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Updated: Jul 18 2022

THA Prosthesis Design

  • Introduction
    • Biomechanics of total hip arthroplasty depend on
      • prosthesis design
      • bearing surface and lubrication characteristics
      • fixation method
    • Designs include
      • femoral component
        • cemented
        • press-fit (uncemented)
          • tapered stems
          • extensively porous coated stems
          • modular stems
      • acetabular components
        • cemented
          • polyethylene
          • metal
        • press-fit (uncemented)
          • metal
        • Modular dual-mobility liners
          • Decreased risk of dislocation in revision setting
          • Unique possibility of liner malseating in the acetabular cup
      • bearing surfaces
        • polyethylene
        • metal
        • ceramic
  • History
    • 1891
      • Dr. Gluck performs first reported attempt at a hip replacement with ivory used to replace the femoral head
    • 1940
      • Austin Moore performs first metallic hip replacement surgery (hemiarthroplasty) with a proximal femoral replacement bolted to the femur
    • 1952
      • Austin Moore prosthesis developed
    • 1960s
      • Sir John Charnley introduces concept of low friction arthroplasty
        • concept
          • termed "low friction" as a small femoral head was used to reduce wear
        • components
          • metal femoral stem
          • polyethylene acetabular component
          • acrylic bone cement
  • Press-fit Femoral Stems
    • Overview
      • rely on biologic fixation
      • compression hoop stresses provide initial stability
    • Types
      • tapered stems
        • most are proximally coated stems that taper distally
        • examples
          • Tri-Lock (DePuy)
          • M/L Taper (Zimmer)
      • extensively coated stems
        • porous coating extends into the diaphysis for distal engagement
        • examples
          • AML (DePuy)
          • VerSys Full Coat (Zimmer)
      • modular stems
        • distal stem and proximal body can be "mixed-and-matched"
        • examples
          • S-ROM (DePuy)
          • ZMR (Zimmer)
      • collared vs. collarless stems
        • pros and cons to both designs 
    • Unique complications
      • intraoperative fracture
        • more likely in press-fit through lateral approach
        • typically due to underreaming
      • loosening
        • high loosening rate when used in irradiated bone (due to lack of ingrowth)
      • junctional corrosion
        • seen in modular components (including cemented modular components)
  • Cemented Femoral Stems
    • Overview
      • rely on cement fixation
        • cement is a grout that provides initial and long-term stability
        • limited remodeling potential
        • preferred for irradiated bone due to the bone's limited ability for ingrowth
        • preferred for poor proximal femoral bone stock (i.e. Dorr C femurs)
      • composition
        • cobalt-chrome or stainless steel
          • most common
          • reduce cement stresses
        • titanium
          • prone to micromotion and debonding
            • less stiff than cobalt-chrome or stainless steel stems 
            • cemented titanium stems leads to crevice corrosion (use of cemented titanium alloy stems is no longer recommended)
    • Unique complications
      • stem breakage
        • cemented stems are smaller than press-fit stems and unable to tolerate as much cantilever bending
          • may occur in cementless stems as well
  • Bearing Surfaces
    • Metal-on-polyethylene
      • metal (cobalt-chrome) femoral head on polyethylene acetabular liner
      • benefits
        • longest track record of bearing surfaces
        • lowest cost
        • most modularity
      • disadvantages
        • higher wear and osteolysis rates compared to metal-on-metal and ceramics
        • smaller head (compared to metal-on-metal) leads to higher risk of impingement
    • Metal-on-metal
      • benefits
        • better wear properties than metal-on-polyethylene
          • lower linear wear rate
          • debris particles much smaller (but more numerous) than those of metal-on-poly
            • overall smaller volume of particles
        • larger head allows for increased ROM before impingement
      • disadvantages
        • more expensive than metal-on-polyethylene
        • increased metal ions in serum and urine (5-10x normal)
          • serum metal ion concentration highest at 12-24 months
            • correlates with the initial "wear in" or "run-in" phase of increased particle generation, but then followed by a "steady state" phase of decreased particle generation
          • no proven cancer link
        • may form pseudotumors
        • hypersensitivity (Type IV delayed type hypersensitvity)
          • mediated by T-cells
            • metals sensitize and activate T-cells (nickel > cobalt and chromium)
            • however, most participating cells are macrophages (only 5% are lymphocytes)
          • antigen-activated T-cells secrete cytokines that activate macrophages
            • activated macrophages have increased ability to present class II MHC and IL-2, leads to increased T-cell activation
            • the cycle continues
        • contraindications
          • pregnant women
          • renal disease
          • metal hypersensitivity due to metal ions
    • Ceramic on Ceramic
      • benefits
        • best wear properties of all bearing surfaces
        • lowest coefficient of friction of all bearing surfaces
        • inert particles
          • no concern for cancer risk
      • disadvantages
        • more expensive than metal-on-polyethylene
        • worst mechanical properties (alumina is brittle, low fracture toughness)
          • small 28mm heads only exist in zirconia because of alumina's inferior mechanical properties
        • squeaking
          • increased risk with
            • edge loading
            • impingement and acetabular malposition
            • third-body wear
            • loss of fluid film lubrication
            • thin, flexible (titanium) stems
        • less modularity with fewer neck length options
        • stripe wear
          • caused by contact between the femoral head and rim of the cup during partial subluxation
          • results in a crescent shaped line on the femoral head
    • Ceramic on polyethylene
      • benefits
        • standard of care
        • alumina ceramic heads 
        • results in less polyethylene wear than metal-on-polyethylene (MoP) bearings
      • disadvantages
        • zirconia undergoes tetragonal to monoclinic phase transformation with time
          • increased with
            • prolonged in vivo implantation >8yr
            • pressure
            • temperature
              • has lower heat conductivity than alumina (joint temperature can reach 99oC for zirconia, and 50oC for alumina)
    • Titanium on Polyethylene
      • not recommended due to high wear rates
  • disadvantages
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