Acetabular Cup Insertion

(see also: Acetabular Component Menu)

Acetabular Component

press fit components »

• note whether acetabular component is hemispheric or non-hemispheric;
  • non hemispheric components (ie over sized components or components with a peripheral flare) may be more appropriate for protrusio or for situations where acetabulum will be medialized where as hemisperic components may be more appropriate for shallow or osteoporotic acetabuli;

acetabular liner / femoral head size

• Jasty M, et al (1997): thickness of polyethylene component is especially important in metal backed press fit components;
• w/ smaller acetabular components consider using a 26 mm rather than a 28 mm head;
• Uhl RL, et al (2000): a bench top study to evaluate the consquences of mixing different modular THR components;
  • combination of a skirted neck, smaller femoral head, and a posterior lipped liner can haave effect of causing impingement of the neck on the raised liner (which can cause the femoral head to lever out of socket);
• references:
  • Larger diameter femoral heads used in conjunction with a highly cross-linked ultra-high molecular weight polyethylene: a new concept.
  • Wear of polyethylene acetabular components in total hip arthroplasty. An analysis of one hundred and twenty-eight components retrieved at autopsy or revision operations.
  • A bench-top method for evaluating modular total hip component combinations

over-sizing the acetabular component »

Technical Considerations in Component Insertion

acetabular reaming »

acetabular component position »

• component is inserted in same direction of reaming;

trial component considerations

• insert trial component and note the distance between the end of the prosthesis and the bottom of acetabulum;
• with firm hand pressure usually this distance is about 5-6 mm;
• note quality of subchondral bone:
  • hard sclertic bone:
    • with hard circumferential subchondral bone only 1 mm oversized component may be sufficient;
    • ensure that the 1 mm undersized component trial can be fully seated (reamer may be used as a trial)
    • consider re-reaming with 1-2 mm undersized reamer to disrupt the circumferential subcondral rim;
    • soft bone:
      • may need to undersize by 2 mm to ensure that the hand pressed trial is about 5-6 mm from the medial edge of the acetabulum (which means that the real component will advance with good peripheral contact);

component insertion

• w/ proper acetabular alignment, impact acetabular cup into place;
• component impaction
  • as the component is impacted into place, the component should advance with each blow of mallet;
  • listen for change in pitch as component seats down;
  • probe screw holes and if gap is present then impact further;
  • references:
    • Fluctuation of Cup Orientation During Press-Fit Insertion: A Possible Cause of Malpositioning
    • Periprosthetic Occult Fractures of the Acetabulum Occur Frequently During Primary THA.
• failure to advance:
  • if the component fails to properly advance with blows of the mallet, then gentle peripheral reaming is required;
  • the component is then reinserted;
• assess component stability:
  • need to determine whether stability is optimal without screw placement;
  • kocher clamp is applied to lip of component - vigorous shaking without component motion indicates component stability;
    • with inadequate stability, it will be necessary to insert screws or switch to a large component;
    • reference: Cup Press Fit in Uncemented THA Depends on Sex, Acetabular Shape, and Surgical Technique
  • acetabular cup insertion without screw insertion: (see also: components with screws)
    • advantages:
      • shorter operative time;
      • ability to adjust cup position if hip instability is found to be present;
      • absence of screw related vascular complications;
      • easier cup removal at the time of revision (should it be necessary);
    • technical considerations:
      • cup should be oversized (or acetabulum should be under-reamed);
      • note that insertion of oversized cups may result in fracture, especially in over-sized bone;
    • component insertion with screws and without forced impaction:
      • acetabular components can be inserted with use of a line-to-line technique without impaction, relying only on the screws to obtain the initial fixation;
      • Craig, et al noted 96% survivorship with technique;
      • impaction is not a prerequisite for bone ingrowth or for long-term implant fixation
      • references:
        • Primary total hip arthroplasty with a porous-coated acetabular component. A concise follow-up, at a minimum of twenty years, of previous reports.
        • Successful long-term fixation and progression of osteolysis associated with first-generation cementless acetabular components retrieved post mortem.
    • references:
      • Impact biomechanics and pelvic deformation during insertion of press-fit acetabular cups.
      • Incomplete seating of press-fit porous-coated acetabular components: the fate of zone 2 lucencies.
      • Extra-large press-fit cups without screws for acetabular revision.

screw placement »

anterior & inferior osteophytes

• if the acetabulum is retroverted then anterior wall itself will project beyond edges of cup if it is set in anteversion;
• reflected head of the rectus may be attached to osteophyte;
• anterio-inferior wall osteophytes must be removed w/ osteotome, cutting along the edge of the cup;
  • failure to heed this may cause posterior dislocation in flexion and internal rotation;

Polyethylene insertion »

References

• Measuring the Femoral Head Size — An Additional Real-Time Intraoperative Monitoring Tool for the Accuracy of the Preoperative Process and Implant Selection
• Load transfer and fixation mode of press-fit acetabular sockets.