Radioulnar Synostosis

Embryological Development

Radioulnar synostosis occurs due to failure of longitudinal segmentation of the cartilaginous anlage:

Molecular Mechanisms

Risk Factors for Post-Traumatic Synostosis

• Monteggia fractures - Highest risk
• Both-bone forearm fractures - Especially same-level fractures
• High-energy trauma - Extensive soft tissue injury
• Delayed surgery - More periosteal reaction
• Open approach to both bones through same incision
• Bone grafting - Graft material in interosseous space
• Head injury patients - Increased heterotopic ossification risk

History

• Age: Usually noted at 2-5 years when complex hand tasks accumulate.
• Complaint: "Difficulty holding soup bowl" (supination deficit) or "awkward running style".
• Pain: Unusual. Pain suggests radial head instability or post-traumatic etiology.
• Function: Difficulty typing, using cutlery, hygiene (wiping requires supination).

Physical Exam

• Forearm Position: Usually fixed in 15-60° of pronation.
• ROM: Block to rotation. Assess elbow flexion/extension (often normal).
• Compensatory Motion: Assess shoulder ROM. High demand on shoulder rotation.
• Radial Head: Palpate for dislocation (posterior prominence).
• Neurology: Rule out associated neurological conditions.

• X-ray: PA and Lateral extended forearm.

  • Proximal radius/ulna fusion.
  • Radial head position (Types III/IV).
  • Radial bowing.

• CT: Rarely needed for diagnosis but useful for surgical planning (osteotomy site).

• MRI: Identifying fibrous band in Type I (rarely indicated).

The hallmark of synostosis is a fixed block to forearm rotation with preserved elbow flexion/extension. Distinguish it from other causes of limited supination/pronation:

The safety alerts, the controversies and the quiz all recommend "staged/gradual correction with an external fixator" for severe deformity, but the technique is never described - even though it is the principal way to correct a large deformity safely.

The problem it solves. The interosseous membrane, vessels and nerves are chronically shortened in a forearm fixed in hyperpronation. Twisting that forearm acutely through a large arc (the topic's quoted danger zone of roughly 60-80 degrees and beyond) wrings out the vessels and raises compartment pressure - the mechanism of the compartment syndrome / Volkmann's ischaemia warned about throughout.

How gradual correction works.

• A single-bone osteotomy (usually the ulna) is made distal to the synostosis and an external fixator (monolateral or a ring/circular frame) is applied across it.
• After a short latency period, the forearm is derotated a few degrees per day (typically over one to three weeks) until the target position is reached, then held in the frame until union.
• The slow rotation lets the shortened soft tissues, nerves and vessels accommodate gradually, so perfusion is maintained and a larger total correction can be achieved than would be safe acutely.

Trade-offs. It avoids acute neurovascular catastrophe and the need for a prophylactic fasciotomy, but at the cost of a frame on a child's arm for weeks, pin-site infection, a longer treatment course and family burden. It is therefore reserved for the severe/high-risk corrections; moderate deformities are still corrected acutely with internal fixation under vigilant compartment monitoring.

• Recurrence (Loss of Correction):

  • Excision: Recurrence rates are greater than 90% and osteotomy is preferred.
  • Derotation Osteotomy: Re-ankylosis in the new position is the goal, but loss of rotational correction can occur if fixation is inadequate or removed early.
  • Non-union: Rare at osteotomy site due to robust periosteum in children.

• Compartment Syndrome (Volkmann's Ischemia):

  • Mechanism: Severe derotation (over 60-80°) wrings out the interosseous membrane and vessels.
  • Prevention: Prophylactic fasciotomy or staged correction.
  • Monitoring: Vigilant post-operative checks. Low threshold for splitting casts.

• Neurovascular Injury:

  • Posterior Interosseous Nerve (PIN): Anatomy is distorted. The nerve may run adjacent to or through the synostosis mass. Distal osteotomy avoids this zone.
  • Radial Artery: Kinking can occur. Radial pulse must be documented pre- and post-correction.
  • Median Nerve: Can be compressed under the pronator teres during pronation-to-supination correction.

• Cosmetic:

  • "Back of Hand": Fixed pronation leads to unsightly eating mechanics.
  • Scars: Forearm scars can hypertrophy.

• Immobilization: Long arm cast for 6-8 weeks until bony union.
• Monitoring: Overnight admission for compartment checks mandatory.
• Follow-up: Check union. Hardware removal often required.

• Functional: Excellent improvement in ADLs (eating, hygiene). Most parents report high satisfaction with the functional gains.
• Cosmetic: Improved. The "awkward" arm position while running or at rest is resolved.
• ROM: Rotation is NOT restored (it is a fusion in a better position). Patients adapt well using shoulder motion.
• Satisfaction: Generally high if target angles are met. Bilateral severe cases derive the most benefit.
• Long-term: No long-term data suggests increased risk of elbow or wrist arthritis, as the articular surfaces are generally spared (unless Type IV/anterior dislocation).

The topic lists the risk factors for post-traumatic synostosis, carries the Jupiter resection card, and notes resection "once cold" - but the prevention and the timing/technique of resection are never pulled together. This matters because, unlike the congenital form, post-traumatic synostosis is largely preventable and genuinely resectable.

Prevention is the first principle. Most post-traumatic radioulnar synostosis is iatrogenic or technique-related and avoidable:

The highest-risk settings - Monteggia injuries, same-level both-bone fractures, high-energy trauma and the head-injured patient - warrant the most meticulous technique.

Resection - and when. A discrete, mature post-traumatic bridge can be excised to restore rotation (the opposite of the rule in congenital disease):

• Wait for maturity before resecting - a well-defined cortical margin on CT and a metabolically quiescent ("cold") bone scan with a plateaued/normalising alkaline phosphatase, classically around 6-12 months. (Jupiter & Ring resected at a mean of 19 months; in their series, resecting before versus after 12 months did not significantly change the rotation regained.)
• Excise the bridge and restore the interosseous space; free-fat interposition did not significantly change outcome in the Jupiter series.
• Outcome: non-recurring limbs regained an average of 139 degrees of rotation; the single recurrence was the only patient with a closed head injury - the key recurrence risk factor.

Global Epidemiology

• Proximal radioulnar synostosis is the most common congenital anomaly of the proximal forearm, but is rare overall (a few hundred cases reported worldwide).
• Bilateral in roughly 60% of cases; both sexes affected, with most non-syndromic isolated cases linked to SMAD6/NOG variants or sex-chromosome aneuploidy.
• Largest contemporary series (97 patients, 122 forearms; European multicentre) confirms Type III as the most frequent radiographic pattern.

Side-by-Side Guidance

There is no dedicated society guideline for this rare condition; evidence is case-series level. Principles converge across bodies:

Registries

• No arthroplasty/implant registry captures this condition. Outcome data derive from single-centre and pooled case series (e.g. the 2022 meta-analysis of 383 forearms and the 2025 European multicentre cohort).

High- vs Limited-Resource Practice

• Well-resourced settings: early occupational therapy, genetic testing (SMAD6/NOG, karyotype for Klinefelter), CT planning, and plate-fixed proximal osteotomy with overnight compartment monitoring.
• Limited-resource settings: diagnosis often delayed until school age; plain radiographs suffice; single-bone derotation held with K-wires and a long-arm cast is a pragmatic, low-cost alternative to plating. Genetic services may be unavailable, so clinical syndrome screening (whole-patient examination) is essential.