During the primary surgery to replace a hip with a Durom Cup, the surgeon uses a device to ream the pelvis socket to the shape of the cup to be implanted. Zimmer expected the cup to fit snugly into the reamed pelvic socket and bone from the socket would then grow into the back side of the cup to stabilize it.
The failure mode reported with the Durom Cup is cup loosening due to lack of bone in-growth on the back side of the cup at the cup-bone interface. There are specific design features of the Durom components that allow for this failure mode to occur.
1.Peripheral press-fit of a non-hemispherical shaped cup and flattened head
The Durom cup itself is not hemispherical but rather an ellipse-shape with flared sides. The non-hemispherical peripheral press-fit cup design prevents contact with the prepared acetabular bone particularly when the preparation is performed with a hemispheric reamer, preventing bone in-growth leading to poor cup fixation. Fixation is concentrated at the rim by the flare design and exacerbated by the sharp peripheral fins. In areas where the cup and bone are in contact, the cup design allows for micromotion of the fixation surface with maximum motion at the dome. Little micromotion (>40um) could impair or prevent bone in-growth. Additionally, the pole of the Durom femoral head has a flattened dome. This reduced sphericity leads to an increased frictional torque at the cup-bone interface, again causing micromotion and disrupting osseointegration into the cup.
The backside coating of the Durom cup has a fine porous structure with 20-50um pore size. For effective secondary bone fixation, the acceptable pore size for the coating has been reported to be between 100-400um. Fixation was reported to be the weakest with a pore size between 20-50um, which is exhibited by the Durom Cup.
3. Cup Deformation
The Durom cup deforms upon impaction during implantation. This deformation changes the clearance and sphericity which affects the lubrication of the articulation. Equatorial contact occurs which increases frictional torque at the cup-bone interface causing micro-motion and preventing bone in-growth.
4. Cup Installation Difficulty
The sharp peripheral fins of the flared cup with a head coverage of 165 degrees does not permit predictable, repeatable bone preparation especially with the recommended hemispheric reamers and hemispheric preparation. Additionally, the design of the Durom Cup made it difficult to orientate and seat resulting in unpredictable fixation.
Helping Defective Hip Replacement Recipients In Wisconsin