Where Are We Now?
Chronic, sometimes even disabling pain is a cardinal feature of knee osteoarthritis (OA) and a major reason why people with OA seek joint replacement surgery. Increasing evidence supports a biopsychosocial model of pain, whereby cognitive and emotional processes are acknowledged as important contributors to the pain experience [9, 12]. This may help explain why patients with knee OA demonstrate such enormous variability in pain perception and its impact on life. Individual cognitive processes, including thoughts, beliefs, expectations, and coping strategies can influence the pain experience . Pain catastrophizing, a multidimensional construct characterized by the tendency to ruminate about and magnify pain, to feel helpless in the context of pain, and by a relative inability to inhibit pain-related thoughts [9, 12], appears to be one of the most-important cognitive processes.
Unfortunately, knee arthroplasty is not successful for all people with knee OA, and between 7.5% and 28% of patients report dissatisfaction after knee arthroplasty . In people undergoing knee arthroplasty, pain catastrophizing has emerged as a predictor of chronic pain for up to 2 years after surgery. Specifically, higher levels of presurgical pain catastrophizing are associated with more pain postoperatively, and may also contribute to poorer functional outcomes (although this evidence is somewhat conflicting) [1, 5, 18]. Preoperative coping strategies also appear to have a role, with less problem-solving coping and more dysfunctional coping associated with more pain and worse knee function 6 months after surgery .
In the current study, Riddle and colleagues report on a cross-sectional analysis of a large sample of people awaiting knee arthroplasty and who have moderate to high levels of pain catastrophizing. Their results seem to suggest that maladaptive responses may have a greater impact on pain and function in people with end-stage knee OA than adaptive responses.
A growing body of research has evaluated the efficacy of cognitive behavioral therapy for OA symptoms. Most studies have focused on pain coping skills training, which aims to teach patients practical skills for managing their pain. A number of studies have shown that pain coping skills training can reduce pain and improve physical function in people with OA [4, 8, 15, 17]. However, there is little research evaluating the efficacy of cognitive behavioral therapies in the context of joint arthroplasty. Preliminary quasi-experimental data from Riddle and colleagues  suggests that psychologist-directed pain coping skills training leads to reductions in pain severity and pain catastrophizing, and improvements in function, compared to usual care at 2 months postsurgery.
Where Do We Need To Go?
Knee arthroplasty usage is rapidly increasing . Although it is cost-effective, we need to reduce the proportion of patients who experience persistent pain and disability postoperatively. Surgeons need to identify individuals at risk of persistent pain and functional disability following arthroplasty, so that effective pre and perioperative interventions can be deployed in a timely manner for maximum benefit.
To achieve these aims, the following questions must be answered: (1) How do we accurately screen patients in the clinical setting to identify those at increased risk of persistent pain and/or disability postoperatively? (2) Which interventions are most effective for reducing pain catastrophizing and maladaptive coping strategies? (3) Do reductions in pain catastrophizing and maladaptive coping strategies mediate post-operative improvements in pain, physical function and satisfaction with outcomes following arthroplasty? (4) Are there other important psychological moderators of arthroplasty outcomes? (5) What is the right time to intervene when a patient is at increased risk of poor postoperative outcomes? (6) Can we make psychological interventions more accessible to the relatively large numbers of people with knee OA who might benefit from treatment?
Riddle and colleagues selected a patient sample with moderate-to-high levels of pain catastrophizing. They showed that relationships between pain catastrophizing and maladaptive coping strategies and pain and physical function preoperatively, but could not establish causation due to the cross-sectional nature of their study. It remains unclear whether cognitive behavioral therapy can reduce pain catastrophizing and maladaptive pain coping strategies in people with these psychological traits undergoing knee arthroplasty, and whether such changes lead to improved outcomes following arthroplasty. Given that research in people with chronic low back pain has showed that reductions in pain catastrophizing with cognitive behavioral and physical treatments mediated reductions in disability and pain intensity , further research in the context of knee arthroplasty is warranted.
How Do We Get There?
In order to answer the above questions, researchers should conduct longitudinal, experimental research designs. Only randomized controlled trials allow causation between preoperative psychological traits and arthroplasty outcomes to be established. Not only do they allow the efficacy of psychological interventions in improving arthroplasty outcomes to be tested, well-designed trials that include process measures (such as pain catastrophizing, coping strategies, mood disturbances) can also determine whether changes in these psychological parameters mediate improvements in pain, function, and satisfaction following arthroplasty. Further, randomized controlled trials that also characterize patients at baseline according to psychological, mood, and personality traits will allow treatment effect moderators to be determined. Trials should include measures of pain catastrophizing and pain coping, which are relatively straight forward to assess using the Pain Catastrophizing Scale and Chronic Pain Coping Inventory, respectively. Analysis of treatment effect moderators will help identify the patient subgroups for which treatments are most, or least, effective and can help direct sensible allocation of resources in the clinical setting to those who will most benefit.
Traditionally, cognitive behavioral therapy is delivered by psychologists, however access to psychologists for people with knee OA awaiting knee arthroplasty may be limited. Consideration needs to be given to testing alternative models of delivering psychological interventions to this patient group. There is some evidence that physiotherapists  and nurses  can be trained to effectively to deliver pain coping skills training to people with chronic OA pain. In addition, emerging evidence suggests that a novel online pain coping skills program designed to translate key therapeutic elements of clinician-delivered training can be effective [3, 15]. Qualitative research exploring the acceptability of these new models of service delivery to both patients and clinicians is required, including an exploration of potential barriers to implementing them into clinical practice.
Excitingly, some of this research is already underway and soon to be completed. In the United States, Riddle and colleagues are conducting  a randomized controlled trial to determine if a physical therapist-delivered pain coping skills training program, delivered prior to knee arthroplasty, can reduce pain at 12 months following surgery in patients with high levels of pain catastrophizing. In Australia, researchers  are conducting a randomized controlled trial evaluating whether a mindfulness-based psychological intervention can enhance outcomes in people undergoing total joint arthroplasty. As the results of these clinical trials come to light, and the body of evidence grows, the challenge will be how best to incorporate research findings into clinical practice.
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