Overview Osteolysis represents a histiocytic response to wear debris Steps in the process include: particulate debris formation macrophage activated osteoclastogenesis and osteolysis prosthesis micromotion particulate debris dissemination Evaluation radiostereometric analysis most accurate and precise technique to evaluate polyethylene wear uses radiopaque tantalum beads planted in the bone to follow the position of the components (relative to the beads) on radiographs Step 1: Particulate Debris Formation Types of wear adhesive wear most important in osteolytic process microscopically, PE sticks to prosthesis and debris gets pulled off abrasive wear cheese grater effect of prosthesis scraping off particles third body wear particles in joint space cause abrasion and wear volumetric wear main determinant of number of particles created directly related to the radius² of the femoral head volumetric wear more or less creates a cylinder V = 3.14r² x w V is volumetric wear r is the radius of femoral head w is linear head wear head size is most important factor in predicting particles generated linear wear measured by the distance the prosthesis has penetrated into the liner Wear leads to particulate debris formation wear rates by material polyethylene non-cross-linked UHMWPE wear rate is 0.1-0.2 mm/year linear wear rates >0.1 mm/year have been associated with osteolysis and subsequent component loosening highly cross-linked UHMWPE generates smaller wear particles and is more resistant to wear (but has reduced mechanical properties compared to conventional non-highly cross-linked UHMWPE) factors increasing wear in THA thickness <6 mm malalignment of components patients <50 y/o men higher activity level femoral head size between 22-46 mm in diameter does not influence wear rates of UHMWPE ceramics ceramic bearings have the lowest wear rates of any bearing combination (0.5-2.5 µ per component per year) ceramic-on-polyethylene bearings have varied wear rates, ranging from 0-150 µ unique complication of stripe wear occurring from gait with lift-off separation of the head recurrent dislocations or incidental contact of femoral head with a metallic shell can cause "lead pencil-like" markings that lead to increased femoral head roughness and polyethylene wear rates metals metal-on-metal produces smaller wear particles and lower wear rates compared to metal-on-polyethylene bearings (ranging from 2.5-5.0 µ per year) titanium used for bearing surfaces has a high failure rate because of poor resistance to wear and notch sensitivity metal-on-metal wear stimulates lymphocytes metal-on-metal serum ion levels are greater with cup abduction angle >55° and smaller component size Particulate type UHMWPE most common PMMA Co-Cr Ti third body Particulate size <1 µm Step 2: Macrophage Activated Osteoclastogenesis and Osteolysis Macrophage activation results in macrophage activation and further macrophage recruitment macrophage releases osteolytic factors (cytokines) including: TNF-alpha osteoclast activating factor oxide radicals hydrogen peroxide acid phosphatase interleukins (IL-1, IL-6) prostaglandins Osteoclast activation and osteolysis increase of TNF-alpha increases RANK increase of VEGF with UHMWPE enhances RANK and RANKL activation RANKL mediated bone resorption an increase in production of RANK and RANKL gene transcripts leads to osteolysis Step 3: Prosthesis Micromotion Osteolysis surrounding the prosthesis leads to micromotion micromotion leads to an increase in particle wear and further prosthesis loosening N-telopeptide urine level is a marker for bone turnover and is elevated in osteolysis Step 4: Debris Dissemination Increase in hydrostatic pressure leads to dissemination of debris into effective joint space increased hydrostatic pressure is the result of inflammatory response dissemination of debris into effective joint space further propagates osteolysis a circumferentially coated prosthesis limits osteolysis in the distal femur