In layman’s English, the wrist is considered to be the joint between the forearm and the hand. In fact, the wrist is more complicated than that.
The wrist joint proper, is the articulation between the radius and ulna, and the lunate and scaphoid from the proximal row of carpal bones.
There are numerous carpal joints between the various carpal bones.
The distal four carpal bones articulate with the five metacarpals at the carpometacarpal joints.
The carpal bones are arranged in two rows:
The proximal (from medial to lateral)
The distal row (from medial to lateral)
They are not quite as neat as the classification might suggest. The pisiform is located posterior to the triquetrum and they are actually arranged in a curve, the carpal arch.
There are five metacarpals – one for each digit. They articulate at the metacarpophalangeal joints with the proximal phalanges.
The proximal phalanges articulate with the middle phalanges, which in turn articulate with distal phalanges, all at synovial interphalangeal joints. The only exception is the thumb which only has two phalanges.
The small muscles of the hand are responsible for fine movements They comprise of the anterior and posterior interosseous muscles which run between metacarpal bones and phalanges. They cause most adduction and abduction of the digits. Like most of the small muscles of the hand, innervation is from the ulnar nerve and the movements are used as a test for the motor component of T1.
The exception to ulnar nerve innervation are the muscles of the thenar eminence. They are supplied by the median nerve (which is why you see wasting in carpal tunnel syndrome).
Each digit has a digital nerve on either side and these divide into dorsal and ventral branches about midway up the first phalanx. The finger can be anaesthetised by blocking the digital nerves at the level of the metacarpophalangeal joint in the appropriate web space on either side of the finger.
As well as the position of nerves and blood vessels, there is also another point near the wrist that is of interest to the anaesthetist. This is the acupuncture point, Neiguan or Inner Gate. It is the sixth point on the pericardium channel and has been shown to decrease the incidence of postoperative nausea and vomiting.
P6 can be located two cun above the transverse skin crease of the wrist, between the tendons of palmaris longus and flexor carpi radialis.
One cun is the width of the patient’s thumb at the interphalangeal joint and is the unit of measurement used in acupuncture.
To locate the palmaris longus, ask the patient to flex their wrist. The tendon of palmaris longus is the most prominent, as it is superficial to the flexor retinaculum. If you can’t find it, don’t assume that it’s your ability that is at fault – 15% of the population do not have this muscle. In these cases you have to estimate where the acupuncture point is.
The ulnar nerve passes posterior to the ulnar styloid process, lateral to the ulnar artery. It then passes on the radial side of the pisiform bone at the level of the wrist skin crease. The nerve can be used for assessment of the train of four at this point – stimulation produces contraction of adductor pollicis and the hypothenar muscles. The ulnar nerve also crosses the hook of the hamate where pressure gives a dull neuropathic pain.
Branches of the radial nerve pass through the anatomical snuff box, as does the radial artery. The snuff box, found on the posterior aspect of the wrist, at the base of the extended thumb, is well known to doctors in emergency medicine. Palpation here may produce pain indicating a fracture of the scaphoid, that may otherwise not show on x-ray for up to 2 weeks.
The radial nerve is with the radial artery on the lateral side of the wrist and the median nerve is in the middle of the wrist, deep between tendons of palmaris longus and flexor carpi radialis.