Surgical Treatment of Carpal Tunnel Syndrome

The carpal tunnel is a bony canal covered on the palmar side by the flexor retinaculum. On the ulnar border the flexor retinaculum attaches to the pisiform and hook of the hamate and on the radial border to the scaphoid tubercle and ridge of the trapezium. The median nerve passes through the carpal tunnel together with eight flexor tendons of the long fingers and the tendon of the flexor pollicis longus. The bony walls of the carpal canal and the rigid flexor retinaculum have no capacity for expansion. Anything that reduces the dimensions of the tunnel or increases the volume of its contents will predispose to carpal tunnel syndrome.^1,2

Many conditions have been associated with carpal tunnel syndrome, but most cases are idiopathic.² Carpal tunnel syndrome can be caused by abnormal anatomical structures such as anomalous muscles³ or a persistent median artery. ⁴ Other causes are trauma, swellings (ganglion, lipoma), inflammatory diseases (rheumatoid arthritis) and metabolic disturbances (diabetes, thyroid disorders, pregnancy).¹ Obesity is a risk factor for carpal tunnel syndrome.^5,6 Carpal tunnel syndrome can also be associated with certain occupations involving handgrip with high forces or vibrating tools, especially when those tasks require repetitive movements of the hands.⁷ However, in another study, working in repetitive or non-repetitive occupations did not cause, accelerate or aggravate carpal tunnel syndrome. ⁸ A genetic predisposition for carpal tunnel syndrome has also been reported. ⁹

In the general population, the prevalence of carpal tunnel syndrome ranges between 2 and 4%.^5,10 Carpal tunnel syndrome is more common in women. The incidence peaks in women in their late 50s and in men between 45 and 54 years of age. In both sexes, the incidence peaks again in the late 70s.^5,11

Symptoms of carpal tunnel syndrome can range in severity from transient sensory disturbances to irreversible thenar wasting and sensory loss. Patients woken by paraesthesias or pain in the distribution of the median nerve have carpal tunnel syndrome until proved otherwise.² On physical examination, wasting of thenar muscles should be looked for and the strength of the abductor pollicis brevis muscle should be tested. It should be examined whether the area of sensory disturbance involves the distribution of the median nerve. Several clinical tests can be used to diagnose carpal tunnel syndrome, such as Tinel’s sign, Phalen’s test, reversed Phalen’s test,¹ Durkan’s compression test¹² and the hand elevation test. ¹³ The diagnosis can be made based on clinical findings and electromyography. ¹

Open Carpal Tunnel Release
The patient is placed supine with the arm extended on a padded arm table and the forearm supinated. General, endovenous, plexus or, preferably, local anaesthesia can be administered.¹⁷ A pneumatic tourniquet can be applied at the upper arm or forearm. With local anaesthesia, patient comfort is greater when a forearm tourniquet is used.¹⁸

When planning the surgical incision, a few landmarks are useful in order to cut the flexor retinaculum on the ulnar side of the median nerve and to avoid injury to neurovascular structures. Kaplan’s cardinal line is drawn from the apex of the thumb–index web to the hook of the hamate. The deep palmar arch lies under the cardinal line and the superficial palmar arch is located between the cardinal line and the proximal palmar crease. Palpation of the hook of the hamate will prevent an excessively ulnar incision. The surgical incision is in the palm, in line with the central axis or radial border of the ring finger. It starts proximal to Kaplan’s cardinal line and ends distal to the distal wrist crease. ^17,19,20 The incision can be extended into the distal forearm if needed, but a 2–4cm-long incision confined to the palm is now recommended for standard open carpal tunnel release (see Figure 1 ).^21,22

When making the skin incision, care must be taken to protect the terminal cutaneous branches.¹⁹ Because there is no internervous plane in the palmar region, four cutaneous nerves are at risk, i.e. the palmar cutaneous branch of the median nerve, the palmar cutaneous branch of the ulnar nerve, branches from the nerve of Henle and transverse branches of the palmar cutaneous branch of the ulnar nerve.²³

After incision of the skin and subcutaneous fat, the underlying flexor retinaculum is identified as a transversely orientated fibrous tissue layer that is contiguous radially with the thenar muscles and ulnarly with the hypothenar fat pad. ¹⁹ Anatomical variations of thenar and hypothenar muscles exist and muscle fibres frequently cross the line of incision on the flexor retinaculum. The flexor retinaculum can have a muscular aspect without a clear separation between the thenar and hypothenar muscles. In these circumstances muscle fibres have to be cut in order to visualise the flexor retinaculum (see Figure 2).^24,25

At the distal edge of the flexor retinaculum is a fat pad containing the branches of the median nerve and the superficial palmar arterial arch. The flexor retinaculum is cut along its ulnar border (see Figure 3). The proximal portion, near the distal wrist flexion crease, is less thick and can be divided with scissors after first freeing the adjacent tissues palmarly and dorsally (see Figure 4). The flexor retinaculum should be incised approximately 3cm distal of the flexor crease. It is advised to inspect the motor branch of the median nerve and the superficial palmar arterial arch before releasing the tourniquet and closing the wound.¹⁹ Some do not explore the motor branch during open carpal tunnel release and find it is not necessary to release the tourniquet and obtain haemostasis prior to wound closure.^26,27