Design of Acetabular Components : Wheeless' Textbook of Orthopaedics

- Cementless Design:
- acetabular components that are to be inserted w/o cement should be designed to be portions of spheres so that spherical reaming will optimize contact;
- sufficient initial fixation is a requirement for osseointegration and secondary stability of press fit cups;
- surface coating:
    - load transfer and points of fixation:
            - Load transfer and fixation mode of press-fit acetabular sockets.
            - [Contact surface and pressure load at implant-bone interface in press-fit cups compared to natural hip joints]
            - Comparative in vitro assessment of the primary stability of cementless press-fit acetabular cups.
    - normal bone ingrowth:
- generally for bone ingrowth to occur, the component must lie with 50 micrometers of the acetabulum, inorder for host bone ingrowht to occur;
- as noted by Bloebaum RD, et al (1997), bone ingrowth into the component averages only 12%, even though 84% of the cup surface
   was in contact with 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;
            - Postmortem analysis of bone growth into porous-coated acetabular components.
            - [Cementless socket fixation based on the "press-fit" concept in total hip joint arthroplasty]
    - hemispere vs peripheral flare components:
- it has also been demonstrated by Kim YS, et al (1995) that underreaming by more than 2 mm leaves
gaps in the prosthesis-bone interval in the polar portion of the acetabulum;
- because of this some surgeons have gone back to line to line reaming with hemispherical components;
- this reduces the likelihood of fracture and maximizes bony contact;
                  - Reamed surface topography and component seating in press-fit cementless acetabular fixation.
    - role of screw fixation:
- these components should be thick enough so that adequate strength is obtained despite holes for fixation;
- role of polyethylene:
- another important design consideration is congruence of inner surface of metal shell to outer surface of ultra-high molecular wt polyethylene liner;
- inadequate congruence of these surfaces can lead to excessive stress and frx of ultra-high molecular wt poly or, alternatively, to
                  motion and wear between the components;
- titanium may be an inappropriate bearing surface against poly
    - offset:
- references:
- Acetabular Revision With a 7-mm Lateral Offset Component Inserted Without Cement in Patients Who Have Acetabular Deficiency.
- Offset acetabular components introduce torsion on the implant and may increase the risk of fixation failure.