[Progressive intra-acetabular dislocation of bipolar hip prostheses: four cases]

Area of Science:

• Orthopedic surgery
• Biomedical engineering
• Implant design

Background:

• Bipolar prostheses are used for femoral neck fractures.
• Complications include acetabular erosion and cup dissociation.
• Progressive intra-acetabular dislocation is a previously unreported complication.

Observation:

• Four elderly women developed progressive intra-acetabular dislocation after bipolar prosthesis implantation.
• The metal-backed cup verticalized, and the ball dislocated into the acetabulum, causing erosion.
• This differs from acute dislocations and is not related to neck-head angles or modular head issues.

Findings:

• The primary cause identified is poor bipolar cup design and a weak retention system.
• Superimposed inner and outer sphere centers lead to cup varus, exacerbated by medial outer sphere center placement.
• Optimal design requires the outer sphere center to be lateral to the inner sphere center for valgus torque.

Implications:

• Highlights the critical importance of bipolar cup design and rotational center positioning.
• Suggests a need for improved implant designs to prevent this specific complication.
• Revision surgery with hemispheric bone ingrowth cups can effectively manage this condition.