A trigonometric analysis of needle redirection and needle position during neural block.

Abstract

Successful regional anesthesia involves a three-dimensional visualization of anatomy and an informed approach to needle placement and repositioning. This study trigonometrically examines the relationship between needle insertion angle and resultant needle position. Needle position during advancement of 2-inch (5.1-cm), 3 1/2-inch (8.9-cm), and 5-inch (12.7-cm) needles was trigonometrically determined. Starting at 90 degrees, the angle of needle insertion theta (measured from the perpendicular) was decreased in 5 degrees and 10 degrees increments, and the resultant change in needle position was calculated by using a computer model. Needle depths relevant to the three needle lengths were studied and ranged from 0.5 to 11.0 cm. The effects of needle insertion angle, redirection angle, and depth on resultant needle position were evaluated. The mean change in needle position was significantly less with a 5 degree redirection of the needle than with a 10 degree needle redirection. In addition, the mean change in needle position was highly correlated with increasing needle depth and acute angle of needle insertion. Incremental needle redirection of 5 degrees allows a precise survey of neural and adjacent anatomy and results in approximately one half the change in needle position occurring with a 10 degree angle of redirection. However, a 10 degree angle of redirection may result in walking over the desired neural structure.