Rationale for Using Local Anaesthetics

• Analgesic: although the effect is temporary, it can make the overall procedure less unpleasant for the patient, break the pain cycle (by reducing nociceptive input to the 'gate' in the dorsal horn of the spinal cord), and increase the confidence of the patient in the clinician, the diagnosis and the treatment. In one study, pain inhibition was better with bupivacaine than lidocaine during the first 6 hours, because of its longer half-life; in later evaluations no differences in outcomes were observed9 3 . In another study, bupivacaine was superior to lidocaine at 2 weeks, but not at 3 and 12 months9 4 . Some practitioners inject
a mixture of short- and long-acting local anaesthetic to obtain both the immediate diagnostic effect plus more prolonged pain relief

• Diagnostic: pain relief following an injection confirms the diagnosis and correct placement of the solution9 2 . Sometimes even the most experienced practitioner will be unsure exactly which tissue is at fault; in this situation, inject a small amount of local anaesthetic into the most likely tissue, wait a few minutes, and re-examine. If the pain is relieved then the source of the problem has been identified and further treatment can be accurately directed; if not, further testing should follow until sure of the cause of the pain

• Dilution: the internal surface area of joints and bursae is surprisingly large, due to the highly convoluted synovial lining with its many villae, so increased volume of the injected solution helps to spread the steroid around this surface

• Distension: a beneficial volume effect in joints and bursae might be the stretching of the capsule or bursa with physical disruption of adhesions9 5 , 9 6 , 1 7 8 , 2 1 7 . Distension is not required at entheses, so use the smallest volume that is practicable; distension in tendons by bolus injection of a relatively large volume of solution may physically disrupt the tendon fibres and compress the relatively poor arterial supply. It can also give rise to distension pain