Unilateral vs bilateral symptomatic knee osteoarthritis: associations between pain intensity and function

Abstract

Objective. The objective of this study was to determine if associations between pain distribution (unilateral vs bilateral) and measures of function (self-report vs performance-based) were influenced by knee pain intensity of the painful knee(s) in persons with moderate to severe symptomatic knee OA.

Methods. Data from persons in the Osteoarthritis Initiative (OAI) dataset (n = 852) with symptomatic knee OA were studied. Key dependent variables were the WOMAC physical function, Knee Injury and Osteoarthritis Outcome Score (KOOS) quality of life, the repeated chair stand test and the 20-m walk test. In addition to covariates, the independent variables were the presence of unilateral or bilateral OA involvement (either Kellgren and Lawrence grade 3 or 4 or a grade <3) and pain category (mild, moderate or severe).

Results. WOMAC physical function scores consistently showed the strongest association with pain intensity for persons with unilateral vs bilateral knee pain. For example, in persons with unilateral severe knee pain, WOMAC scores averaged 19.9 (s.d. = 12.0) points while persons with bilateral knee pain with at least one knee rated as severe had WOMAC scores ranging from 25.3 to 28.9, depending on pain severity of the contralateral knee. These differences were statistically significant (P < 0.001) as was the test for trend (P = 0.001). Self-report measures generally showed larger effect sizes than performance-based measures.

Conclusion. Knee pain intensity influences self-report and performance-based tests differently depending on whether knee pain is unilateral or bilateral. WOMAC scores are most strongly associated with pain intensity in persons with unilateral vs bilateral pain while walking tests are least influenced by pain intensity.

Introduction

Osteoarthritic knee pain occurs in 7–17% [1, 2] of adults age ≥45 years in the USA. Greater knee pain severity is associated with both a greater self-reported functional loss [3] and worse physical performance [4]. Self-reported functional status measures capture a person’s perception of his/her ability to do activities, while performance measures indicate a person’s actual ability to perform activities.

Quantifying pain in patients with knee OA is standard clinical practice, but the impact of unilateral vs bilateral knee pain severity on functional status or performance measures have received surprisingly little attention by researchers. As noted by White et al. [5], when examining knee OA epidemiology, researchers have measured knee pain during daily activities without considering whether the pain was unilateral or bilateral [6–8]. Not accounting for unilateral vs bilateral pain and pain intensity of both limbs in studies of persons with knee OA may influence study results.

Many daily activities such as stair climbing, walking and rising from a chair require bilateral limb movement. Therefore one might logically conclude that a person with bilateral knee pain would have greater difficulty with daily activities than a person with unilateral knee pain. For example, a person with bilateral knee pain of a given intensity would experience a greater pain load as compared with a person with unilateral pain of the same intensity. This greater pain load could theoretically compound functional deficits, particularly for activities like walking that require bilateral knee movements. Research to date on this issue, however, has come to divergent conclusions, with some suggesting that the presence of unilateral vs bilateral knee pain has no association with functional status [9], while others have found that persons with bilateral knee pain display greater functional loss or greater worsening than persons with unilateral knee pain [5, 10, 11]. None of this prior evidence accounted for pain severity of both limbs, which we suspect may help to explain the divergent findings. Given that pain and functional status are associated in persons with knee OA, pain severity of both limbs may need to be accounted for when studying persons with bilateral vs unilateral knee pain. This issue is also important for clinical practice, given that both self-report and performance-based functional status measure are in common use. Self-reported functional status is substantially driven by pain [12–14] and it is unknown if self-report and performance-based functional measures equally capture functional loss that may be associated with bilateral vs unilateral knee pain.

The main purpose of our study was to determine if associations between pain distribution (unilateral vs bilateral) and measures of function were influenced by differing levels of knee pain intensity in persons with moderate to severe symptomatic knee OA. The potential interaction between pain intensity and distribution was examined for both self-report functional status measures and performance-based measures. We hypothesized that when pain intensity of both limbs was taken into account, functional status limitations would be greater for persons with bilateral vs unilateral knee pain. In addition, because self-report and physical performance measures are influenced by different phenomena [15, 16] and are only moderately correlated [4, 16–18], we hypothesized that associations between unilateral/bilateral pain severity and function would be greater for self-report measures vs performance-based measures. The WOMAC physical function scale, for example, has been shown to be highly correlated with pain [16, 19] and unable to differentiate among pain and function in construct validation studies [20, 21].

Materials and methods

The Osteoarthritis Initiative

The Osteoarthritis Initiative (OAI) is a National Institutes of Health (NIH) and privately funded longitudinal cohort study or persons with or at risk of developing knee OA. A total of three subcohorts (i.e. control, incidence and progression) have been defined in the OAI with each subcohort having racially and ethnically diverse mixes of persons between the ages of 45 and 79 years at baseline. Subjects were recruited to four clinic sites using a variety of approaches, including mailings to clinical populations in the four recruitment sites, advertisements in local newspapers, presentations to churches, community and civic organizations and via a website. Subjects were recruited from (i) the University of Maryland School of Medicine in Baltimore, Maryland, (ii) Ohio State University in Columbus, Ohio, (iii) the University of Pittsburgh in Pittsburgh, Pennsylvania and (iv) Memorial Hospital of Rhode Island, in Pawtucket, Rhode Island. The study was approved by the institutional review board at the OAI coordinating centre, the University of California at San Francisco, in accordance with the Declaration of Helsinki. Data are available for public use at http://oai.epi-ucsf.org/datarelease/About.asp.

Persons were excluded from the OAI if the following were present: RA, bilateral knee arthroplasty or pre-existing plans to undergo bilateral (not unilateral) knee arthroplasty in the next 3 years, bilateral Osteoarthritis Research Society International (OARSI) stage 3 (severe) knee OA [22], positive pregnancy test, inability to provide a blood sample, use of ambulatory aids other than a single straight cane for >50% of the time, comorbid conditions that might interfere with 4-year participation, unlikely to reside in the clinic area for at least 3 years, current participation in a double-blind randomized controlled trial and being unwilling to sign an informed consent. Because of MRI requirements, men weighing >130 kg and women weighing >114 kg were excluded because they were unable to undergo 3.0 T MRI. In total, 27% (n = 4796) of those screened (n = 17 457) were admitted to the study. The study design protocol is available at http://www.oai.ucsf.edu/datarelease/docs/StudyDesignProtocol.pdf

Sample for current study

To be included in the current study, persons had to have either unilateral or bilateral knee pain and a Kellgren and Lawrence (KL) centrally adjudicated grade of 3 or 4 on at least one knee. We chose these criteria for two reasons. First, persons with KL grades of 3 or 4 are much more likely to report more severe and more frequent pain [23], and thus to seek medical care, than persons with either no or mild knee OA. Second, a recently published study by Marmon et al. [9] used similar criteria and we were interested in making comparisons with their findings. Because the OAI study is longitudinal, we also recruited persons from the 2- and 4-year follow-up sessions that met the inclusion criteria but were not included in the baseline dataset. The study sample comprised 852 unique persons, 404 of whom had bilateral knee pain and KL grades of 3 or 4 in both knees, 247 of whom had a right-sided knee OA grade of 3 or 4 and right knee pain and 202 of whom had left knee OA and pain (see Fig. 1). A total of 494 persons were recruited from the baseline dataset, 204 were recruited from the 2-year follow-up dataset and 155 were recruited from the 4-year follow-up dataset. Data from release versions 0.2.2, 3.2.1 and 6.2.1 were used in our study.

Fig. 1.

Flow of patients through the study.

Variables of interest

For the function measures we chose the following self-report functional status measures: WOMAC physical function [24], Knee Injury and Osteoarthritis Outcome Score (KOOS) function, sports and recreational activities scale and KOOS quality of life (QOL) scale [25]. We chose these three self-report scales to capture the full depth and breadth of self-reported function from typical everyday activities (WOMAC) to higher-order activities (KOOS function and sports) to more global judgments of knee function (KOOS QOL). The self-report scales are commonly used in research and practice and have undergone extensive validation [24–30]. Physical performance measures were the five repetitions of the chair stand test, measured in stands per second, the 20-m walk test, measured in metres per second, and the 400-m walk test, measured in seconds. The performance-based tests were chosen to reflect the variety of physical activities performed by persons with knee OA and are highly reliable [14, 31, 32]. These performance tests also require differing amounts of endurance, strength and balance. All self-report and performance measurements are thoroughly described in the OAI operations manuals, which are available online (http://www.oai.ucsf.edu/datarelease/OperationsManuals.asp).

The pain measure used in the OAI was a verbal pain rating scale. The scale was the following: ‘Please rate the pain that you’ve had in your right (or left) knee during the past 7 days that best describes the pain at its worst. “0” means “No pain” and “10” means “Pain as bad as you can imagine”.’ Persons reporting unilateral pain ratings from 1 to 3 were labelled mild, 4–6 moderate and 7–10 severe. Using the same pain classification categories, persons with bilateral involvement were labelled mild–mild, moderate–mild, moderate–moderate, severe–mild, severe–moderate and severe–severe. For example, a person with bilateral knee OA reporting moderate pain on the more involved limb and mild pain on the less involved limb was labelled moderate–mild. Pautex et al. [33] examined persons with primarily arthritic pain and reported high reliability [Intraclass Correlation Coefficient (ICC) = 0.97] and strong associations between a similar verbal rating scale to that used in the OAI study and more traditional visual analogue scales of pain measurement. Jensen [34] reported strong associations with very similar scales to those used in the OAI and more traditional pain intensity scales as well as responsiveness to changes following treatment.

Covariates

We adjusted for age [35], gender [36], comorbidity [37, 38] and BMI. The modified Charlson comorbidity index asks patients a series of questions regarding the presence and impact of 13 diseases, such as heart attack, diabetes and cancer. Scores in our OAI ranged from 0 to 10, with 70% of the sample scoring a 0. We therefore dichotomized the score to either 0 or ≥1. The covariates were chosen because they have been shown to be related to knee OA incidence or progression in persons with knee pain and arthritis [39–43].

Statistical analysis

We calculated descriptive statistics and these are summarized in Table 1. The dependent variables were the self-report functional status measures (WOMAC and KOOS measures) and the physical performance measures (20-m walk test, 400-m walk test and repeated chair stand test). The independent variable was the level of OA involvement (either grade 3 or 4 KL or KL grade <3) and pain. Nine combinations or levels of involvement were created. These levels were unilateral mild, unilateral moderate, unilateral severe, bilateral mild–mild, bilateral moderate–mild, bilateral moderate–moderate, bilateral severe–mild, bilateral severe–moderate and bilateral severe–severe. We included the following covariates in all analyses: sex, age, BMI and number of comorbidities. For each dependent variable we performed three analyses of variance (ANOVAs) that included the following independent variable levels: (i) severe unilateral, severe–mild bilateral, severe–moderate bilateral, severe–severe bilateral; (ii) moderate unilateral, moderate–mild bilateral, moderate–moderate bilateral and (iii) mild unilateral, mild–mild bilateral. For the moderate and severe involvement categories, two non-orthogonal contrasts were performed. The first contrast tested for a difference in the dependent variable’s mean score for the unilateral and weighted mean of the comparable bilateral involvement group. For example, the severe unilateral mean score was compared to the weighted mean of the severe–mild, severe–moderate and severe–severe scores. The second contrast tested for a linear trend specific to involvement status. Using the severe category as an example, we used the following hierarchy of involvement: bilateral severe–severe > bilateral severe–moderate > bilateral severe–mild > unilateral severe. Given that we performed five analyses for each dependent variable, we controlled for these multiple comparisons by dividing our chosen family-wise error rate of 0.05 by 5. Accordingly, an effect was considered statistically significant if P < 0.01. Data were analysed using STATA release 12 (StataCorp, College Station, TX, USA).

Table 1.

Characteristics of the 853 persons in the study

Variable	Mean (s.d., min, max) or n (%)	Missing data, n
Demographic
Age in years, mean (s.d.)	64.7 (8.9, 45, 83)	0
Female, total (%)	433 (50.8)	0
Race, total (%)		0
Other non-white	20 (2.3)
White or Caucasian	660 (77.4)
Black or African American	167 (19.6)
Asian	6 (0.7)
Comorbidity score, mean (s.d.)	0.49 (1.0, 0, 12)	12
BMI, mean (s.d.)	29.9 (4.7, 18.2, 48.7)
Sub-cohort at baseline, total (%)
Incidence	353 (41.4)
Progression	500 (58.6)
Self-reported function and performance
WOMAC Physical Function Score, mean	15.6 (12.3, 0, 67)	11
KOOS QOL Score, mean (s.d.)	55.4 (20.0, 0, 100)
KOOS Function, Sports and Recreational Activities	59.0 (25.3, 0, 100)	322
20-m walk pace, m/s	1.28 (0.22, 0.32, 2.02)	18
400-m walk, s	317.6 (62.3, 57.9, 685.9)	52
Repeated chair stand, stands/s	0.48 (0.14, 0.14, 1.14)	66

Results

Tables 2–5 summarize the results for the chair stand test, the 20-m walk test, the WOMAC physical function test and the KOOS QOL scale. Reported in these tables are the unadjusted means and means adjusted for sex, age, BMI and number of covariates. These tables also report the ANOVA contrast results. For the unilateral mild pain comparison there is only one possible bilateral comparison group (mild–mild), while there are two comparisons for the moderate pain groups (e.g. unilateral mild to bilateral mild and moderate) and three comparisons for the severe pain groups.

Table 2.

Analysis for the unilateral–bilateral pain comparisons for the repeated chair stand test (stands/s)^a

Severe	Unilateral, severe	Bilateral, severe–mild	Bilateral, severe–moderate	Bilateral, severe–severe		Unilateral–bilateral contrast P*		Trend P
Unadjusted	0.50, 0.15, 88	0.43, 0.11, 42	0.44, 0.13, 60		0.39, 0.12, 39
Adjusted	0.49, 87	0.43, 42	0.44, 60		0.40, 39		<0.001	0.003

Moderate	Unilateral, moderate	Bilateral, moderate–mild	Bilateral, moderate–moderate				Unilateral–bilateral contrast P*	Trend P
Unadjusted	0.48, 0.14, 130	0.49, 0.14, 67	0.43, 0.11, 71
Adjusted	0.47, 126	0.49, 65	0.45, 70				0.27, 0.61	0.13

Mild	Unilateral, mild	Bilateral, mild–mild					Unilateral–bilateral contrast P*	Trend P
Unadjusted	0.52, 0.13, 201	0.48, 0.14, 89
Adjusted	0.52, 199	0.49, 89					0.21

*All P-values are adjusted for age, gender, comorbidity and BMI.

^aAll data are reported in the following order: mean, s.d. and sample size for unadjusted analyses and mean and sample size for the adjusted analyses.

Table 3.

Analysis for the unilateral–bilateral comparisons for the 20-m walk (m/s)^a

Severe	Unilateral, severe	Bilateral, severe–mild	Bilateral, severe–moderate		Bilateral, severe–severe		Unilateral–bilateral contrast P*	Trend P
Unadjusted	1.25, 0.21, 93	1.24, 0.20, 49	1.20, 0.20, 65		1.17, 0.21, 43
Adjusted	1.23, 93	1.25, 49	1.21, 64		1.19, 43		0.21	0.47

Moderate	Unilateral, moderate	Bilateral, moderate–mild	Bilateral, moderate–moderate				Unilateral–bilateral contrast P*	Trend P
Unadjusted	1.28, 0.20, 138	1.27, 0.23, 73	1.22, 0.21, 75
Adjusted	1.27, 133	1.27, 71	1.25, 74				0.78	0.47

Mild	Unilateral, mild	Bilateral, mild–mild					Unilateral–bilateral contrast P*	Trend P
Unadjusted	1.36, 0.21, 209	1.32, 0.22, 90
Adjusted	1.35, 207	1.35, 90					0.98

*All P-values are adjusted for age, gender, comorbidity and BMI.

^aAll data are reported in the following order: mean, s.d. and sample size for unadjusted analyses and mean and sample size for the adjusted analyses.

Table 4.

Analysis for the unilateral–bilateral comparisons for the WOMAC physical function scale^a

Severe	Unilateral, severe	Bilateral, severe–mild	Bilateral, severe–moderate	Bilateral, severe–severe	Unilateral–bilateral contrast P*	Trend P
Unadjusted	19.9, 12.0, 93	25.3, 11.5, 48	28.7, 12.0, 67	28.9, 13.4, 41
Adjusted	20.7, 92	25.6, 48	28.1, 67	28.1, 41	<0.001	0.001

Moderate	Unilateral, moderate	Bilateral, moderate–mild	Bilateral, moderate–moderate		Unilateral–bilateral contrast P*	Trend P
Unadjusted	13.3, 9.9, 139	17.5, 9.3, 74	20.2, 9.6, 78
Adjusted	13.6, 134	17.3, 72	19.9, 77		<0.001	<0.001

Mild	Unilateral, mild	Bilateral, mild–mild			Unilateral–bilateral contrast P*	Trend P
Unadjusted	5.7, 6.0, 211	10.9, 8.5, 91
Adjusted	6.0, 209	10.4, 91			<0.001

*All P-values are adjusted for age, gender, comorbidity and BMI.

^aAll data are reported in the following order: mean, s.d. and sample size for unadjusted analyses and mean and sample size for the adjusted analyses.

Table 5.

Analysis for the unilateral–bilateral comparisons for the KOOS QOL scale^a

Severe	Unilateral, severe	Bilateral, severe–mild			Bilateral, severe–moderate	Bilateral, severe–severe	Unilateral–bilateral contrast P*		Trend P
Unadjusted	47.3, 19.6, 96	43.4, 17.7, 49	40.0, 15.7, 68			37.4, 20.7, 44
Adjusted	46.5, 95	42.4, 49	41.7, 67			37.9, 44		0.07	0.014

Moderate	Unilateral, moderate	Bilateral, moderate–mild		Bilateral, moderate–moderate				Unilateral–bilateral contrast P*	Trend P
Unadjusted	57.9, 19.4, 141	54.1, 16.8, 74	51.5, 13.8, 78
Adjusted	57.9, 136	54.3, 72	51.4, 77					0.035	0.011

Mild	Unilateral, mild	Bilateral, mild–mild						Unilateral–bilateral contrast P*
Unadjusted	68.3, 16.5, 212	61.2, 17.0, 91
Adjusted	68.4, 210	61.0, 91						0.005

*All P-values are adjusted for age, gender, comorbidity and BMI.

^aAll data in the table are reported in the following order: mean, s.d. and sample size for unadjusted analyses and mean and sample size for the adjusted analyses.

Chair stand test

Table 2 presents the results for the chair stand test. Inspection of the unadjusted and adjusted means reveals they are similar. The contrast analyses were statistically significant for the severe group only. The interpretations of these contrasts are that the unilateral severe group performed more stands per second than the severe bilateral groups combined and there was a trend towards fewer chair stands per second as the level of involvement increased.

20-m walk test

Table 3 presents the results for the 20-m walk test. None of the contrast analyses were statistically significant for these performance tests. The 400-m walk test also had no statistically significant differences among the contrasts (data not shown).

WOMAC physical function

Table 4 summarizes results for the WOMAC physical function scale. All comparisons were statistically significant for all unilateral–bilateral contrast comparisons. For example, patients with bilateral severe pain had WOMAC physical function scores that were ∼7 points higher than persons with unilateral severe pain after adjustment for covariates.

KOOS QOL scale

For the KOOS QOL scale, all comparisons with the exception of the severe unilateral–bilateral comparisons were significant (see Table 5). The results for the KOOS function, sports and recreational activities scale (data not shown) were very similar to those found for the KOOS QOL scale. For example, KOOS QOL scores for persons with unilateral moderate pain were ∼6 points higher (better QOL) than persons with bilateral moderate pain after covariate adjustment.

Based on the results for the performance-based and self-report measures, we calculated the following effect sizes [44] for comparisons of the subgroups with unilateral vs bilateral symptomatic knee OA: 0.10–0.26 for the chair stand, 0.02–0.10 for the 20-m walk test, 0.26–0.28 for the WOMAC physical function scale and 0.16–0.20 for the KOOS QOL measure.

Discussion

Our hypothesis that pain intensity is associated with measures of function when symptomatic knee OA pain severity in one knee vs in both knees is accounted for was generally supported. Differences in effect sizes in the hypothesized direction also were found for the self-report measures compared with the performance measures. This was particularly true for the WOMAC physical function scale, but was also true for the KOOS QOL and function and sports scales, relative to most of the performance measures.

We speculate that one explanation for this more noticeable effect of pain on self-report vs performance measures is that persons have a limited capacity to distinguish between the ability to complete activities such as walking and stair climbing and what they experience (e.g. pain) when doing these types of activities. This has been well substantiated for the WOMAC scale, with multiple studies indicating strong associations with pain and an inability to differentiate between pain and function constructs [16, 19–21]. Our data would suggest that the KOOS QOL and sports scales are less vulnerable to this limitation.

A substantial literature also exists to demonstrate that associations between self-report and performance measures are weakly to moderately correlated [4, 15, 45–47]. For example, after accounting for the repeated chair stand test score, variations in self-reports of rising from a chair were explained by age, BMI, greater comorbidity and serum albumin levels in persons age ≥60 years, 44% of whom were diagnosed with arthritis [15]. This body of literature suggests that while self-report and performance measures are related, a person’s mental and physical health characteristics (including pain) and idiosyncratic views of their abilities likely influence self-report measures. This evidence indicates that self-report and performance-based measures capture different phenomena, and our study supports this assertion.

Persons with bilateral vs unilateral symptomatic knee OA of varying pain intensities perform essentially equally during walking tests when controlling for pain severity. For the repeated chair stand, a more demanding performance test requiring substantial quadriceps and hip extensor muscle forces, we found statistically significant differences among persons with unilateral vs bilateral pain that was severe in at least one limb. Pain reported as severe in at least one limb impacts repeated chair stand performance and the extent of impact varies depending on whether the contralateral limb pain is mild, moderate or severe. Evidence of a dose-response also was found (test for trend P < 0.003). However, bilateral pain that is moderate or mild at its worst does not appear to influence repeated chair stand performance relative to unilateral moderate or mild pain. These data suggest that the knee pain–chair stand relationship is complex and likely non-linear when considering the effects of unilateral vs bilateral knee pain on chair stand performance.

Our study is consistent with previously published evidence [5, 10, 11] but contradicts a recent paper published by Marmon et al. [9]. We suspect that because our study had a substantially larger sample than that of the Marmon et al. [9] study (n = 853 in our study vs n = 152) and because we stratified by pain severity levels of both limbs, we found potentially meaningful differences in persons with unilateral vs bilateral pain.

According to Cohen’s interpretive guidelines [44], the effect sizes observed for our self-report and repeated chair stand data would be considered small. Some interpretation of the meaningfulness of these effect sizes would provide some context. With the exception of the WOMAC physical function scale, there is a paucity of information concerning important between group (level) differences for the outcome measures reported in our study. A substantial amount of evidence suggests that a within-person change of 6–9 WOMAC physical function scale points is a meaningful difference [48–51]. Goldsmith et al. [52], in a novel study that was not specific to the WOMAC, reported that an important between-group difference (i.e. the difference between a drug and placebo) is ∼50% of an important within-patient change. Applying this recommendation from Goldsmith et al., a between-group difference of ∼3 or 4 points would be considered important for the WOMAC physical function scale. This standard was met for our data. More limited data are available to judge the magnitude of the effect for the repeated chair stand test results. French et al. [53] reported effect sizes for performance-based measures including the repeated chair stand test. A total of 39 patients with knee OA completed a course of physical therapy emphasizing exercise, an intervention known to be at least mildly effective for patients with knee OA [54]. The effect size reported by French et al. [53] for the repeated chair stand was 0.33. The French et al. effect size is similar to what we estimated for the severe group comparison (0.26), but was larger compared with our moderate (0.11) and mild (0.10) pain groups.

Our study has some important limitations. We examined associations between pain severity in one or both limbs and function using a cross-sectional design. Further study using follow-up data would provide additional important information about the potential influence of pain severity and unilateral vs bilateral pain distribution on function over time. In addition, it is known that self-report measures are strongly influenced by pain [12–14, 16], and while we adjusted for pain severity for both limbs, it is possible that the differences we found in function between persons with unilateral vs bilateral pain were simply a reflection of differences in pain severity in the contralateral limb and not an actual difference in function. The data indicate consistent trends of worsening with increasing pain of the contralateral limb for both self-report and performance-based measures, suggesting that function was actually worse, but the exact role of pain in our study is not entirely clear.

In conclusion, we found that pain severity in the contralateral limb of persons with symptomatic knee OA is associated with the extent of functional loss. Self-report measures are susceptible to influences of a contralateral painful knee and the WOMAC physical function scale showed the strongest associations. The only performance-based measure that was influenced by contralateral knee pain was the repeated chair stand test and for this test only persons with severe knee pain had potentially meaningful differences in performance depending on the severity of pain in the contralateral limb. Clinicians should be aware that self-report measures and performance measures are affected by pain differently. Self-report measures are prone to being influenced by the severity of pain in the contralateral limb. It is likely that these variations most often reflect idiosyncratic differences in persons’ mental and physical health profiles and not actual performance-based differences. Future research should explore the bases for differences in self-report and performance-based measures in samples of persons with symptomatic knee OA.

Rheumatology key messages.

• Function with symptomatic knee OA is influenced by whether the pain is unilateral or bilateral.

• In knee OA, self-report measures are influenced more by unilateral vs bilateral knee pain distribution than performance-based measures.