The bulk of the injuries occurring in Operation Iraqi Freedom (OIF) and Operation Enduring Freedom (OEF) involve the extremities, and are associated with severe acute pain. After the injury has occurred, the wounded warrior is cared for at a forward operating base hospital where stabilizing treatment occurs, which often includes surgery. After the injured are stabilized, they are transported to a military treatment facility for definitive care. From the time of injury to the time of definitive care, the injured person is typically treated with intravenous opiates, such as morphine or fentanyl to control their pain. In some instances, local anesthetic medicine can be injected next to the nerves that supply the injured area to help control the pain, a technique called a peripheral nerve block. While opiate use is fairly effective for treating acute pain, there are a number of draw backs to relying on opiate-based pain control including excessive sedation, respiratory depression (loss of drive to breathe), dependence, and even a worsening of the pain (hyperalgesia) or development of a pain syndrome where the patient feels pain even without a typical painful stimulus (allodynia). Nerve blocks can be very useful for treating acute pain, but are limited to treatment of a relatively short duration. Additionally, the medicine available to use in nerve blocks is not selective for the different types of nerve fibers and makes it so that the motor and tactile sensory fibers are also blocked, potentially interfering with early rehabilitation. Patients whose acute pain is inadequately treated can go on to develop chronic pain syndromes that interfere with their ability to rehabilitate and return to active duty or civilian work.

As an alternative to the currently available treatments for acute pain, much focus has been given to a special type of receptor channel (transient receptor potential vanilloid I or TRPV1) that is located on the small nerve fibers that conduct noxious and painful stimuli, but is not located on nerves that control motor movement or tactile sensation like light touch. By activating this TRPV1 receptor with a drug like capsaicin, the noxious stimuli from injured tissues can be blocked, thus providing profound pain relief without affecting motor or tactile sensory nerves. Pain control through this modality can potentially last for weeks after a single treatment, instead of hours as seen with opiates. Additionally, since capsaicin is applied directly to the injured tissues, systemic side effects may be avoided. We hypothesize that injecting capsaicin into the fracture injury site and surrounding tissue will provide analgesia for weeks and reduce local inflammation without affecting motor or tactile sensory nerve function or inhibiting bone repair. This would allow for unencumbered rehabilitation and possibly eliminate the development of chronic pain syndromes. In this study, we will develop a model of battlefield long bone injury and repair in anesthetized rats. We will study the effects of different doses of capsaicin injected directly into the fracture site and surrounding injured tissue to achieve maximal pain control. We will also study the effects of capsaicin on bone healing and local inflammation. The results are expected to help achieve effective long-term treatment of acute pain with a reduction in long-term pain and improved functional capacity for wounded warriors. As capsaicin already has a long history of safe use in humans, translation of positive results from this study to human trials should be rapid.