A higher than expected failure rate of the Zimmer Durom acetabular component has been reported. A study by Zimmer did not reveal a design defect. This study investigated impaction deformation of two cup designs.

Eleven Durom cups and modular heads (Zimmer, Warsaw, IN) were retrieved at an average of 13.9 months. The Birmingham Hip Resurfacing (Smith & Nephew, Memphis, TN) served as a control. Cups were impacted into a two-point acetabular loading model made of 30 grade urethane foam (Sawbones, Vashon, WA). A coordinate measuring machine with 2 micron (um) accuracy was used to map the inside diameters of the cups before, during and after impaction. Machinist’s dye was used to check head-cup contact.

The Durom porous coating was essentially devoid of tissue ingrowth. Two heads used with size 62/56 Duroms had equatorial wear stripes. The outside diameter of the Durom was 2.93±0.03mm larger than the nominal diameter. Dome wall thickness was 3.23±0.07mm for the Durom and 6.08±0.65mm for the BHR (n=11). Inside diameters of all cups had less than 10um deviation from roundness before impaction and after removal from the model. The mean diametral deformation of the Durom was 89.8±14.8um, significantly greater than the BHR, 57.2±25.0um (p< 0.002). Non-impacted cups exhibited polar contact—circular areas of dye at the dome with no contact near the rim. Duroms with greater deformation exhibited linear contact—a 2cm band of dye extending from rim to rim with no contact on either side of the band.

The Durom is a relatively thin-walled acetabular component with low clearance and an aggressive rim flare. Impaction of this cup into an acetabular model resulted in deformation which approached the diametral clearance. Maximal deformation with larger cups and warping of the articular surface correlated with observed wear stripes. The absence of residual cup deformation indicated deformation is a dynamic phenomenon which can be detected only under conditions simulating in vivo use. It is likely that impaction deformation, with consequent friction and wear, contributed to the early failure of the Durom acetabular component.