Mechanisms & Levels of Injury

Avulsion, Ganglion, Roots

Mechanisms of Injury

Stretching injuries are the more frequent lesions of brachial plexus since motorcycle accidents is the main etiology in the majority of cases.
Usually the violent shock of the head against the ground provokes a sudden and abnormal augmentation of the angle formed by the cervical spine and the shoulder. In this situation the brachial plexus, more frequently at the level of the spinal foramina, is stretched and depending from different factors a variety of nerve lesions can occur.

The nerve lesions vary from a simply stretching without a real rupture to a complete avulsion of the roots from the spine. In the same plexus we can find an association of rupture and avulsion lesions at different root level.
The retaining system of roots at the foramen is stretched to the point that his rupture leads to a direct stretching to the nerve root itself. When no retaining is present the roots can be avulsed from the spine (see figure below).

High velocity accident, associated bone and vascular trauma may aggravate the nerve lesions.
Generally the upper roots have stronger protective anatomical structure as compared to lower roots and this can explain, in total palsies, the quite constant avulsion of lower roots from the spine as compared to upper roots more frequently ruptured outside the spine.

Frequently the accidental impact of the upper limb with the obstacle can provoke severe bone lesion at the level of the shoulder and the nerve lesion will be situated more distally from the spine, at the level of the cords. The association with scapula, clavicle, and humerus fractures is very frequent in these type of lesions. This fact leads to the possibility of a double level of nerve lesions that can complicate either the diagnosis and the surgical treatment. Less frequently the plexus can be directly damaged by a cutting mechanism which is more likely provoked by a work accident or by voluntary infliction. In these cases the nerve lesions will be less extended even if the frequent association with vascular lesions (subclavian and/or axillary artery) can seriously aggravate the prognosis rendering more urgent the surgical treatment.

Level of Injury

Faced to a traumatic brachial plexus lesion the questions we must answer to are the following:

  • Which are the damaged roots?
  • Which is the level of lesion? (pre or postganglionic?)
  • Is there a double level lesion?

Which part of the plexus has been damaged?

The answer to the first question is quite easy after a correct clinical examination. Even if in every roots the nerve fibres destined to the distal function are variably mixed, in a schematic way we can assess that:

  • C5 root prevalently gives fibres destined to shoulder function
  • C6 root gives fibre destined to elbow flexion
  • C7 root to elbow and wrist extension
  • C8 root to flexors of the wrist and fingers
  • T1 root to intrinsic muscles of the hand

After a correct clinical examination we are generally able to assess the level of lesions as supraclavicular (root’s level) or infraclavicular (cord level) or more distal (nerves)
We will classify the supraclavicular palsies as:

  • C5 C6 palsy
  • C5 C6 C7 palsy
  • Total palsy (all the roots involved)
  • Less frequently we can find lesions of all the roots except T1 or isolated lesions of the lower roots (C8 and T1)

We will classify the infraclavicular palsies, that is lesions located under the clavicle into lateral cord, posterior cord or medial cord lesions. In many occasions as already said, the lesions are combined retro and infraclavicular, with more cords involved and with possible double level of nerve damage.
The more frequent involved nerve lesions associated to roots and/or cord lesions are: musculocutaneous, axillary, suprascapular and radial nerves.

Which is the level of root’s lesion?

This is a crucial point in BP palsies since from this can depend the possibility and the strategy of repair and of course the quality of functional results we can obtain from the repair itself. The roots can be damaged outside the foramina (postganglionic or rupture lesions) or inside (preganglionic or avulsion lesions). The presence of root's stump will allow a repair of the nerve gap with grafts, while the absence of suitable stumps (avulsion) will require nerve transfer (neurotizations). The diagnosis of pre or postganglionic lesions could be clinical even if, as we will see, MRI will help in a substantial way in definition of the real level of lesion.

The upper roots are less frequently avulsed as compared to lower roots. This fact depends on anatomical conditions that allow a better defence from traction injuries of the upper roots while lower roots lack of this type of anatomical structures.

MRI image of avulsion of C8 root
MRI image of avulsion of C8 root