accumulate in the body. In implants which involve metal-on-metal contact, microscopic fragments of cobalt and chromium can be absorbed into the person's bloodstream. There are reports of cobalt toxicity with hip replacement, particularly metal-on-metal hip replacements, which are no longer in use. Use of metal-on-metal hip replacements from the 1970s was discontinued in the 1980s and 1990s, particularly after the discovery of aseptic lymphocyte-dominant vasculitis-associated lesions (ALVAL). However, the FDA's 510k approval process allowed companies to have new and "improved" metal-on-metal hips approved without much clinical testing. Some people with these prostheses experienced similar reactions to the metal debris as occurred in the 20th century; some devices were recalled. Nerve palsy Post operative sciatic nerve palsy is another possible complication. The frequency of this complication is low. Femoral nerve palsy is another, but much rarer, complication. Both of these will typically resolve over time, but the healing process is slow. Patients with pre-existing nerve injury are at greater risk of experiencing this complication and are also slower to recover. Chronic pain A few patients who have had a hip replacement suffer chronic pain after the surgery. Groin pain can develop if the muscle that raises the hip (iliopsoas) rubs against the edge of the acetabular cup. Bursitis can develop at the trochanter where a surgical scar crosses the bone, or if the femoral component used pushes the leg out to the side too far. Also some patients can experience pain in cold or damp weather. Incision made in the front of the hip (anterior approach) can cut a nerve running down the thigh leading to numbness in the thigh and occasionally chronic pain at the point where the nerve was cut (a neuroma). Death The rate of perioperative mortality for elective hip replacements is significantly less than 1%. Metal-on-metal hip implant failure By 2010, reports in the orthopaedic literature increasingly cited the problem of early failure of metal-on-metal prostheses in a small percentage of patients. Failures may have related to the release of minute metallic particles or metal ions from wear on the implants, causing pain and disability severe enough to require revision surgery in 1–3% of patients. Design deficits of some prothesis models, especially with heat-treated alloys and a lack of specialized surgical experience, accounted for most of the failures. In 2010, surgeons at medical centers such as the Mayo Clinic reported curtailing their use of metal-on-metal implants by