Chronic pain was originally defined as pain lasting 3-6 months after onset, but has since been described as pain that extends beyond the healing period, disrupts sleep or normal activities, and is not explained by the low levels of pathology that characterize the disease or condition (JCAHO 2001 ). Patients with lower back pain, myofascial pain, and osteoarthritis (OA) are the most likely to suffer from chronic pain, which is one of the leading causes of disability within the work force (Yelin and Callahan 1995 ; CDC 2001 ; APF 2002 ). Over 40% of patients with musculoskeletal disease reported some form of disability, and more than half of working age people with musculoskeletal conditions were unable to work (Yelin and Callahan 1995 ; CDC 2001 ). According to the American College of Rheumatologists (ACR), 21 million Americans are affected by OA, which is associated with annual losses of 36 million workdays (Babul et al 2004 ; ACR 2005 ). The impact of chronic pain may be even greater; recent estimates by the Center for Disease Control place the number of adults with arthritis and chronic joint symptoms at around 70 million (CDC 2002 ). Living with chronic pain significantly reduces patients’ quality of life. In a study of 306 patients aged 55-74 years, patients with chronic pain in the hip or knee reported a significantly lower quality of life than a reference group not suffering from chronic pain (p < 0.045) (Hopman-Rock et al 1997 ). As outlined in Table 1 , untreated pain increases anxiety and depression, and is commonly associated with a decreased ability to cope (Eisendrath 1995 ; Yelin and Callahan 1995 ; APS 1996 ; Cohen et al 2000 ).
The effects of chronic pain on patients’ quality of life are also reflected in the low degree of life satisfaction in patients with this condition (Laborde and Powers 1980 ). According to the American Pain Foundation, two thirds of chronic pain sufferers were unable to perform routine physical tasks or to enjoy their hobbies, even though they were taking pain medication (APF 2006). The impact of chronic pain is underscored by the finding that past, present, and future satisfaction scores (assessed on Cantril’s self-anchoring scale) showed that patients with severe OA had significantly lower life satisfaction scores than patients on hemodialysis (p < 0.05) (Laborde and Powers 1980 ). Sleep disturbance is another major concern of patients with noncancer chronic pain; poor sleep has been reported in 70% of patients in chronic pain clinics and in 60% of patients suffering from arthritis (Menefee, Cohen, et al 2000 ). A cross-sectional survey of 167 patients with chronic spinal pain showed that high sleep quality and low sleep latency correlated positively with a shorter duration of pain and improved physical functioning (Menefee, Frank, et al 2000 ). High pain scores were independent indicators of overall sleep quality and sleep latency (Menefee, Frank, et al 2000 ). A small comparative study between 16 healthy subjects and 14 patients with OA showed an association between chronic pain and changes in EEG sleep patterns (Leigh et al 1988 ). Significant increases in stage I sleep (drowsiness), accompanied by decreases in stage II sleep (sleep onset), were observed in patients with OA, compared with normal subjects (Leigh et al 1988 ). Chronic pain is also associated with considerable economic costs. In the US in 2002, total lost productivity costs due to arthritis and lower back pain were estimated at US $10.3 billion and $19.8 billion, respectively (Stewart et al 2003 ).