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This study examined the degree to which pain catastrophizing and pain-related fear explain pain, psychological disability, physical disability, and walking speed in patients with osteoarthritis (OA) of the knee. Participants in this study were 106 individuals diagnosed as having OA of at least one knee, who reported knee pain persisting six months or longer. Results suggest that pain catastrophizing explained a significant proportion (all P's ≤ 0.05) of variance in measures of pain (partial r2 [pr2] = 0.10), psychological disability (pr2 = 0.20), physical disability (pr2 = 0.11), and gait velocity at normal (pr2 = 0.04), fast (pr2 = 0.04), and intermediate speeds (pr2 = 0.04). Pain-related fear explained a significant proportion of the variance in measures of psychological disability (pr2 = 0.07) and walking at a fast speed (pr2 = 0.05). Pain cognitions, particularly pain catastrophizing, appear to be important variables in understanding pain, disability, and walking at normal, fast, and intermediate speeds in knee OA patients. Clinicians interested in understanding variations in pain and disability in this population may benefit by expanding the focus of their inquiries beyond traditional medical and demographic variables to include an assessment of pain catastrophizing and pain-related fear.
Recent advances in cognitive-behavioral pain theory and research suggest that pain cognitions, that is the way an individual thinks about and interprets pain, may be very important in explaining variations in pain and disability in patients with osteoarthritis (OA). Recent studies of OA patients suggest that two pain-related cognitions in particular may be especially important predictors of pain and disability: pain catastrophizing and pain-related fear. Pain castrophizing refers to the tendency to focus on and magnify pain sensations, and to feel helpless in the face of pain (1). Pain catastrophizing has been studied in arthritis patients suffering from both OA and rheumatoid arthritis (2,3). Studies of OA samples have found that patients who engage in pain catastrophizing not only report higher levels of pain, but also have much higher levels of psychological disability (2,4). OA patients who catastrophize about pain also have been found to have higher levels of physical disability (2), though these findings are not as strong and consistent as those regarding the relation of catastrophizing to pain and psychological disability (2,4). What makes these findings regarding catastrophizing impressive is that they have been obtained even after controlling for demographic variables and medical status variables that are important in explaining pain and disability.
The term pain-related fear refers to an excessive and debilitating fear of physical movement and activity resulting from a feeling of vulnerability to pain (2). The impact of pain-related fear on pain and disability has been shown in studies of patients with chronic non-malignant pain conditions (e.g., low back pain) (6,7), but has only recently been investigated in patients with disease-related pain conditions such as arthritis. Individuals who experience pain-related fear are likely to engage in avoidance behaviors, specifically avoidance of movement and physical activity. Maintaining a sufficient level of activity and exercise is critical to managing the pain and disability associated with OA (e.g., 8), however, OA patients who fear engaging in painful movements may be hesitant to initiate or engage in daily activity or exercise. In this way, pain-related fear contributes to a vicious cycle of a more restricted and physically inactive lifestyle adopted to avoid pain, paradoxically leading to increased pain and disability. To date, only one study has examined the relationship of pain-related fear to pain and disability in persons with OA (9). In this study, 254 patients with radiographic evidence of OA were administered measures of fear of pain, pain, and physical disability. Results indicated that pain-related fear explained a significant proportion of variance in physical disability even after controlling for pain and radiographic findings. Interestingly, radiographic findings of OA were not predictive of disability.
In order to understand the role of pain cognitions in OA patients several basic questions need to be addressed. First, there is a need to determine whether these cognitions can explain pain and adjustment over and above what can be explained on the basis of demographic variables (age, sex, race) and medical status variables (disease severity, body mass index [BMI]) believed to be important in understanding adjustment to OA. A second basic question that arises when studying pain-related cognitions is their relative importance in explaining indices of pain and adjustment. To our knowledge, no studies have simultaneously tested the effects of pain catastrophizing and pain-related fear in OA patients, which would help to identify the unique effects of each variable. Finally, it is unknown how important pain cognitions such as catastrophizing and pain-related fear are in explaining variations in objective measures of walking speed. The effects of pain cognitions on objective markers such as walking speed are important since physical activities such as walking are important in managing pain and disability in OA patients (10,11). Understanding the relative importance of these two pain cognitions is important for two reasons. First, it may help focus psychosocial assessment efforts on the most critical cognitions. Second, it could have implications for treatment interventions that could be used to improve pain and adjustment.
The goal of the present study was twofold. First, we wanted to examine the degree to which pain catastrophizing and pain-related fear explained pain and disability in overweight or obese OA patients after controlling for demographic and medical status variables. We hypothesized that both pain catastrophizing and pain-related fear would be related to pain severity, psychological disability, physical disability, and gait velocity assessments (e.g., walking speeds) after controlling for demographic and medical status variables. Second, we wanted to examine the unique predictive value of each of these two types of pain-related cognitions. Based on findings from prior research we hypothesized that 1) pain catastrophizing would be more strongly related to pain severity and psychological disability, and 2) that pain-related fear would be more strongly related to physical disability and gait velocity assessments (e.g., walking speeds).
An interesting aspect of the present study was that it focused on a sample of overweight and obese patients with OA. People with OA who are overweight (Body Mass Index [BMI] = 25-29) or obese (BMI ≥ 30) are much more likely to experience higher levels of pain (12) which may increase the likelihood that they will experience overly negative pain cognitions such as pain catastrophizing. In addition, since overweight or obese OA patients are especially likely to experience pain with movement (13), they may be more likely to develop pain-related fears of movement that may serve as obstacles or barriers to involvement in daily activities or exercises that can help them control their weight.
Participants in this study were recruited through the Rheumatology, Orthopedic Surgery, and Pain Management clinics at Duke University Medical Center, through flyers posted in the community, and from advertisements in local newspapers. All participants were volunteers for a randomized trial known as OALife (#NCT00305890). All participants provided informed consent and study procedures were approved by the Duke University Institutional Review Board. Data presented in this paper were collected at the baseline evaluation prior to randomization to treatment conditions. To determine if they were eligible for the study, all participants were weighed, received bilateral knee radiographs, and met with a study rheumatologist (who took a medical history and did a physical examination). Patients were included if: 1) they reported knee pain on most days of the month for at least the prior 6 months, 2) were overweight or obese (BMI between 25 and 42), 3) met the American College of Rheumatology criteria for OA and had radiographic evidence of OA affecting one or both knees, 4) had no other major weight bearing joint affected by OA, and 5) if OA of the knee(s) was the medical condition that contributed most to limitations in their daily function. Patients were excluded if: 1) they had a significant medical condition that increased their risk of a significant adverse health event during physical activity (e.g., myocardial infarction in the previous six months, an abnormal cardiac response to exercise such as exercise induced BT or abnormal blood pressure), 2) had another known organic disease that would contraindicate safe participation in the study, 3) had a non-OA inflammatory anthropathy/another arthritic disorder (e.g., rheumatoid arthritis), 4) engaged in regular oral corticosteroid use, or 5) already were involved in a regular exercise or weight loss program.
All patients completed a series of questionnaires to assess pain cognitions and arthritis-related disability, and psychological disability. Patients also provided demographic and medical information.
Table 1 summarizes the demographic and medical characteristics of this sample. The study sample included 106 overweight or obese OA patients (24 men and 82 women) who had a mean age of 58.7 (SD = 9.2). Participants self-reported to be White (60%), African-American (37%), or another race (3%); 62% of the sample had a college degree or higher. The average BMI of the sample was 34.7 (SD = 4.3) and the average Kellgren Lawrence OA disease rating (e.g., 15) was 2.8 (SD = 1.0). Most (85%) participants were recruited through community and local advertisements, and 15% were recruited from Duke University Medical Center clinics. There were no statistically significant (P-values >0.10) differences between participants recruited from the community versus clinics on demographic, medical, psychological, disability, or gait variables.
The diagnosis of knee OA was based on the American College or Rheumatology clinical criteria for classification of knee OA. Radiographic assessment of both knees was used to confirm OA diagnosis. A fixed-flexion PA knee radiograph was taken with the SynaFlex™ X-ray positioning frame (Synarc, San Francisco, CA). With this platform, the feet were externally rotated 10 degrees, the knees and thighs touched the vertical platform anteriorly, and the X-ray beam was angulated 10 caudally. Skyline views of both patellae were taken with the participant in the seated position, knees bent, and the beam angled from the feet toward the knees. One of the study rheumatologists (VBK) graded each x-ray on the basis of Kellgren-Lawrence criteria (14). The most severe Kellgren-Lawrence score (range 1-4) was used to measure disease severity in this study, with higher scores indicating greater disease severity.
Catastrophizing was assessed using the Catastrophizing Scale of the Coping Strategies Questionnaire (CSQ; 15) This scale contains six items that ask patients to rate the frequency with which they engage in catastrophizing when they experience pain using a six-point scale on which 0 = never and 6 = always. The CSQ Catastrophizing Scale has been shown to have good internal reliability (15) as well as a high degree of stability over time (16). For the current study sample, a Cronbach alpha revealed that the scale had good internal reliability (Cronbach's alpha = 0.81).
Pain-related fear was measured with the Tampa Scale of Kinesiophobia (TSK) (5). The TSK is a self-report measure assessing the fear of (re)injury due to movement. The instrument consists of 17 items scored on a 4-point scale from “strongly agree” to “strongly disagree.” Prior studies have supported the reliability and validity of this scale (17). Internal consistency of the TSK for this study sample was adequate (Cronbach's alpha = 0.73).
The Arthritis Impact Measurement Scales (AIMS) (18) is a widely used 45-item self-report questionnaire designed to measure health status in arthritis patients. The AIMS provides three component summary scales (i.e., pain, psychological disability, physical disability). Reliability and validity of the three summary scales of the AIMS have been demonstrated in prior research (19,20). In this sample each AIMS scale (pain, psychological disability, physical disability) demonstrated high internal consistency (Cronbach's alpha = 0.93 0.74, and 0.81, respectively).
Normal walking speed, fast walking speed, and intermediate walking speed were used as dependent variables in this study. Three-dimensional kinematic data were collected using a motion analysis system (Motion Analysis Inc, Santa Rosa, CA). In preparation for data collection, patients completed three practice trials along a 30 meter walkway at three speeds: the speed at which they normally perform their daily walking activities (normal), the maximum speed they felt comfortable achieving (fast), and a speed in between those two speeds (intermediate). These three speeds were chosen in order to get a sense of the speed at which the participants were most comfortable (normal speed) and to see how their gait mechanics change when they were presented with a moderate (intermediate speed) or intense (fast speed) challenge. Gait velocity was measured using two wireless infrared photocell timing devices (Brower Timing Systems, Draper Utah) positioned five meters apart and the patient's target walking velocity for each speed was determined. Following the practice trials, kinematic data were collected at 60Hz. Reflective markers were placed bilaterally at the following landmarks: acromion process, lateral epicondyle of the humerus, wrist, anterior superior iliac spine, thigh, lateral knee (at the joint line), shank, lateral malleolus, calcaneus, and foot (2nd webspace). A marker was also placed at the superior aspect of the L5-sacral interface to aid in defining the pelvis. In addition, markers were placed bilaterally on the medial femoral condyle and medial malleolus for collection of a static trial. Once the static trial was completed, the four medial markers were removed. Patients performed five walking trials along the walkway at each of the self-selected speeds. Time synchronized ground reaction force data were collected at 1200Hz using AMTI force plates (Advanced Medical Technologies Inc., Watertown, MA). Variability in walking velocity for each speed was restricted to ±5%; trials outside of this range or trials during which the subject did not contact at least one of the force plates cleanly were repeated. EvaRT (Motion Analysis Inc, Santa Rosa CA) software was used to track the reflective markers and condition the data. The raw data were smoothed using a 4th order, recursive Butterworth filter with a 6Hz cutoff frequency. Three trials at each speed in which all markers were identified and the subject had clean contact with the force plate were averaged to yield kinetic and kinematic data.
Table 1 provides descriptive data for the demographic variables and medical status variables, and the measures of pain catastrophizing, pain-related fear, pain, psychological disability, and physical disability.
Correlational analyses were performed to examine the associations between demographic variables and medical status variables and outcome variables (e.g., pain, psychological disability, physical disability, walking speeds). Demographic and medical status variables that were related with the outcome variables (P ≤ 0.10) were retained in further analyses.
Hierarchical linear regression (HLR; 21) was used to assess the unique contribution of pain catastrophizing and pain-related fear to the outcomes. In each HLR, pain catastrophizing and pain-related fear were entered on the final step (Step 3) after controlling for demographic variables entered on Step 1 and medical status variables entered on Step 2. Finally, to provide an estimate of the unique proportion of the variance accounted by pain catastrophizing and pain-related fear (23) we reported the individual partial r2 (pr2) for each of these variables. The current study had adequate power (0.80 with a two-tailed alpha=0.05) to test HLR models with up to ten predictors and a small to moderate total R2 (0.12 to 0.15).
Correlational analyses revealed that age (r = -0.21, P < 0.05) was correlated with pain. Age (r = -0.30, P < 0.01), sex (r = -0.28, P < 0.01), and disease severity (r = -0.20, P < 0.05) were correlated with psychological disability. Disease severity was correlated with walking at all three speeds (normal, fast, intermediate) (r's = -0.20 – -0.24, P's < 0.05) as was BMI (r's = -0.19 – -0.26; P's < 0.05).
Table 2 summarizes the HLR results. HLR was conducted to test the association between pain catastrophizing and pain-related fear and the outcomes (pain severity, psychological disability, physical disability, and walking speeds) after controlling for demographic and medical status variables.
As shown in Table 2, demographic and medical status variables accounted for 6% of the variance in pain severity, whereas pain catastrophizing (pr2 = 0.10) and pain-related fear (pr2 < 0.01) accounted for an additional 13% of the variance in pain severity. Of these two pain cognitions, only pain catastrophizing was a significant independent predictor of pain severity (β = 0.35, t = 3.15, P = 0.002).
Demographic and medical status variables explained 18% of the variance in psychological disability. Pain catastrophizing (pr2 = 0.20) and pain-related fear (pr2 = 0.07) accounted for an additional 30% of the variance. Both of these pain cognitions were significant predictors of psychological disability with pain catastrophizing (β = 0.47, t = 4.87, P < 0.001) being a slightly stronger predictor than pain-related fear (β = 0.25, t = 2.80, P = 0.006).
Demographic and medical status variables accounted for 25% of the variance in physical disability. Pain catastrophizing (pr2 = 0.11) and pain-related fear (pr2 = 0.01) accounted for 12% additional variance in physical disability. Of pain catastrophizing and pain-related fear, only pain catastrophizing was a significant independent predictor of physical disability (β = 0.35, t = 3.24, P = 0.002).
Demographic and medical status variables accounted for 22% of the variance when predicting walking at normal speed. Pain catastrophizing (pr2 = 0.04) and pain-related fear (pr2 = 0.02) accounted for an additional 7% of the variance in walking at normal speed with pain catastrophizing being a significant individual predictor (β = -0.22, t = -2.02, P = 0.05). When predicting walking at fast speed, demographic and medical status variables predicted 21% of the variance and pain catastrophizing (pr2 = 0.04) and pain-related fear (pr2 < 0.05) accounted for an additional 11% of the variance of the variance. Pain-related fear was a significant independent predictor of walking at fast speed (β = -0.22, t = -2.20, P = 0.03) as was pain catastrophizing (β = -0.22, t = -2.04, P = 0.04). When predicting walking at an intermediate speed, demographic and medical status variables accounted for 24% of the variance. Pain catastrophizing (pr2 = 0.04) and pain-related fear (pr2 < 0.02) accounted for an additional 9% of the variance in walking at intermediate speed. Pain catastrophizing was a significant independent predictor of walking at intermediate speed (β = -0.25, t = -2.30, P = 0.02) while pain-related fear was not.
This study found in a sample of patients with OA of the knee that pain catastrophizing explained a significant proportion of variance in pain (10%), psychological disability (20%), physical disability (11%), and walking at all speeds (4%). Pain-related fear also explained a significant proportion of variance in measures of psychological disability (7%) and walking at fast speed (5%). These results are noteworthy in that they were obtained after controlling for demographic (e.g., age, sex, race) and medical status variables (e.g., disease severity, BMI) believed to be important in explaining pain and disability in OA patients. In fact, pain catastrophizing and pain-related fear accounted for more overall variance in pain and psychological disability than demographic and medical status variables combined. These findings suggest that, in understanding pain and adjustment to OA, pain catastrophizing and pain-related fear have something additional to offer above and beyond what might be explained by traditional disease related factors. They also suggest that clinicians interested in understanding variations in adjustment to knee OA pain might benefit by expanding the focus of their inquiries beyond traditional demographic and medical status variables to include an assessment of pain catastrophizing and pain-related fear.
Consistent with our hypotheses, pain catastrophizing explained a higher proportion of variance in pain and psychological disability than pain-related fear. This finding agrees with prior studies of OA patients as well as prior studies of patients with other persistent pain conditions (e.g., low back pain, pain due to rheumatoid arthritis) (3,23,24). Our study provides new data in that it demonstrates that findings regarding pain catastrophizing can be generalized to overweight and obese OA patients. One reason that pain catastrophizing may have similar effects across diverse pain conditions is that it may affect fundamental processes involved in the processing of pain stimuli. Along these lines, pain catastrophizing has been linked in imaging studies to abnormal processing of pain stimuli (25,26), suggesting it may have important effects on neural processes related to pain perception.
This study found that both pain catastrophizing and pain-related fear were significantly related to psychological disability. Although pain catastrophizing explained almost twice the amount of variance in psychological disability compared to pain-related fear, both pain cognitions explained unique and significant variance in psychological disability. One concern that could be raised regarding these findings is that the effects of pain catastrophizing and pain related fear on psychological disability simply reflect the presence of higher pain, that is persons who have more pain are more likely to report higher levels of psychological disability. However, in this study the findings regarding pain catastrophizing and pain-related fear were evident even after controlling for pain severity.
Contrary to our hypotheses, pain-related fear did not explain a significant proportion of variance in self-reported physical disability in OA patients whereas pain catastrophizing did. This suggests that tendencies to ruminate upon pain and feel helpless in the face of pain (e.g., pain catastrophizing) may be more important in explaining physical disability in person with OA than pain-related fear. Pain-related fear, however, did explain a significant portion of variance in one of the objective walking speed measures (walking fast). Although this suggests that pain-related fear may show stronger relationships to objective markers of knee-related disability, it should be noted that pain catastrophizing (and not pain-related fear) explained a significant portion of variance in walking at all speeds. The fact that pain catastrophizing and pain-related fear explained significant proportions of variance in walking speed in OA patients is important. As noted earlier, these pain cognitions may serve as obstacles to OA patients willingness to engage in demanding physical activities (e.g., walking fast), even though such activities are important in managing pain and disability. Interventions to address and modify these cognitions may enable OA patients to feel more confident about and begin engaging in activities they are capable of doing, but may not otherwise engage in. By becoming more involved, for example, in treatment efforts to increase activity (e.g., cardiovascular exercise, weight training) OA patients may be able to strengthen muscles, decrease weight, and reduce their pain.
Crombez et al. (27) reported that patients with a sudden traumatic pain onset exhibit greater pain-related fear than patients whose pain symptoms begin gradually. This may partially explain why we found pain-related fear to have limited relationships to physical disability in our sample. OA patients typically do not experience an abrupt onset of symptoms but rather experience gradual progression of their symptoms and the degree to which these symptoms interfere with activities. In any event, future studies are needed to examine the relationship of pain cognitions to self-report measures and other objective markers of physical disability (e.g., stair climbing, transfers from sitting to standing, and performance of standard exercises) in patients with OA.
Disease severity (as assessed using Kellgren-Lawrence grading of x-rays; 14) and body weight (as assessed using BMI) are commonly believed to be very important factors explaining pain in patients with OA (28). Interestingly, this study found that neither of these factors explained a significant proportion of variance in patients' pain ratings. Pain catastrophizing, however, was a highly significant predictor of pain in this study sample. These findings underscore the importance of pain catastrophizing and suggest that clinicians interested in understanding pain in OA patients need to consider pain catastrophizing along with more traditional risk factors for OA pain.
The results of this study can inform future treatments efforts that address pain and disability in patients with OA or other disease-related pain. They suggest that pain catastrophizing plays a larger role than pain-related fear in explaining pain and disability in OA patients. We had expected that pain-related fear might have accounted for more variance in our outcomes particularly given that overweight and obese OA patients are likely to experience greater pain upon movement than their normal weight counterparts. It is important to distinguish between these two pain cognitions as pain catastrophizing may be best addressed through cognitive restructuring of maladaptive thought patterns while pain-related fear may be more impacted by in vivo exposure techniques. Thorn et al. (29) have described an intensive cognitive restructuring protocol for modifying pain catastrophizing in patients with chronic headaches that could be adapted for patients with OA. Vlaeyen and his colleagues (30,31) have developed in vivo exposure techniques that enable patients with pain to learn that they can successfully experience and habituate to movements or activities that they might normally avoid. These techniques have been found to be effective in decreasing pain and disability in patients with other persistent pain conditions and could be adaptive for use in OA patients.
While it should be noted that this is a cross-sectional study which limits the ability to make causal assumptions between our predictors and outcomes, the results of this study suggest that pain cognitions, in particular pain catastrophizing, are important in understanding pain, disability, and walking speed in persons having knee OA. Taken together, these findings support the utility of pain cognitions when attempting to understand OA pain and disability.
This study was supported by NIH Grant # 1PO1AR50245-03.
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