The location of the radial nerve in the humeral shaft between the prone and lateral decubitus positions with different elbow positions and its implications: A cadaveric study

SITTHIPHONG SUWANNAPHISIT Department of Orthopedics, Faculty of Medicine, Prince of Songkla University WACHIRAKORN AONSONG Department of Orthopedics, Faculty of Medicine, Prince of Songkla University PORAMES SUWANNO Department of Orthopedics, Faculty of Medicine, Prince of Songkla University CHAIWAT CHUAYCHOOSAKOON (  chaiwat.c@psu.ac.th ) Department of Orthopedics, Faculty of Medicine, Prince of Songkla University


Introduction
Surgical intervention for a humerus fracture is common, with the posterior approach mostly preferred for fractures of the mid-to distal humerus. However, one study reported primary palsy of the radial nerve after humeral fracture repairs in 11.8% of the cases 1 . The incidence of iatrogenic injury to the radial nerve has been reported as 6% 2 . Injury to the radial nerve resulting in radial neuropathy or radial nerve palsy leads to wrist drop with a poor outcome 3 . To decrease the incidence of radial nerve palsy, identi cation of the exact location of the radial nerve in relation to the osseous structures is essential when preparing a posterior approach to the humerus fracture.
Various studies have examined the location of the radial nerve in relation to the common anatomical landmarks, the olecranon fossa 3,4 , the intersection of the long lateral head of the triceps and triceps aponeurosis 5,6 , and four ngerbreadths from the lateral epicondyle 7 . All of these studies, however, evaluated the radial nerve in only one position, which does not accurately re ect the real clinical situation in which the patient can be placed in the prone, lateral decubitus or supine positions, depending on surgeon preference, and the distance from the radial nerve to the osseous structures will be different in different positions.
To date, there have been no studies investigating the distance from the radial nerve to the important landmarks in these common surgical positions, the lateral decubitus and prone positions, with the elbow exed at different angles. Hence, the purpose of this study was to identify the safety zones of the various patient and elbow exion positions, to help reduce the risk of radial nerve injuries.

Methods
Ten fresh-frozen cadavers (8 males and 2 females) from the Faculty of Science, Prince of Songkla University, were acquired for this study. The study was carried out in accordance with the guidelines and local regulations. Approval from the research and development o ce from the local Prince of Songkla University was granted for this experimental protocol. The donors did not have to sign a statement about this speci c study but they did express their consent to anonymously give part of their body to science for ethically approved experiments. The mean age at death was 74.10 ± 5.74 years, and no specimens showed evidence of arm surgery. There were no signi cant anatomical variations and no evidence of previous traumatic upper extremity injuries in any of the cadaveric arms and elbows.
The upper arms of all cadavers were dissected and measured using a standardized protocol by a single experienced surgeon using a triceps-splitting approach. The skin and subcutaneous tissues were . Differences between distances were compared using paired t-test. A p-value of 0.05 was considered signi cant due to multiple comparisons.

Results
The humeral length averaged 28.87 cm (± 2.71 cm, range 24.50 to 32.60 cm). The mean distances from the upper part of the olecranon to the radial nerve in the radial groove in different elbow exion angles in the prone and lateral decubitus positions are summarized in Table 1. The mean distances from the upper part of the olecranon to the radial nerve in the lateral intermuscular septum in different elbow exion angles in the prone and lateral decubitus positions are summarized in Table 2. The mean distances from the lateral epicondyle to the radial nerve in the lateral intermuscular septum in different elbow exion angles in the prone and lateral decubitus positions are summarized in Table 3. From the upper part of the olecranon to the central posterior point where the radial nerve crosses the humerus, the distances were signi cantly lower with increasing elbow exion angles, as shown in Figure 3. There was excellent intraobserver correlation between the two orthopedists, with correlation coe cients between 0.77 and 0.99.
The important nding in this study was that the different distances from where the radial nerve There was very little change in the measurements until the elbow was exed to 120 degrees, a position that is rarely used with a posterior approach to the elbow. When limiting ROM to 0 to 90 degrees of elbow exion the variation was less than 3 mm and not signi cant. shoulder abduction from 30 degrees to 110 degrees required 4 mm of radial nerve excursion at the elbow. The greatest radial nerve migration was 11.4 mm with shoulder abduction of 110 degrees and elbow exion of 90 degrees 11 . However, this study investigated radial nerve excursion at a point approximately 4-6 cm proximal to the lateral epicondyle, and the ndings are not relevant in the situation of a midshaft of humerus fracture. Another study 12 suggested that radial nerve excursion can be improved with elbow exion and lateral intermuscular septum release by approximately 3 times. Our current study aimed to provide accurate information concerning the points of entry and exit of the radial nerve around the elbow which can be applied in the case of a humerus fracture. We found that there was a little change of nerve location between the two positions in different elbow exions from 0 to 90 degrees, except when the exion exceeded 120-degrees, when the distance was ± 3 mm from the mid-point of the humerus to the lateral intermuscular septum. The reason to explain a little change of radial nerve distance due to its anatomic that the course of radial nerve run posterior to spiral groove, exits to anterior via lateral intermuscular septum with usually course tethering the distal part of radial nerve of arm, consequently minimal dynamic change.

Discussion
In terms of clinical implications, our ndings show that the posterior approach in either the prone or lateral decubitus position can be used in diaphyseal middle-third to distal-third procedures 13,14 . Our study identi es the safe zones for the radial nerve in each position. The surgeon can use our ndings concerning the safe zone in different elbow exion angles in their preferred position. In the case of a distal humerus fracture with intact olecranon fossa or a simple fracture of the distal humerus according to the AO classi cations (AO-OTA 13 type A,B and C1) 15 , we found that the olecranon fossa was a reliable landmark to estimate the location of the radial nerve when using the posterior approach.
The study had potential limitation. The length of the nerve in the lateral intermuscular bundle may have been changed by dissecting the triceps muscle, although we attempted to minimize this possible problem by dissecting only the triceps and identifying and leaving intact the radial nerve.
There were no signi cant differences in the measurements and no important different distances of radial nerve location until the elbow was exed to greater than 120 degrees, which was not signi cant as this exion is rarely used. Understanding the distance of the radial nerve in both lateral decubitus and prone positions from these common landmarks in the various common operative positions is helpful in avoiding iatrogenic radial nerve injury during a surgical approach to the mid-to distal third of the shaft of the humerus.

Declarations
Compliance with ethical standards Con icts of interest: None Disclosures All authors -SITTHIPHONG SUWANNAPHISIT, WACHIRAKORN AONSONG, PORAMES SUWANNO, and CHAIWAT CHUAYCHOOSAKOON -declare they have no funding or commercial associations that might pose a con ict of interest in connection with the submitted article.