The problem with large diameter metal-on-metal acetabular cup inclination

Area of Science:

• Orthopedic surgery
• Biomedical engineering
• Materials science

Background:

• Large diameter metal-on-metal (MoM) hip bearings offer clinical success for active patients.
• A subset of MoM hip implants experience elevated wear and metal ion levels.
• This adverse outcome is linked to specific surgical positioning: steep cup inclination and extreme version angles.

Purpose of the Study:

• To investigate the geometric relationship between acetabular component orientation and critical bearing surface angles.
• To determine if perceived cup inclination by surgeons accurately reflects the critical bearing angle.
• To provide evidence-based recommendations for optimizing MoM hip implant positioning to minimize wear.

Main Methods:

• Analysis of the geometry of six distinct commercial large diameter MoM acetabular components.
• Calculation of the critical bearing surface angle relative to the cup face inclination.
• Comparison of geometric findings with typical surgical perceptions of cup inclination.

Main Results:

• The critical bearing surface operates at an angle up to 16 degrees greater than the cup face inclination.
• Cup inclination, as measured geometrically, differs from surgeon perception.
• Component geometry significantly influences the effective bearing angle.

Conclusions:

• Steep cup inclination and extreme version angles are critical factors in MoM hip bearing failure.
• Surgeons' perception of cup inclination may not align with the actual critical bearing angle.
• Employing lower inclination and version angles is strongly recommended for large diameter MoM bearings to prevent excessive wear and ion release.