Osteoarthritis is the most important underlying condition that creates the need for an artificial hip implant. Pain is the primary symptom of this noninflammatory arthritis of the hip. The pain increases with exercise and is often diminished by rest. Patients may complain of stiffness after prolonged sitting. Radiographic studies may reveal narrowing of the hip joint space caused by loss of cartilage.
The prevalence of osteoarthritis increases with age. Other risk factors for osteoarthritis include trauma and repetitive joint use. Obesity is ariskfactor for osteoarthritis of the hip, but the magnitude of the risk is much lower than for osteoarthritis of the knee .
Body mass index [BMI] is directly associated with the risk of total hip replacement. For example, a study by Karlson et al.  found that higher BMI was significantly associated with an increased risk of hip replacement because of osteoarthritis (p for trend = 0.0001). Women with BMI > 35 were twice as likely as more lean women with a BMI < 22 to undergo hip replacement. In a study by Flugsrud et al. , 50 034 patients were followed for 9 years, and a dose-response relationship was found between BMI and total hip replacement for primary osteoarthritis. The highest quartile of BMI subjects had a relative risk of 2.0 (95% CI = 1.4-2.9) among men, and 3.0 (95% = CI2.1-4.1) among women, compared in both cases to the lowest quartile of BMI subjects.
Aseptic necrosis, also referred to as osteonecrosis or avascular necrosis, of the femoral head is another important underlying pathology that can result in the need for artificial hip replacement. The causes of aseptic necrosis are truly legion, including anemia, corticosteroid use, ethanol abuse, pancreatitis, and trauma. Many cases of aseptic necrosis are idiopathic as well . Pavelka  has observed that the femoral head is a common site of osteonecrosis and that almost one-half of all cases of femoral head osteonecrosis require hip arthroplasty. The mean age of patients with aseptic necrosis of the femoral head is far lower than that of their counterparts with severe osteoarthritis of the hip. Aseptic necrosis patients are often near 40 years of age . The life expectancy of such patients is substantially greater than the typical patient who receives an artificial hip secondary to osteoarthritis. Aseptic necrosis patients pose a greater clinical challenge, as they will be more likely to require revision of their artificial hip implant because of their generally younger age at initial implantation.
Total hip arthroplasty can yield very substantial clinical dividends. These benefits were well quantified in a study by Bachmeier et al. . A total of 86 patients with osteoarthritis of the hip who received artificial hip arthroplasty were followed for 1 year. Dramatic improvements in function and quality of life were documentedby the use of the Western Ontario and McMaster Universities Osteoarthritis Index, a well established and recognized tool. One year following hip replacement, the following observations were made: a 71% reduction in pain, a 55% reduction in stiffness, and a 68% increase in physical function. The study also employed the Medical Outcomes Study SF-36 Health Survey to assess the impact of the hip replacement. Again, the observed clinical changes were dramatic. The measures for pain improved 22%, indicating a decrease in pain, and there were improvements in physical function (247%), physical role functioning (402%), general health (110%), vitality (143%), social functioning (275%), and mental health (114%).
An examination of the medical literature in the aggregate for evidence of improvement of the patient's health-related quality of life following total hip arthroplasty was conducted by Ethgen et al. . This examination included studies published in French or English during January 1980-June 2003 that utilized a prospective cohort study design and what the authors considered to be a validated self-reported quality of life measurement tool. The authors included 32 studies of both total hip and knee arthroplasty and 26 studies limited to total hip arthroplasty. They concluded that total hip arthroplasty was highly successful in improving health-related quality of life parameters. The failure of age to present a barrier to favorable total hip arthroplasty outcomes was another noteworthy conclusion.
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