Why should the hip remain in abduction after reduction?

Hip Abduction After Reduction: Prevention of Redislocation and Vascular Compromise

The hip should remain in abduction after reduction to maintain joint stability and prevent redislocation while simultaneously reducing the risk of avascular necrosis (AVN) of the femoral head by avoiding excessive abduction angles that compromise femoral head vascularity. 1, 2

Primary Rationale: Balancing Stability and Vascularity

The fundamental principle is that abduction provides mechanical stability to the reduced hip by maintaining the femoral head centered within the acetabulum, but this must be balanced against vascular risk. 1

Optimal Abduction Angle

• 45° of hip abduction (range 38-50°) is the evidence-based target following closed or open reduction, as this angle provides adequate stability without significantly increasing AVN risk 1
• Studies comparing 60° abduction versus 45° abduction showed no significant difference in AVN rates (19% vs 16%, p=0.78), but the lower angle maintained stability with only one redislocation in 44 patients 1
• Excessive abduction (>60°) increases tension on the femoral head blood supply, particularly the medial circumflex femoral artery, leading to higher AVN rates 3, 1

Mechanism of Stability

Abduction positioning works through several mechanisms:

• Maintains concentric reduction of the femoral head within the acetabulum by preventing posterior or superior migration 2
• Reduces tension on the hip capsule and surrounding soft tissues that could pull the femoral head out of position 3
• Prevents impingement between the femoral neck and acetabular rim during the healing phase 1

Duration and Position Specifics

The standard immobilization protocol is 12 weeks in a hip spica cast in the "Fettweis position" (approximately 90° flexion, 45° abduction, 0-10° internal rotation), followed by 3 months in an abduction splint 3

Post-Casting Bracing Considerations

• Abduction bracing after cast removal may reduce early secondary surgery rates (2.5% vs 11.4% without bracing, p=0.019), though it does not significantly improve acetabular index at 2-4 years 4
• The decision to use post-casting bracing should consider the severity of initial dislocation (IHDI grade) and age at reduction 4

Critical Complications Prevented by Proper Abduction

Redislocation Prevention

• Inadequate abduction is the primary modifiable risk factor for redislocation after reduction 1
• Redislocation rates are <1% when proper abduction angles are maintained 1, 2
• Severe dislocations (Suzuki type C) require longer duration of abduction positioning to allow soft tissue healing 2

AVN Risk Mitigation

• AVN rates as low as 1.0% can be achieved with gradual reduction techniques and appropriate abduction angles 2
• The key is avoiding both extremes: insufficient abduction (leading to instability) and excessive abduction (compromising vascularity) 1

Common Pitfalls to Avoid

Do not use abduction angles >60° in an attempt to maximize stability, as this significantly increases AVN risk without providing additional stability benefit 1

Do not allow the hip to fall into adduction or neutral position during the immobilization period, as this is the most common cause of redislocation 3, 1

Do not shorten the immobilization period below 12 weeks in spica cast, as premature mobilization increases redislocation risk even with proper initial positioning 3

Monitor for signs of excessive pressure including skin breakdown or neurovascular compromise, which may indicate the need for cast adjustment while maintaining abduction 3

Age-Specific Considerations

• For infants under 12 months, closed reduction with 45° abduction is typically sufficient 3, 1
• For children 12-24 months, the same abduction principles apply but may require longer immobilization periods 3
• For children over 24 months, open reduction is often necessary, but the same abduction positioning principles apply postoperatively 3