Metasul® Metal-on-Metal Technology

Geometry

Geometric Key Factor: Clearance

The diametrical clearance of a metal-on-metal articulation is the geometric key factor controlling its wear behavior. The diametrical clearance is the difference in articulation diameter between the acetabular cup and femoral head.

Minimizing Contact Stress

The wear of a metal-on-metal bearing is proportional to the contact stresses of the articulation, therefore the contact stresses should be kept to a minimum. To minimize these contact stresses, the clearance should be as small as possible without increasing the potential of component clamping.

For the Metasul sandwich design, contact stresses were determined by finite element analysis (FEA)1, and illustrated by the graph below. This graph depicts the influence of the diametrical clearance on the contact stresses for the Metasul articulation.

The Clearance and Its Limits

While it has been shown that decreasing diametrical clearance results in decreasing contact stresses or wear, the optimal clearance may not be the lowest possible clearance that can be manufactured. Due to deformation of the acetabular component under load, a minimum clearance is mandatory to avoid equatorial clamping of the articulation, which can lead to excessive frictional torque and eventual loosening of the acetabular component.

All other variables being equal, as the articulation diameter increases, the construct become more flexible. Therefore larger diameter articulations require larger diametrical clearance values. The deformation of the Metasul Alpha insert as a representative sandwich design and Durom ® acetabular components was investigated by FEA. These investigations demonstrated an optimal range of clearance values corresponding to the articulation diameter that is large enough to avoid potential risk of clamping. A minimal diametrical clearance of 70 μm is large enough for the 28mm or 32mm Metasul Alpha and Epsilon inserts and a 120 μm clearance is large enough for Durom acetabular components with an articulation diameter ≥38mm.

Clearance Influences Run-in Wear  

Clearance is also a key factor controlling the run-in wear behavior in metal-on-metal articulations. A small clearance minimizes the amount of wear and the number of cycles during the run-in period2 and thus  would reduce metallic debris.

References

  1. Data on file at Zimmer
  2. Rieker CB, et al: Influence of the clearance on in-vitro tribology of large diameter metal-on-metal articulations pertaining to resurfacing hip implants. Orthop Clin N Am 36, 2005, 135–142