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
). 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
), 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
) 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.