Unlike acute pain (pain after surgery or injury), chronic pain is a disease driven by a self-reinforcing cyclic process between the nerves, the brain, & the muscles,
Injury, Surgery, & Inflammation
Initial tissue injury triggers the chronic pain cycle. In the setting of chronic pain, the initial injury may resolve while the chronic pain cycle persists. This is because the chronic pain cycle is self-reinforcing.
Pain signal starts at the site of injury/surgery/inflammation and arrives at the midbrain, the seat of consciousness. This generates pain experience.
The pure pain experience is interpreted by the cortex by cognitive processes and then colored by mood. In the pathologic setting, the pain experience is interpreted as being malignant, which then triggers protective responses. While these protective responses are meant to be beneficial, they are maladaptive. In the case of the brain, studies have found that the brain structure is altered by chronic pain.
If pain signal is interpreted as malignant, the musculoskeletal system is altered, leading to avoidance and muscle spasms. Ultimately this leads to deconditioning. These musculoskeletal changes then lead to secondary injury, completing the pain cycle.