Knee osteoarthritis is considered to be a progressive disorder of articular cartilage within the knee joint, often related to age or activity. The symptomatic presentation of knee osteoarthritis varies considerably and often fails to correlate with the magnitude of joint dysfunction as observed in imaging studies. New evidence suggests that changes in pain processing within the peripheral and central nervous system could be essential as to why such variations in patient-reported symptoms and clinical presentation exist.
Similar to a number of chronic pain conditions, including fibromyalgia, chronic whiplash, and chronic low back pain, it appears as though knee osteoarthritis may also be influenced by central sensitization.
This brings to mind several clinical questions regarding the nervous system’s influence on pain processing as it relates to knee osteoarthritis:
- What is sensitization?
- What is the difference between peripheral and central sensitization?
- How is pain perception measured?
- Can knee osteoarthritis lead to central sensitization?
- What are the clinical implications?
- How do we apply this to clinical practice to alleviate pain?
Pain Sensitization
Both peripheral and central sensitization contributes to pain associated with knee osteoarthritis. Peripheral Sensitization results from repeated activation of the local nociceptors within the knee during inflammation and causes hyperalgesia (a heightened response to painful stimuli). Due to the inflammation caused by injury, local pain receptors become much more sensitive to detecting neurochemical messengers. This heightened state drastically increases the likelihood of nociceptive neurons deeming a particular sensation as painful, causing them to send pain signals upstream through the central nervous system to the brain. Danger signals delivered to the brain are heightened which causes pain to increase or last.
While peripheral sensitization and central sensitization may appear on the surface, they actually represent distinct processes and clinical presentations. As its name suggests, peripheral sensitization is limited to localized tissues and is caused by the heightened status of pain receptors.
In Central Sensitization, physical changes within the central nervous system due to prolonged neural signaling causes new stimuli to be “co-opted” into pain-receiving neural pathways. It is believed that pain facilitation and inhibition within the nervous system are affected, over-activating the pain facilitatory pathways. To put it simply, too many danger messages are flowing up to the brain with not enough stop commands being sent back. Your brain has the ability to override these sensations. This is what is known as descanting inhibition of pain and one of the key ways the brain can stop your pain.
Within healthy tissue, central sensitization is characterized by:
- Hyperalgesia – a heightened response to painful stimuli.
- Allodynia – a painful response to what should be a harmless stimulus.
- An expanded receptive field – increased pain response to stimuli outside the area of injury.
How is Pain Perception Measured?
Currently, there is no single gold standard test to identify and measure pain sensitization. Therefore, a battery of tests is often used to assess the potential for pain sensitization in people with knee osteoarthritis. The most commonly used method involves quantitative sensory testing (QST), which assesses sensitivity to noxious or innocuous stimuli using standardized mechanical, thermal, and electrical means.
Typically, pressure, heat, cold, electrical, and pinprick are used to examine peripheral sensitization. To examine central sensitization specifically, studies have used tests of different neural phenomena, including:
- Temporal summation (TS) – “wind up,” repeated pain receptor stimulation causes a heightened response
- Conditioned pain modulation (CPM) – the nervous system’s ability to inhibit pain
- Flexor withdrawal response (FWR) – a reflex related to the quick removal from a painful stimulus
Temporal summation is tested via repeated noxious stimulation. Conditioned pain modulation testing involves evaluating a painful test stimulus with and without a second painful stimulus applied to a different area. This second stimulus serves to “condition” the nervous system, challenging the ability to inhibit pain perception. Flexor withdrawal response testing looks at spinal excitability threshold, with lower thresholds leading to a more rapid response.
Is Knee Arthritis Influenced by Central Sensitization?
Central sensitization may very well play a role in the pain associated with knee osteoarthritis. The evidence shows widespread hyperalgesia due to pressure, pinprick, and electrical stimuli in areas isolated from the symptomatic knee. This suggests an involvement of the central nervous system and these central changes are believed to be related to the ongoing pathological neural signals originated from the arthritic knee joint. Additionally, there is evidence to support spinal hyperexcitability in people with osteoarthritis, as seen in temporal summation and flexor withdrawal reflex testing, as well as dysfunctional conditioned pain modulation.
The results regarding knee arthritis and central sensitization are largely similar to what is seen in many chronic pain conditions, such as fibromyalgia and chronic low back pain. While the mechanisms at play are analogous, their respective causes are very different and it appears as though central sensitization may form a subgroup of people with knee osteoarthritis. this may hold especially true for individuals experiencing very intense pain associated with what appears to be relatively low disorder severity.
Clinical Implications in Pain Care
Similar to a chronic pain condition, central sensitization should be considered when evaluating an individual for knee arthritis. There are numerous methods for measuring sensitization that can be cheaply and easily applied in the clinic. This includes assessing pressure pain threshold at a local and remote site to test for widespread hyperalgesia. Conditioned pain modulation can be tested using pressure pain threshold as the test stimulus. Temporal summation can also be tested using successive applications of pressure pain threshold stimuli.
In addition to these examination techniques, when working with an individual who has knee arthritis, average pain over the previous month and radiographic severity should be recorded and compared. Identifying those at risk for arthritis-related central sensitization could allow for targeted pharmacological interventions, in addition to skilled physical therapy practice.
Education for those with pain is paramount. It is important to work with these clients to:
- Change maladaptive disease perceptions. Changing thoughts, mood and emotions changes pain.
- Change any misunderstandings related to pain. Pain science intervention is important.
- Reframe how pain is perceived. Chronic pain is much more about the joint.
Additionally, manual therapy and an individualized exercise program can be very effective in combating central sensitization. I’ve spoken about the importance of exercise for pain relief here.
Take Home Points
- Central sensitization is characterized by hyperalgesia, allodynia, and an expanded receptive field
- Sensitization can be measured in the clinic through pressure pain threshold, temporal summation, conditioned pain modulation, and flexor withdrawal response
- Central sensitization may be responsible for a subgroup of individuals with knee osteoarthritis and can be contributing to their heightened pain response
- Consider education to reframe how pain is cognitively perceived
- Consider the use of manual therapy, exercise, pain education therapy and rely on targeted pharmacological interventions only after all else has failed
Pain is like a puzzle, it’s complex. But the human body and brain have an incredible and innate ability to heal. Often we rely on imaging studies to tell us the cause of pain. Nowhere is that more prevalent than in osteoarthritis. How do you change the perception in your knee that may hurt?
Pain is like a puzzle, it's complex. Click To Tweet
How Is Osteoarthritis Treated?
Combine treatments that focus on lifestyle. Osteoarthritis treatments have 5 main goals.
- Improve joint function.
- Keep a healthy body weight.
- Learn about pain.
- Achieve a healthy lifestyle.
- Avoid inflammatory foods.
Osteoarthritis treatment plans include:
- Exercise
- Weight control
- Nutrition
- Relative rest and joint care
- Nondrug pain relief techniques to control pain
- Complementary and alternative therapies
- Surgery as a last resort, only when the above has been exhausted
Self-Care Can Save Your Knee
Osteoarthritis self-management programs help you:
- Understand the disease and empower you with strategies
- Reduce pain while staying active
- Cope with your body, mind, and emotions
- Have more control over the disease
- Live an active, independent life.
People with good coping skills can overcome osteoarthritis pain
- Focus on what you can do, not what you can’t do
- Focus on your strengths, not your weakness
- Break down activities into small tasks.
- Pace yourself
- Now which foods trigger your pain such as sugar, gluten or nightshades
- Develop ways to lower and manage stress
- Connect emotions such as fear, anger, sadness or guilt to your pain
- Balance rest with activity
- Develop a support system of family, friends, and healthcare providers.
