The radial nerve danger zone : A cadaver study

Distal humerus fractures are uncommon, comprising 2% of all fractures and a third of all humerus fractures. Open fractures, comminuted fractures and fracture dislocation of the elbow are often accompanied by severe soft tissue damage. External fixation of the humerus is indicated when severe soft tissue damage or the presence of infection warrants initial alternative immobilisation of the fractures around the elbow. External fixation allows for easy access for wound cleaning and dressing, stabilises fractures where vascular and nerve repair was done and allows for frame adjustments to improve alignment. External fixation allows for early mobilisation of adjacent joints and mobilisation of the patient. External fixation can cause pin tract sepsis, and injury to nerves and blood vessels and the surrounding soft tissue. The radial nerve is situated close to the humerus, and placing external fixation around the distal humerus may lead to nerve damage. The upper limb is innervated by a plexus of nerves arising from the ventral rami of the C5–T1 nerve roots. The ventral rami of the upper, middle and lower trunks divide into anterior and posterior divisions. The posterior divisions of all three trunks form the posterior cord. In the axilla, the radial nerve is located posterior to the axillary artery from where it runs inferiorly along the medial aspect of the proximal humerus. The radial nerve then descends along the radial groove to pierce the lateral intermuscular septum proximal to the lateral epicondyle where it runs between the brachialis and brachioradialis muscles. Abstract


Introduction
Distal humerus fractures are uncommon, comprising 2% of all fractures and a third of all humerus fractures. 1Open fractures, comminuted fractures and fracture dislocation of the elbow are often accompanied by severe soft tissue damage.External fixation of the humerus is indicated when severe soft tissue damage or the presence of infection warrants initial alternative immobilisation of the fractures around the elbow. 2 External fixation allows for easy access for wound cleaning and dressing, 3 stabilises fractures where vascular and nerve repair was done 4 and allows for frame adjustments to improve alignment.External fixation allows for early mobilisation of adjacent joints and mobilisation of the patient. 5External fixation can cause pin tract sepsis, 6 and injury to nerves and blood vessels and the surrounding soft tissue. 7The radial nerve is situated close to the humerus, and placing external fixation around the distal humerus may lead to nerve damage.
The upper limb is innervated by a plexus of nerves arising from the ventral rami of the C5-T1 nerve roots.The ventral rami of the upper, middle and lower trunks divide into anterior and posterior divisions.The posterior divisions of all three trunks form the posterior cord. 8In the axilla, the radial nerve is located posterior to the axillary artery from where it runs inferiorly along the medial aspect of the proximal humerus.The radial nerve then descends along the radial groove to pierce the lateral intermuscular septum proximal to the lateral epicondyle where it runs between the brachialis and brachioradialis muscles.
As the radial nerve approaches the lateral epicondyle it divides into the superficial radial and posterior interosseous nerves. 8he highest risk of injury to the radial nerve is at the point where the nerve pierces the lateral intermuscular septum. 9Bodner et al. 10 identified the radial nerve at 100 mm proximal to the epicondyle using ultrasonography.Artico et al. 11 performed a study on fresh cadavers and found that the mean distance between the lateral epicondyle and the point where the nerve pierces the lateral intermuscular septum was 110 mm. 11Kamineni et al. 12 described the safe zone for placing pins in relation to the trans-epicondylar distance.They concluded that 100% of the trans-epicondylar distance along the lateral border of the humerus was a safe zone for external fixation. 12lement et al. 13 stated the risk of radial nerve damage by external fixation may be due to the variation in the course of the nerve and that wide incision and blunt dissection to the cortex was necessary to prevent nerve damage.
Our study aims to identify a safe zone for the surgical placement of pins and records the location of the radial nerve in relation to the two pins placed.

Method
Our sample consisted of 39 cadavers (28 male and 11 female), between 18 and 99 years of age.Cadavers were dissected by second year medical students in the Department of Anatomy at the University of Pretoria.The use of cadavers for research is covered under the South African National Health Act 41 of 2003.
The cadavers were positioned supine with the palms of both hands facing up.The lateral epicondyle was palpated and the two half pins (4 mm in diameter) were inserted at 100 mm (proximal pin) and 70 mm (distal pin) to the epicondyle.A hand drill and a multi-pin clamp (Figures 1a  and 1b), similar to those used in most external fixators around the elbow, was used to insert the pins.Once the pins were placed, the upper limbs were dissected.The radial nerve was identified at the lateral border of the humerus and the incidence of nerve damage caused by the pins and the relation of the nerve to the pins was recorded.
Statistical analysis was done using the chi-square and mixed model test estimated along a 95% confidence interval.The overall results adjusted dependence between left and right sides and this proportion, together with its confidence interval was analysed using the statistical software Stata.Testing was carried out at the 0.05 level of significance.A Fisher's exact test was used to identify the incidence of radial nerve damage relative to pin insertion.

Results
The radial nerve was damaged by the proximal pin in 56.4% of cases and by the distal pin in 20.5% of cases (Table I).
Results of bilateral radial nerve damage by the proximal and distal pin for males and females are shown in Table II.
The radial nerve was anterior to the proximal pin on the left humerus (43.5%) compared to the right (38.5%) for both males and females.The radial nerve was anterior to the proximal pin more often in male cadavers (50.0%) than in females (27.3%).These findings were not statistically significant (p=0.29).
The radial nerve was anterior to the distal pin in 79.5% of cases irrespective of side.
The radial nerve was damaged by the proximal pin more often in female right sides (81.8% of cases) The radial nerve was posterior the proximal pin on the left side more often in females (9.1%) compared to males (3.6%), in 5.1% of cases on the left sides only.The radial nerve was never encountered posterior to the distal pin.

Figure 1b. insertion of the proximal and distal pin with the use into the left arm of a cadaver table i: incidence (%) of radial nerve damage in a South African cadaver sample (n = number of cases out of total sample)
Table III indicates the location of the radial nerve in relation to the proximal and distal pin bilaterally.

Discussion
In the present study, the incidence of nerve damage at the proximal pin (100 mm) was significantly higher than the distal pin (70 mm).This suggests that the risk of radial nerve damage is greater at 100 mm than at 70 mm.The lower incidence of nerve damage at the distal pin relates to the anterior course of the nerve.Clement et al. 13 inserted pins into 20 cadaver arms at 50 mm and 30 mm proximal to the lateral epicondyle.The proximal pin (50 mm) damaged the radial nerve in five out of 20 cases (25.0%). 13he distal pin damaged the radial nerve in four out of 20 cases (20.0%). 13According to our results and those of Clement et al., 13 the radial nerve is more likely to be damaged if the pin is inserted between 70 and 100 mm proximal to the lateral epicondyle.Although not clinically significant, we found nerve damage caused by the proximal pin in more female than male cadavers and more on the right than left side.No other studies have reported sex or bilateral differences.The higher incidence of nerve damage on the right side indicates that the proximal pin position is crucial to avoid hitting the nerve.The nerve was anterior to both pins in most cases but more so to the distal pin.The nerve changes course as it travels distally, eventually wrapping around the lateral epicondyle.The radial nerve was found at distances (our study) similar to Artico et al., 11 who examined the topographical relation of the radial nerve to different anatomical landmarks in 20 fresh cadavers.They reported that the mean distance between the entry point of the nerve in the lateral intermuscular septum and the lateral epicondyle was 110 (±23) mm.Our findings reinforce that the high-risk area for pin insertion is 100 mm proximal to the lateral epicondyle.
Clement et al. 13 similarly placed pins in upper limbs and after dissection found that the radial nerve was anterior to the proximal pin in 13 cases and anterior to the distal pin in 14 cases.In three arms the nerve was posterior to the distal pin.We found similar results and conclude that pins should be placed more posteriorly as the radial nerve runs more anteriorly.