Background and Objectives We wished to determine which long-acting local anesthetic would produce the least pain on injection for treatment of myofascial pain disorders. We compared the pain on intramuscular injection of bupivacaine, ropivacaine, bupivacaine with dexamethasone, ropivacaine with dexamethasone, and needle placement alone.

Methods Thirty volunteers received 5 injections each: (1) needle only, (2) bupivacaine 0.5%, (3) ropivacaine 0.5%, (4) bupivacaine 0.5% with dexamethasone 0.13 mg/mL, and (5) ropivacaine 0.5% with dexamethasone 0.13 mg/mL. The injections were made in the volunteers’ upper trapezius muscles; there was a 15-minute interval between injections. The sequence of injections was randomized by Latin square design. The intensity of pain was rated on a 0 to 10 cm visual analogue scale (VAS) score. Neither the investigator nor the volunteer was aware of the nature of the injectate. The pH of the injected solutions was checked to determine if differences in the intensity of pain on injection were due to differences in the pH of the solutions.

Results The VAS pain scores were 3.1 ± 2.4 for needle only, 4.4 ± 2.8 for bupivacaine, 2.5 ± 2.0 for ropivacaine, 4.7 ± 2.7 for bupivacaine/dexamethasone, and 3.7 ± 2.2 for ropivacaine/dexamethasone. The pain on injection of ropivacaine was significantly less than the pain on injection of bupivacaine or bupivacaine/dexamethasone. The pH values of the solutions were as follows: (1) bupivacaine, 5.50; (2) ropivacaine, 5.57; (3) bupivacaine/dexamethasone, 6.64; and (4) ropivacaine/dexamethasone, 6.60.

Conclusions The pain on intramuscular injection of bupivacaine is significantly more intense than with ropivacaine. The difference in the intensity of the pain on injection between bupivacaine and ropivacaine does not appear to be related to differences in pH. The results of our study have implications on the choice of the local anesthetic used in trigger point injections.