The hip joint is a “ball and socket”-type synovial joint. The “ball” is actually the head of the femur bone, and the “socket” is an area of the pelvis called the acetabulum. Osteoarthritis (OA) of the hip has recently been described as a “disease process” with tissue destruction and a poor ability to repair the synovial joint (Bissonault & Goodman, 2003). Fifty percent of hip OA is caused by a previous injury or genetic predisposition (Gelber et al., 2000; Spector et al., 1996); other risk factors include obesity, inactivity, and structural abnormalities of the hip. Hip OA results in damage to the joint capsule, cartilage, eventually bone. In addition to causing pain, these degenerative changes lead to characteristic losses in range of motion, muscle imbalances, and gait abnormalities.
Leading Cause of Disability
Arthritis and other rheumatic conditions are the leading cause of disability in older adults (MMWR, 2009). Conservatively, OA affects 13.9% of adults aged 25 and older and 33.6% (12.4 million) of adults over 65 (Lawrence et al., 2008). Osteoarthritis of the hip occurs in approximately 4.4% per 100 population over age 55 (Lawrence et al., 2008). The incidence of hip arthritis increases rapidly with age, however, affecting more women than men, particularly after age 60 (Olivera et al., 1995). Hip OA patients have a 78% chance of developing OA in their opposite hip within 10 years (Ritter et al., 1996).
Hip OA is usually managed conservatively with medication, assistive devices, and physical therapy. While there is no cure for osteoarthritis, exercise has been shown to be effective at reducing pain and improving function in patients with hip OA (Fransen et al., 2002; Tak et al., 2005; Van Baar et al. 1998, 1999), perhaps even influencing the disease progression (Bennell et al., 2005).
Patients with hip OA often display muscle imbalances. Most commonly, hip OA is associated with tightness of the hip flexors and adductors, and weakness of the hip extensors and abductors (Arokoski et al., 2002; Long et al., 1993). Researchers note that arthritic joints exhibit “arthrogenous muscle inhibition” (AMI) (Hurley and Newham, 1993), where muscles are weakened due to a lack of normal joint proprioception due to sensitized joint receptors in arthritic joints. While OA patients typically exhibit similar patterns of muscle imbalance, a cause-and-effect relationship remains yet to be established. Nonetheless, exercises should be prescribed in patients with hip OA based on the presence of muscle imbalance (Page et al., 2010). Simply put, stretch muscles that are tight, and strengthen muscles that are weak. (For more information on muscle imbalance, read my book Assessment and Treatment of Muscle Imbalance: The Janda Approach.)
Severe hip OA often requires hip replacement surgery, replacing the ball and/or socket with an implant. Total hip replacements (THR) replace both the ball and socket, while partial hip replacements replace either the ball or socket (usually the ball). The orthopedic surgeon decides the specific implant to use, which can be comprised of any combination of metal, plastic, or ceramic parts. The American Academy of Orthopedic Surgeons reported a total of 231,000 total hip replacements in the United States in 2006 (AAOA, 2009), and expects an increase of nearly 150% by 2030 to 572,000 (AAOA, 2008) because of the increase in the aging population.
Pre-operative exercises before hip replacement (“prehabilitation”) have been shown to improve outcomes (Gilbey et al., 2003; Wang et al., 2002; Whitney et al., 2002) and decrease the length of hospital stays (Suetta, 2004). Hip replacement patients typically receive six to eight weeks of home health therapy after surgery. Not surprisingly, many patients continue to have cardiovascular and musculoskeletal impairments due to long-standing limitations prior to surgery. Horstmann et al. (2002) found limitations in aerobic and exercise capacity at six months post-operatively. Hip replacement patients also exhibit strength deficits up to 80% from 6 to 12 months after surgery (Horstmann et al., 1994, 2001; Long et al., 1993; Reardon et al., 2001; Shih et al., 1994; Sicard-Rosenbaum et al., 2002) and remain significantly weaker than other individuals at their age (Sicard-Rosenbaum et al., 2002).
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