Ultrasound imaging of the thenar motor branch of the median nerve: a cadaveric study
Ultrasound imaging of the thenar motor branch of the median nerve: a cadaveric study
David Petrover 0 1 2
Jonathan Bellity 0 1 2
Marie Vigan 0 1 2
Remy Nizard 0 1 2
Antoine Hakime 0 1 2
0 Association pour la Recherche en Chirurgie de l'Epaule et du Coude (ARCEC) , 20 rue Laffitte, 75009 Paris , France
1 Service de chirurgie orthopédique, Hôpital Lariboisière, AP-HP , 2 rue Ambroise Paré, 75475 Paris Cedex 10 , France
2 Centre Imagerie Medicale Bachaumont Paris Centre (IMPC Bachaumont-Blomet Ramsay GDS) , 6 rue Bachaumont, 75002 Paris , France
Introduction Anatomic variations of the median nerve (MN) increase the risk of iatrogenic injury during carpal tunnel release surgery. We investigated whether high-frequency ultrasonography could identify anatomic variations of the MN and its thenar motor branch (MBMN) in the carpal tunnel. Methods For each volar wrist of healthy non-embalmed cadavers, the type of MN variant (Lanz classification), course and orientation of the MBMN, and presence of hypertrophic muscles were scored by 18-MHz ultrasound and then by dissection. Result MBMN was identified by ultrasound in all 30 wrists (15 subjects). By dissection, type 1, 2 and 3 variants were found in 84%, 3%, and 13% of wrists, respectively. Ultrasound had good agreement with dissection in identifying the variant type (kappa =0.9). With both techniques, extra-, Jonathan Bellity sub-, and transligamentous courses were recorded in 65%, 31%, and 4% of cases, respectively. With both techniques, the bifid nerve, hypertrophic muscles, and bilateral symmetry for variant type were identified in 13.3%, 13.3%, and 86.7% of wrists, respectively. Agreement between ultrasound and dissection was excellent for the MBMN course and orientation (kappa =1). Conclusion Ultrasound can be used reliably to identify anatomic variations of the MN and MBMN. It could be a useful tool before carpal tunnel release surgery. Key Points Ultrasound can identify variations of the motor branch of the median nerve. Ultrasound mapping should be used prior to carpal tunnel release surgery. All sub-, extra-, and transligamentous courses were accurately identified. Type 3 variants (bifid nerve), hypertrophic muscles, and bilateral symmetry were accurately identified.
Carpal tunnel release surgery; Anatomy; Ultrasound; Median nerve; Dissection
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Surgery for carpal tunnel syndrome involves sectioning the
transverse carpal ligament to release the compressed median
nerve. One of the main challenges of this surgery is avoiding
damaging small structures, such as the thenar motor branch of
the median nerve (Fig. 1), which exists in a number of
anatomical variations [1–3]. Some authors report that over 50%
of hands exhibit deviations from the standard anatomy [2].
The thenar motor branch supplies the thenar muscles. It
provides motor innervation to the abductor pollicis brevis,
Fig. 1 Schematic of the anatomy of the wrist. Standard anatomy. Black
arrow: classic variation [median nerve type 1 with an extraligamentous
radial course of the motor branch of the median nerve (MBMN)].
Figure provided courtesy of Virginie Denis
opponens pollicis, and superficial part of the flexor pollicis
brevis. This recurrent branch is called the Bmillion dollar
nerve^ because of the litigation-related compensation that
may occur if it is transected resulting in loss of function of
the hand muscles.
As we know of few clinical or electrophysiological signs
that denote anatomical variations of the median nerve,
surgeons have relied on visualisation during surgery to avoid
iatrogenic injury. Open carpal tunnel release surgery lends
itself well to the visualisation of the anatomical structures,
but it is associated with sizeable scars and lengthy recovery
periods [4]. Endoscopic techniques are less invasive and are
associated with faster and easier recoveries than open wrist
surgery. They, however, are associated with a higher risk of
nerve injury due to the restricted field of vision of the
endoscope and the inability to anticipate anatomic variations [5, 6].
Continuous sonography combined with minimally invasive
percutaneous carpal tunnel release [7–9] enables physicians
to demarcate superficial soft tissues and to identify very small
anatomic and pathologic details before and during surgery.
This relatively new technique relies on the idea that ultrasound
can accurately map small anatomical variations.
Few studies have investigated the reliability of ultrasound
in the identification and characterisation of the median nerve
and its motor branch. In one recently published study,
ultrasound was used successfully to identify the thenar motor
branch in a cohort of ten cadaveric limbs. Anatomic
variations, however, were not formally evaluated [10]. In this
article, we present the results of a cadaveric study in which we
specifically evaluated the concordance between
ultrasonography and dissection in the identification of anatomical
variations of the median nerve and its thenar mot (...truncated)