The Significance of Peripheral Input

The Significance of Peripheral Input in Patients With Post Surgery Neuropathic Pain: Evaluation of Trigger Point Anesthesia. A Double-blind, Randomized, Controlled Exploratory Study

The investigators want to examine trigger point injections of local anesthetics, in patients with post operative neuropathic pain, determine changes in sensory disturbances such as allodynia, hyperalgesia and thermal sensitivity and examine whether trigger point injections will attenuate pain in rest and activity.

The presence of cutaneous trigger points in the surgical incision of patients with post-surgery pain has been shown in our pain clinic, all patients have persistent pain for more than 3 month and different sensory disturbances. Examinations with trigger point injections of local anesthetics have been used to determine the influence of peripheral changes and the significance of afferent input on persistent post-surgery neuropathic pain. In the present study the investigators will document the influence of cutaneous trigger point injections for both subjective pain experience and objective findings in the area such as allodynia, hyperalgesia and thermal sensitivity. The investigators expect reduction in spontaneous pain intensity and de-crease in peripheral sensory disturbances and will determine the significance of peripheral input in persistent neuropathic pain. The investigators aim to investigate whether injection of local anesthesia into trigger points compared with placebo (saline) injection will attenuate pain and normalize abnormal sensations, such as hyperalgesia and allodynia. The investigators believe this to be an important investigation to understand the mechanisms behind chronic post-surgery pain.

Ropivacaine injection 1 ml in all trigger points in and around scar after operation in knee, shoulder and foot, one time, duration 3 minutes

Saline injection 1 ml in all trigger points in and around scar after operation in knee, shoulder and foot, one time, duration 3 minutes

Injection of 1 ml Ropivacaine in all trigger points in and around scar. Injection of 1 ml saline in all trigger points in and around scar.

Change in worst pain assessed on a visual analogue scale (VAS, 0-100 mm) at rest and after stair walking/abduction of shoulder, before and after trigger point injections compared to baseline pain score

Area of pin-prick hyperalgesia, using von Frey filament 60 g and brush allodynia, using Somdic brush, assessed pre- and post-injection. Areas identified at a control day between the two intervention days.

Change in warm and cold detection, threshold and tolerance before and after intervention in area with hyperalgesia compared to normal skin

Detection, thresholds and tolerance for cold and heat within the hyperalgesia area compared to normal skin at the contralateral site, before and after trigger point injections. All measures made with Somedic MSA Thermal Stimulator

Inclusion Criteria: Age > 18 years old Patients with chronic pain (daily VAS pain scores >30 mm) for at least 3 months after knee, shoulder or foot surgery Patients with at least one identifiable trigger point in relation to incision from the knee, shoulder or foot surgery. A trigger point is defined as an ar-ea in relation to the incision from surgery, in which a light pressure from a cotton pin radiate pain to the nearby area, and exudes a motor reflex causing withdrawal of the leg A written informed consent to participate in the study after having fully understood the contents of the protocol and restrictions Exclusion Criteria: Patients who cannot cooperate Patients who cannot understand or speak Danish. Patients with allergy to the drugs used in the study Patients in treatment with vitamin K antagonists. Patients with other pain conditions Patients with diabetes

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