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Acetabular Component Loosening

(see also: acetabular component

Loosening of Press Fit Acetabular Components

(press fit acetabular components)

normal bone ingrowth

radiographic signs of ingrowth fixation

(from Moore MS, et al.)

  • 97 percent of the cups with three or more of these signs were determined to be stable at the time of revision surgery
  • in contrast, 83% of the cups with two or fewer signs were loose.
  • five radiographic signs of stable cup fixation:
    • absence of radiolucent lines
    • presence of a superolateral buttress
    • medial bone stress-shielding
    • radial trabeculae
    • inferomedial buttress
  • Radiographic Signs of Osseointegration in Porous-coated Acetabular Components.

radiographic signs of loosening

(see also: radiographic techniques for eval of acetabulum)

other considerations

  • consider presence of occult infection; (see infected total hip replacement)
  • Is aseptic loosening of the prosthetic cup after total hip replacement due to nonculturable bacterial pathogens in patients with low grade infection?

Loosening of Cemented Acetablar Components

(cemented acetabular component)

  • radiolucency upto 2 mm wide with or without a surrounding fine line of density may develop in one or more of the the three zones about cement mass in the pelvis;
  • as in the femur, radiolucency is produced by the dense fibrous membrane and in some areas, fibrocartilage that forms about the surface of the cement and the surrounding shell of reactive bone;
  • loosening commonly occurs at the stem cement interface, but it rarely occurs at the cup cement interface;

risk factors for loosening

  • as noted by Kobayashi S, et al (1997), the most important factor for loosening is rapid wear of the polyethylene (more than 0.2 mm per year), followed by atrophic osteoarthritis (DJD w/o osteophytes);

definite loosening

  • migration of component of more than 5 mm;
  • crack in cement mantle;

probable loosening

  • continuous radiolucent line > 1-2 mm wide at the bone cement interface;
  • as pointed out by Hodgkins et al 1988, the finding of a continuous radiolucent line about the acetabular component indicates loosening in 94% of patients;

possible loosening

  • non circumferential radiolucency which extends at least 50% around the bone cement interface;

implications of loosening

  • even w/ radiographic loosening, only 10% of these hips will require revision;

Effect of Femoral Head Size

  • higher frequency of isolated acetabular loosening was associated with use of a 32-mm femoral head.
  • increased loosening rate of acetabulum w/ larger femoral head diameter may reflect thinner poly and higher frictional torsional stresses transmitted to the acetabular component;
    • w/ smaller femoral heads, the frictional forces are less and the polyethylene is thicker