Pain Phenotypes in Knee Osteoarthritis

In a 2011 statement to the Food and Drug Administration, the Osteoarthritis Research Society International (OARSI) identified the "phenotyping" of OA pain as a research priority to "better target pain therapies to individual patients." Successful identification of pain phenotypes will allow new interventions to be tested in homogeneous populations of patients presenting with similar pain pathophysiology, ultimately enhancing treatment effects in defined populations for whom interventions are determined safe and effective. Clinical populations of knee OA are clearly heterogeneous, spanning wide age ranges and encompassing patients with a wide variety of functional abilities. The pain experience in knee OA may be similarly individualized and complex; some patients may present with pain that appears attributable to classic signs of joint damage, while others may present with pain due to psychological distress or central mechanisms. Changes in somatosensory processing and pain threshold are also known to occur with aging. However, the scientific community has yet to examine these variables concurrently in the same study population. Therefore, the relative importance of each of these measures in determining pain severity across the lifespan is unknown. It is also unknown whether these variables (or interactions between variables) are representative of different pain phenotypes in knee OA. This is an important yet unresolved question; a patient with high levels of psychological distress and low levels of joint damage may warrant a different intervention strategy than the traditional knee-directed approach. On the other hand, someone with increased psychological distress in addition to severe joint damage may benefit from traditional interventions that are further augmented with other impairment-specific interventions. This sort of targeted approach is the topic of current research in other chronic pain populations, where a similar conceptual model, composed of peripheral, psychological and central components to the pain experience (among others), is recognized.