A New Scoring System for Detailed Visual Inspection of Retrieved Metal on Metal Hip Arthroplasties

Abstract

Metal-on-metal (MOM) total hip arthroplasty and hip resurfacing using large diameter femoral heads offer clinical advantages, however the failure rates of these hips are unacceptably high. Retrieved MOM hips have a wide range of wear rates of their bearing surfaces and there is ongoing research to identify the causes of failure.

Detailed visual inspection is the first step in the forensic examination of failed hip components, but there is no universally accepted description or process. Visible features may help explain the mechanism of failure of MOM hips.

During our analysis of 2000 MOM hip components, we have developed protocols to undertake detailed, non-destructive macroscopic and stereomicroscopic examinations of each component, using quantitative assessment to document the presence of types of damage.

We have established a systematic terminology to describe the types of damage that are observed, allowing for consistency and clarity in the vocabulary used. These include (but are not limited to):

• Scratching – when there is an increase in the number and/or magnitude of scratches present on the surface, typically increasing measured roughness parameters. The severity of scratching is determined by rubbing a 0.18 mm thickness acetate gage or fingernail over the surface:

  • Light –visible but not detectable with gage.

  • Moderate – visible and detectable with gage.

  • Heavy – depth clearly visible and will catch a fingernail or gage.

• Pitting – indentations in the surface for which the dimensions are similar in all directions.

• Embedded Particles – hard particles that have become embedded in the bearing surface.

• Discolouration – observed as a change in the appearance of the surface, often as colour diffraction patterns.

• Polishing, gouges, etching (from corrosion), surface films, surface deposits and tribochemical reaction layers.

To assess the distribution of these types of damage on the components, each surface is considered in terms of zones defined by quadrants (cup, head, stem and taper) and sub-quadrants (cup and head only), Figure 1. Each zone is scored on a scale of 0 to 3 by determining the percentage of the surface area of the quadrant that exhibits the feature in question:

• a score of 0 indicates 0%

  1. 1

     indicates greater than 0% but less than 25%

  2. 2

     indicates between 25% and 75% inclusive

  3. 3

     indicates greater than 75%.

The use of zones to differentiate between the polar and equatorial regions of the cup and head surfaces can offer important information relating to the articulation of the bearing surfaces. For example in a cohort of 90 inspections, a score of 3 for light scratching was recorded in all areas of the bearing surface in 40% of cups, whilst approximately 5% had evidence of pitting, occurring near the rim.

Current research at our retrieval centre involves correlating the results of detailed inspections with a range of variables, including implant design, size and surgical position.