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 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