A randomized controlled crossover trial: Lidocaine injected at a 90-degree angle causes less pain than lidocaine injected at a 45-degree angle

Abstract

To the Editor: Local anesthetic injection results in discomfort to patients.1Long C.C. Motley R.J. Holt P.J. How to reduce the discomfort caused by local anaesthetics.Clin Exp Dermatol. 1993; 18: 291Crossref PubMed Scopus (1) Google Scholar, 2Zilinsky I. Bar-Meir E. Zaslansky R. Mendes D. Winkler E. Orenstein A. Ten commandments for minimal pain during administration of local anesthetics.J Drugs Dermatol. 2005; 4: 212-216PubMed Google Scholar Anecdotal reports suggest that injecting at a 90-degree angle reduces pain.2Zilinsky I. Bar-Meir E. Zaslansky R. Mendes D. Winkler E. Orenstein A. Ten commandments for minimal pain during administration of local anesthetics.J Drugs Dermatol. 2005; 4: 212-216PubMed Google Scholar Controlled studies examining this approach are lacking. This study examines the effect of angle of needle insertion (90 vs 45 degrees) during local administration of anesthetic on subjects’ pain. An institutional review board–approved randomized controlled crossover trial was conducted in volunteers aged 18 to 70 years without history of neuropathy, severe skin disease in injection areas, allergy to lidocaine, or anxiety to needle sticks. The study used room-temperature lidocaine HCl 1%, containing 1:100,000 epinephrine, which is the standard anesthetic in our clinics for biopsies. A 0.5-inch, 30-gauge needle was inserted bevel-up into the skin to a depth of 0.25 inch. Each subject received a 1.0-cc injection in each forearm utilizing a 90-degree angle in one, and a 45-degree angle in the other. Injections were made at a rate of 0.1 mL per second by the same investigator with a 1-minute wash-out period between injections. Subjects were blinded as to the initial angle of insertion. After each lidocaine infusion, subjects reported whether the 45-degree or the 90-degree angle injection hurt more, or if the pain was the same. Subjects reported maximum pain felt during the procedure by using a validated 10-point scale,3Wong D.L. Baker C.M. Pain in children: comparison of assessment scales.Pediatr Nurs. 1988; 14: 9-17PubMed Google Scholar with 0 indicating “no pain” and 10 indicating “worst possible pain”. Data were compared by means of Kruskal-Wallis, Fisher’s exact, and Wilcoxon-rank sum tests. Regression was used to control for age, gender, and body mass index (BMI). Sixty-five subjects enrolled in this study. The mean age was 36.3 years (standard deviation [SD], 13.8). Thirty-five percent were male. Twenty-nine subjects (45%) perceived the 45-degree angle to be more painful, 22 (34%) reported the 90-degree angle as more painful, and 14 (22%) reported the same pain score for both (P < .0001). The median pain score for the 45-degree angle was 3 (interquartile range [IQR] = 2.0-4.0), and the mean pain score was 2.9 (SD = 1.6). For the 90-degree angle, the median was 2 (IQR = 2.0-3.0), and the mean was 2.5 (SD = 1.3). The differences were significant (P = .0002, P = .0295, respectively) (Fig 1). Age (P = .589), BMI (P = .526), and sex (P = .0985) did not influence pain. Our results suggest that a 90-degree angle reduces pain during lidocaine injection. While the difference we found was clinically small, we excluded patients with anxiety associated with needle sticks, and a larger difference may be appreciated in this population. Local lidocaine administration results in pain from the needle stick and the infusion. Egekvist et al4Egekvist H. Bjerring P. Arendt-Nielsen L. Pain and mechanical injury of human skin following needle insertions.Eur J Pain. 1999; 3: 41-49Crossref PubMed Scopus (95) Google Scholar found no difference in pain between 45- and 90-degree angles of injection, but that the energy required to inject was lower with insertion at 90 degrees. Lower injection energy may lead to less tissue distension and thus less pain. In addition, 90-degree angle insertion may result in less direct transection of nerve endings than 45-degree angle insertion.2Zilinsky I. Bar-Meir E. Zaslansky R. Mendes D. Winkler E. Orenstein A. Ten commandments for minimal pain during administration of local anesthetics.J Drugs Dermatol. 2005; 4: 212-216PubMed Google Scholar Up to 21% of adults express anxiety from needles,5Nir Y. Paz A. Sabo E. Potasman I. Fear of injections in young adults: prevalence and associations.Am J Trop Med Hyg. 2003; 68: 341-344PubMed Google Scholar which may lead to syncope or avoidance of necessary procedures. To reduce the pain of lidocaine injections, clinicians use warm, buffered solutions, small needles, and inject slowly.1Long C.C. Motley R.J. Holt P.J. How to reduce the discomfort caused by local anaesthetics.Clin Exp Dermatol. 1993; 18: 291Crossref PubMed Scopus (1) Google Scholar, 2Zilinsky I. Bar-Meir E. Zaslansky R. Mendes D. Winkler E. Orenstein A. Ten commandments for minimal pain during administration of local anesthetics.J Drugs Dermatol. 2005; 4: 212-216PubMed Google Scholar, 5Nir Y. Paz A. Sabo E. Potasman I. Fear of injections in young adults: prevalence and associations.Am J Trop Med Hyg. 2003; 68: 341-344PubMed Google Scholar This study supports use of a 90-degree needle angle as an additional method of pain reduction.