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

Clinical Presentation

• 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).

• Referral Pathways:

  • Primary Care: GP referral to Paediatric Orthopaedics usually occurs at age 3-5 when functional deficits become apparent.
  • Tertiary Centers: Cases are centralized to major children's hospitals (e.g., Royal Children's Hospital Melbourne, Sydney Children's Hospital Network, Queensland Children's Hospital).
  • Specialist Surgeons: Complex osteotomies (especially staged) require surgeons with specific fellowship training in paediatric upper limb reconstruction.

• Support Services:

  • National Disability Insurance Scheme (NDIS): Access for children with significant functional impairment. Funding covers occupational therapy, adaptive cutlery, and writing aids.
  • School Support: Education integration support for scribing or laptop use during exams if writing speed is impaired.

• Genetic Counseling:

  • Referral to clinical genetics (e.g., VCGS) is recommended for all cases, even if apparently isolated, to screen for X-linked conditions (Klinefelter) or subtle syndromic features (Apert, Carpenter).
  • Family screening may be indicated for autosomal dominant variants.

• Guidelines:

  • Management follows principles from the Australian Paediatric Orthopaedic Society (APOS).
  • Alignment with POSNA (Pediatric Orthopaedic Society of North America) clinical practice guidelines.