can lead to catastrophic failure. This occurs in about 2% of implants. They may also cause an audible, high pitched squeaking noise with activity. Metal-on-metal arthroplasty can release metal debris into the body. Highly cross linked polyethylene is not as strong as regular polyethylene. These plastic liners can crack or break free of the metal shell that holds them. Loosening On radiography, it is normal to see thin radiolucent areas of less than 2 mm around hip prosthesis components, or between a cement mantle and bone. These may indicate loosening of the prosthesis if they are new or changing, while areas greater than 2 mm may be harmless if they are stable. The most important prognostic factors of cemented cups are absence of radiolucent lines in DeLee and Charnley zone I, as well as adequate cement mantle thickness. In the first year after insertion of uncemented femoral stems, it is normal to have mild subsidence (less than 10 mm). The direct anterior approach has been shown to itself be a risk factor for early femoral component loosening. Metal sensitivity Concerns were raised in the early 2000s regarding metal sensitivity and the potential dangers of metal particulate debris from hip prostheses, including the development of pseudotumors, soft tissue masses containing necrotic tissue, around the hip joint. It appears these masses were more common in women, and these patients showed a higher level of iron in the blood. The cause was then unknown, and was probably multifactorial. There may have been a toxic reaction to an excess of particulate metal wear debris or a hypersensitivity reaction to a "normal" amount of metal debris. Metal hypersensitivity is a well-established phenomenon and is not uncommon, affecting about 10–15% of the population. Skin contact with certain metals can cause immune reactions such as hives, eczema, redness and itching. Although little is known about the short- and long-term pharmacodynamics and bioavailability of circulating metal degradation products in vivo, there have been many reports of immunologic-type responses temporally associated with implantation of metal components. Individual case reports link immune hypersensitivity reactions with adverse performance of metallic cardiovascular, orthopedic and plastic surgical and dental implants. Metal toxicity Most hip replacements consist of cobalt and chromium alloys, or titanium. Stainless steel is no longer used. Any metal implant releases its constituent ions into the blood. Typically, these are excreted in the urine, but in certain individuals the ions can