What is a false acetabulum?

False Acetabulum in Developmental Dysplasia of the Hip

A false acetabulum is a secondary articulation point that forms when a dislocated femoral head creates a new socket against the ilium outside the true (original) acetabulum, typically seen in untreated developmental dysplasia of the hip (DDH).

Definition and Formation

A false acetabulum develops as a pathological adaptation when the femoral head remains chronically dislocated from its normal position. This occurs most commonly in:

• Untreated developmental dysplasia of the hip (DDH)
• Chronic dislocations of total hip arthroplasty 1
• High congenital hip dislocations 2

In DDH, when the femoral head is not properly contained within the true acetabulum during development, it migrates superoposteriorly and creates pressure against the ilium. Over time, the body forms a secondary socket (false acetabulum) at this abnormal contact point.

Classification in Congenital Hip Disease

According to research, congenital hip disease in adults can be classified into three types 2:

1. Dysplasia: The femoral head remains contained within the original true acetabulum
2. Low dislocation: The femoral head articulates with a false acetabulum where:
   • The inferior lip of the false acetabulum contacts or overlaps the superior lip of the true acetabulum
   • This creates the appearance of two overlapping acetabula on imaging
3. High dislocation: The femoral head has migrated superoposteriorly with:
   • No contact between the true and false acetabulum
   • Complete separation of the two structures

Radiographic Identification

False acetabula can be identified on imaging through several features:

• A secondary concave depression in the ilium above the true acetabulum
• In chronic cases, a well-defined, concentric, radio-opaque shadow may form around the dislocated femoral head 1
• The presence of heterotopic ossification in surrounding soft tissues
• Disruption of Shenton's line on pelvic radiographs 3
• The false acetabulum may appear more shallow and less developed than the true acetabulum

Clinical Significance

The presence of a false acetabulum has important clinical implications:

• Indicates long-standing hip dislocation that was either untreated or inadequately treated
• Associated with abnormal gait patterns, including:
  • Typical limp with toe-walking on the affected side in unilateral cases
  • Increased lumbar lordosis, prominent buttocks, and waddling gait in bilateral cases 3
• Predisposes to early degenerative joint disease and impaired function
• Complicates surgical planning for hip reconstruction or arthroplasty
• May require specialized surgical techniques such as cotyloplasty (medial advancement of the acetabular floor) in adult reconstruction 2

Prevention

Early detection and treatment of DDH is crucial to prevent the formation of a false acetabulum:

• Regular hip examinations during well-baby visits (at 1-2 weeks, 2,4,6,9, and 12 months of age) 3
• Selective ultrasound screening for infants with risk factors or abnormal physical examination at 4-6 weeks of age 3
• Radiographic evaluation after 4-6 months when the femoral head begins to ossify 3

Conclusion

A false acetabulum represents a significant pathological adaptation to chronic hip dislocation. Its presence indicates long-standing displacement of the femoral head and requires careful evaluation and specialized management approaches to restore proper hip biomechanics and function.