Indications
• Median nerve compression neuropathy at the wrist
• As part of a fasciotomy for compartment syndrome/decompression after distal radial fracture
• Drainage of sepsis.
Contraindications
• Active overlying skin infection
• Uncertainty over diagnosis – may warrant further investigation before proceeding.
Consent and risks
• Nerve injury: median nerve injury <1 per cent; palmar cutaneous nerve injury <1 per cent
• Radial artery injury: <1 per cent
• Failure to relieve symptoms: 1–10 per cent; the incidence is highest in heavy/repetitive manual workers
• Pillar pain: quoted at up to 10 per cent, this is tenderness around the site of ligament release
• Scar tenderness: the incidence is reduced by massage in the postoperative period
• Complex regional pain syndrome (rare)
• Infection
Operative planning
History and clinical examination remain the mainstay of diagnosis. It is essential to examine the entire limb as well as the cervical spine to exclude a ‘double-crush’ lesion. Nerve conduction studies are useful and should be available on the day of surgery. They are considered essential in cases of recurrent carpal tunnel syndrome and complex upper limb lesions. Prolonged sensory latency is the earliest and most reliable nerve conduction abnormality. Magnetic resonance imaging (MRI) is rarely indicated, unless there is clinical evidence of a space-occupying lesion causing the symptoms. Conventional radiography is not generally indicated. Consideration should be given to extraneous causes such as diabetes mellitus, rheumatoid and other arthritides, amyloidosis and thyroid dysfunction; where appropriate these may also require investigation prior to operation.
Anaesthesia and positioning
The procedure may be carried out under local, regional or general anaesthesia. Most primary decompressions are performed under local anaesthesia. A local anaesthetic consisting of 1 per cent lidocaine and 0.5 per cent bupivacaine in a 1:1 mixture is infiltrated into the wound prior to surgical draping. General anaesthesia is usually reserved for revision procedures.
The patient is positioned supine on an operating table and the arm is positioned on an arm table in supination, with a padded lead hand
used to maintain finger extension. A tourniquet is inflated to 250 mmHg. In obese patients, a forearm tourniquet is recommended.
SURGICAL TECHNIQUE
Landmarks
The tendon of palmaris longus (absent in about 10 per cent) is easily seen and palpated by opposing the thumb and little finger and then flexing the wrist to around 30°. The distal end of the tendon bisects the anterior surface of the carpal tunnel. Other useful landmarks include the thenar skin crease (running at the base of the thenar eminence) and the transverse skin crease of the wrist joint (running parallel to the joint line). The transverse wrist crease marks the proximal border of the flexor retinaculum. If the thumb is outstretched to 90° a parallel line drawn across the palm in line with its distal border represents the surface marking of the superficial palmar arch: this is known as Kaplan’s cardinal line (Fig. 5.1).
Incision
The incision runs a few millimetres to the ulnar side of the thenar skin crease, in the line of the long axis of the ring finger. This ensures that any scarring is well away from the median nerve and
ensures that proximal extension avoids the palmar cutaneous branch of the median nerve. The extent is from the distal volar wrist up to a few millimetres proximal to the superficial palmar arch. In revision surgery, the proximal extent is increased: this is curved to run along the ulnar side of the palmaris longus tendon (Fig. 5.2). This avoids crossing the wrist joint crease at a right angle and, once again, minimizes any damage to the palmar cutaneous branch of the median nerve.
Dissection
The exposure continues in line with the skin incision until the superficial palmar fascia is exposed deep to subcutaneous fat. Occasionally the belly of flexor pollicis brevis (FPB) is superficial to the fascia and is divided. The fibres of the superficial palmar fascia are incised in the same line.
Retraction of the skin flaps will reveal the
Structures at risk
• Palmar cutaneous branch of the median nerve is at risk if the skin incision is angled to the radial side of the forearm
• Deep motor branch of the median nerve (due to variation in its course) – staying on the ulnar side of the median nerve minimizes the risk of damaging the structure
• Superficial palmar arch
• Median nerve FRC PL PCN R M K H U
Figure 5.1Surface anatomy of the wrist and hand. K, Kaplan’s cardinal line; M, median nerve; R, recurrent motor branch; PCN, palmar cutaneous nerve; U, ulnar nerve; H, hook of hamate; PL, palmaris longus tendon; FCR, flexor carpi radialis tendon
Motor branch
of medial nerve Palmar cutaneous branch of median nerve Palmaris longus Median nerve
Figure 5.2Extended incision for revision/complex carpal tunnel decompression
insertion of palmaris longus into the flexor retinaculum. If it is in the way, it can be retracted to the radial side: this exposes the median nerve. Careful dissection through the flexor retinaculum is recommended until the nerve is visualized. A McDonald tissue dissector is passed between the plane of the flexor retinaculum and the median nerve. The dissector must be used with caution and should elevate the retinaculum and not press down on the nerve. The flexor retinaculum is incised with a scalpel, cutting down onto the McDonald tissue dissector, which lies over the nerve and protects it (Fig. 5.3).
The nerve is released from proximal to distal. In revision surgery the nerve should be dissected out proximal to the wrist crease. The perivascular fat pad is the distal border of the flexor retinaculum. This must be retracted to visualize the distal end of the ligament to ensure complete decom - pression. The proximal end of the wound should also be retracted to ensure complete release under direct vision with either tenotomy scissors or a blade.
The deep motor branch of the median nerve can have a variable course. Usually, it arises on the radial side of the median nerve as the nerve exits the carpal tunnel. The nerve continues radially, entering the thenar muscles between abductor pollicis brevis and FPB. However, variations may include a motor branch arising from the median nerve within the carpal tunnel, running distally to pierce the retinaculum supplying the thenar muscles. Bearing this in mind during the
Carpal tunnel decompression 47
dissection, it is prudent to stay on the ulnar side of the median nerve to prevent damage to the motor branch.
External neurolysis need only be performed if the nerve is adherent to adjacent structures. Internal neurolysis is not performed.
The tourniquet should be released prior to wound closure. It is important to check for reperfusion of the nerve and to ensure adequate haemostasis before skin closure.