The Anatomy of the Carpal Tunnel

The carpal tunnel is an anatomical structure in your wrist and hand that protects the median nerve and several tendons. It is often the source of a common repetitive strain injury called carpal tunnel syndrome (CTS).

The bones, ligaments, and structures around your wrist and hand form an anatomical tunnel. Injury here may cause pain, tingling, and weakness in your wrist and hand and may limit functional use of your upper extremity.

Anatomy

The anatomical borders of the carpal tunnel are the carpal (wrist) bones and the flexor retinaculum, also known as the transverse carpal ligament.

The concave carpal bones—there are eight of them—form the base of the carpal tunnel. These bones are called:

• Scaphoid
• Lunate
• Triquetrum
• Pisiform
• Trapezium
• Trapezoid
• Capitate
• Hamate

Atop these bones is the transverse ligament, spanning from the scaphoid and trapezium on the thumb side to the hamate and pisiform on the pinky side of your wrist. The fibers of the transverse carpal ligament run across your wrist joint at the base of your hand.

Researchers have found significant variation in thickness of the transverse carpal ligament between individuals. This may account for why some people suffer from carpal tunnel syndrome, while others, exposed to similar repetitive strain, do not experience symptoms.

Three small muscles originate from the transverse carpal ligament as well. These are:

• Abductor pollicis brevis
• Flexor pollicis brevis
• Flexor digiti minimi

These small muscles move your thumb and pinky finger in each hand and form the natural contours of the palm of your hand.

Function

The function of the carpal tunnel is to provide an anatomical tunnel and protection for various structures that pass from your forearm and into your hand. There are nine tendons (tendons connect muscles to bones) and a nerve that travel through the passageway formed by the carpal tunnel.

The tendons that travel through the carpal tunnel are:

• Flexor pollicis longus
• Four tendons of flexor digitorum superficialis
• Four tendons of flexor digitorum profundus

The tendon of the flexor pollicis longus is contained within its own synovial sheath as it passes through the carpal tunnel. The eight flexor digitorum tendons are all contained within one synovial sheath.

A synovial sheath is a casing around a tendon that contains synovial fluid, a natural lubricant. This allows the tendons to slide and glide freely as they pass through the carpal tunnel.

Some people also classify the flexor carpii radialis tendon as one that passes through the carpal tunnel. This tendon passes through the bands of the bands of the flexor retinaculum instead of through the actual carpal tunnel itself.

One of the most important functions of the carpal tunnel is to protect and provide a passageway for the median nerve. The median nerve arises from your neck from cervical level six through thoracic level one. It then courses down your arm and into your hand via the carpal tunnel.

Once the median nerve crosses through the carpal tunnel, it divides into two distinct branches called the recurrent branch and the palmar digital nerves. The recurrent branch of the median nerve innervates the muscles around your thumb in an area also called the thenar branch.

The palmar digital nerves provides sensory information from your thumb, index, and middle finger. It also innervates small muscles in your first and second finger called lumbricals.

Associated Conditions

The most common condition associated with the carpal tunnel is called carpal tunnel syndrome (CTS). This problem affects approximately eight million people each year.

Symptoms of carpal tunnel syndrome include:

• Wrist pain
• Pain in the palm of your hand near your thumb
• Pain in your thumb, first, and second fingers
• Tingling in your thumb and first two fingers
• Weakness in your hand
• Loss of muscle bulk in your thumb (an area called the thenar eminence)

Symptoms of carpal tunnel may come on gradually and range from mild to severe. Carpal tunnel syndrome is classified as a repetitive strain injury (RSI), meaning that it occurs as a result of repeated stress to your wrist and hand joints.

Carpal tunnel syndrome occurs when the transverse ligaments thicken, or by inflammation or swelling in any tissues in the tunnel. This happens when you repeatedly place stress and strain on the front of your wrist.

As the ligament is repeatedly injured, your body's repair systems attempt to heal it; cells that make collagen are brought to the area, and the ligament becomes thicker. Thickening of the transverse carpal ligament causes a decrease in space in the carpal tunnel, and the median nerve and tendons there can become pinched, leading to pain, limited mobility, tingling, and weakness in the hand.

Other conditions may affect the carpal tunnel and lead to pain or loss of hand and wrist function. These may include:

• A Colles' fracture of the wrist
• A Smith's fracture of the wrist
• Arthritis of the wrist

Rehabilitation

There are several treatments available to rehab an injury to your carpal tunnel. These range from conservative to invasive.

Diagnosis of carpal tunnel syndrome is done by clinical examination and by a test called an electromyographical (EMG) test. Your physician may gently tap your wrist near the carpal tunnel. If your median nerve is irritated, pain or tingling may occur, indicating possible CTS.

An EMG test involves using small needles placed near the median nerve and measuring the speed of an electrical signal that is sent down the nerve. Slowing of the signal as it crosses the carpal tunnel is also a sign of CTS.

Treatment for carpal tunnel syndrome may range from conservative to more invasive.

Exercise for CTS

Your doctor or physical therapist may recommend exercises for managing carpal tunnel syndrome. Exercises may include:

• Tendon gliding exercises
• Median nerve glides
• Wrist muscle stretching
• Wrist and hand strengthening exercises

Splinting

People with carpal tunnel syndrome may benefit from using a wrist cock-up splint. This splint, worn at night while sleeping, keeps your wrist and hand in a neutral position, taking pressure off the tendons and median nerve in the carpal tunnel.

Injections

Your physician may recommend an injection for your CTS. Injection of a strong anti-inflammatory corticosteroid may be used to decrease pain, decrease swelling to tissues in the carpal tunnel, and improve overall wrist mobility.

Surgery

Surgery for CTS involves making a small incision in the palm of your hand near your wrist. Your surgeon will then cut away the thickened transverse ligament, taking pressure off the pinched median nerve and its neighboring tendons.

After surgery, the palm of your hand will be sutured and you may be required to wear a protective brace for a few weeks. You can expect to slowly increase wrist and range of motion and strength. Most patients return to full function about eight weeks after surgery.

If your symptoms are severe and do not remit with conservative treatment, then considering invasive treatments like injections or surgery may be warranted.