Introduction

In 1989, Chow et al. described a two-portal endoscopic carpal tunnel release (ECTR) technique.¹ This method has undergone a number of modifications since that time and is described in a separate chapter. Agee et al. published a prospective randomized clinical trial on his uniportal technique in 1992.² Other investigators have published their preferred technique of using a one- or two-portal approach. The decision as to which technique to use is largely based on personal preference and comfort with the procedure. This chapter discusses the uniportal technique using the Agee endoscope.

Indications

The indications for ECTR are the same as for an open carpal tunnel release (CTR). The patient should have the appropriate symptoms and signs of carpal tunnel syndrome and should have failed a trial of conservative treatment with splinting, NSAIDs, and activity modification. Positive electrodiagnostic studies are not a requirement but may be useful in ruling out associated neuropathies.

Contraindications

Absolute contraindications include any distortion of the carpal canal due to tumor, previous surgery, carpal fracture/dislocation, or mal-union of the radius. A loss of wrist extension of 20 to 30 degrees either from bony fusion or wrist contracture would hamper this procedure by impeding the correct placement of the endosocope. Unfamiliarity with the regional anatomy is another contraindication. Due to the high learning curve, it is recommended that anyone contemplating this procedure should take a cadaver training course prior to attempting it clinically—unless training has been provided in a teaching environment.³

Relative contraindications abound and are not universally agreed upon. Flexor tendon thickening due to synovial hyperplasia or adhesions from previous flexor tendon repairs will complicate the procedure. Recurrent or persistent carpal tunnel syndrome following a previous release often includes a component of traction neuropathy due to scar, which may thwart attempts at ECTR. However, successful cases have been reported.⁴ Any suspicion of separate entrapment of the recurrent motor branch mandates an open procedure.

Equipment

The use of the Agee endoscope (3M, Orthopedic Products, St. Paul, MN) is integral to this procedure. A 2.7-mm 30-degree angle arthroscope along with a fiberoptic light source and camera setup are interfaced with the Agee system. The instrumentation includes two dilators and a soft-tissue elevator.

Relevant Anatomy

The flexor retinaculum (FR) consists of three distinct and continuous segments that extend from the distal radius to the base of the long finger metacarpal.⁵ The proximal segment is continuous with the deep forearm fascia and is inseparable from the thickened antebrachial fascia. The transverse carpal ligament (TCL) makes up the second part of the FR. The TCL arises from the scaphoid tuberosity and trapezial beak radially, and from the pisiform and hook of the hamate ulnarly. The distal segment of the FR consists of the aponeurosis between the thenar and hypothenar muscles. The median nerve becomes superficial in the forearm approximately 5 cm proximal to the wrist. It lies between the tendons of the flexor digitorum superficialis (FDS) and the flexor carpi radialis (FCR), and is dorsal or dorsoradial to the palmaris longus (PL).

It passes under the FR in the radiopalmar portion of the carpal tunnel at a level that corresponds to the volar flexion crease of the wrist. At the distal edge of the retinaculum, the nerve normally divides into six branches: the recurrent motor branch, three proper digital nerves, and two common digital nerves. The motor branch typically passes through a separate fascial tunnel immediately before entering the thenar muscles. The palmar cutaneous branch (PCN) originates 5 cm proximal to the proximal wrist crease. It travels with the median nerve for 1 to 2 cm and then separates and enters a short tunnel medial to the FCR tendon to innervate the skin of the thenar eminence.

Anatomically, two areas of the carpal tunnel may cause median nerve compression. The first is at the proximal edge of the TCL, where compression may be produced by acute flexion of the wrist—which is the basis for a positive Phalen’s test. The second is adjacent to the hook of the hamate, which is the usual location for any hourglass deformity of the median nerve.

Surgical Considerations

The position of the ulnar nerve and artery are of particular significance for endoscopic carpal tunnel release. Most endoscopic devices are designed to divide the flexor retinaculum just to the radial aspect of the hamate hook. In an anatomical study of the boundaries of Guyon’s canal, however, Cobb et al. showed that the confines of this space do not extend from the pisiform to the hook of the hamate (as previously accepted).⁶ The fascial roof extends radial to the hook of the hamate, which allows the ulnar neurovascular bundle to course radial to the hamate hook. Utilizing cross-sectional analysis of nine cadaver specimens, they found the ulnar artery to course radial to the hamate hook in five and palmar to it in four. Therefore, the ulnar artery may be at greater risk of injury during endoscopic procedures than previously recognized.

The safest path for release of the TCL has been studied extensively. Rotman and Manske investigated the anatomical relationships of an endoscopic carpal tunnel device to surrounding soft-tissue structures along the ring finger and the long-ring interspace axis.⁷ The average distance from the center of the device to the median nerve in the carpal tunnel averaged 3.3 mm in the ring finger axis and 2.5 mm in the long-ring interspace axis. The average distance from the distal edge of the transverse carpal ligament to the superficial palmar arch was 4.8 mm in the ring finger axis and 5.5 mm in the long-ring interspace axis. These and other more subtle anatomical observations indicate the greater safety of using the ring finger axis for endoscopic carpal tunnel release.

Surgical Technique

The patient is positioned in the supine position, with the arm abducted and lying on an arm board. The procedure is performed under tourniquet control after limb exsanguination. The author’s preference is to use a general anesthetic due to the not-infrequent difficulties with instrumentation, including fogging of the camera lens. However, regional anesthesia will also suffice.

The landmarks to localize the hook of the hamate are as follows.^8,9 First, the pisiform is palpated and marked. A second mark is placed on the proximal palmar skin crease, in line with the mid portion of the index finger. These two points are then connected, forming the index-pisiform line. A second line is drawn from the midpoint of the base of the ring finger, proximally to the wrist crease at the junction of its middle and ulnar third—forming the fourth metacarpal line. The intersection of the index-pisiform line and the fourth metacarpal line directly overlies the hook of the hamate (Figure 30.1). The palmar longus tendon (if present) is traced. The dissection should stay medial to the PL, which protects the median nerve.

FIGURE 30.1 Landmarks for locating the hamate hook (*) (p = pisiform, M4 = fourth metacarpal line, I-P = index pisiform line).

A 2-cm transverse incision is placed in the proximal wrist crease between the PL and pisiform. As the surgeon’s comfort level increases, this incision can be placed in the distal wrist crease—although there is more subcutaneous fat to contend with (Figure 30.2). The plane between the subcutaneous fat and the deep forearm fascia or flexor retinaculum is identified and developed. A distally based U-shaped flap of the flexor retinaculum is elevated by making two parallel incisions approximately 1 cm apart and 1 to 2 cm long through the deep fascia between the FCU and PL/median nerve (Figure 30.3A). The incisions are then connected by a proximal transverse cut. Care is taken to avoid penetration of the underlying flexor synovium to lessen inadvertent tendon laceration. The distally based flap is elevated (Figure 30.3b) while developing the plane between the FDS tendons and the undersurface of the TCL. Establishing this plane is integral to proper scope placement.

FIGURE 30.2 Skin incision at distal wrist crease (in red).

FIGURE 30.3 (A) Outlining a distally based fascial flap. (B) Elevation of the fascial flap.

The wrist is then hyperextended over two folded towels. A small dilator is gently inserted, superficial to the plane of the flexor tendons but underneath the TCL—keeping it aligned with the ring metacarpal line and radial to the hamate hook. With proper placement, the dilator will abut against the hook of the hamate when sweeping it in an ulnar direction. If there is an obstruction to ulnar passage of the dilator, it should be removed and reinserted. Otherwise, there is a risk of grievous injury to the ulnar nerve and artery.

A larger dilator is next inserted to enlarge the gliding path for the scope. The distal tip of the dilators can be easily palpated in the midpalm by levering the tip anteriorly. Multiple passes of the dilators increases the risk of a median neuropraxia, especially in small wrists, and should be avoided. The soft-tissue elevator is then inserted and used to gently scrape any synovium from the undersurface of the TCL. While keeping the wrist hyperextended, the tip of the Agee endoscope is then gently inserted into the canal. The surface line marking the axis of the ring finger is used as a guide (Figure 30.4 and b).

FIGURE 30.4 (A) Insertion of the Agee endoscope, which is aligned with the ring finger axis. (B) End on view of proper scope placement.

At this point, the shiny transverse fibers of the undersurface of the TCL should be visualized. Often, fogging of the scope, fluid from fatty tissue, or synovial remnants will block the view. In this event, the scope should be removed and defogged and the synovial elevator used once more. Because the scope is often colder than the carpal canal, keeping the scope in warm water until just before use may minimize the fogging. The distal edge of the TCL must be completely visualized (Figure 30.5a). This can be identified by a change of the transverse fibers to an ill-defined fat pad, which contains the superficial palmar arch and the common digital branch to the third web space.

FIGURE 30.5 (A) View through the endoscope of the white undersurface of the transverse carpal ligament (TCL). The appearance of the midpalmar fat pad demarcates the distal edge of the TCL (*). (B) Partial release of the TCL results in a minor separation of the ligament edges (arrows). (C) After complete release of the TCL. Note the increased separation of the ligament edges (arrows) with exposure of the palmar brevis, which denotes a complete ligament release.

Ballottement of the fat pad can aid in the demarcation zone. The hook blade should not be engaged until the distal edge is well seen. It is prudent to start the blade 2 to 3 mm from the distal edge on the first pass (Figure 30.5b). The hook blade is then maintained upright while the scope is slowly withdrawn proximally, keeping the blade in sight at all times. The scope is then reinserted to visualize the cut. The TCL is under tension and will usually split apart, allowing identification of the muscle fibers of the overlying palmar brevis or the longitudinally oriented fibers of the palmar aponeurosis. A second pass may be made at this point to release the distal-most fibers of the TCL (Figure 30.5c). The scope is then removed.

The proximal skin flap is next retracted to allow visualization of the deep flexor retinaculum. Tenotomy scissors are used to divide the retinaculum for an additional 2 cm. The scope is then reinserted into the carpal canal, and the tourniquet is deflated. Pulsatile bleeding is suggestive of an injury to the superficial palmar arch, which is explored through an open incision. Digital nerve injury cannot be recognized by any means, but a high level of suspicion and due diligence will dictate the need for exploration.

It is possible for the mesoneural tissue surrounding the median nerve to become interposed between the scope and the undersurface of the TCL. It will not have the appearance of nerve tissue and can be drawn in by the hook blade, which may lead to a partial or complete median nerve laceration. If there is any doubt, the scope should be removed and the field cleared. If the view cannot be improved, conversion to an open procedure is necessary. The wound is infiltrated with Marcaine and closed with an absorbable subcuticular suture and Steri-Strips.

Postoperative

The patient is placed in a large bulky hand dressing with a volar wrist splint for comfort. Finger motion begins immediately, which also accomplishes some nerve gliding. The splint is removed at one week, and the patient is started on a wrist range-of-motion program. The sutures are removed at two weeks, followed by gradual strengthening.

Complications

There have been significant and devastating complications reported, which are often related to poor scope placement or inadequate visualization of the TCL. This includes partial and complete median nerve lacerations, common digital nerve lacerations, and injuries to the superficial palmar arch.¹⁰ The author has noted a neurapraxia of the third common digital nerve branch in small women patients with small carpal canals that has lasted up to six months. Flexor tendon lacerations have also been reported. If there is anything less than complete visualization, the procedure should be converted to an open CTR. Placing the scope in Guyon’s canal due to inadequate identification of the hook of the hamate has resulted in ulnar nerve and artery lacerations.¹¹ Incomplete release of the TCL is commonly reported and is a major criticism of this technique.^12,13

Results

Controversy continues to dog any method of ECTR. Advocates proclaim less postoperative pain, quicker recovery of grip strength, and a faster return to work.^14–16 Critics cite the higher occurrence of incomplete release of the transverse carpal ligament, disturbing reports of neurovascular injury, and a similar incidence of pillar pain.^10,17–22 Agee et al. carried out a 10-center randomized prospective multicenter study of endoscopic release using his technique. There were 122 patients in the study. Twenty-five had carpal tunnel surgery on both hands and 97 had surgery on one hand. Of the surgical procedures, 65 were in the control group and 82 were in the device group.

For patients in the device group with one affected hand, the median time for return to work was 21.5 days less than that for the control group. Another large multicenter prospective study of 192 cases demonstrated at least in the short term that the patients treated with the endoscopic method had significantly greater grip strength, pinch strength, and hand dexterity. The open technique resulted in greater scar tenderness during the first three months after surgery, as well as a longer time until the patients could return to work.²³

Everyone is not in agreement on this point, though. In a small study of 25 patients with bilateral carpal tunnel syndrome who underwent an endoscopic release by the Agee technique on one hand and open release on the other, the investigators found no significant advantages in terms of return of grip strength, manual dexterity, or sensation.²² Foucher et al. examined his initial experience with this technique in 95 hands at 4.5 years.²⁴ Seventy-two percent of hands were free of symptoms and 94% were described by the patients as functionally normal. Seventeen hands (out of 27) with residual or recurrent symptoms were examined. Nine hands (nine patients) were only partially improved (mean 6.7 on a 10-point scale) and in eight hands (seven patients) some symptoms had recurred after a mean delay of 3.8 years.

One utility of the endoscopic method is the ability to perform simultaneous bilateral CTR without significantly increasing patient morbidity.²⁵ This, however, has been replicated with simultaneous bilateral open release as well.^26,27