Disassembly and dislocation of a bipolar hip prosthesis

Area of Science:

• Orthopedic surgery
• Biomedical engineering

Background:

• Hip prosthesis dislocation is a frequent complication following arthroplasty.
• Bipolar hip prostheses are used to address hip joint issues, but can also fail.
• Dislocation typically involves the femoral head exiting the acetabulum or prosthetic cup.

Observation:

• A 72-year-old woman experienced an unusual hip prosthesis failure after a fall.
• Examinations revealed a disassembly-dislocation of the bipolar hip prosthesis, specifically the inner head detaching from the outer prosthesis.
• This was classified as a type II failure, with the outer head in a varus position and a detached locking ring.

Findings:

• The case presented a rare instance of bipolar hip prosthesis inner head dissociation from the outer prosthesis.
• Osteoarthritis of the acetabulum was identified as a contributing factor, leading to a varus position of the outer head.
• This varus positioning caused impingement on the femoral stem neck, dislodging the locking ring and resulting in disassembly-dislocation.

Implications:

• This case highlights a rare failure mechanism in bipolar hip prostheses, particularly in patients with pre-existing acetabular osteoarthritis.
• Understanding this specific failure mode is crucial for improving implant design and surgical techniques to prevent such complications.
• Further research may be needed to explore the long-term implications of acetabular changes on bipolar prosthesis stability.