Vol. 19 Issue 5
An Examination of the WOMAC Index
Can pain and treatment effects for hip and/or knee osteoarthritis be measured?
Let's take a closer look at the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC).
Osteoarthritis (OA), also known as degenerative arthritis or degenerative joint disease, is a condition in which low-grade inflammation results in pain in the joints, caused by abnormal wearing of the cartilage that acts as a cushion inside joints, and decrease of synovial fluid that lubricates those joints.1 It is not a single disease but rather the end result of a variety of disorders leading to the structural or functional failure of one or more joints, most commonly the hands, knees, hips or spine.2
Osteoarthritis involves the entire joint, including the nearby muscles, underlying bone, ligaments, joint lining (synovium) and the joint capsule. It is the most common form of arthritis and causes pain, swelling and reduced motion in the joints. Factors that may cause osteoarthritis include being overweight, advancing age and joint injury.
Exams and Tests for Osteoarthritis
Radiographs: Approximately one-third of people with osteoarthritis on X-rays have symptoms such as pain or swelling. X-rays can show narrowing of the space between the joint (articular surface), osteophytes, cyst formation and hardening of the underlying bone.
Scoring systems have been used by physicians to assess the extent of the bony changes on X-rays. Separate scoring systems for the different joints have been studied and found to be predictive of disease status.
An important finding from these studies was that the presence of osteoarthritis of the hands was a predictive sign of deterioration of the knee joint. In other words, people with finger joint osteoarthritis were more at risk to show a rapid progression in their knee.
MRI. MRI is capable of visualizing all structures within the joint.
CT scan. This study may be used to image a joint. CT scanning mainly provides information on the bony structures of the joint but in greater detail than plain radiographs.
Synovial fluid analysis. Fluid may be aspirated from the knee with a needle in cases in which the diagnosis is uncertain or if an infection is suspected.
Blood tests. No currently accepted blood test or marker for this disease exists. Blood tests may be drawn in cases in which infection is suspected.
Self-report measurement procedures for quantifying pain and other symptoms from OA of the hip and knee prior to 1981 were nondescript, not standardized and were unavailable in languages other than English.3
In 1982, the medical community sought innovative ways to measure the debilitating affects of OA focusing on a disease-specific, self-report health questionnaire. The outcome of this quest was the WOMAC.
The WOMAC was originally devised to evaluate three dimensions including pain, stiffness and physical function in people with osteoarthritis of the hip and knee.4 It is intended to assess change in symptoms of patients who have received therapeutic intervention.5 The pain scale has five questions and evaluates ability to walk and stair-climb. Nocturnal pain and pain with weight bearing and at rest is also assessed.
There are two questions for stiffness (morning or stiffness occurring later in the day), and 17 questions were allotted for assessing physical function (e.g., descending/ascending stairs, rising from sitting, standing, bending to floor, walking on flat, getting in/out of a car, going shopping, putting on socks, rising from bed, taking off socks, lying in bed, sitting, getting on or off the toilet, heavy domestic duties, light domestic duties).
Since its development, the WOMAC has undergone extensive validation studies and has been translated into numerous languages including German, Swedish, Spanish and Hebrew.6-9
The WOMAC uses a Likert ordinal scale ranging from 0-4 to rate patient symptoms, with lower scores indicating lower levels of symptoms of physical disability.
A score of 0 indicates no symptoms; 1 = slight; 2 = moderate; 3 = severe; 4 = extreme. Maximum scores for the pain, stiffness and physical function scales are 20, 8, and 68 respectively, for a total score of 96.5,10 A global score may be calculated by summing the scores for the three subscales.11
A visual analog version (VAS) of the WOMAC is also available for use with scores ranging from 0-10.3,5 Of note is the development of social and emotional function scales that were not included in the final version of the WOMAC.
Reliability, or the "consistency" or "repeatability" of measures, has been extensively evaluated in both the Likert and VAS version of the WOMAC. Test-retest reliability of the Likert version had the following values for pain (Kendall's tau c = 0.68), stiffness (Kendall's tau c = 0.48) and physical function (Kendall's tau c = 0.68). The VAS version had Kendall's tau c values of 0.64, 0.61 and 0.72 for pain, stiffness and physical function, respectively. The global score was 0.68 (Kendall's tau c) for the Likert version and 0.64 (Kendall's tau c) for the VAS version.
In a study of arthroscopically assessed patients with knee OA, the testÐretest reliability of the WOMAC had ICC values of 0.74, 0.58 and 0.92 for the pain, stiffness and physical function subscales respectively.12
Internal consistency, or the extent to which all items in a scale or test measure the same concept, has also been scrutinized in both Likert and VAS versions of the WOMAC.
The pain subscale had a Cronbach's = 0.86 for the Likert version and 0.89 for the VAS version.10 Cronbach's for the stiffness subscale was 0.90 for the Likert version and not determined for the VAS, with the physical function subscale having a Cronbach's value of 0.95 for the Likert version and 0.89 for the VAS version.10 Internal consistency at presurgery, six weeks and six months postsurgery for pain in patients undergoing THA, ranged from 0.78 to 0.93; for stiffness from 0.75 to 0.88; and for physical function from 0.92 to 0.97 for the Likert version.5,10
Similar values were obtained with the VAS version.13 Internal consistency in a study of arthroscopically assessed knee patients was found to be 0.83 (pain), 0.87 (stiffness), and 0.96 (physical function) (Cronbach's alpha).5,7 Overall, these studies conclude that the WOMAC, both Likert and VAS versions, are internally consistent.
Convergent validity, which consists of providing evidence that two tests that are believed to measure closely related skills or types of knowledge correlate, has been reported for the WOMAC.
Moderate to strong correlations between the WOMAC and impairment measures such as range of motion, radiology Kellegren rating, and disability measures such as the SF-36 physical function and the Nottingham Health Profile scale, have been found.5
Responsiveness of the WOMAC has been examined in patients with knee OA, total knee arthroplasty (TKA), anterior cruciate ligament and meniscal tears, as well as in patients who with total hip arthroplasty (THA). It has been found that across interventions, the subscales of the WOMAC have demonstrated varying degrees of responsiveness.
The physical function scale tends to be the most responsive, particularly in patients with THA and TKA. The pain subcale is next, followed by the stiffness subscale. A detailing of the SRMs for the WOMAC across interventions may be found in a review by McConnell et al.5
A short version of the WOMAC has been developed in order to enhance the usability of the scale in clinical practice and research. As compared to the long version of the WOMAC, the short form has a standardized response mean (SRM) of 0.73 (long form =0.61), intraclass correlation coefficients (ICC) is 0.68 (long form = 0.76), with Cronbach's = 0.85 (long form = 0.93).
The short form of the WOMAC may be a reliable, responsive and valid alternative for clinical use and research endeavors for individuals who have hip and/or knee osteoarthritis.14
When compared with the short form -36 (SF-36) in patients with OA in the lower extremties undergoing rehabilitation, the function scale were shown to be more responsive than the pain scales in both instruments, SRM = 0.72 and 0.53 for the WOMAC and SF-36 respectively. The WOMAC demonstrated greater responsiveness than the SF-36 on the function scale at the end of treatment SRM = 0.68 versus 0.25.
The authors conclude that the function scale of the WOMAC was a better indicator of functional improvement in individuals with LE OA than the SF-36.15
The WOMAC is a self-administered assessment of pain, stiffness and function in people with hip and/or knee OA that takes approximately five to 10 minutes to complete.
As mentioned previously, numerous studies have evaluated the reliability, responsiveness and the validity of the instrument in its various forms and languages and found it to be an acceptable instrument for clinical and research use.
So to answer the question, "can pain and treatment effects from hip and/or knee ostearthritis be measured?" The evidence suggests that yes, with the proper administration of the WOMAC, treatment outcomes and pain and function levels can be accounted for with this instrument.
References available at www.advanceweb.com/pt or by request.
Dr. Lewis is a private practice and consulting clinical specialist for ProfessionalSportsCare and Rehab. She lectures exclusively for GREAT Seminars and Books, Inc. Dr. Lewis is also the author of numerous textbooks. Her Website address is www.greatseminarsandbooks.com. Dr. Shaw is an assistant professor in the physical therapy program at the University of South Florida dedicated to the area of geriatric rehabilitation. She lectures exclusively for GREAT Seminars and Books in the area of geriatric function.