MRCS Part B (OSCE) - Online Resources

"The ulnar nerve originates from the medial cord of the brachial plexus. "

"The root value of the ulnar nerve is the ventral rami of C8 and T1 nerve roots "

 Descends along the posteromedial aspect of the humerus and runs medial to the brachial artery.

 In the arm it pierces the medial intermuscular septum and then passes posterior to the medial epicondyle and medial to the olecranon to enter the forearm.

 Passes between the two heads of flexor carpi ulnaris to enter the flexor compartment of the forearm.

 Runs alongside the ulnar border of the forearm deep to flexor carpi ulnaris.

 In the distal forearm it lies lateral to FCU and medial to the ulnar artery.

 Approximately 5 cm proximal to the wrist it gives off the dorsal cutaneous branch.

 Passes superficial to the flexor retinaculum in Guyon’s canal where it divides into the superficial sensory branch and a deep motor branch.

"Flexor carpi ulnaris and ulnar (medial) half of flexor digitorum profundus. "

"The dorsal cutaneous branch of the ulnar nerve innervates the sensation to the dorsum of the ulnar 1½ digits to the level of the DIPJs. "

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• Palmaris brevis.
• Sensation to the ulnar 1½ digits.

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• Hypothenar muscles: Opponens digiti minimi (ODM); abductor digiti minimi (AbDM); flexor digiti minimi brevis (FDM).
• Third and fourth lumbricals.
• 4 x dorsal interossei (DI) and 4 x palmar interossei (PI).
• Adductor pollicis (AdP).
• Deep head of flexor pollicis brevis (FPB).

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• Firstly, check for muscle wasting in the first dorsal webspace, hypothenar eminence and between the digits.
• Ask patient to abduct fingers against resistance to test the dorsal interossei.
• Ask patient to grip paper placed between the fingers to test the palmar interossei (this can also be tested by asking the patient to ‘scissor’ the fingers).
• Ask patient to abduct little finger against resistance to test abductor digiti minimi.
• Test the adductor pollicis by adducting the thumb against the index finger (Froment’s test).

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• The ulnar nerve is vulnerable as it runs superficial behind the medial epicondyle of the humerus and may be damaged in fractures or dislocations of the elbow. It can also be injured during deep soft tissue lacerations of the forearm and wrist or fractures of the pisiform and hamate bones.
• In distal ulnar nerve injuries (at the level of the wrist) all the intrinsic muscles of the hand (excluding the radial two lumbricals) are paralysed.
• The lumbricals flex the MCPJs and extend the interphalangeal joint in addition to balancing the actions of the long flexors and extensors.

• The clawing is mainly due to the loss of innervation to the medial (ulnar) two lumbricals, which result in hyperextension of the MCPJs and flexion of the IPJs.
• The clawing does not affect the middle and index fingers as the lumbricals to the radial half of the hand are supplied by the median nerve.

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"Clawing of the little and ring fingers is more marked when the ulnar nerve is divided near the wrist than it is if the ulnar nerve is injured higher at the level of the elbow. This is because the flexor digitorum profundus tendons to the little and ring fingers are still functioning and the two digits can be flexed. When the FDP tendons to the little and ring fingers are non-functional the two digits cannot be flexed and hence the clawing is less marked. "

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Each nerve should be tested individually using isolated movements. Function and strength of the muscles supplied by the nerve should also be assessed.

 

Radial nerve:

• Hand :           No motor innervation to the intrinsic muscles.
• Forearm :           Test wrist, thumb and finger MCPJ and IPJ extension.

 

Median nerve:

• Hand :           Test thumb opposition.
• Forearm :  Text flexion of the thumb at the IPJ, wrist flexion                        (FCR), PIPJs of all 4 digits and, DIPJs of the index and middle fingers.

Ulnar nerve:

• Hand : See above.
• Forearm : Text flexion of the wrist (FCU) and DIPJs of the ring and little fingers.

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