Anatomy
Upper Limb
A patient presents to ED complaining of weakness in her left arm. She tells you she sustained a fracture to this arm about 2 months ago, but is unable to give you more detail. Examination reveals loss of wrist extension and weakness of grasp. Extension at the elbow joint is normal, and there is no loss of sensation. Which of the following nerves is most likely affected:
Answer:
The posterior interosseous nerve is the continuation of the deep branch of the radial nerve which innervates the muscles of the posterior compartment of the forearm. Damage to this nerve may occur due to fracture of the radial head or posterior dislocation of radius and results in weakness of wrist extension and finger extension (resulting in weak finger flexion and grip strength as the long flexor tendons are not under tension). The superficial branch of the radial nerve supplies skin over the dorsum of the lateral hand and lateral three and a half fingers and thus is not affected in this case. The anterior interosseous nerve is a branch of the median nerve supplying muscles in the anterior forearm and the recurrent branch innervates the thenar muscles.Radial Nerve
Anatomy / Upper Limb / Innervation of Upper Limb / Posterior Arm / Posterior Forearm
Last Updated: 8th April 2019
The radial nerve is a continuation of the posterior cord, containing fibres from C5 - T1.
Table: Anatomical Overview of the Radial Nerve
| Nerve | Radial |
| Nerve roots | C5 – T1 |
| Plexus cords | Posterior cord |
| Motor Supply | Triceps brachii, posterior compartment of forearm: superficial muscles (brachioradialis, extensor carpi radialis longus and brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris) and deep muscles (supinator, abductor pollicis longus, extensor pollicis longus and brevis, extensor indicis) |
| Sensory supply | Lower lateral arm, posterior arm, posterior forearm, dorsum of lateral hand and three and a half fingers |
Anatomical course
The radial nerve enters the arm by crossing the lower margin of the teres major muscle, where it lies posterior to the brachial artery. It enters the posterior compartment of the arm, where it descends obliquely passing from medial to lateral in the radial (spiral) groove of the humerus. After emerging from the spiral groove, the radial nerve pierces the lateral intermuscular septum and enters the anterior compartment of the arm, descending into the cubital fossa where it lies between the brachialis and brachioradialis muscles.
The radial nerve enters the forearm after passing over the lateral epicondyle of the humerus. Within the proximal forearm the nerve terminates by bifurcating into the deep branch and the superficial branch.
N.B. Once the deep branch of the radial nerve penetrates between the two heads of the supinator muscle to access the posterior compartment of the forearm, it becomes known as the posterior interosseous nerve.
![By Henry Vandyke Carter [Public domain], via Wikimedia Commons](https://mrcemsuccess.com/wp-content/uploads/2016/12/Radial-Nerve.png)
Radial Nerve. (Image by Henry Vandyke Carter [Public domain], via Wikimedia Commons)
Branches
Table: Branches of the Radial Nerve
| Branches of Radial Nerve | Origin | Supply |
| Radial nerve | Axilla | Triceps brachii, extensor carpi radialis longus, brachioradialis |
| Posterior cutaneous nerve of the arm | Axilla | Skin of posterior arm |
| Inferior lateral cutaneous nerve of the arm | Arm | Skin over lateral aspect of lower arm |
| Posterior cutaneous nerve of the forearm | Arm | Strip of skin down middle of posterior forearm |
| Deep branch which continues as the posterior interosseous nerve | Forearm | Posterior compartment of forearm: superficial muscles (extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris) and deep muscles (supinator, abductor pollicis longus, extensor pollicis longus and brevis, extensor indicis) |
| Superficial branch | Forearm | Skin of dorsum of the hand and lateral three and a half fingers |
In the arm, the radial nerve directly innervates the triceps brachii, the extensor carpi radialis longus and the brachioradialis. In the forearm, the deep branch, which continues as the posterior interosseous nerve, innervates the muscles of the posterior compartment of the forearm and the superficial branch supplies the skin of the dorsum of the hand and lateral three and a half fingers.
The radial nerve also gives rise to several cutaneous branches; the posterior cutaneous nerve of the arm originating in the axilla, and the inferior lateral cutaneous nerve of the arm and the posterior cutaneous nerve of the forearm originating in the arm. These cutaneous branches supply skin over the posterior surface of the arm, the lateral aspect of the arm and the skin down the middle of the posterior forearm respectively.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
