Defining the cross-sectional anatomy important to interscalene brachial plexus block with magnetic resonance imaging

Abstract

Background and Objectives. Interscalene brachial plexus block is a useful technique to provide anesthesia and analgesia for the shoulder and proximal upper extremity. The initial needle direction at the interscalene groove has been described as being “perpendicular to the skin in every plane” (1). A cross-sectional (axial) approach may offer a more easily conceptualized directed needle placement. The purpose of this study is to define the cross-sectional anatomy and idealized needle angles important to interscalene brachial plexus block. Methods. Following IRB approval, 50 patients were studied. Cross-sectional volume coil T1-weighted magnetic resonance images (MRI) were obtained from 50 patients undergoing cervical region imaging for other reasons. At the interscalene groove, a simulated needle path to contact the ventral rami or trunks of the brachial plexus was approximated at the level of C6 or C6–C7 interspace. The angle of this needle path intersecting the sagittal plane was recorded for each patient. Results. The mean angle of the simulated needle path relative to sagittal plane was determined to be 61.1 ± 6.1° (range, 50–78°). In 13 of 50 (26%) MRI scans, the cervical nerve roots were not visualized at the level of C6 and were measured at the C6–C7 level. Conclusions. These findings suggest initial needle placement at the interscalene groove should be angled less perpendicularly relative to the sagittal plane than is often observed. A cross-sectional approach enables more practical visualization of initial needle placement. A more accurate initial needle placement may minimize the number of needle passes necessary to contact the nerve roots, thereby more efficiently obtaining a successful block.