An in vivo investigation of ulnar nerve sliding during upper limb movements

Abstract

Background

Peripheral nerves straighten and stretch in order to accommodate increases in bed length during joint movements. The ulnar nerve is predicted to show large bed length changes, particularly on elbow flexion. The present study examines sliding of the ulnar nerve during limb movements, to determine how far these changes are accommodated by straightening and stretch.

Methods

Ultrasound imaging was used to measure longitudinal nerve sliding in the forearm and upper arm during 40° wrist extension, 90° elbow flexion and 50° shoulder abduction. Nerve trunk folding in the upper arm was measured from still ultrasound images taken in a series of limb positions from 40° shoulder abduction, elbow extended and wrist neutral to full elbow flexion, 90° shoulder abduction and wrist extension, a position designed to stretch the ulnar nerve.

Findings

Wrist extension led to clear nerve sliding in the forearm with movements of up to 4 mm. However, shoulder abduction and elbow flexion caused remarkably little nerve movement. Images of the ulnar nerve showed considerable curvature with 40° shoulder abduction and elbow extension but a much straighter path with the elbow flexed.

Interpretation

The ulnar nerve appears unloaded and follows a wavy path in most functional upper limb positions. During elbow and shoulder movements, changes in bed length appear to be accommodated largely by straightening of the nerve path, with only modest stretch of the nerve itself when the elbow flexes. The ulnar nerve is thus well adapted for the large changes in bed length that occur during limb movements.

Introduction

Peripheral nerves must slide and stretch in order to accommodate changes in nerve bed length during joint movements. For example the median nerve is stretched when the shoulder is abducted and the elbow and wrist are extended (Wright et al., 1996, Dilley et al., 2003). However, when the upper limb is in a more flexed posture, the median nerve behaves as though unloaded and adopts a curved appearance (Dilley et al., 2003). In fact, it has been proposed that when the nerve bed length is reduced, peripheral nerves will become slack (Sunderland, 1978). Therefore, straightening of the nerve during the initial increases in bed length accounts for much of the way the median nerve adjusts to elbow and wrist extension, and has the important consequence of limiting the stretch of the nerve itself.

Ulnar nerve sliding has been examined in cadavers (Apfelberg and Larson, 1973, Grewal et al., 2000, Wright et al., 2001, Byl et al., 2002), but not in vivo. Flexion of the elbow joint causes large increases in bed length of the ulnar nerve. Interestingly, this nerve follows a curved, wavy, path at the elbow (Patel et al., 1998). It is important to establish the normal pattern of ulnar nerve movement in vivo so that it can be compared to movements in painful conditions that affect the ulnar nerve, for example, cubital tunnel syndrome. In patients with cubital tunnel syndrome, flexion of the elbow joint frequently causes exacerbation of painful symptoms (Rich and McKay, 2002), which is considered to be due to excessive stretch of the ulnar nerve across the elbow (Toby and Hanesworth, 1998, Byl et al., 2002, Aoki et al., 2005).

The present study uses ultrasound imaging to determine (a) the extent of ulnar nerve longitudinal sliding during upper limb movements in healthy subjects and (b) whether the ulnar nerve is unloaded in limb positions where the nerve bed length is relatively short.