Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage (The International Association for the Study of Pain). Different categories of pain can be defined according to the duration, etiology, or perception of the painful experience.

1. Acute pain is caused by damage, ongoing inflammation, or physiological malfunction after a physical injury to body tissues and usually resolves as the wound heals. The intensity and duration of acute postoperative pain is a major risk factor in the development of chronic postsurgical pain. Improved control of acute pain following surgery has become a focus for practice improvement measures leading to a more timely and diversified approach to pain. Studies demonstrate that techniques that effectively reduce acute pain, including “prehabilitation,” multimodal preventative analgesia, and the use of regional anesthetics are also associated with a lower incidence of subsequent chronic postsurgical pain and is a target as a primary prevention measure.
2. Chronic pain is pain that persists beyond the time of healing, often defined as pain persisting beyond 3 to 6 months. Common chronic pain conditions include chronic postsurgical pain, chronic low back pain, complex regional pain syndrome, postherpetic neuralgia, temporomandibular disorders, cancer pain, and myofascial pain. Current research suggests that mechanisms in the development of chronic pain involve signaling at both the site of tissue trauma and central sensitization at the level of the spinal cord and above.
3. Neuropathic pain results from pathologic function of the somatosensory system, either in peripheral elements (receptors or peripheral nerves) or in the central nervous system. The abnormal somatosensory function of neuropathic pain is a direct result of injury to the nervous system, and this type of pain persists even after tissue healing appears to be complete. It is most frequently described as burning, radiating, lancinating, or shooting in nature and can be accompanied by allodynia, or the perception of pain from a normally innocuous stimulus.
4. Nociceptive pain results from an injury that activates peripheral nociceptors, which can be somatic or visceral in origin, and is the pain associated with most acute injuries. Somatic pain typically reflects injury to the superficial structures of the musculoskeletal system and skin. Somatic pain is typically well localized in contrast to that of visceral pain, which arises from distention or injury to the viscera, owing to the less dense sensory innervation of the organs as compared with other tissues.
5. Inflammatory pain is categorized as nociceptive pain in the presence of acute inflammation, but chronic inflammatory states can play mechanistic roles in neuropathic pain states (Loeser & Treede, Kyoto). Inflammation resulting from tissue damage can lead to hyperalgesia, which is the exaggerated painful perception of a known noxious stimulus.