Overview Osteolysis represents a histiocytic response to wear debris, resulting in bone loss Basic steps of osteolysis 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 Imaging Plain radiographs to monitor for progression underestimates lesion size and may miss small lesions various methods for monitoring osteolysis progression manual methods utilize distances from the superior and inferior aspects of femoral heads to the acetabular cup to monitor for progression Livermore method Measures maximum head penetration by identifying the shortest distance between the femoral head center and a point on the acetabular surface computer-assisted techniques utilizes two-dimensional radiographs to create a 3D rendering that estimates wear Radiostereometric analysis most accurate and precise method for wear measurement spherical tantalum markers are used to physically define the implant's location radiographs from two different angles are then taken and processed in a computer program to geospatially define the object in 3D space osteolysis progression is measured by comparing prior radiostereometric measurements to current measurements to determine component migration Computed Tomography creates true 3D rendering of components, allowing for monitoring of osteolytic defect progression useful for surgical planning helpful with determining component positioning possible future utility in establishing the degree of linear and volumetric wear Step 1: Particulate Debris Formation Mechanisms of wear adhesive wear caused by two surfaces of different materials bonding together and subsequent peeling of the softer material during motion abrasive wear occurs between two dissimilar materials harder non-smooth material cuts into softer material (akin to a cheese grater) delaminating wear formation of fissures within polyethylene, which leads to large particle formation third-body wear particles in the joint space cause abrasion and wear Sub-types of wear linear wear can occur with adhesive or abrasive wear reduction in polyethylene component thickness due to prosthesis penetration into the liner linear wear rates are independent of femoral head size volumetric wear can occur with adhesive or abrasive wear main determinant of the number of particles created directly related to the radius² of the femoral head volumetric wear more or less creates a cylinder head size is most important factor in volumetric wear Wear leads to particulate debris formation wear rates by material polyethylene colorless and vary in shape size (spheroid is most commonly seen shape) linear wear rates >0.1 mm/year have been associated with osteolysis and subsequent component loosening non-cross-linked UHMWPE wear rate is 0.1-0.2 mm/year 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), however, it is considered more bio-reactive 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 typically alumina or zirconia 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 debris from ceramic-on-ceramic implants are thought to be the result of mechanical issues with implantation (i.e. malpositioning, instability) as opposed be the cause of implant failure unique complication of stripe wear occurring from contact between the femoral head and the edge of the liner etiology unclear, though surmised to be caused by gait or edge loading during deep hip flexion recurrent dislocations or incidental contact of the 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 (30 nm to 200 um and lower wear rates compared to metal-on-polyethylene bearings (ranging from 2.5-5.0 µ per year) 4-5% risk of aseptic loosening with metal on metal prosthesis at 6 years post implantation 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 and pseudotumor development via ALVALreaction seen more frequently in Titanium alloy compared to Cobalt/Chrome alloy or stainless steel ALTR creates a local reaction characterized by watery, yellow or gray appearing joint effusion basic (opposed to acidic) pH low cell count metal-on-metal serum ion levels are greater with cup abduction angle >55° and smaller component size metal on metal implants also have risk of electro-chemical corrosion leading to additional debris formation metal ions chromium oxide Particulate type UHMWPE most common PMMA Aluminum oxide Zirconium oxide Co-Cr Ti third body Particulate size <1 µm particle size is more important than particle type with regard to influencing biologic activity. Risk factors for wear activity level femoral head size (> 32 mm) increased contact stresses male high BMI acetabular inclination > 45 degrees Step 2: Macrophage-Activated Osteoclastogenesis and Osteolysis Macrophage activation occurs via phagocytosis or cell contact activation (via toll-like receptors) leads to increased transcription of Nuclear Factor Kappa B, which leads to increased production of: TNF-alpha increases RANK production and RANKL mediated bone resorption stimulates the release of granulocyte macrophage-colony stimulating factor (GCSF), which leads to the production of reactive oxygen species such as: oxide radicals hydrogen peroxide monocyte chemoattractant molecule 1 recruits additional macrophage/monocytes osteoclast activating factor acid phosphatase interleukins (IL-1, IL-6, IL-8) prostaglandins Osteoclast activation and osteolysis increase in TNF-alpha levels increases RANK presence increase of VEGF specifically with UHMWPE enhances RANK and RANKL activation RANKL-mediated bone resorption an increase in production of RANK and RANKL gene transcripts leads to osteolysis via osteoclast activation reactive oxygen species produced as a result of increased Nuclear Kappa Factor B levels also stimulate osteoclast-mediated bone resorption Step 3: Prosthesis Micromotion Osteolysis surrounding the prosthesis leads to micromotion micromotion leads to an increase in particle wear and further prosthesis loosening mechanical forces caused by joint fluid/implant can additionally lead to bone resorption, though these mechanisms are not well characterized hypothesized mechanisms include pressure induced bone resorption, osteoclastogenesis modulation, inflammatory processes involving cell mediated 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 the dissemination of debris into effective joint space increased hydrostatic pressure is the result of an inflammatory response dissemination of debris into effective joint space further propagates osteolysis circumferentially coated prosthesis limits osteolysis in the distal femur\ Systemic Dissemination increased levels of metal ions may be measured in the serum in patients with metal on metal implants no clear evidence of association between metal debris and neoplastic, toxic, or metabolic conditions