​Chronic pain, chronic primary pain, chronic secondary musculoskeletal pain, chronic postsurgical or posttraumatic pain,… What is the difference between these terms and which one contains knee osteoarthritis pain? 

Definitions

First of all, chronic pain is defined as pain that lasts longer than the normal healing time, or in more specific time-related terms: pain that persists longer than three months (1). Often, altered central pain processing symptoms and psychosocial factors can be seen in chronic pain patients (2). Recently, the International Classification of Diseases (ICD) decided to classify this term further into sub-terms and introduced various definitions regarding chronic pain. In this blog we will focus on ‘chronic primary pain’, ‘chronic secondary musculoskeletal pain’ and ‘chronic postsurgical pain’, as these are the most related with knee osteoarthritis pain (3). 

Chronic primary pain refers to pain as the disease in its own right of which the pain sensation is driven by central mechanisms. Chronic secondary musculoskeletal pain means that pain is caused by an underlying disease (e.g. peripheral damage) that drives the pain sensation. Finally, chronic postsurgical pain refers to pain sensation for longer than three months after surgery due to surgical trauma  (3). 

Chronic pain and knee osteoarthritis

Knee osteoarthritis is a degenerative chronic disease known as a common cause of joint pain, disability, and loss of quality of life (4). To date, an efficient treatment for end-stage knee osteoarthritis is total knee replacement surgery with a success rate of about 80% (5). The ICD classifies knee osteoarthritis pain as chronic secondary musculoskeletal pain, because the structural changes affecting the joint are assumed as the peripheral source of nociception for chronic pain experienced by the patients. About 20% of knee osteoarthritis patients experience chronic pain after total knee replacement surgery, and are therefore classified into the chronic secondary postsurgical pain category (3).

Is this division correct for all knee osteoarthritis patients?

If this classification would be completely correct, it could be assumed that pain decreases and central pain processing signs normalize after eliminating the peripheral source of nociception (for example by performing a total knee replacement surgery). However, some studies showed a normalization of the central pain processing signs after total knee replacement surgery in combination with pain relief (6,7), while others indicated that abnormal signs remained present and that patients experienced no pain relief (8-10). The inconsistent conclusions of different studies in combination with 20% of patients experiencing chronic pain after total knee replacement indicate that this current classification is not applicable to all knee osteoarthritis patients experiencing chronic pain. It is postulated that a substantial part - the group in which the pain and abnormal signs of central pain processing after surgery will remain present - needs to be classified as chronic primary pain.

Future research goals

In order to identify these chronic primary pain knee osteoarthritis patients and thus the group of 20% that experience chronic postsurgical pain already preoperatively, predictive studies regarding central pain processing signs and psychosocial parameters have been performed, but more research is highly needed (11,12). Also, pre- and postsurgical signs and parameters need to be addressed to study the evolution of these signs and parameters over time in relation to the presence of chronic postsurgical pain. 

Sophie Vervullens

Physiotherapist and PhD student at the University of Antwerp (MOVANT) and the University of Maastricht (CAPHRI).

2022  Pain in Motion

References and further reading:  

1 Treede, R.-D. et al. Pain 156, 1003–1007 (2015) 
2 Clauw, D. J. et al. Postgrad. Med. 131, 185–198 (2019)
3 Treede, R.-D. et al. Pain 160, 19–27 (2019) 
4 Reginster, J. Y. Rheumatol. Oxf. 41 Supp 1, 3–6 (2002)
5 Sakellariou, V. I. et al. Orthopedics 39, 55–62 (2016)
6 Graven-Nielsen, T. et al. Arthritis Rheum. 64, 2907–2916 (2012)
7 Kurien, T. et al. J. Pain Off. J. Am. Pain Soc. 19, 1329–1341 (2018)
8 Arendt-Nielsen, L. et al. Curr. Osteoporos. Rep. 13, 225–234 (2015)
9 Skou, S. T. et al. Eur. J. Pain Lond. Engl. 18, 1024–1031 (2014)
10 Kosek, E. et al. Osteoarthritis Cartilage 21, 1299–1307 (2013)
11 Petersen, K. K. et al. Pain 162, 31–44 (2021)
12 Braun, M. et al. Curr. Pain Headache Rep. 25, 3 (2021)