Deep branch of the radial nerve: effect of pronation/supination on longitudinal nerve alignment

The deep branch of the radial nerve (DBRN) is one of the major divisions of the radial nerve in the elbow region. DBRN entrapment occurs primarily at the level of the superior arcade of the supinator muscle (SASM), which is also known as the arcade of Frohse [1]. An association between DBRN entrapment, SASM, and repetitive rotational forearm movements has been previously proposed [1‐4]. Paralytic and painful entrapment syndromes of the DBRN, i.e., posterior interosseous nerve (PIN) and radial tunnel syndromes, have been associated with sports activities and heavy manual work requiring forceful and repetitive forearm pronation and supination with the elbow extended [5]. PIN syndrome has been associated with sports, such as bodybuilding/weightlifting, frisbee, gymnastics, tennis/racquetball, and swimming [6], and has been reported in violinists, orchestral conductor, bartender, corsetiere, and dairy workers [7]. In addition, maximal pronation has been shown to increase pressure on the nerve beneath the SASM and the supinator in cadavers [5].

Several imaging methods can be applied to assess peripheral nerves. The role of ultrasonography has grown in the last decades with the advent of higher-frequency transducers and software improvements [8, 9]. High-resolution ultrasonography (HRUS) is now considered the first-line imaging modality for peripheral nerve evaluation because of, but not limited to, the high spatial resolution for visualization of tiny nerves, its cost, the ease of exam for the patient, and the capability of real-time dynamic images [10, 11]. However, a knowledge gap persists for several aspects of the neurodynamics of the DBRN by non-invasive imaging methods, including HRUS.

Our study aimed to evaluate how the extremes of rotational movement of the forearm affect the longitudinal alignment of the DBRN at the SASM by using HRUS. We investigated alterations in the long axis configuration of DBRN by measuring the angulation of the nerve. The hypothesis is that the DBRN has a more pronounced convergence towards the SASM when the forearm is in maximal pronation compared to maximal supination. We aim to understand the neurodynamics of the DBRN by imaging and to correlate the findings with the pathophysiology of the entrapment syndromes.

This study evaluated the effect of maximal pronation and maximal supination of the forearm on the longitudinal alignment of the DBRN at the most frequent site of nerve impingement. We identified patterns of the DBRN deflection at the level of the SASM and explored our initial hypothesis regarding nerve alignment in pronation. Our results demonstrated DBRN convergence towards the SASM in maximal pronation in more than 75% of the participants and divergence in maximal supination in more than 65%. In addition, we identified that the DBRN has, on average, a ROM of approximately 7° of deflection in the long axis.

Traditional US markers for peripheral nerve abnormalities include cross-sectional area, nerve swelling ratio, side-to-side cross-sectional area asymmetry, and changes in echogenicity [9, 13]. However, nerve neurodynamic is gaining recognition as an essential part of imaging evaluation, in addition to nerve morphology and structural changes [9, 14, 15]. Despite that, few reports evaluated the neurodynamic of the radial nerve and its branches with HRUS [16‐18]. Martinoli et al. commented that the PIN (described as DBRN in our study) appeared to follow an angulated course at the arcade of Frohse, which should not be mistaken for a pathologic finding on ultrasound [16]. We agree with this statement, and in our study, we went further, quantifying the nerve’s angulation and demonstrating the existence of different patterns of nerve deflection with forearm rotation. Chen and Wang demonstrated the clinical application of forearm pronation and supination with HRUS in a case report of supinator syndrome [17].

Therefore, to the best of our knowledge, we were the first prospective observational study to investigate by imaging and in an in vivo human cohort the long-axis angulation of the DBRN in the SASM. It is tempting to hypothesize that a higher deflection angle would have implications on DBRN entrapment. However, considering several confounders, our study design does not support this assumption. For future studies, interesting insights can be provided by comparing the results of our normal population with a group of normal athletes and patients with DBRN entrapment syndrome.

Despite that, another hypothesis can be made based on our data, which may shed light on the pathophysiology of the DBRN entrapment syndromes. We consider the concept proposed by Shacklock that joint angulation increases the length of the nerve bed on the side of the axis of rotation that opens [19] and the engineering theory that when an isotropic rod bends, the outside material is tensioned and the inside is compressed [20]. We hypothesize that the DBRN lengthens in most cases on the deep side (furthest from the SASM) in pronation and on the superficial side (closer to the SASM) in supination. Therefore, both sides of the nerve will be under some degree of tension with full forearm ROM, particularly under the extremes of sports activities, such as baseball pitching, racket sports, and overhead throwing in football. Knowledge of the neurodynamics may have implications in sports practice and training programs. For instance, our data support injury prevention by reducing the number of repetitions of forearm pronation and supination, which reduces friction and tension on the deep and superficial surfaces of the DBRN. Considering our study and the bulk of information in the literature [2, 5], special attention should be given to the reduction of overload in pronation during athletic training regimens.

Factors traditionally associated with entrapment of the DBRN are the arcade of Frohse, i.e., a SASM with tendinous consistency as opposed to muscular or membranous, and the increased pressure under the SASM in pronation [1‐5]. However, more than this is needed to fully explain the dynamic nature of the entrapment of the DBRN at the level of the SASM. For instance, according to a recent meta-analysis, the arcade of Frohse is commonly found in adults, with a pooled prevalence of approximately 66% in this population [1]. On the other hand, increased pressure under the SASM seems to be a physiological finding, like the increased intrathoracic pressure observed with the Valsalva maneuver [21]. We speculate that there is a multifactorial nature of the dynamic entrapment of the DBRN, with our findings representing the missing piece of the puzzle. In summary, we believe that to trigger clinical symptoms of entrapment, in addition to anatomical (arcade of Frohse) and physiological factors (increased pressure in pronation at the SASM level), supraphysiological biomechanical stress on the nerve (repetitive flexion and friction) must be added, increasing the local level of neuroinflammation.

In our normal population, systemic biometric features, sports activities with the upper limb, hours of computer work, and forearm rotation during work showed no correlation with the DBRN angles and the net ROM. However, some characteristics of our population should be considered for comparison with future studies. Although most of our participants (61.8%) were engaged in sports activities with the upper limb, none were at the professional level or in preparation for a competition. In addition, rotational movements at work were reported by a minority of participants (25.4%), and in no case was there any association with continuous and repetitive heavy activities or the use of machinery. The majority of participants, i.e., 92.7%, were right-handed, a proportion very close to that estimated for the general population (81.9 to 90.7%) [22]. Hand dominance and muscle tension may have accounted for more positive deflections in the dominant right side in supination compared to the left side.

Our study had a very good inter and intraobserver agreement in assessing the DBRN angle, which we attributed to several factors, such as readers’ experience and ROI selection. Despite that, we observed a few discrepancies in the angle measurements, illustrating potential pitfalls with this technique. These challenges were related to a low amount of echogenic perineural fat associated with muscle hypertrophy (N = 3), adjacent vessel simulating the nerve path (N = 1), and nerve branching (N = 1). The analysis can be facilitated in the real world with the use of a cine clip to review the images, by correlation with the short axis of the nerve, and with the aid of Doppler. In addition, knowledge of positional changes in the DBRN alignment is important for the radiologist to avoid pitfalls during imaging interpretation and interventional procedures.

The study has several limitations. First, we did not consider the longitudinal excursion of the nerve with movements of other parts of the body [19, 23]. However, to mitigate the longitudinal nerve excursion, we partially immobilized the participants’ hands and wrists in the neutral position and oriented them to keep their heads straight, avoiding side rotation. It is worth mentioning that DBRN excursions can also occur in the transverse plane, and other nerve deflections may happen with movement. However, their dedicated analysis is out of the scope of our study. These other components of DBRN neurodynamics were at least partially incorporated into our angle analysis, as the transducer was positioned in a way to visualize the longest longitudinal segment of the nerve, regardless of forearm position and positional nerve changes. Second, our population comprised South Americans, primarily Brazilians, and, therefore, results may vary in different populations, with different biometrical features, social behaviors, and distinct physical, work, and sports activities. Third, the nerve was only assessed at the SASM, and the neurodynamics of the nerve at other areas have also been implicated in entrapment syndromes. Furthermore, the ROI used in the study focused on one specific nerve area, but other regions of the nerve and adjacent structures may have an additional role in the angulation and entrapment of the DBRN [24].

In conclusion, our study provides additional information on the neurodynamics of the DBRN at the level of the SASM. A dominant pattern of angulation of the DBRN with a more convergent deflection in maximal pronation and divergent deflection in maximal supination was observed. Knowledge of the patterns of positional changes of the DBRN is important for the radiologist during imaging interpretation and interventional procedures. It may also have implications in sports practice and training programs.