Our findings support our hypothesis that the RN’s sinuous pathway predisposes it to potential compression. This could be further exacerbated by the presence of other associated pathology or dysfunction of myofascial or connective tissue surrounding the nerve: densification, fibrosis, shortening, contracture, adhesions or scars. A deeper knowledge of the course of the peripheral nerves and their relationships with intermuscular septa, fibrous bands, muscle margins, and interneural planes is therefore crucial to understand how and where nerve compression can occur [1]. This study shows the causes and possible sites where RN compression can occur. We will discuss, working from proximal to distal, the following critical sites.
Lateral head of the triceps brachii muscle & lateral intermuscular septum
Different anatomical variants, mainly in the LHTB muscle and LIMS, have been observed to interfere with the normal course of the nerve and cause entrapment neuropathy [12]. Building on these observations, our study found, in 53% of the specimens, a fibrous arch coming off the LHTB muscle, similar to other studies that found this in 40% of specimens [17]. The RN glides below this arch to enter the longitudinal aponeurosis of the LHTB. This aponeurosis may be generated as we described previously, either from the LHTB muscle itself or from the LIMS, or both (Fig. 4). Division of the LIMS in the posterior compartment of the arm was found in 100% of the specimens. These divisions reinforce the fascia, and may have a protective role for the nerve, as well as being points of insertion of the surrounding muscles. We observed that with greater densification of the fascia there was greater muscular insertion. Several studies support this theory and observe that the RN can be trapped either by the muscular fibres of the LHTB muscle [18–20]) or by a fibrous arch [10, 12]. These arches are of variable tightness, making them capable of creating local compression [10].
The mean thickness of the LIMS where the radial nerve pierced it was 1.1 mm, similar to the 1.0 mm thickness reported by Tubbs et al [21]. The present study also aimed to determine the relationship between LIMS thickness and RN diameter as a possible indicator of nerve damage. We observed that with an LIMS thickness > 1.1 mm there was an increase of 0.4 mm in RN diameter, both at the space between the B/BR muscles and at the SA, compared with those with an LIMS < 1.1 mm (Table 3). It could therefore be suggested that the RN diameter may be due to epineurial damage when it passes through a thicker LIMS. Likewise, it was observed that, when the LIMS was > 1.1 mm, the difference between the RN diameter at the level of the LIMS and that at the space between B and BR was 0.2 mm larger distally, when it would have been expected to get smaller (Table 3). Unfortunately, possibly as a consequence of the small sample size, non-significant differences were found between the two categories of LIMS thickness. Nonetheless, reinforcing the concept of entrapment by the LIMS, other authors have described that the RN is anatomically tethered in the brachium by the LIMS and has limited excursion compared with the median and ulnar nerves [22]. It seems possible then that the adjacent muscles could create traction in the fibrous canal leading to a smaller lumen [12]; in our opinion, this would be mainly be due to contraction of the muscles of the posterior compartment of the arm.
Table 3
RN Diameter (mm) regarding LIMS thickness
| LIMS > 1.1 mm | LIMS < 1.1 mm |
Before LIMS | 2.9 ± 0.3 | 2.8 ± 0.3 |
After LIMS | 2.8 ± 0.3 | 2.9 ± 0.2 |
BR-B | 3.0 ± 0.5 | 2.7 ± 0.4 |
SA | 2.3 ± 0.5 | 1.9 ± 0.2 |
Abbreviations: LIMS, lateral intermuscular septum; B, brachialis muscle; BR, brachioradialis muscle; SA, supinator arch. Values are presented as mean (standard deviation) |
Space between brachialis and brachioradialis muscles
In this space, some authors describe a different entrapment at the origin of the BR muscle, a tight angle of the BR muscle fibers compressing the nerve against the humerus [6]. Our findings suggest predisposition to entrapment may be affected by the type of tissue surrounding the RN as it glides through this space. We observed that in 79% of the specimens, there was a close relationship between the RN and the epimysium of BR. However, this could also be interpreted as a form of stabilizing the nerve, to avoid nerve displacement with the flexion-extension movements of the elbow. In the remaining 21%, where the relationship between the B and BR muscles was vascular or muscular, this could also predispose to nerve compression. RN compression between the B and BR muscles can also induce a sensory-motor radial deficit [13]
Supinator arch (Arcade of Fröhse)
The SA is the most common compression site of the RN motor branch (6). In some cases, however, compression may occur due to the medial edge of the extensor carpi radialis brevis muscle or adhesions binding the nerve to the capsule of the radio-humeral joint [23]. As Roles also found, we saw that the extensor carpi radialis brevis muscle may be involved in compression, but more attention must be paid to the direct relationship of the nerve with the aponeurotic arch generated by the lateral septum of the forearm (Fig. 8). Moreover, in this region, posterior interosseous nerve compression may coexist with lateral epicondylitis [1].
With movement of the upper limb, the RN glides and varies its tension in the areas of interest that we studied. As the elbow is flexed from 0º to 90º, the excursion of the RN is approximately doubled [22]and strain increases by approximately 15% or more [16]. Based on these features, it may be concluded that people with a lot of muscle exertion and/or professions that involve a repetitive movement at the elbow are predisposed to peripheral RN neuropathy [6, 10, 12, 20].
Therefore, any factor that limits excursion at these points could cause repetitive traction of the nerve that may manifest as pain [16]. This study demonstrates that the possible mechanical limiting factors to bear in mind would be connective tissue, or fascial elements that surround the nerve, and that the fascia (as generalized connective tissue including epimysium and perimysium, capable of proprioception and nociception) [24] is a plausible source of pain.
This study provides a description with ultrasound guidance to help identify the RN at different points. Further clinical studies are necessary to demonstrate that the manual treatment of the fascia and connective tissue, at these points, could improve RN neuropathy.