Anatomic Landmarks to Locate the Median Nerve for Safe Wrist Block or Carpal Tunnel Steroid Injection

Ron Brooks, MD; Amanda Kistler, MD; Saeed Chowdhry, MD; Andrew Swiergosz, BS; Katharina Perlin, BS; Morton L. Kasdan, MD; Bradon J. Wilhelmi, MD

Disclosures

ePlasty. 2019;19(e19) 

In This Article

Abstract and Introduction

Abstract

Introduction: Carpal tunnel syndrome is the most common entrapment neuropathy involving the upper extremity. As such, various nonoperative techniques have been developed to aid in management of mild to moderate disease, including local steroid injection. However, definitive guidelines for needle/injection location have not been defined, especially in relation to diminishment of iatrogenic injury to the median nerve.

Methods: A cadaveric study was designed to determine the average width of the median nerve, as well as its location specifically in relation to the palmaris longus (if present), the flexor carpi radialis, and the midpoint of the wrist. All measurements were obtained at the radial tip of the interstyloid line.

Results: Data demonstrated that the average width of the median nerve was 7.85 mm and that it lies ulnar in location to the palmaris longus (3 mm), as well as the midpoint of the wrist at the radial tip of the interstyloid line (0.43 mm). Furthermore, the distance between the median nerve and the flexor carpi radialis was measured to be 9.57 mm.

Outcomes: Therefore, injection location should be radial with respect to the palmaris longus and the midline of the wrist. It can be just ulnar to the flexor carpi radialis tendon or between 8 and 10 mm radial to the midpoint of the wrist in order to prevent median nerve injection and direct trauma to the nerve.

Introduction

Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy involving the upper extremity. While estimates of prevalence and incidence vary widely in literature, data from pooled analyses suggest that the prevalence in the general population is about 1% to 5%, where an increase was demonstrated in populations whose occupations demanded repetitive hand and wrist movements. Greater than 450,000 carpal tunnel release procedures are performed annually in the United States, totaling a cost of $2 billion. Furthermore, CTS was responsible for 1.6% of injuries that required time off work and demonstrated the longest time away from work, only surpassed by automobile accidents, fracture/crush/dislocation, and other traumas.[1,2] As the commonality and economic impact of CTS is immense, a multitude of preventive and conservative management techniques have emerged.

The carpal tunnel is a fibro-osseous channel on the anterior portion of the wrist that is located within the concave arch of the carpus and enclosed by the transverse carpal ligament (TCL).[3] Ten structures traverse this channel including the 4 flexor digitorum superficialis tendons, 4 flexor digitorum profundus tendons, the flexor pollicis longus tendon, and the median nerve (MN). Under typical situations, these structures are able to slide across one another as well as bony protuberances with little friction relating to their synovial sheaths. However, inflammation and edema within the channel can result in compression of tunnel structures, namely, the MN. Specifically, the narrowest part of the tunnel is located at the level of the hook of the hamate with the overlying TCL and is located about 2 to 2.5 cm distal to the origin of the canal. This is the typical location of compression of the MN seen in patients with CTS.[3]

As the MN enters the wrist through the carpal tunnel, it bifurcates into the digital cutaneous branches and the muscular branch. The digital cutaneous branches terminate as 2 common plantar digital nerves that are responsible for motor innervation to the second lumbrical and sensory innervation to the palm and fingers where the thenar motor branch, known as the "million dollar nerve," innervates the opponens pollicis, the abductor pollicis brevis, and the superficial part of the flexor pollicis brevis before giving rise to the proper palmar digital branch nerves that provide motor innervations to the first lumbrical and sensory innervations to the ulnar side of the hand.[4] In relation to CTS, both motor and sensory deficits become apparent including myasthenia most noticeably in abduction and opposition of the thumb, which can lead to atrophy of the thenar eminence in advanced stages as well as varying degrees of paresthesia, hypoesthesia, or anesthesia of the volar side of the radial 3½ digits with retention of sensation to the central palm including the thenar eminence.[5]

Current treatment modalities include physical therapy, anti-inflammatory medications, lifestyle modification, splinting, steroids (oral or local), and surgery. Specifically, local injection of steroids not only has been used to treat mild to moderate CTS, with as many as 50% having a good long-term effect for more than 15 months, but also serves as a diagnostic and prognostic tool in the evaluation of potential efficacy of future surgery.[6,7] In fact, Green[6] in "Diagnostic and Therapeutic Value of the Carpal Tunnel Injection" demonstrates a correlation between results of steroid injections with respect to symptom alleviation or resolution and efficacy of future surgery. However, inappropriate needle placement can lead to injury to the medial nerve, the ulnar neurovascular bundle, or the radial artery. In efforts to prevent associated injuries, various techniques have been suggested including injection just radial to the palmaris longus (PL) tendon, in line with the fourth digit, between the PL and flexor carpi radialis (FCR) tendon, between the PL and flexor carpi ulnaris (FCU) tendon, and through the FCR tendon; however, injury prevention has been inconsistent.[8] One consideration is anatomic variability in the course and branches of the MN. Henry et al in "The Prevalence of Anatomical Variations of the Median Nerve in the Carpal Tunnel: A Systematic Review and Meta-Analysis" classified variations in MN in the hand, including variations in the course of the thenar motor branch, accessory branches of the MN at the distal carpal tunnel, high bifurcation of the MN, and branching of the MN proximal to the carpal tunnel, finding prevalence of 11% to 75%, 4.6%, 2.6%, and 2.3%, respectively.[4] As such various methods have been established to attempt to standardize injection location. Our aim was to identify a method of local steroid injection that would diminish if not eliminate iatrogenic injury to the MN, especially in the setting of the absence of relevant superficial landmarks such as the PL tendon.

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