hold the foot in the corrected position (plantarflexion and inversion).
- Drive a K-wire retrograde from the dorsum of the 1st MT, through the navicular, into the Talus.
- Confirm on image intensifier.
- Cast for 6-8 weeks.
(Note: Ensure list items are not directly before closing tag)
Complications
| Complication | Risk Factors | Prevention/Management |
|---|---|---|
| Progression to CVT | Failure of treatment, severe laxity. | Prevention: Close monitoring. If becoming rigid, treat as CVT. |
| Painful Flatfoot | Subluxation causing arthritis. | Management: Orthotics first, then realignment surgery (osteotomies), finally triple arthrodesis (salvage). |
| Recurrence | Undercorrection of Equinus. | Prevention: Aggressive Achilles lengthening. Management: Repeat casting or surgery. |
| AVN of Talus | Excessive dissection during open reduction. | Prevention: Use Dobbs (minimally invasive) technique where possible. Avoid stripping dorsal neck. |
| Overcorrection | Varus deformity. | Prevention: Careful molding. Don't over-lengthen lateral column. |
Postoperative Care
- 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".
Outcomes/Prognosis
- 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.
Evidence Base
- Defined the spectrum of congenital vertical talus
- Group 2 (Oblique Talus) distinguished by reducibility
- Advocated for early casting treatment
- Revolutionized treatment of CVT with reverse Ponseti casting
- Applicable to Oblique Talus as well
- Drastically reduced the rate of extensive soft tissue releases (and AVN risk)
- Evans procedure corrects the deformity in three planes
- Can convert a subluxated TN joint to a congruent one
- High success rate in flexible/semi-rigid deformities
- Orthotics control the hindfoot position
- Reduce strain on the spring ligament
- Do not permanently alter bony architecture
- Defined Meary's Angle (Talus-1st Metatarsal)
- Standard for assessing sagittal plane deformity
- Broken line indicates subluxation/sag
Viva Scenarios
Practice these scenarios to excel in your viva examination
The Rocker Bottom Foot
"A newborn presents with a rocker bottom foot. How do you distinguish between CVT and Oblique Talus?"
The key differentiator is rigidity. I would perform a physical exam focusing on the midfoot. I would attempt to reduce the navicular onto the talus by plantarflexing and inverting the foot. If it reduces (slides back), it is an Oblique Talus. If it is rigid and reducible, it is a Congenital Vertical Talus. I would confirm this with a forced plantarflexion lateral radiograph.
The Painful Teenager
"14-year-old with painful bilateral planovalgus feet. Meary's angle is 25 degrees. Navicular subluxed. Treatment?"
This represents a symptomatic Oblique Talus or severe Flexible Flatfoot. Since conservative measures (orthotics) have likely failed given the severity, I would consider surgical reconstruction. My aim is joint-preserving. I would propose a Lateral Column Lengthening (Evans) to correct the hindfoot valgus and reduce the TN joint, possibly combined with a Medial Cuneiform Osteotomy (Cotton) to restore the arch. I would also assess for Achilles tightness.
The Neuromuscular Foot
"Child with Cerebral Palsy and Oblique Talus. Does this change your management?"
Yes. In neuromuscular conditions, the deforming forces (spasticity, muscle imbalance) are persistent. Soft tissue procedures alone invariably fail. Recurrence is high. I would be more aggressive with stabilization. While I would still attempt reconstruction (osteotomies + tendon balancing), I would have a lower threshold for bony stabilization (e.g., Grice arthrodesis or subtalar fusion) if the deformity is severe/unstable, although definitive fusion is best delayed until maturity.
MCQ Practice Points
Radiology MCQ
Q: Which view best differentiates Oblique Talus from Vertical Talus? A: Forced Plantarflexion Lateral. Oblique Talus reduces (axis restores); CVT does not.
Natural History
Q: What is the natural history of untreated oblique talus? A: Most cases remain asymptomatic or have mild flatfoot symptoms. Unlike CVT, it does not cause severe disability.
Radiographic Key
Q: What is the key radiographic view to distinguish Oblique Talus from CVT? A: Forced Plantarflexion Lateral View. In Oblique Talus, the TN joint reduces (lines up). In CVT, it remains dislocated.
Anatomy MCQ
Q: Where does the navicular articulate in Oblique Talus? A: Dorsal aspect of the Talar Head. It is subluxed, not dislocated (dislocation = CVT).
Association MCQ
Q: What is a strong risk factor for Oblique Talus? A: Generalised Ligamentous Laxity (or Neuromuscular conditions).
Treatment MCQ
Q: Why is serial casting successful in Oblique Talus but not rigid CVT? A: Because the TN joint is capable of reduction. Casting stretches the dorsal tissues to allow the navicular to remain reduced. In rigid CVT, the dislocation is fixed.
Australian Context
- Terminology: Often referred to as "Severe Flexible Flatfoot" in some clinics, though "Oblique Talus" implies the TN subluxation.
- Management: Conservative management (orthotics) is very popular in Australia for asymptomatic cases. Surgery is reserved for pain.
OBLIQUE TALUS
High-Yield Exam Summary
Key Features
- •Flexible Deformity
- •Reducible TN Joint
- •Talar Head Prominence
- •Painless in Child
X-ray Findings
- •Plantarflexed Talus
- •Normal Calcaneal Pitch
- •Reduces on PF View
- •Meary's Angle Broken
Differentiation
- •CVT is Rigid
- •Oblique is Flexible
- •Flatfoot is variant
- •Neuromuscular (Exclude)
Management
- •Observation (Most)
- •Orthotics (Symptomatic)
- •Stretching (Achilles)
- •Surgery (Rare)