WHAT CAN ULTRASOUND SEE IN THE CARPAL TUNNEL REGION? Jay Smith, M.D. CMO, Sonex Health LLC June 2017 Modern day ultrasound (US) machines provide a powerful combination of submillimeter resolution and dynamic imaging capabilities. Due to continued advances in US technology, we can now detect and evaluate all major anatomical structures in the carpal tunnel region. Consequently, the role of diagnostic US in carpal tunnel syndrome is well-established, and advanced procedures such as precisely guided injections and ultrasound guided carpal tunnel release (USCTR) are being performed to enhance patient care. The following is a demonstration of key anatomical structures in the carpal tunnel region as they appear on currently available US machines: Slides Carpal Tunnel Anatomy Median Nerve: The median nerve is the primary neurological structure in the carpal tunnel region. It courses superficially within the carpal tunnel, just deep to the transverse carpal ligament (TCL). Modern day ultrasound machines can easily detect the normal fascicular architecture of the median nerve and examine the nerve from the elbow to its terminal branching in the distal carpal tunnel region. The ability of ultrasound to detect median nerve abnormalities and diagnose carpal tunnel syndrome is well-established in the literature. Although typically appearing as an ovoid, mixed echogenicity structure, the median nerve may be bifid or trifid, and may be associated with a persistent median artery of variable size. The relationship between these variations and carpal tunnel syndrome or surgical risk during carpal tunnel release remains uncertain. Reassuringly, ultrasound can detect bifid and trifid median nerves, as well as persistent median arteries. Slides Median Nerve Palmar Cutaneous Branch of the Median Nerve: The palmar cutaneous branch of the median nerve (PCB) arises from the radial aspect of the median nerve in the distal forearm approximately 5 cm proximal to the wrist. It typically separates from the median nerve at the point where the median nerve moves from its deeper location between the superficial and deep forearm flexors to a more superficial position deep to the transverse carpal ligament (TCL). It then penetrates the distal antebrachial fascia or the proximal osseous TCL (see Transverse Carpal Ligament for discussion regarding TCL anatomy and terminology) and usually courses between the palmaris longus and flexor carpi radialis tendons, terminating within the TCL, palmar fascia, and/or subcutaneous tissue. The PCB can be entrapped at its site of fascial penetration, or injured iatrogenically during surgical procedures in the radial wrist-palm region, including open carpal tunnel release. Under normal circumstances, injury to the PCB would not be expected during endoscopic carpal
tunnel release (ECTR) procedures since ECTR is typically performed on the ulnar side of the palmaris longus and TCL transection occurs from inside-out versus outside-in. Variations in the course of the PCB have been reported, including an ulnar origin from the median nerve following by a horizontal/transverse passage in a radial direction superficial to the median nerve (and just deep to the TCL), travel within the flexor carpi radialis tendon sheath, or fascial/ligamentous penetration into the subcutaneous tissues more proximal or distal than usual. Ultrasound can identify the PCB as well as detect abnormalities such as neuroma. Slides Palmar cutaneous branch of the median nerve Thenar Motor Branch/Recurrent Motor Branch of the Median Nerve: The thenar motor/recurrent motor branch (TMB/RMB) of the median nerve is perhaps the most important distal branch of the median nerve since it provides the primary motor innervation to the thenar muscles. It typically arises from the radial aspect of the median nerve at the distal carpal tunnel and passes superficially into the thenar muscles. TMB/RMB neuropathy can occur in isolation or in the setting of carpal tunnel syndrome. Iatrogenic injury has also been reported in the setting of open and endoscopic carpal tunnel release. Anatomic variations such as transligamentous branches and ulnar origins have been reported and theoretically increase the risk of injury during carpal tunnel surgery, regardless of technique. Reassuringly, recent publications have documented the ability to sonographically identify the TMB/RMB, as well as detect ulnar origin variants. Transligamentous branches may also be detected on ultrasound (see Transverse Carpal Ligament, TCL). Slides Thenar Motor branch of the median nerve Third Common Palmar Digital Nerve: The third common palmar digital nerve (TCPDN) arises from the median nerve in the distal carpal tunnel region and courses ulnarly to enter the 3 rd intermetacarpal space (between the long and ring fingers). Presumably due to its ulnar-ward course into the 3 rd intermetacarpal space, injury to the TCPDN has been reported during endoscopic carpal tunnel release. The TCPDN may also receive palmar communicating branches from the ulnar nerve, known as Beretinni branches. The TCPDN can be easily identified on ultrasound from its median nerve origin to its division into proper palmar digital nerves. Fourth Common Palmar Digital Nerve: The fourth common palmar digital nerve (FCPDN) arises from the ulnar nerve near the hook of the hamate and courses radially to enter the 4 th intermetacarpal space (between the ring and pinky fingers). Although its radial-ward course into the 4 th intermetacarpal space may place the FCPDN at risk during endoscopic carpal tunnel release procedures, injury to this nerve is apparently rare (compared to the third common palmar digital nerve, TCPDN). Similar to the TCPDN, the FCPDN can be easily identified on ultrasound and can be traced distally to the 4 th
interspace, including examination for major communicating branches between the FCPDN and TCPDN. Slides Common Palmar, Third, and Fourth Palmar Digital Nerves Ulnar Artery: The ulnar artery (UA) and accompanying ulnar veins enter Guyon s canal in the proximal wrist region, where they are bordered ulnarly by the ulnar nerve and pisiform. More distally, the ulnar neurovascular bundle lies palmar to the hamate before dividing into its superficial and deep branches. In this region the ulnar vessels, and more importantly the UA, may lie superficial to the transverse carpal ligament (TCL) at the ulnar limit of the carpal tunnel. Studies have demonstrated that depending on wrist position, the UA may encroach up to 4 mm into the carpal tunnel, placing it at risk during endoscopic carpal tunnel release procedures since the position of the UA superficial to the cutting path cannot be seen until the TCL is transected. Ultrasound can easily identify the ulnar vessels throughout the carpal tunnel region and more importantly determine their position relative to the ulnar limit of the carpal tunnel during carpal tunnel procedures. Slide Ulnar Artery Superficial Palmar Arterial Arch: The superficial palmar arterial arch (SPA) is the distal continuation of the superficial branch of the ulnar artery distal to the osseous carpal tunnel. The anatomy of the SPA and its position relative to the distal transverse carpal ligament are variable, and therefore the SPA may be at risk during carpal tunnel procedures relying only on anatomical landmarks. Similar to the case of the ulnar artery, ultrasound can reliably identify the SPA and characterize its relationship to the distal TCL. Slides superficial palmar arterial arch Transverse Carpal Ligament (TCL): Transection of the TCL is the primary goal during carpal tunnel release (CTR), regardless of technique. The TCL forms the roof of the osseous carpal tunnel, spanning between the scaphoid and pisiform proximally and trapezium and hamate distally. However, anatomical studies have demonstrated that TCL transection as performed during CTR not only divides the osseous TCL but also the more distally located aponeurotic fibers connecting the thenar and hypothenar muscles, with which the osseous TCL is continuous. This combination of the osseous TCL and distal aponeurotic fibers has been called the classic TCL by Cobb and colleagues since this is the target for CTR. These distal aponeurotic fibers extend an average of 11-12 mm distal to the hook of the hamate and incomplete release of the distal TCL is a well-documented cause of failed CTR. Ultrasound clearly demonstrates the osseous TCL using transverse views of the carpal tunnel, and the distal TCL (i.e. the distal aponeurotic fibers) using longitudinal views of the carpal tunnel. In the latter case, the distal TCL is identified by a tapering of the TCL, referred to as the duck s beak by Rojo-Manaute and colleagues. In the longitudinal view of the distal carpal tunnel, the duck s beak of the distal TCL and its relationship to the superficial palmar arterial arch can be clearly defined, providing precision and safety
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