Archives of Physical Medicine and Rehabilitation
Volume 89, Issue 6 , Pages 1066-1073, June 2008

Factors Affecting Self-Reported Pain and Physical Function in Patients With Hip Osteoarthritis

  • Riikka Juhakoski, MD

      Affiliations

    • Department of Physical and Rehabilitation Medicine, Mikkeli Central Hospital, Mikkeli, Finland
    • Corresponding Author InformationReprint requests to Riikka Juhakoski, MD, Dept of Physical and Rehabilitation Medicine, Mikkeli Central Hospital, Porrassalmenkatu 35-37, FIN-50100 Mikkeli, Finland
    • ,
  • Seppo Tenhonen, MD

      Affiliations

    • Department of Physical and Rehabilitation Medicine, Mikkeli Central Hospital, Mikkeli, Finland
    • ,
  • Tapio Anttonen, MD

      Affiliations

    • Mikkeli Health Centre, Mikkeli, Finland
    • ,
  • Timo Kauppinen, MD

      Affiliations

    • Department of Clinical Radiology, Mikkeli Central Hospital, Mikkeli, Finland
    • ,
  • Jari P. Arokoski, MD, DMedSc

      Affiliations

    • Department of Physical and Rehabilitation Medicine, Kuopio University Hospital, and Institute of Clinical Medicine, University of Kuopio, Kuopio, Finland.

Article Outline

Abstract 

Juhakoski R, Tenhonen S, Anttonen T, Kauppinen T, Arokoski JP. Factors affecting self-reported pain and physical function in patients with hip osteoarthritis.

Objective

To determine the factors associated with self-reported pain and physical function in patients with hip osteoarthritis (OA).

Design

Cross-sectional study.

Setting

Rehabilitation clinic in a Finnish hospital.

Participants

Participants with hip OA (N=118; 35 men, 83 women; age, 66.7±6.5y; range, 55–80y).

Interventions

Not applicable.

Main Outcome Measures

Self-reported pain and self-reported disease-specific physical function were recorded by using the Western Ontario McMaster Universities Osteoarthritis Index. Self-reported generic physical function was analyzed by using the Finnish version of the RAND 36-Item Short-Form Health Survey. As listed in the International Classification of Functioning, Disability and Health model, the effects of personal factors (age, sex, education, depression, life satisfaction, smoking, years of sporting activities), pathophysiologic factors (radiologic score of hip OA, body mass index [BMI], comorbidities, duration of knee pain) and body functions and structures (measurement of leg extensor power, passive internal rotation and flexion of the hip joint, the six-minute walk test [6MWT], Timed Up & Go [TUG] test, ten-meter walk test, sock test) were analyzed.

Results

The educational level (r=−.264, P<.001), comorbidities (r=.313, P<.001), and BMI (r=.252, P<.001) were identified as significant factors for self-reported disease-specific physical function as well as the educational level (r=.291, P<.001), life-satisfaction (r=−.319, P<.001), BMI (r=−.290, P<.001), and comorbidities (r=−.220, P<.005) for the self-reported generic physical function. No direct relationship with the pain and psychologic factors was detected. The number of comorbidities and duration of knee pain and life satisfaction explained 22% of self-reported pain. The number of comorbidities, passive hip flexion, and the TUG test explained 20% of self-reported disease-specific physical function whereas the passive hip flexion, 6MWT, and educational level explained 25% of self-reported generic physical function.

Conclusions

Educational level, life satisfaction, and number of comorbidities were identified as significant factors for both self-reported pain and physical functioning in hip OA. Performance measures are better predictors of physical function than pain in hip OA. Factors explaining disability and pain in hip OA are multidimensional and no single predicting factor was found to be superior to any other.

Key Words: Osteoarthritis, hip, Pain, Rehabilitation

 

HIP OSTEOARTHRITIS (OA) is among the most common joint diseases and is therefore a major social and health problem.1 It is one of the main causes of disability in the elderly and its prevalence will increase with the aging of Western society.2, 3 Clinically, the hip OA is characterized by joint pain, tenderness, limitation of movement, and variable degrees of local inflammation, but without evoking any systematic effects. The disease processes affect not only the articular cartilage, but also involve the entire joint, including the subchondral bone, ligaments, capsule, synovial membrane, and periarticular muscles.

Disability is a major sequela from hip OA and continues to have a tremendous economic impact. According to contemporary disablement models, such as the International Classification of Functioning, Disability and Health (ICF),4 health conditions including disability and a response to a disease reflect dynamic and interrelated processes of biologic functioning, psychosocial, and environmental factors, and social support systems. In the case of hip OA, little is known about the factors contributing to the patients' responses to disease, including pain and disability. A clear understanding of the relationship of these factors would help health care professionals to develop more effective preventive and intervention strategies.

Several studies have evaluated the factors influencing pain and physical function in patients with knee OA.5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 Surprisingly, there are only a few studies that have analyzed determinants of pain and physical function in patients with hip OA.19, 20, 21, 22, 23, 24 The pain in hip OA is typically felt at its worst in the anterior groin, but it may radiate over a wider area including also the anterior thigh and knee. Due to the fact that the source of pain (hip or knee) is difficult to determine by self-report, many studies have analyzed hip and knee pain together.22, 23, 24 It is not known whether knee symptoms should be considered as potential factors for disability.

In OA, disability may include poor mobility, difficulty with activities associated with daily life, social isolation and loss of employment opportunities with related financial concerns.25(p13) A number of validated instruments are available to assess self-reported disability and dimensions relating to general health status and quality of life.25(p14,198,206) It is alarming that these tools have been only occasionally adapted to clinical practice. The reasons for disability and functional loss are not always clear in OA. People generally believe that their limitations in functional capacity are related to the severity of their OA symptoms and to the degenerative changes occurring in articular cartilage, but this assumption is only partially supported by research findings. Van Baar et al5 showed that in hip OA, disability is associated with muscle strength, joint range of motion (ROM), pain, ability to cope with pain, and psychologic well-being, but the effect of articulate status was insignificant. It is notable that those results were based on studying videos of patients' performance and no actual performance measurements of observed disability were used.

Pain is the main clinical feature of hip OA and is also the usual reason for seeking advice. Pain severity has been shown to be associated with physical and psychosocial disability,23 with muscle weakness and also with the ability to cope with that pain.5 In addition to these 2 studies, socioeconomic and psychosocial factors influencing on either pain or physical function have been evaluated in Asian patients with hip or knee OA22 with a result that lower age, employment, and longer education were indicative of lower pain scores. In conclusion, there is little research concerning factors associated with pain in hip OA and the research design and interpretation of the results in all these studies has varied, hindering a reliable comparison between studies.

Thus, even though some factors that contribute to disability and pain in hip OA have been identified, our understanding of these issues is incomplete. The purpose of this study was to identify factors that could explain the experience of disability and health in this specific population and to clarify the interactions between the components of pain and disability in hip OA. The ICF model was used in this study to provide an organizational framework for the tests in the hope that this would provide a deeper understanding of the experience of disability and health in this specific population. Interactions between the components of ICF and locations of the studied variables of the present study in the ICF framework are shown in figure 1. The main hypotheses of the study were: (1) there is a direct relationship between the objective measurements and self-reported scores of physical functioning; (2) pain is more strongly related to psychologic factors in addition to disability in impairing performance measurements; and (3) the presence of knee pain with hip OA would significantly aggravate pain experience and disability.

  • View full-size image.
  • Fig 1. 

    Interactions between the components of ICF and locations of the studied variables of the present study in the ICF framework. See details in Methods. Abbreviations: RAND-36, RAND 36-Item Short-Form Health Survey; WOMAC, Western Ontario McMaster Universities Osteoarthritis Index.

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Methods 

Participants 

We recruited the participants (N=118; age range, 55–80y) for a randomized study from Mikkeli, Finland, and the surrounding areas, via a newspaper advertisement (n=113) with a small number being selected from specialists' clinics (n=2) or by general practitioners (n=5). The criteria for their inclusion were unilateral or bilateral hip OA with Kellgren-Lawrence26, 27 grade of 1 or higher (radiographs <3 years old) and the pain experience in the hip region within the preceding month as indicated in the clinical criteria of the American College of Rheumatology.28 The exclusion criteria included: total hip replacement; rheumatoid arthritis; cognitive impairment; major surgical operation within the preceding 6 months in the lower limb or lower back area; acute or subacute lower back pain; cardiovascular or pulmonary disease; or some other chronic disease that would prevent full participation in the training program.

Data Collection 

Data collection was accomplished during the time period from August 2005 to February 2006. A cross-sectional research technique was used. All data were collected in a single section. Participants completed a demographic questionnaire, Western Ontario McMaster Universities Osteoarthritis Index (WOMAC) and the RAND 36-Item Short-Form Health Survey (RAND-36) of self-reported pain and physical function and objective assessments of physical function (passive internal rotation and flexion of the hip joint, extensor power of lower limb, six-minute walk test [6MWT], Timed Up & Go [TUG] test, ten-meter walk test [10MWT], sock test) were recorded. The data that we used in this study were part of an ongoing, randomized clinical trial of the long-term effects of exercise therapy on patients with hip OA (a baseline analysis, data collection accomplished before a randomization to exercise therapy groups).

Dependent (Outcome) Variables 

Self-reported pain and disease-specific physical function 

We assessed self-reported pain and disease-specific physical function by using the visual analog scale (VAS) format of the WOMAC, designed specifically for people with hip or knee OA.29 The VAS version allows the patients to estimate the symptoms by marking an X on a 100-mm long line (0mm is no symptoms, 100mm is worst possible symptoms). Subscale pain consists of 5 questions and the subscale of physical function consists of 17 questions concerning physical function. Summing the coded responses and then dividing by the number of items generated the mean score of hip pain and physical function.

Self-reported generic physical function 

Self-reported generic physical function was determined by using the physical function part of the RAND-36. The instrument contains exactly the same questions as the Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36)30 and the reliability and construct validity of the RAND-36, as a measurement of the health-related quality of life (HRQOL) in the Finnish general population, have been established.31 The RAND-36 survey is composed of 8 multi-item dimensions: general health, physical functioning, mental health, social function, vitality, bodily pain, and physical and emotional role function. There is a range from 0 to 100 in each subscale with higher scores indicative of a better HRQOL.

Independent (Explanatory) Variables 

Radiographic measurements and grading 

The same physician analyzed the degree of hip OA from the radiographs (supine and Lauenstein or anteroposterior pelvis). The mean age of the radiographs was 8.7±10.3 months. The Kellgren-Lawrence method of classification was used: grade 0 as normal; 1 as possible narrowing of joint space and possible osteophytes; 2 as definite narrowing of joint space and definite osteophytes; 3 as marked narrowing of joint space, definite osteophytes, and some deformity of femoral head; and 4 as gross loss of joint space, large osteophytes, and marked deformity of femoral head.26 The intrarater (.85–.91) reliability of Kellgren-Lawrence grading has been established as being high.27, 32, 33

Body mass index 

Body mass index (BMI) was calculated from measured height and weight and overweight was classified as a BMI of 25kg/m2 or heavier.34

Comorbidities 

Comorbidities were determined with the following questions: Do you have any of the following comorbidities: (1) Heart or coronary diseases (eg, coronary disease, hypertension, cardiac failure)? (2) Pulmonary diseases (eg, asthma, chronic obstructive pulmonary disease)? (3) Diabetes? (4) Other, what?

Existence and duration of knee pain 

Existence (yes, no) and duration of knee pain (in years) were considered as an explanatory variable, but knee pain was not counted as comorbidity. In all, 33.9% of participants suffered from knee pain in addition to hip OA (n=40).

Personal Factors 

Questions concerning sociodemographic background (age in years), educational level (years of schooling), sporting activities (in years), and smoking (in years) were extracted from a special questionnaire designed for the study.

Beck Depression Inventory 

The Beck Depression Inventory (BDI) with scores from 0 to 63 was used to assess possible depression.35 The cutoff point for clinically significant depression was set at 14 to 15, which has been found to have good sensitivity (.835), specificity (.813), positive predictive value (.968), and negative predictive value (.419) in screening major depression during all phase of the depressive disease.36, 37, 38 The sum score was analyzed continuously or dichotomously with 0 to 14 indicating normal mood and 15 or more indicating clinically significant depression.

Self-reported Life Satisfaction Scale 

The Life Satisfaction Scale was included to assess mental well-being.39 For each item, participants chose the statement that best described their experience. The sum scores were analyzed continuously as well as dichotomously (the satisfied group had scores 4 to 11 and the dissatisfied group had scores 12 to 20).40, 41

Objective Body Function Measurements 

We measured physical function by using a battery of tests performed in a random order. Four physiotherapists performed tests in 4 different measuring points during 1 day. The participants required approximately 45 minutes to complete these tests. The same physiotherapists completed the same tests for each participant with the tests being completed in a random order.

The passive hip flexion and the hip internal rotation were determined because hip OA subjects have a lower ROM in hip flexion and internal rotation21 and the hip internal rotation is considered to be the first restriction problem in hip OA.25(p14,206) The hip internal rotation was measured in a sitting position, hands placed on the pelvis, as described earlier.21(p14,206) The hip flexion was measured in the supine position of the participant, following the earlier described methodology.21(p14,206) The ROM measurements were determined by using 2-arm goniometry.21 The passive movement, including stretching, was done until the firm (or bony) end was reached and/or discomfort limited the motion or compensatory movement. The intra- and interday, intra- and intertester intraclass correlation coefficient, and coefficient of variation (in percent) of the passive ROM of the hip flexion and hip internal rotation were in the ranges of .655 to .988 and 2.1 to 12.6, respectively.21

Extensor power of lower limb was recorded by using the leg extensor power rig (Concept2, dynamic strength traininga). The measure of leg extensor power was expressed as relative power (absolute power divided by body weight [in W/kg]). The 1-week interday intertester and intratester reproducibilities were .982 and .996, respectively. A comparable device has been shown to be reliable for assessing patients with OA of the knee and prior surgery.42, 43 The measurement of power derived using the leg extensor power rig and those of quadriceps strength measured by maximum voluntary contraction have shown a positive correlation.42 It has been postulated that the measurement of power is more sensitive to age- and disease-related loss.44

The sock test,45 for evaluating activity limitation in patients with musculoskeletal pain, was used to describe the functional loss caused by hip OA. Scores are awarded in ordinal values from 0 (patient can grab toes with fingertips and perform the action with ease) to 1 (patient can grab toes with fingertips, but performs the action with effort) to 3 (patient can hardly, if at all, reach as far as the malleoli).

The 6MWT, a reliable, valid, and safe test, was used to quantify each participant's walking ability.46 The score recorded was the total distance traveled (in meters) during 6 minutes. The 10MWT, a measure of gait velocity, was performed indoors in a 10-m long track and the time spent to complete the walk (in seconds) was measured.47 The 6MWT was chosen because it is considered as a useful measure of both gait speed and functional and endurance capacity, whereas the 10MWT measures gait speed.

The TUG test is a performance-based measure that is clinically well established as a measurement of function for knee OA.48 The time (in seconds) required to complete the test was recorded.

Statistical Analyses 

We performed all statistical analyses using SPSSb for Windows. Descriptive statistics (means, standard deviations, ranges) of the continuous data and frequencies for categorical variables were calculated. Of those who had bilateral hip OA, the radiographic more severely affected hip was chosen as the “index hip,” which was subsequently used in further analysis. In order to determine the factors having an effect on pain and physical function in patients with hip OA, the bivariate correlations between all dependent and independent variables were counted. Correlations between dependent and independent variables were computed by using the Pearson correlation coefficient and Spearman correlation coefficient for the ordinally scaled educational level. The analysis of covariance (ANCOVA) was used to create a model for self-reported pain, self-reported disease-specific physical function, and self-reported generic physical function. Independent variables showing a statistically significant correlation (P<.01) with dependent variables were included into the ANCOVA. The RAND-3631 general health, mental health, social function, vitality, bodily pain, and physical and emotional role functioning and the WOMAC subscale stiffness29 were not entered into our models of self-reported mobility measures because of possible colinearity with pain and functioning in self-reported measures.

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Results 

Patient Characteristics 

The baseline characteristics of the participants are shown in Table 1, Table 2. A total of 120 patients agreed to participate in the study. One of the participants withdrew voluntarily on personal grounds and one died before the collection of baseline documentation, so the data from 118 participants were used for analysis. Of this sample, 35 were men and 83 women, with a mean age of 66.7±6.5 years (range, 55–80y); 71 participants had bilateral and 49 had unilateral hip OA. No statistically significant difference was found in pain and physical function between unilateral and bilateral hip OA groups (data not shown). According to Kellgren-Lawrence grading 1 to 4, the distribution of radiographic severity was as follows: 41.7% (n=50) of participants had Kellgren-Lawrence grade 1, 42.5% (n=51) had Kellgren-Lawrence grade 2, 13.3% (n=16) had Kellgren-Lawrence grade 3 radiographic degree of hip OA, and 3 of the patients (2.5%) were determined to have Kellgren-Lawrence grade 4 OA in the more afflicted side. The median duration of hip pain was 8.6±9.2 years and the median duration of diagnosed hip OA was 4.1±6.7 years.

Table 1. Descriptive Data of Participant Characteristics
VariablesnValid %
Sex
Male3630.0
Female8470.0
Working status
No longer employed8572.0
Part-time employment1311.0
Employed2016.9
Radiographic grades⁎
Kellgren-Lawrence 15041.7
Kellgren-Lawrence 25142.5
Kellgren-Lawrence 31613.3
Kellgren-Lawrence 432.5
Knee pain (yes/no)4033.9
Comorbidities
No chronic disease4941.5
1 chronic diseases5344.9
≥2 chronic diseases1613.6
BMI (kg/m2)
<252319.7
≥259480.3
BDI scores†
Normal mood (BDI ≤14)7088.6
Clinically important depression (BDI ≥15)911.4
Life Satisfaction Scale‡
Satisfied with life6379.7
Dissatisfied with life1620.3

⁎Kellgren-Lawrence grade 0 (normal), 1 (questionable osteophytes), 2 (definite osteophytes without joint space narrowing), 3 (definite osteophytes with moderate joint space narrowing), and 4 (definite osteophytes with severe joint space narrowing).

†Scoring range: 0–63. Cutoff point for clinically important depression was 14/15.

‡Scoring range: 4–11 (satisfied group scores); 12–20 (dissatisfied group scores).

Table 2. WOMAC, RAND-36, and Physical Performance Measures
VariablesMean ± SDRange
WOMAC (mm)⁎
Pain25.2±18.02–75
Stiffness35.3±24.40–90
Physical function26.8±19.70–83
RAND-36†
Physical functioning62.3±20.215–100
Role–physical51.3±39.00–100
Bodily pain57.5±18.313–100
General health56.1±17.35–95
Vitality66.0±16.810–100
Social functioning80.5±18.538–100
Role–emotional67.0±39.30–100
Mental health78.4±15.036–100
Performance measures
10MWT (s)5.9±1.43.5–14.3
6MWT (m)480.8±85.2222.0–709.0
TUG test (s)7.0±1.44.5–13.3
Leg extensor power (W/kg)‡1.1±0.30.36–2.26
Passive hip flexion (deg)§97.4±16.860–130
Passive hip internal rotation (deg)§28.8±8.210–47

⁎Scoring range: 0mm (no symptoms) to 100mm (maximal symptoms).

†RAND-36 is the Finnish version of the SF-36. Scoring range: 0 (maximal symptoms) to 100 (no symptoms).

‡Leg extensor power was measured by Concept2 dynamometer.

§Passive hip flexion and internal rotation of index hip was measured by goniometry.

Any participant with a BMI of 25kg/m2 or higher was considered as being overweight; and 80.3% (n=94) of participants met this criterion (see table 1). Chronic comorbidities were absent in 41.5% of participants; 44.9% (n=53) had 1 chronic disease, and 2 or more chronic comorbidities were found in 13.6% (n=16) of participants (see table 1). In this study sample, 11.4% of the patients were found to have clinically significant depression and 20.3% were judged as being dissatisfied with life (see table 1).

The participants tended to report relatively low pain scores (25.2±18.0mm), as well as low scores for stiffness (35.3±24.4mm) and physical function (26.8±19.7mm) (see table 2). The baseline documentation of the RAND-36 is shown in table 2. There was no statistical difference in pain and physical function when the groups were subdivided by sex or age (data not shown). The existence of knee pain, in addition to hip OA had a significant effect on the WOMAC self-reported pain and physical function (fig 2).

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  • Fig 2. 

    Impact (means, 95% confidence intervals) of knee pain on the WOMAC pain and function scores in patients with hip OA. The existence of knee pain, in addition to hip OA has a significant effect on the self-reported pain (t test for equality of means, P=.004) and physical function (t test for equality of means, P=.012).

Bivariate Correlations 

The bivariate correlations of pain, self-reported disease-specific physical function, and self-reported generic physical function with all independent variables are shown in table 3. There were several statistically significant correlations between performance measurements and personal and pathophysiologic factors with self-reported pain and physical function. Pain and self-reported function correlated significantly with each other; bivariate correlations between pain and both disease-specific physical function (r=.853, P<.001) and generic physical function (r=−.530, P<.001) were found to be good.

Table 3. Pearson Correlation Coefficients Between Dependent (outcome) and Independent (explanatory) Variables
VariablesWOMAC PainWOMAC FunctionRAND-36 Physical Function
Pathophysiologic variables
Kellgren-Lawrence grade of hip OA⁎.027.016−.018
BMI (kg/m2).137.252††−.290††
Comorbidities.369††.313††−.220⁎⁎
Duration of knee pain (y).198⁎⁎.195⁎⁎−.104
Personal factors
Age (y).123.149−.138
Education (y)−.276††−.264††.291††
BDI†.196.131−.184
Life Satisfaction Scale‡.299††.204−.319††
Smoking (y)−.102−.097−.019
Duration of sport activities (y).163.079.101
Performance measures
Leg extensor power (W/kg)§−.191⁎⁎−.277††.256††
Passive hip flexion (deg)−.261††−.286††.239††
Passive hip internal rotation (deg).050−.010.090
Sock test#.134.243††−.303††
10MWT (s).233⁎⁎.208⁎⁎−.338††
TUG test (s).251††.282††−.344††
6MWT (m)−.277††−.286††.417††

⁎Kellgren-Lawrence grade 0 (normal), 1 (questionable osteophytes), 2 (definite osteophytes without joint space narrowing), 3 (definite osteophytes with moderate joint space narrowing), and 4 (definite osteophytes with severe joint space narrowing).

†Scoring range: 0–63. Cutoff point for clinically important depression was 14/15.

‡Scoring range: 4–11 (satisfied group scores); 12–20 (dissatisfied group scores).

§Leg extensor power was measured by Concept2 dynamometer.

Passive hip flexion and internal rotation of index hip was measured by goniometry.

#Scoring range: 0 (no difficulties with simulating sock wearing) to 4 (remarkable difficulties with simulating sock wearing).

⁎⁎P <.05;

††P<.01.

Analysis of Covariance 

The predictors for self-reported pain, self-reported disease-specific physical function, and self-reported generic physical function are listed in table 4. The number of comorbidities and duration of knee pain as well as life satisfaction explained 22% of self-reported pain. The number of comorbidities, passive hip flexion, and the TUG test explained 20% of self-reported disease-specific physical function whereas the passive hip flexion, 6MWT, and educational level accounted for 25% of self-reported generic physical function.

Table 4. Predictors of WOMAC Pain and Function and RAND-36 Physical Function: ANCOVA
ModelVariablesβSEPAdjusted R2
WOMAC pain⁎Life Satisfaction Scale†2.1270.710.004.221
Duration of knee pain0.5730.258.029
Comorbidities6.3352.265.007
WOMAC functionComorbidities6.8892.364.026.203
Hip flexion‡−0.3160.098.004
TUG test§3.4901.198.004
RAND-36 functionHip flexion0.2600.099.010.249
6MWT−0.0900.020<.001
Education
Level 1−11.9974.529.009
Level 2−5.2105.490.345
Level 3#RefRefRef

Abbreviation: SE, standard error.

⁎Scoring range: 0mm (no symptoms) to 100mm (maximal symptoms).

†Scoring range: 4–11 (satisfied group scores); 12–20 (dissatisfied group scores).

‡Passive hip flexion of index hip was measured by goniometry.

§Timed in seconds.

Timed in meters.

#Reference (Ref) group.

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Discussion 

This study aimed to evaluate the relation between different categories of factors experienced by patients with hip OA, including personal factors, activities, participation, and body structure and function as described in the ICF model. We found that personal factors, educational level, and life satisfaction were associated with disability. With respect to the category of body functions and structures, BMI, comorbidities, and duration of knee pain were significantly related with disability. All objective body measurements, with the exception of passive hip internal rotation, examined in this study, were found to be associated with disability. The finding that no single predicting factor was superior to any other confirms the opinion that problems arising from hip OA are multidimensional and can predispose patients to disability.

In this study, the performance measures were observed to be better predictors of physical function than pain. Our selected objective measurements are widely used in clinical practice and this study aimed also to evaluate the relation between self-reported outcome measures and performance measures in hip OA population. The passive hip flexion ROM, the 6MWT, the TUG test, and the 10MWT were all significantly associated with all main outcome measures. These results lead us to believe that instead of following the radiographic severity of hip joint we might be better advised to resort to commonly used objective measurements to describe the functional severity experienced by the hip OA.

Specifically, we found that depression and life satisfaction were not explanatory factors for pain. Van Baar et al5 established significant correlations between pain, disability and anxiety in patients with hip OA. Nonetheless, no significant correlations between pain, disability and depression were found. To assess the extent of depression, we chose the BDI, because it evaluates not only motivational, cognitive, and emotional items but also somatic issues relating to weight loss, sleep disturbance, and working capacity. However, these factors may reflect not only depression but also somatic concerns and disabilities. Nevertheless, it is concluded that the pain evoked by hip OA is not necessarily directly influenced by psychologic factors.

With respect to the other personal factors, the educational level of the participants displayed a significant correlation with both pain and physical function. This relation has not been studied in hip OA previously. The results of this study confirm that higher educational levels reflect lower experienced pain scores and also support the conception that there is a clear link between the educational level and physical functioning. Our study extends the inverse association between low level of education and morbidity in general to consequences of hip OA and one logical explanation for this finding could be that lower educational level is usually connected with physically heavy manual work.49

According to pathophysiologic factors, analyzed in this study, the number of comorbidities was associated with all of the main outcome measures. There are no previous studies that have analyzed this association in the hip OA population. An interesting observation was that the BMI had no significant association with pain. Although the BMI did not correlate significantly with the pain, there was a statistically significant correlation between BMI with the self-reported disease-specific physical function (r=.252) and the generic physical function (r=−.290). Therefore obesity, a potentially treatable factor, should be considered as a possible determinant of the functional capacity of the patients with hip OA.

The analysis of the association of radiographic score of hip OA with pain and physical function confirms earlier observations that the radiographic score is not strictly associated with pain or physical function in patients with hip OA.16, 17, 18 It is generally accepted that the radiographic definition of the articular status represents the basis for the diagnosis of hip OA, but considering the fact that the radiographic score did not correspond either with the pain or the physical function, we believe that radiographic score should not be the main tool used to analyze the clinical severity of hip OA; at least this parameter should be supplemented with objective body function measurements.

In this study, one third of the participants experienced knee pain in addition to hip OA and the duration of the knee symptoms was found to exhibit a statistically significant correlation with both pain and disease-specific physical function. In order to dissect the link between these 2 separate pain evoking joints, pain and physical function were analyzed in 2 groups; 1 group had exclusively symptomatic hip OA and the other patients suffered from a combination of hip and knee pain. It was evident that the patients in the group with supplementary knee pain self-reported significantly higher levels of hip pain and disability than the subjects with only hip OA. This result gives us also a possibility to consider that being able to reduce one of these pain-evoking focuses may result in pain relief in other areas. In conclusion, the existence of knee pain in addition to hip OA may be a significant factor when analyzing pain and the consequent disability in patients with hip OA.

Quadriceps strength has been shown to have a strong association with knee pain and disability in patients with knee OA.5, 6, 7, 8 On the contrary, in hip OA, hip muscle strength seems not to correlate with the subjective severity of the hip pain in OA patients.19 The association between hip muscle strength and subjective functional capacity has not been previously reported. Thus, the association of muscle strength with hip OA is not so clear, possibly due to difficulties in isolating or defining muscle groups that are linked to the hip joint function. Barker et al16 observed an association between pain and muscle power in patients with severe OA of the knee. They also used the leg extensor power as a measurement of power. As a single predictor of physical function, leg extensor power explained less than 10% of the experienced pain. In this material no direct relation between leg extensor strength and hip OA impairments was found. It is possible that weak lower extremity strength, and knee OA pain results in compensatory impaired hip movement and deterioration in physical function. This question needs further studies.

Study Limitations 

This study has certain limitations. In this material, the subjects mainly suffered from mild or moderate hip OA, diagnosed by radiography, with relatively low scores of pain and physical function. This may be due to the fact that the participants were recruited mainly via a newspaper advertisement, not being a pure random population sample. Including participants with more severe symptoms of hip OA might have given a more objective picture of this population. Another limitation is directed to the consideration of depression and life satisfaction as the sole identifiers of psychologic status. A further assessment of anxiety and helplessness could have provided additional information.

One other limitation relates to the measurement of muscle power of limbs. The hip joint is the connection between the pelvis and the lower limb, with very complicated muscle systems that can be affected along with the hip joint. The different muscles of the hip have not been analyzed separately. The measurement of extension power of the lower limbs assesses mainly quadriceps femoris, gastrocnemius, and soleus muscles. This measurement provides a general perspective of the whole capacity of the lower limbs, but does not differentiate it into any separate muscle groups. However, the extensor power of lower limb was believed to provide an insight into total functional capacity of the lower limbs and was therefore considered as a relevant measure of total functional capacity of the limbs. The gluteal and abdominal muscle groups are also crucial in the role of the hip joint but these groups were not assessed in our measurements; this omission weakens the value of the study's measurements. This aspect can only be decided by further studies into muscle power and how this is related to pain and the subsequent disability in patients with hip OA.

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Conclusions 

This study provides convincing evidence that the clinical severity of hip OA is a sum of several factors. The educational level, life satisfaction, number of comorbidities, and duration of knee symptoms were found to display the most consistent association with self-reported pain in hip OA. Performance measures are better predictors of physical functioning than pain. The subjects with supplementary knee pain had significantly higher levels of self-reported hip pain and disability than participants with only hip OA. The radiographic score is not associated with pain or physical function in patients with hip OA. The finding that no single predicting factor is superior to any of the others confirms the belief that the factors explaining disability and pain in hip OA are multidimensional. This fact should be taken into account when considering interventions and prevention strategies in hip OA.

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Acknowledgments 

We thank physiotherapists Veli Huopalainen, PT, Outi Kuurne, PT, and Jaana Merivirta, PT, for practical help and for organizing the measurements, to Vesa Kivinemi, PhLic, for statistical advice, and to Nicholas Theakston for proofreading.

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 Supported in part by Mikkeli Central Hospital (EVO grant).

 No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated.

PII: S0003-9993(08)00142-1

doi:10.1016/j.apmr.2007.10.036

Archives of Physical Medicine and Rehabilitation
Volume 89, Issue 6 , Pages 1066-1073, June 2008

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