Metal-metal reactivity: Houston, we have a problem!

Area of Science:

• Orthopedic Surgery
• Biomechanical Engineering

Background:

• Symptomatic pseudotumor is a rare but serious complication following hip arthroplasty.
• Previous studies have suggested associations with patient sex and implant size, but the underlying biomechanics remain unclear.

Purpose of the Study:

• To investigate the biomechanical factors, specifically component positioning, associated with the development of symptomatic pseudotumor after hip arthroplasty.
• To identify optimal parameters for acetabular and femoral component placement to minimize pseudotumor risk.

Main Methods:

• Retrospective analysis of hip arthroplasty cases requiring revision for symptomatic pseudotumor.
• Comparison of acetabular component lateral opening angle, acetabular anteversion, and femoral version between pseudotumor cases and controls.
• Analysis of patient demographics and native hip anatomy, including congenital dysplasia.

Main Results:

• Three hips (0.51% incidence) required revision for pseudotumor, all with 50-mm acetabular components and increased acetabular anteversion (mean 27.1° vs 16.4°).
• Increased combined anteversion was identified as a common mechanical factor in pseudotumor formation.
• Congenital dysplasia, characterized by shallow sockets and high anteversion, may predispose to adverse joint mechanics.

Conclusions:

• Increased combined anteversion, rather than female sex or small component size, is the primary mechanical factor leading to pseudotumor.
• Accurate assessment of acetabular lateral opening angle (<50°), acetabular anteversion, and femoral version is essential for satisfactory component positioning.
• Adjusting acetabular lateral opening angle in dysplastic cases with altered neck-shaft angles is necessary to ensure proper head capture and bearing function.