For hundreds of years, it has been assumed that pain is a direct result of injury. In theory, a greater amount of damage to the tissues of the body should result in a higher degree of pain. This makes intuitive sense, yet has been proven to be overly simplistic.
Consider, for example, a study by Liu et al (1). Subjects who were experiencing major pain in one of their knees or shoulders had an MRI taken on both the painful joint as well as the joint on the other side of the body. Amazingly, doctors were unable to tell based off of the images which side was the one in pain. In both the painful side and non-painful side, a similar degree of tissue damage was present.
This is not an isolated finding. Many additional studies have found that:
- MRIs of the low back are unable to predict which patients are suffering from pain. Many people with no back pain have major disc herniations or other structural damage (2-9)
- MRIs of the knee are unable to predict which patients are suffering from knee pain (10-14)
- MRIs of the shoulder are unable to predict which patients are suffering from shoulder pain (15-18)
Clearly, these studies indicate that the connection between injury and pain is more complicated than many may believe.
What is pain?
Pain is a defense mechanism produced by the brain in response to a perceived threat. Many factors are inputted into our brain that influence the perception of pain. Tissue damage is one of these factors, but there are many psychological factors that play a major role as well (19).
Amazingly, these psychological factors can sometimes play a larger role than the structural factors in determining pain and treatment outcomes. For example:
- For low back pain, patient’s perceptions of their injury may be a better predictor of treatment outcomes than the severity of the injury (20-24)
- For ankle injuries, psychosocial factors can predict pain symptoms better than severity of the injury (25-27)
- For shoulder injury, psychosocial factors predict pain better than severity of injury (28-30)
In all of these studies, the psychological factors tended to outweigh the structural factors in determining who would suffer from pain.
Nervous System Changes to Chronic Pain
Further evidence of the differentiation between structural damage and pain is evident in the neural changes that occur in patients who suffer from chronic pain.
Patients who are in pain for an extended period of time can often suffer from long-term changes to the way their brain processes pain (31-48).
According to Dr. Adriaan Louw in his book Therapeutic Neuroscience Education, “Pain mechanisms due to processing issues, commonly referred to as ‘central sensitization,’ occur due to neuroplastic changes in the dorsal horn of the spinal cord, inappropriate synapsing, neuronal death, sprouting, receptor field expansion, activation of the neuromatrix, changes in neurotransmitters and ion channels in the CNS and brain, and loss of gray matter. In central sensitization, pain is due more to abnormal processing of the nociception by the spinal cord, brainstem and cerebral hemispheres” (49, page 103).
These types of patients will require specialized treatment protocols to address the unfavorable effects that have occurred on the nervous system, and may not respond to typical physical therapy approaches that focus exclusively on biomechanical dysfunction.
Factors that Affect Pain Perception
Many psychological factors have been shown to affect perceptions of pain. For example:
- Expectations – When subjects expect physical therapy to work, they are more likely to experience greater reductions in pain (50-52). In one interesting study, when subjects in an MRI machine were given a placebo and told that it would relieve their pain, decreased brain activity was seen in the areas of the brain associated with pain perceptions (53)
- Social support – When subjects receive social support, perceptions of pain are dramatically decreased (54-62). In one study, subjects in an fMRI machine were subjected to electric shocks. When these subjects were allowed to hold their husbands hands, decreased brain activity was seen in the areas of the brain associated with pain (56)
- Physician empathy – When doctors are communicative with their patients and demonstrate empathy, patients feel less pain compared to less empathetic doctors, even when the treatments are identical (63-67)
- Therapeutic Neuroscience Education – When physical therapists talk with their patients about the science of pain and explain how sometimes pain can be present even in the absence of any serious pathology, this decreases threat perceptions and can lead to better treatment outcomes (68-80)
Taking Pain Science Too Far?
Because pain is created by the brain and can be absent even in the presence of injury or movement dysfunction, I have seen some clinicians claim that injury or movement dysfunction no longer matter. Some have claimed that instead of fixing the way their patients move, they focus on educating them on how there is no “proper way to move” and gradually exposing them to painful movements to help them feel safer and reduce threat perception. I even had one chiropractor tell me that he doesn’t care if all he is doing is creating a placebo effect because pain is all in the brain and as long as his clients feel better when they leave, that is all that matters.
Just because tissue injury and structural damage does not correlate well with pain does not mean that they are completely irrelevant. Imagine a runner with poor ankle dorsiflexion, poor glute strength, poor hip mobility, and other dysfunctions that increase his risk for damage to the knee joint. He goes to his chiropractor to treat his knee pain, and the chiropractor decides to focus all of his efforts towards treating the client’s pain rather than his ankle and hip dysfunction. After a few sessions of therapeutic neuroscience education, joint manipulations, and massage, the client is feeling less pain and decides to continue running regularly.
After many years, because the hip and ankle problems were never addressed, the knee continues to degrade to the point where severe osteoarthritis becomes present. At this point, no amount of psychosocial interventions are enough to reduce the pain he is feeling due to his injury, and now he has no choice but to have his knee replaced.
If his therapist had addressed his hip and ankle issues when he had first started feeling pain, it’s possible that the tissue damage would not have accelerated to this point. Unfortunately, an overemphasis on pain-reducing techniques can sometimes be harmful if it comes at the expense of treating movement dysfunction.
Pain and injury are two separate physiological phenomena. Pain can be present in the absence of injury and injury is often present in the absence of pain. Skilled clinicians must understand how to distinguish between the two and how to balance a treatment protocol to address both the pain and injury that their clients may be experiencing.
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