Prevalence and Characteristics of Widespread Pain in People With Knee Osteoarthritis

Background

Knee osteoarthritis (KOA) affects approximately 14% of the Swedish population over the age of 45 (1). Typical symptoms are pain, stiffness and reduced physical function (2). Most people with KOA (90%) have at least two pain sites (3). One-third of people with chronic knee pain meet criteria for widespread pain, which is associated with impaired self-rated physical function (4, 5). Female gender, ethnic background, synovitis and degree of osteoarthritis (OA) on X-ray are predictors of a deterioration in function in people with OA in the lower extremity (6). Multiple pain sites are associated with fear of movement (3), while increased fear of movement has a moderate correlation with reduced physical function in people with KOA (7). While high self-efficacy can predict an increased physical function (7).

Evaluating leg muscle strength can be critical in identifying people with OA who are at risk of worsening and who may need to strengthen their muscles to improve physical function and relieve pain. This may also help prevent sarcopenia and its metabolic consequences (8). Today, self-assessment forms and performance-based tests are used to assess patients' physical function (9). An advantage of performance-based tests compared to self-assessment forms is that they measure what the patient is actually able to do (10), and performance-based tests are better at differentiating between pain and physical function (11). These tests are usually performed in the clinic under the supervision of healthcare personnel.

For people who do not have the ability to perform functional testing, pain history can be of value in differentiating whether or not a person has lower extremity functional limitations - without being evaluated with tests or forms. Because pain in people with OA can vary, assessment with the ICOAP form can be used to describe whether the pain changes (12). However, lack of time can prevent clinics from using standardized forms or instruments (13).

More knowledge is needed that describes how common it is with widespread pain among people with KOA who seek primary care and examine the relationship between pain intensity, number of pain sites, and its impact on function, activity and quality of life. In addition, it is of interest to examine relationships between demographic data and pain history on physical function in order to assess the functional impairment according to a standardized function test with the help of common anamnesis questions in the clinical meeting with the patient without having to perform the actual test. The knowledge of these correlations is missing today.

Purpose

The aim of this study is to evaluate whether the number of pain sites or pain estimation with ICOAP is related to a functional impairment in the lower extremity in people with KOA.

Research questions:

1. What is the relationship between the number of pain sites and physical function when adjusted for age, sex, education level and origin for people with KOA?
2. What is the relationship between pain experience measured with ICOAP (constant and intermittent pain subscales, as well as total score) and reduced physical function when adjusted for age, sex, education level and origin of people with KOA?
3. What is the relationship between demographic data (age, sex, BMI, origin, level of education) and reduced physical function for people with KOA?

Method: Population/Sample

Inclusion criteria

Participants were recruited from five rehab clinics in primary care within the Västra Götaland region to a former study which this study is based on (NCT03855813). Patients with diagnosed KOA were asked to participate in the study. The patients mostly had problems in the knee joint, but could also have OA in other joints in the lower extremities. They were independent in walking, with or without walking aids, and could understand test instructions both verbally and in writing in Swedish.

Exclusion criteria

• Neurological problems that affected balance or walking ability.
• Severe or serious somatic or mental illnesses that affected the ability to function or the ability to understand and follow the test instructions.
• Had knee or hip replacement in the past six months.
• Other knee surgery in the last six months that could affect functional capacity.

Method: Data collection

Patients with established KOA received written information about the study, consent form and questionnaire about background data, either directly at the clinic or by written letter. When participating in the study, the patients filled in all the forms at home and brought them to the physiotherapist at the next visit.

Background data

Background data included age, sex, body mass index (BMI), other diseases, pain sites, pain intensity, duration of pain in the knee joint and experience of the knee problems.

Pain history

Pain intensity was measured with a numeric rating scale (NRS) 0-10 (14), where 0 corresponds to no pain and 10 the worst imaginable pain. The patient rated the pain at rest, when loaded, and on average over the past week.

ICOAP - the Swedish version, was used to assess the pain experience. The questionnaire contains two subscales with question areas about intermittent and constant pain. The total score for the subscale for intermittent pain varies between 0-24 points and between 0-20 points for constant pain. The total score, 0-44 points, is usually recalculated to a scale of 0-100 (total score/44 x 100), where 0 corresponds to no pain and 100 to extreme pain (12).

The number of pain sites was evaluated with a pain drawing with 18 predefined body areas (15).

Functional test with 30 CST

The chair test "30 seconds chair stand test" (30 CST) is part of a test battery that is recommended both in research and in the clinic for people with OA. This test is a reliable and valid method for measuring the strength and functional capacity of people with OA assessed by clinician (16, 17), and as a self-test (18).

The chair test 30 CST was performed on an approximately 45 cm high chair without armrests. The participant was asked to do as many stands as possible in 30 seconds. The starting position was sitting on the chair. The stands would be to an upright position (fully extended hips and knees), then back to a seated position. The arms were to be crossed over the chest throughout the test. In all measurements with the chair test, the patient had to indicate perceived pain intensity in the knee joint before and during the test. All physiotherapists received training on the study protocol and the chair test.

Method: Data processing/Data analysis

Sample size

The sample size was calculated with the statistical program PASS 16, for the reliability study for which the data for this study is based on (18). Sample size of 117 participants, with two observations per participant for each inter- and intra-rater test, achieved a power of 80% to detect an intraclass correlation coefficient (ICC) of 0.8 with a significance level of 0.05. A total of 114 participants were recruited for the reliability study. All participants in the reliability study (n=114) will be included in this study and are considered to be sufficiently plentiful to be able to answer the purpose and the research questions with good probability.

Statistical analysis

Demographic data will be presented in the form of central and dispersion measures depending on the data level. For nominal data, number and percent are presented.

Logistic regression will be conducted between the independent variables 1) demographic data, 2) number of pain sites, 3) ICOAP to assess the effects on the dependent variable impaired physical function measured by 30 CST. The results from the regression analysis will be presented with the odds ratio with 95% confidence interval and illustrated with graphs of the expected probabilities. The analyzes will be carried out in the statistical program Statistical Package for Social Science (SPSS) (19). The significance level is set at p< 0.05.

Expected meaning

The number of pain sites could be used to detect potential risks in functional impairment and could be helpful to clinicians when deciding how to treat patients with KOA, without having to spend time on functional tests. This patient group with multiple pain sites may need more support to maintain physical activity. An understanding of possible effects between people's background history and physical function could contribute to people with KOA being able to receive more individually tailored and preventive care.