Introduction

The radial nerve (RN) (C5, C6, C7, C8 and T1) is the largest branch of the posterior cord of brachial plexus. In the mid shaft region, the RN courses infero-laterally across the posterior aspect of the upper arm and pierces the lateral intermuscular septum at the lower end of spiral groove to enter into the anterior compartment [8]. It then runs along the lateral border of triceps aponeurosis (TA) between the brachialis and brachioradialis muscles in distal arm. As the RN approaches the lateral epicondyle, it divides into the superficial and deep branch (posterior interosseous nerve) [11]. The long course and close proximity to the periosteum of humerus makes RN very prone to injury in distal humeral fractures. RN palsies are mainly two types, primary and secondary. Primary palsies (seen in 11.80% of all cases reported) are mostly due to fracture humeral shaft, commonly Holstein Lewis fracture involving the middle and middle-distal shaft of the humerus [17]. Secondary or iatrogenic palsies can occur during any kind of operative interventions for unstable/compound humeral fractures, such as plating, open reduction and internal fixation (ORIF), close reduction and Kirschner wire fixation (CRIF) or screw fixation.

Fracture of humeral diaphysis with metaphyseal extension remains a nagging problem [15] for a treating surgeon as RN is very prone to iatrogenic injury over this region, specifically where it pierces the lateral intermuscular septum and runs along the lateral margin of TA [2].

Identification of the nerve is crucial in fracture surgery as it requires the surgeon to mobilize the nerve away from the operative field for the application of lateral pre-contoured plates/screws/Kirschner wire and also in elbow arthroplasty that necessitates safe mobilization of the soft tissue envelope around the dia-metaphyseal segment of the humerus [10].

Compound fracture of distal one-third of humerus (lateral condyle) and fracture dislocations of elbow requires early external fixation through lateral approach. During such approach, straight incision is given over the lateral supracondylar ridge in the intermuscular plane between triceps and brachioradialis and extended proximally. During proximal extension, surgeons need to be very careful as the radial nerve crosses proximally in line of incision. Moreover, the close proximity of the RN with distal humerus makes the zone hazardous for the procedure. Half pins used in external fixation (EF) of the distal humerus can easily injure the radial nerve [5]. No known ‘safe area’ for the placement of an external fixator has been identified on the lateral border of the humerus [12].

With such a facile, the primary aim of the study was to describe the surgical anatomy of the radial nerve with reference to a constant soft tissue land mark such as lateral border of the TA. Secondary aim was to determine a ‘safe area’ for the radial nerve during lateral fixation of fracture distal humerus.

Materials and methods

The present study was done on 40 upper limbs belonging to 20 formalin fixed cadavers (15 males, 5 females) aged between 45 and 60 years. In dissection study formalin is universally used for fixing tissues. To compensate the volume shrinkage effect of formaldehyde, we have used 10% formol solution (contain 4% formaldehyde) as it may be considered as a standardized method of fixation. Arms having any gross malformation or deformity were excluded from the study. The specimens were prepared through scapulo-thoracic disarticulation and mid-clavicular amputation. Each cadaveric upper extremity was placed in prone position with flexed elbow (90°). A direct lateral skin incision was given starting from the tip of lateral epicondyle of humerus to the tip of acromion. Subcutaneous tissue and deep fascia were also incised in the same line exposing brachialis, brachioradialis and triceps with its aponeurosis (TA). The TA represents the convergence of the long and lateral heads of the triceps muscle bellies, forming a shiny layer lying between the deep fascia superficially and periosteum of the distal humeral dia-metaphyseal segment underneath. It can be readily identified by its shiny pearl white color. Proximally, the TA runs obliquely upwards over the posterior aspect of the arm, creating two apices, one proximal or medial, the other distal or lateral (Fig. 1). The lateral border of the TA was identified. The RN was dissected very carefully in distal to proximal direction, starting from the lower end of humerus in front of its lateral epicondyle up to the lower end of spiral groove where it pierces the lateral intermuscular septum. RN was exposed over the distal length of its course by dissecting the overlying muscle with a sharp scalpel. Extra care was taken so as to prevent undue manipulation of the normal course of the nerve. This ensured that the positions of the RN and the TA remained unaltered during the exposure.

Fig. 1
figure 1

The radial nerve after it pierces the lateral intermuscular septum and enters into the anterior compartment (point of entry) and courses along the lateral border of triceps aponeurosis (TA) between the brachialis and brachioradialis muscles (lateral approach view)

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Measurements

The point of entry of RN into the anterior compartment of arm was noted first in relation to the apices of the TA. We have marked the proximal or medial apex as point ‘A’ and distal or lateral apex as point ‘B’. Subsequently the location of the nerve was described in relation to the proximal apex (A) and along the lateral border of TA. The distance between the point ‘A’ and the point of entry of RN into the anterior compartment was measured and recorded as AR. The distance between the lateral margin of the TA and the medial margin of RN was measured at four equally spaced sites starting proximally from distal or lateral apex (B). The straight distance between point ‘A’ and ‘B’ was measured and this straight distance was used to determine three equally spaced sites (point C, D and E) along the lateral margin of TA distal to the lateral apex (B) (Fig. 2).

Fig. 2
figure 2

The distances of radial nerve from the proximal or medial apex and lateral border of triceps aponeurosis (TA). A: proximal or medial apex of triceps aponeurosis (TA), B: distal or lateral apex of TA, C, D and E: three equally distanced points along the lateral border of triceps aponeurosis (TA)

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Three measurements for each distance were taken by the same individual with the help of surgical ruler. The mean of each distances was recorded as AR, BR, CR, DR and ER, respectively, and analyzed.

Statistical analysis

Subsequent statistical analysis through paired, two-tailed Student t test was performed (Microsoft Office Excel software version 16.0 for Windows 10).

Results

The point of entry of RN into the anterior compartment of arm from the posterior compartment was variable. In majority of cases (67.50%), it was piercing the lateral intermuscular septum and entering the anterior compartment at the level of proximal or medial apex followed by distal or lateral apex (17.50%). In three cases each (7.50%), the point of entry was either above the level of proximal apex or in between the proximal and distal apex.

Table 1 shows the distances of the RN from the proximal or medial apex of TA (A) and the lateral border of the TA at four sites (B, C, D and E). The mean distance of the RN from the aponeurosis was 1.98 ± 0.60 cm with a range of 1.00–2.50 cm. The minimum distance (1.00 cm) was found at the level of distal or lateral apex (B) and maximum distance (2.50 cm) was at the level of point ‘E’ in the distal part of TA. RN was not seen to give any branches in the quadrangular area adjacent and parallel to the lateral border of TA and was found to be relatively safe for dissection (Fig. 3).

Table 1 Distances of the radial nerve (RN) from the lateral border of triceps aponeurosis (TA) at different level
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Fig. 3
figure 3

Quadrangular shaped area along the lateral border of triceps aponeurosis as ‘safe zone’ for radial nerve during surgical intervention through the lateral border of distal humerus

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Discussion

Knowledge of the location of the radial nerve and its relationship with a consistent and reliable anatomic/soft tissue landmark are major concerns for treating surgeons.

Several studies have been done on various ethnic populations to determine the relationship between RN and bony landmarks, such as lateral and medial epicondyle [3, 7, 9]. But such relations may not hold true in case of fracture humerus with displacement. Therefore, in case of fracture and displacement of humerus, a non-osseous superficial soft tissue landmark may guide the orthopaedician to identify the radial nerve and prevent potential iatrogenic injury.

Recently few authors have described the relation of RN with superficial soft tissue land marks such as triceps aponeurosis (TA) keeping in mind the posterior or modified posterior triceps splitting approach used for traditional plating. Seigerman et al. [16] described the point of confluence of the long and lateral heads of the triceps and the TA as a visualizable anatomic reference point to identify the RN during the posterior approach to the humerus. Arora et al. [1] described the apex of the TA as a constant and reliable landmark to identify RN during posterior triceps splitting approach. Patra et al. [13] described the point of confluence of the TA to be the most stable and reliable anatomic landmark for localization of the RN during the posterior approach to the humerus.

Such relations may not hold to be useful in the setting of lateral approach used alternatively in fixing fracture distal third of humerus or dia-metaphyseal junction. Depending upon the surgeon’s preference and their familiarity with the muscular and neurovascular anatomy of the upper limb, fracture in this region may need lateral approach for the application of lateral pre-contoured anatomic plates [10]. Submuscular plating, and minimally invasive percutaneous osteosynthesis (MIPO) of the lateral column poses a risk of damage to the RN, specifically during distal to proximal release of the triceps laterally, as the nerve crosses from the posterior to the anterior compartment.

Just distal to the spiral groove, the radial nerve (RN) enters the flexor compartment by piercing the lateral intermuscular septum. At this point, the radial nerve is quite immobile, superficial and prone to injury. The point at which the nerve pierces the lateral intermuscular septum has been studied by few authors. According to Bono et al. [2], the maximum risk of iatrogenic injury is at the point where the nerve pierces the lateral intermuscular septum. They described the location of RN at the level of intermuscular septum in relation to bony anatomical landmarks such as, the proximal margin of the humeral head, the olecranon fossa and the lateral epicondyle. They observed a mean distance of approximately 16 cm in case of lateral epicondyle and found that interval as most relevant one to identify RN intraoperatively.

Fleming et al. [6] described this point in qualitative way rather than quantitative terms. They found the nerve to traverse the septum within 5 mm of the junction between the middle and distal thirds of the humerus.

McCann et al. [10] studied the relation between the lateral or distal apex of the TA and the point of entry of RN into the anterior compartment of arm from posterior compartment. They kept the limb in 90° elbow flexion as in our study and located the nerve 1 mm proximal to the lateral apex. However, in 60% of the cases, the nerve was located at the level of the lateral apex.

In the present study, we have tried to locate the point of entry of RN into the anterior compartment in relation to the apices of TA. In majority of cases (67.50%), it was found to be at the level of proximal apex followed by distal apex (17.50%). Distance of the nerve from these two landmarks also varies. RN was a bit closer to the distal apex whereas away from the proximal apex. The surgical anatomy of the RN at its entry into the anterior compartment of arm may be proven very useful during soft tissue dissection for repair of fracture dislocation of distal humerus.

The lateral border of TA has also been found to be a useful and reliable guide to locate the RN intraoperatively. Chaudhary et al. [4] studied the relationship of the RN with the lateral border of TA in the British population and found the nerve at a distance of 22.00–27.00 (± 2) mm from the lateral border of TA. Traditionally, RN was not found to be closer than 13.00 (± 1) mm to the aponeurosis. The area < 10 mm from the aponeurosis represented a “safe” area for intervention.

Prasad et al. [14] studied the location of RN in relation to the lateral border of TA in the south Indian population and found the nerve at a distance of 12.00–19.50 (± 3.20) mm from the lateral border of the aponeurosis. It was never found to be closer than 4.60 (± 2.54) mm to the aponeurosis. Therefore, the area immediately adjacent to the aponeurosis (< 4.00 mm) was considered “safe” for soft tissue dissection and advancement to bone.

In the present study, we have studied the course of the RN and its location along the lateral border of TA in north Indian population. We found the RN to run along the lateral border of TA before its exit into the cubital fossa in front of the lateral epicondyle. The distance or gap between RN and lateral border of TA gradually increased from proximal to distal. The mean distance of the RN from the lateral border of aponeurosis was found to be 1.98 ± 0.60 cm with a range of 1.00–2.50 cm. The minimum distance (1.00 cm) was found at the level of distal or lateral apex (B) and maximum distance (2.50 cm) was at the level of point ‘E’ in the distal part of TA. For any given point, the nerve was not found closer than 1.00 cm. Therefore, in our suggestion, the rectangular zone with breadth < 1.00 cm can be considered ‘safe’ zone for surgical intervention. Our finding was in consonance with the observations of Chaudhary et al. [4].

Limitations

The small number of female cadavers (5) than that of the males (15) did not allow us to compare our findings between them to study the effect of gender on the measured parameters. Moreover, the interindividual variations in built, body fat and body mass index can effectively influence the value of the measured parameters.

Conclusion

We believe this study provides a novel method for intraoperative identification and protection of the radial nerve on lateral aspect of the humerus. The location of the point of entry of radial nerve into anterior compartment is variable but the relationship of the nerve along the lateral border of TA is constant and easily reproducible. We suggest that, the rectangular zone immediately adjoining the lateral border of TA (< 1.00 cm) can be considered “safe” for soft tissue dissection and application of pre-contoured anatomic plates or fixator pins in management of distal humeral fracture. Knowledge of the “safe zone” could potentially be useful for intraoperative guidance and might contribute to reduce the risk of radial neuropathy.