Pertinent Anatomy

The carpal tunnel (CT) is a fibro-osseous canal on the volar side of the wrist, formed by the carpal bones (floor) and transverse carpal ligament (TCL) (roof) ( Figs. 32.1 and 32.2B and C ). The TCL is approximately 1.5 to 4 mm thick on average. The proximal TCL inserts on the scaphoid bone and traverses medially to the pisiform, while the distal TCL inserts on the trapezium and traverses medial to the hook of the hamate (see Fig. 32.2B and C ). The proximal portion of the TCL is the distal continuation of the antebrachial fascia, with transverse instead of longitudinally oriented fibers. ^{, , ,} The radial side of the TCL divides into two layers, a superficial layer and a deep layer, to accommodate the tendon of the flexor carpi radialis (FCR). ^{, ,}

Anatomy.

Anatomic view of the carpal tunnel.

Ultrasound of the Carpal Tunnel With Anatomic Correlates.

(A) Transducer positioning for sonographic short-axis (SAX) view of the proximal (solid rectangle) and distal (dotted rectangle) carpal tunnel (CT). (B) US SAX correlate, with transducer positioned as shown in Fig. 32.2A, of the proximal CT with ulnar (ULN) to the right. Note the bony landmarks of the scaphoid (S) radially and the pisiform (P) ulnarly. The median nerve (dotted circle) is situated superficial within the CT. (C) US SAX correlate, with transducer positioned as shown in Fig. 32.2A, of the distal CT with ULN to the right. Note the transverse safe zone (TSZ) (double arrow) , in this patient bordered on the ULN side by the hook of the hamate (H). a , Ulnar artery; APB , abductor pollicis brevis; C , capitate; FCR , flexor carpi ulnaris; FP , flexor digitorum profundus; FPB , flexor pollicis brevis; FPL , flexor pollicis longus; FS , flexor digitorum superficialis; L , lunate; solid circles , ulnar nerve; t , trapezoid; T , triquetrum.

The carpal canal contains the median nerve and nine tendons: the flexor pollicis longus (FPL), the four flexor digitorum superficialis (FDS), and the four flexor digitorum profundus (FDP) (see Fig. 32.1 ). The FPL has its own synovial sheath, while the FDS and FDP have a common synovial sheath. The FCR, flexor carpi ulnaris, ulnar nerve, and palmaris tendons are external to the CT. The abductor pollicis brevis (APB) and flexor pollicis brevis (FPB) have anatomic connections on the radial side of the TCL, while ulnarly, the flexor digiti minimi brevis (FDMb) originates from the TCL.

The median nerve is anatomically located in the forearm between the FDS and FDP muscles. As the nerve travels distally, it becomes more superficial just proximal to the TCL by looping radially around the FDS muscles and tendons in the distal forearm. ^{, , ,} The palmar cutaneous branch arises off the radial side of the median nerve, most commonly between the FCR and palmaris longus (if present), providing sensation to the palmar surface of the thumb (see Fig. 32.1 ). From here, the median nerve travels superficially between the tendons of the FPL and FDS to the index and middle fingers. The median nerve normally travels in the CT as a single nerve, prior to dividing into radial and ulnar branches at the distal border of the TCL.

The thenar motor branch (TMB) (also referred to as the recurrent motor branch of the median nerve) typically originates from the antero-ulnar aspect of the radial division at the distal TCL and travels vertically in a palmar direction around the distal TCL to innervate the thenar musculature (see Fig. 32.1 ). The TMB is a critically important structure within the CT region because it is the primary motor innervation to the thenar muscles in most people. A detailed knowledge of anatomic variations of the TMB (mentioned later in this chapter) is recommended prior to performing procedures on or about the CT.

In the palm, the median nerve supplies sensation to the palmar surface of the thumb, index and middle finger, and radial palmar surface of the ring finger, while also providing motor innervation of the thenar eminence as well as the first and second lumbricals via the TMB. ^{, ,}

Anatomic Variants

There are many reported anatomic variants of the wrist, involving vessels, nerves, tendons, and muscle; some of these have significant clinical relevance when it comes to USCTR. ^{, , ,} The operator should be aware that the presence of one anatomic variant should alert to the presence of potential additional anatomic variants.

The most common variant of the median nerve itself is a bifid median nerve with or without a persistent median artery (PMA). ^, A PMA, which is present in 12% to 23% of the population, arises from the ulnar artery and travels distally along the ulnar side of the median nerve and may contribute to symptoms of carpal tunnel syndrome (CTS). ^{, , ,} The superficial location of the PMA in the CT puts the artery at risk for injury during USCTR, and its presence should be noted prior to attempting release. Furthermore, a PMA is often associated with a bifid or trifid median nerve. The use of color Doppler can help to identify this variant. ^,

PMA with bifid median nerve can also often be associated with an anomalous FDS muscle belly that can be seen dynamically with flexion and extension of the fingers. Another variant is proximal bifurcation of the median nerve, which can be seen in 1% to 3% of the population. Early division of the terminal branches is even less common.

Anatomic variants of the TMB are common and present in 46% to 90% of the population. There are four main anatomic variants of the TMB: the distal ligamentous (most common); the subligamentous branch (33%); the transligamentous branch (8% to 12%); and the ulnar division take off (rare). ^{, , , ,} Hypertrophy of the thenar muscles has been associated with transligamentous TMB. A firm understanding of normal and anatomic variants of the TMB and identification of the TMB and possible variants with ultrasound (US) prior to any USG procedures about the TCL is highly recommended.

There are two main variants of the palmar cutaneous nerve described in the literature. The first variant arises on the radial side of the median nerve, and the second, on the ulnar side of the median nerve. The latter crosses the nerve prior to going through the fibro-osseous tunnel before becoming subcutaneous. ^, Damage to this nerve during surgery has been shown to cause the development of a neuroma, and therefore, demands consideration when performing USCTR.

There is one anatomic variation of the ulnar nerve that warrants mention. In 1% to 3% of the population, the nerve travels in an osseo-fibrous canal (different from Guyon’s canal) situated on the anteromedial portion of the TCL, extending approximately 4 cm from the pisiform to the hook of the hamate bifurcation. ^,

The palmaris longus (PL) muscle has the greatest number of possible variants in the human body. ^, Two variants are particularly important when considering USCTR and must be identified before surgery. The first variant involves the PL traversing the CT, possibly contributing to symptomatology. The second, referred to as a reverse PL, involves the muscle belly in a distal rather than proximal position. In the case of a reverse PL, the muscle belly may extend into the CT, possibly increasing or contributing to compression. ^, The low-lying FDS muscle bellies, as well as more proximally located lumbrical muscle bellies, may also dynamically impinge on the median nerve with dynamic testing. There are also reports at the wrist of other anatomic variants to the tendons, including accessory FDS to the index finger and the palmar surface. ^{, , ,}

The CT can also be compromised by any number of cystic or mass-like structures. If discovered, care should be taken to note these structures and make appropriate referrals for either diagnosis or treatment.

The sonographer should always report the presence of anatomic variants of the nerves, ligaments, and tendons about the TCL, which might otherwise be damaged or require additional treatment during any surgical release of the TCL.

Pathophysiology/Diagnosis

Median nerve entrapment at the TCL, leading to CTS, is the most common peripheral entrapment neuropathy, with an incidence of 3.5% to 6.2%. Approximately 450,000 surgical releases are performed annually in the United States, with a total cost of over 2 billion dollars. More than 90% of patients report clinical improvement following release.

The diagnosis of CTS is primarily clinical, with most patients complaining of numbness and tingling in the lateral three digits of the hand with or without associated weakness. Patients may have a positive Tinel’s sign over the median nerve at the TCL or a positive Phalen’s test. Electrodiagnostic (EDX) testing may be obtained to assess the severity of injury to the median nerve and differentiate CTS from competing diagnoses, such as peripheral neuropathy or cervical radiculopathy. ^, Isolated or predominant TMB involvement has been implicated in some patients with CTS who present with disproportionate thenar atrophy or weakness, particularly in the setting of a relatively normal-appearing median nerve.

Sonographic Evaluation/Findings

Recently, the use of musculoskeletal US has increasingly been used to evaluate and manage patients with CTS. The most commonly reported findings on US in the setting of CTS are that of median nerve enlargement, focal median nerve constriction, reduced median nerve gliding (dynamically), TCL bowing, flexor tenosynovitis, or space-occupying lesions (e.g., ganglia, tumors, thrombosed or anomalous arteries, abnormal muscle slips, or supernumerary muscles or tendons). Other studies have indicated cross-sectional area (CSA) at the proximal TCL may provide more conventional diagnostic parameters. ^, The normal CSA can vary from 6.5 to 12 mm ² , depending on the study. ^, Enlargement of the median nerve CSA (with contralateral comparison) in conjunction with EDX testing and clinical examination should lead to an accurate diagnosis. In postoperative evaluation, US may identify incomplete TCL transection, TCL regrowth, scarring and adhesions, improvement of CSA, and injury to the median nerve or its palmar cutaneous or TMB in patients with continued symptoms. ^{, ,}

Sonographic evaluation of the CT typically begins in the anatomic axial plane and eventually in the anatomic sagittal plane. In the axial view, the nerve has a “honeycomb” appearance, with individual hypoechoic fascicles separated by echogenic septa representing the interfascicular perineurium (see Fig. 32.2B and C). The nerve bundles are less susceptible to anisotropy, unlike other structures, such as muscles and ligaments. Differential anisotropy with toggling of the transducer can be used to make the nerve more conspicuous for identification. The nerve is followed from the pronator quadratus in the forearm through the proximal and eventually distal CT. Dynamic maneuvers, with flexion and extension of the fingers, is recommended to identify possible muscle variants. Color Doppler should be used to identify vascular anatomy. Sagittal plane (longitudinal images) is useful in cases of focal compression, showing the compressed portion of the median nerve as well as the proximal and/or distal swollen portion of the nerve (“notch sign”). ^{, ,} Special care is taken to note the aforementioned anatomic variants as they pertain to the TCL.

It is suggested that the following structures are viewed as part of a pre-screening protocol: ^,

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  The median nerve:

  
  
  
  
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    With CSA measurements at the distal border pronator quadratus, the proximal TCL, and the distal TCL

  
  
  
  
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  The palmar cutaneous branch of the median nerve

  
  
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  The TMB of the median nerve

  
  
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  The ulnar nerve and artery:

  
  
  
  
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    Location of the superficial palmar arch to the hook of the hamate

    
    
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    Location of the ulnar nerve to the hook of the hamate

  
  
  
  
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  The vasculature and presence of anatomic variants, including persistent median artery (if present)

  
  
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  Measurement of length of the TCL in sagittal plane

  
  
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  The native transverse safe zone

  
  
  
  
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    Distance between the median nerve and ulnar artery or hamate

  
  
  
  
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  The FCR, FDS, FDP, flexor carpi ulnaris (FCU), and FPL tendons

  
  
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  Superficial palmar artery and palmar fat pad

  
  
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  The bony acoustic landmarks of scaphoid, pisiform, trapezium, and hook of hamate

  
  
  
  
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    Contraindications to USCTRS

  
  
  
  
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  Inability to adequately visualize at-risk structures:

  
  
  
  
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    TMB/recurrent motor branch of the median nerve

    
    
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    Palmar cutaneous branch of the median nerve

    
    
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    Ulnar vessels

    
    
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    Superficial palmar arterial arch

    
    
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    Median and ulnar palmar digital nerves

  
  
  
  
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  Variant anatomy that would preclude establishment of a safe transverse safe zone

  
  
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  Presence of mass lesion or other process that requires treatment beyond CTR

Treatment Options

Conservative treatment options for CTS typically include activity modification, occupational therapy, bracing, steroid injections, and alternative medicine (i.e., acupuncture, low-level laser, yoga, and static magnetic field therapy). For patients who fail conservative treatment, surgical release is considered the definitive treatment. Although historically performed via a large (3 to 5 cm) palmar incision, CTR techniques have evolved to reduce trauma with less-invasive treatments. ^, The goal of less-invasive treatments is improving cosmesis, reducing postoperative pain, and promoting faster recovery. ^{, , , ,} The currently available CTR techniques include mini-open CTR (mOCTR) via a single 1 to 3 cm palmar incision, endoscopic CTR (ECTR) via one (wrist) or two (wrist and palm) 1 to 2 cm incisions, and more recently, ultrasound-guided CTR (USCTR) via a single less than 1 cm wrist or palmar incision. ^{, , ,}

Ultrasound-Guided Release

The primary goal of CTR is to transect the TCL while avoiding injury to the neurovascular structures. ^{, ,} Although ECTR may promote a faster recovery than mOCTR, there is a concern that limited visualization of the surrounding structures during ECTR may increase complications. ^{, , ,} USCTR combines a single or dual incision (Manos procedure) with direct US visualization of at-risk structures. Despite many different devices being used to perform USCTR, review of the current publications on USCTR report a greater than 98% clinical success rate without any documented neurovascular injuries. Rojo-Maudate et al published their randomized clinical trial comparing mOCTR versus USCTR, demonstrating a five times faster functional recovery, pain reduction, and pain medication discontinuation with USCTR.

ECTR techniques and most USCTR techniques place the surgical device within the TSZ. The TSZ is defined as a region bordered radially by the median nerve and ulnarly by the hook of the hamate or ulnar vessels, whichever lies more radial (see Fig. 32.2C ). ^, The TSZ has been shown to vary from approximately 4.4 to 12.4 mm, and was statistically larger in men when compared to women, but exhibits significant inter-individual variability. In some cases, a 0 mm TSZ has been documented. It can be therefore technically challenging to establish a consistent TSZ while performing CTR. During ECTR, the size of the TSZ is determined by the size of the introducer and endoscopic device. ^{, , , ,} This is contrasted to USCTR, in which the cutting instrument is smaller, requiring a small incision, but also presenting the same challenge of establishing a TSZ. ^{, , ,}

More recently, the SX-One MicroKnife (Sonex Health LLC, Rochester, MN), a disposable device with ability to protect the TSZ, has become available. A cadaveric investigation using the SX-One device to perform USCTR on cadaveric specimens documented a 100% rate of TCL release and no neurovascular injury. Initial studies on this device have shown promising results, with faster recovery and return to normal function with no reported neurovascular injuries to date. ^{, , , ,}

Equipment

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  High-frequency linear transducer (e.g., 12 to 5 MHz)

  
  
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  Arm board

  
  
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  Surgical drapes

  
  
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  Lidocaine ∼8 to 10 mL

  
  
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  Dilator

  
  
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  21-gauge, 2-inch needle

  
  
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  A #15 blade

  
  
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  Skin glue vs 4-0 sutures for closure

  
  
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  Steri-strips

  
  
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  Sterile towels and gauze

  
  
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  Sterile saline (10 mL)

  
  
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  Cutting device (SX-One MicroKnife by Sonex, hook knife, Gou wire)

Authors’ Preferred Technique