Can we prevent chronic osteoarthritis pain? A view from the bench

Molecular pathogenesis of OA pain: Past, present, and future

OBJECTIVE

To provide a historical perspective and narrative review on research into the molecular pathogenesis of osteoarthritis pain.

DESIGN

PubMed databases were searched for combinations of "osteoarthritis", "pain" and "animal models" for papers that represented key phases in the history of osteoarthritis pain discovery research including epidemiology, pathology, imaging, preclinical modeling and clinical trials.

RESULTS

The possible anatomical sources of osteoarthritis pain were identified over 50 years ago, but relatively slow progress has been made in understanding the apparent disconnect between structural changes captured by radiography and symptom severity. Translationally relevant animal models of osteoarthritis have aided in our understanding of the structural and molecular drivers of osteoarthritis pain, including molecules such as nerve growth factor and C-C motif chemokine ligand 2. Events leading to persistent osteoarthritis pain appear to involve a two-step process involving changes in joint innervation, including neo-innervation of the articular cartilage, as well as sensitization at the level of the joint, dorsal root ganglion and central nervous system.

CONCLUSIONS

There remains a great need for the development of treatments to reduce osteoarthritis pain in patients. Harnessing all that we have learned over the past several decades is helping us to appreciate the important interaction between structural disease and pain, and this is likely to facilitate development of new disease modifying therapies in the future.

The Genesis of Pain in Osteoarthritis: Inflammation as a Mediator of Osteoarthritis Pain

Chronic pain is a substantial personal and societal burden worldwide. Osteoarthritis (OA) is one of the leading causes of chronic pain and is increasing in prevalence in accordance with a global aging population. In addition to affecting patients' physical lives, chronic pain also adversely affects patients' mental wellbeing. However, there remain no pharmacologic interventions to slow down the progression of OA and pain-alleviating therapies are largely unsuccessful. The presence of low-level inflammation in OA has been recognized for many years as a major pathogenic driver of joint damage. Inflammatory mechanisms can occur locally in joint tissues, such as the synovium, within the sensory nervous system, as well as systemically, caused by modifiable and unmodifiable factors. Understanding how inflammation may contribute to, and modify pain in OA will be instrumental in identifying new druggable targets for analgesic therapies. In this narrative review, we discuss recent insights into inflammatory mechanisms in OA pain. We discuss how local inflammation in the joint can contribute to mechanical sensitization and to the structural neuroplasticity of joint nociceptors, through pro-inflammatory factors such as nerve growth factor, cytokines, and chemokines. We consider the role of synovitis, and the amplifying mechanisms of neuroimmune interactions. We then explore emerging evidence around the role of neuroinflammation in the dorsal root ganglia and dorsal horn. Finally, we discuss how systemic inflammation associated with obesity may modify OA pain and suggest future research directions.