Adult brachial plexus injuries frequently lead to significant physical disabilities. It could be caused by various mechanisms including falls, powerful sports activities, and motor vehicle trauma. Different mechanisms of injury lead to different injury patterns with different levels of plexus damage (1). Since severe injuries often lead to permanent disabilities, prevention of the injury is significant. Investigating the mechanism of the injury by biomechanical approaches should provide aid with regard to preventing injuries.

However, there are no reports of a complex three-dimensional finite element model (3D-FEM) of the spine, dura mater, root, and the brachial plexus all in one model till date. The first objective of this study is to construct a complex 3D-FEM of the spine, dura mater, root, and the brachial plexus. In this study, to assess the validity of the model, we analyzed the patterns of clinically reported brachial plexus injuries by applying stress to the model and verified weather the results of the analysis and clinically reported results correspond. Secondly, we inspected the proposed model's availability in studying the prevention of brachial plexus injury.

Figs. 3–6 illustrates the distribution and size of strain applied to the brachial plexus in each case. In case 1, the strain was focused on the root of C5. Contact of the root and the roof of the intervertebral foramen were observed that led to an increase of strain in one spot. The maximum level of strain was 21% (Fig. 3). In case 2, the strain was focused on the root of C5 and the upper trunk of the plexus. Lateroflexion of the cervical led to a stretch of the upper trunk of the brachial plexus, applying stress diffusely throughout the upper trunk. The maximum level of strain was 15% (Fig. 4). In case 3, no focus of strain was observed and the stress was applied diffusely throughout the brachial plexus (Fig. 5). In case 4, strain was focused on the lower trunk of the brachial plexus and the roots of C7 and C8. Abduction of the upper limb led to a stretch of the middle and lower trunk, with strain strongly focused in the middle and lower trunk region. The maximum level of strain was 16% (Fig. 6).

Injuries of the brachial plexus frequently lead to significant physical disabilities, psychological distress, and socioeconomic hardship. Excluding brachial plexus injuries in infants during delivery, adult brachial plexus injuries predominantly occur in young men or boys with an average age of 20 to 30 years (1,2,12–14). Injuries could be caused by various mechanisms, including penetrating injuries, falls, powerful sports activity, and motor vehicle accidents. In closed injury, pathological types of injury include avulsion of the nerve at root level or rupture or significant stretch at various levels of the brachial plexus (1,7).

Doi et al have reported successful outcomes with double free muscle transfer against complete avulsion of the brachial plexus (14). Bertelli et al have reported satisfactory results in abduction of the shoulder with spinal accessory nerve transfer to the suprascapular nerve in patients with complete brachial plexus injury (15). However, even though several surgical treatment options exist, perfect recovery after complete brachial plexus injury is impossible and physical difficulty is inevitable even after surgery (14–16). Therefore, prevention of the injury becomes important. Analyzing the mechanism of injury could provide further knowledge for preventing the injury.

Considerable number of brachial plexus injuries are caused by closed trauma (77–91%) (12,17). Nerve injury in these cases is from traction and compression, with traction accounting for 95% of closed injuries (1). Following traction, any combination of avulsion, rupture, or significant stretch might occur throughout the brachial plexus. However, certain patterns of injuries seem to be substantially prevalent. The supraclavicular region is affected considerably frequently (70–91%) than the retroclavicular or infraclavicular regions (1,2). The roots and trunks are considerably commonly injured compared to the divisions, cords, or branches. Clinically, traction to the brachial plexus occurs when the head and neck are violently moved away from the ipsilateral shoulder or when the upper limb is abducted violently over the head with significant force. The former results in injury to the upper elements of the brachial plexus (C5, C6 roots or upper trunk), and the latter results in injury to the lower elements (C8, T1 roots or lower trunk) (1,7,13). Similar results were reported in cadaver experiments (7). It is assumed that panplexal injuries occur when the force of injury is significantly high. Bertelli et al reported the frequency of panplexal injury, upper brachial plexus injury, and lower brachial plexus injury within supraclavicular injuries were 50, 47, and 3% respectively (2).

Based on this prior knowledge, we conducted stress analysis using 3D-FEM of the brachial plexus. We conducted four patterns of stress based on the patterns of mechanisms of injury described clinically. Nishida et al have reported a 3D-FEM of the spinal cord to conduct stress analysis of various clinical conditions, such as ossification of the posterior longitudinal ligament (OPLL) and cervical spondylotic myelopathy (CSM) (18,19). Imajo et al have constructed 3D-FEM of the spinal column in different studies (3). However, there are no reports of a complex 3D-FEM of the spine, dura mater, root, and the brachial plexus all in one model till date. The overall objective of this study was to develop a complex 3D-FEM of the spine, dura mater, root, brachial plexus, and other components that stimulate the clinical situation appropriately.

In previous studies of 3D-FEM of the spinal cord by Nishida et al and Kato et al, the mechanical property of a bovine spinal cord was used in the model since it was impossible to obtain fresh human spinal cord (18–20). Li and Dai noted that it was reasonable to employ the mechanical properties of bovine spinal cord because the brain and spinal cord of cattle and humans demonstrate similar injury changes (21). In this study, for the spinal nerve root, we used the mechanical property of a porcine nerve root because it was impossible to obtain fresh human spinal nerve root and there were no previous reports that obtained mechanical property of a bovine spinal nerve root. We believe this is reasonable because Olmarker et al reported that human and porcine spinal nerve roots demonstrated significant resemblance in both neural and vascular anatomy (22).

In our analysis results, retroflexion and lateroflexion of the cervical, simulating a clinical situation of moving the head and neck away from the shoulder, resulted in the focus of strain in the upper trunk and the roots of C5 and C6. From these results, traction of upper region of the brachial plexus is suggested that corresponds to the clinical findings as reported. Abduction of the upper limb resulted in focus of strain in the lower trunk and the roots of C7 and C8. These results, suggesting traction of lower region, also corresponds to clinical findings. Rotation of the cervical resulted in no focus of strain. We believe this result is appropriate because rotation of the neck applies less force of traction to the nerves compared to the other three situations. Overall, we believe that the validity of this model was demonstrated. From our results, discussing approaches of protection from strong force leading to retroflexion or lateroflexion of the neck, or abduction of the upper limb might be acceptable strategies of preventing brachial plexus injuries.

There are certain limitations in this study. Firstly, since there is no knowledge of threshold value of strain when symptoms of brachial plexus injury appear, a quantitative evaluation using the model was impossible. Thus, this is a qualitative study. Subsequently, we could not evaluate upper limb abduction further than 30°. To express further abduction, we require reproducing the upward rotation of the scapula and the motion of other components of the shoulder which is a future task of this model. Reproducing other tissues around the brachial plexus and the cervical spine, such as muscles and vessels might render the model considerably precise and substantially accurate results might be expected. Finally, the lack of assessment of blood flow of the nerves is a major limitation, because loss of blood flow could be a cause of neurological disorder. Significant further tests might include examination of a considerably complicated combination of motion of the cervical and the upper limb and applying further stress to examine the type of motion that leads to panplexal brachial plexus injury with the smallest amount of stress.

Although certain limitations are mentioned above, overall in this study, we were able to illustrate the pathology of brachial plexus injuries with this complex 3D-FEM. The results of the analysis supported the conventional clinical reports. We believe that this model has a potential of being used in various analysis in the future, including studies to prevent brachial plexus injuries.

We constructed a complex 3D-FEM of the spine, dura mater, spinal nerve root, and the brachial plexus all in one model. Retroflexion and lateroflexion of the cervical resulted in upper region brachial plexus injury and abduction of the upper limb resulted in lower region brachial plexus injury.

We were able to illustrate certain pathology of brachial plexus injury, demonstrating the validity of this model. There is a future potential of using this model for analyzing pathological conditions including the spine and spinal nerve roots.