- 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;
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surface coating:
- load transfer and points of fixation:
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Load transfer and fixation mode of press-fit acetabular sockets.
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Contact surface and pressure load at implant-bone interface in press-fit cups compared to natural hip joints
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Comparative in vitro assessment of the primary stability of cementless press-fit acetabular cups.
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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 RD Bloebaum et al 1997, bone ingrowth into the component averages only 12%, eventhough 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;
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Cementless socket fixation based on the "press-fit" concept in total hip joint arthroplasty
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hemispere vs peripheral flare components:
- it has also been demonstrated by
Kim YS et al (J. Arthroplasty, 10 [Supplement]: S17, 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;
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role of screw fixation:
- these components should be thick enough so that adequate strength is obtained despite holes for fixation;
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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;
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offset:
- references:
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Acetabular Revision With a 7-mm Lateral Offset Component Inserted Without Cement in Patients Who Have Acetabular Deficiency.
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Offset acetabular components introduce torsion on the implant and may increase the risk of fixation failure.
Postmortem Analysis of Bone Ingrowth into Porous Coated Acetabular Components.
RD Bloebaum PhD, NL Mihalopoulus, JW Jensen, and LD Dorr MD. JBJS Vol 79-A, No 7, Jul. 1997. p 1013.
Effect of cementless acetabular cup geometry on strain distribution and press-fit stability.
Histological and radiographic assessment of well functioning porous-coated acetabular components. A human postmortem retrieval study.
Load transfer and fixation mode of press-fit acetabular sockets.
Initial stability of a cementless acetabular cup design: experimental investigation on the effect of adding fins to the rim of the cup.
Photoelastic analysis of stresses produced by different acetabular cups.
The ability of various acetabular components to resist protrusio migration.
Micromotion of cementless hemispherical acetabular components. Does press-fit need adjunctive screw fixation?
Review of the evolution of the cementless acetabular cup.
