Radioulnar Synostosis

Embryological Development

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

Normal Development:

• Week 5-7 gestation: Common cartilaginous mass differentiates into radius and ulna
• Segmentation begins proximally and proceeds distally
• Interzone apoptosis separates the two bones
• Complete separation by week 8

Pathological Process:

• Congenital: Failure of interzone formation → bones remain fused
• Post-traumatic: Bone bridging across the interosseous space after fracture healing

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).

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

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.