Acetabular Component Loosening : Wheeless' Textbook of Orthopaedics

Loosening of Press Fit Acetabular Components

generally for bone ingrowth to occur, the component must lie with 50 micrometers of the acetabulum, inorder for host bone ingrowth to occur;
as noted by Bloebaum RD, et al (1997), bone ingrowth into component averages only 12%, even though 84% of the cup surface was in contact w/ periprosthetic bone;
- bone ingrowth was found to be uniform in all zones, most likely due to the uniform distribution of stresses from the metal backing;
references:
- Postmortem Analysis of Bone Ingrowth into Porous Coated Acetabular Components.
- Incomplete seating of press-fit porous-coated acetabular components: the fate of zone 2 lucencies.

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.

radiolucent lines that initially appeared after two years
- The histology of the radiolucent line.
progression of radiolucent lines after two years
radiolucent lines in all three zones
radiolucent lines 2 mm or wider in any zone
migration
- loosening is present w/ more than 2 mm of migration (either horizontal or vertical);
- references:
  - The acetabular teardrop and its relevance to acetabular migration.
  - Migration of Acetabular Components, Inserted with and without Cement, in One-Stage Bilateral Hip Arthroplasty.
continuous radiolucent line;
- note that peripheral radiolucent lines which are non-continuous are commonly found in press fit acetabular components and are often not progressive;
of note, postoperative gaps are net necessarily associated with subsequent presence of radiolucent lines, progressive radiolucent lines, or socket-loosening;
reference:
- Comparison of Preoperative Radiographs and Intraoperative Findings of Fixation of Hemispheric Porous-Coated Sockets.

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?

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;

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);

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;

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

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