Clinical anatomy of the neurofascial sheath

Abstract

Background: The peripheral nerve blockade (PNB) and minimally invasive surgery are central to the “Fast Track Surgery” strategy which uses multimodal perioperative rehabilitation programs and aims to facilitate early discharge from the hospital and more rapid resumption of normal activities of daily living after surgery. The anatomical basis for the PNB is one that conceives of the nerve to be enclosed by a tubular fascial sheath, called the neurofascial sheath (NFS), into which the local anaesthetic (LA) solution is injected. The confinement of the LA in the NFS can prolong the effective time of anaesthetic and reduce drug dosage and complications. However, in the literature, the anaesthetic effects of the intra-NFS injection vary significantly and have shown significant regional variations.

The NFS is composed of the endoneurium, perineurium, epineurium and extraneural fascia, from innermost to outermost, respectively. It has been suggested that during the PNB, the optimal needle placement is in a potential space between the extraneural fascia and the epineurium. An intraneural injection is defined as the needle tip penetrates the epineurium. The intraneural injection may damage the nerve and thus is not recommended. However, some studies argue that the intraneural injection may not cause nerve injury but help to more precisely localise the injected LA. This debate is mainly because the origin and architecture of the NFS and its anatomical relationship among its sublayers and with the neighbouring structures remain unclear. For example, it is generally believed that the perineurium extends from the meninges of the brain or spinal cord and serves as the blood-nerve barrier to protect the axons from mechanical and chemical attacks. However, some recent studies found that both arachnoid and dura maters (parts of the meninges) do not extend beyond the skull base or vertebrae in the human, suggesting that the NFS may have different origins, particularly for the distal segment of a peripheral nerve. Several questions remain to be clarified. Does an intact NFS exist along a peripheral nerve? Where do the different sublayers of the NFS originate? Does the architecture of the NFS vary at different locations? What is the anatomical relationship between the NFS and its neighbouring structures? Therefore, the central hypothesis of this thesis is that the NFS of a peripheral nerve has a consistent fibrous configuration along its whole course.

Objectives: The overall aim of this thesis is to identify the origin and configuration of the NFS of various peripheral nerves. The specific objectives are to investigate the origin and configuration of the NFS of (1) the distal segment of the spinal nerve, (2) the proximal segment of the spinal nerve, and (3) the proximal (intracranial) segment of the cranial nerve.

Material and Methods: A total of 80 cadavers (39 females, 41 males; age range, 38-97 years) were used in this thesis. The coloured latex injection, micro-dissection, epoxy sheet plastination, confocal microscopy, 3-dimensional reconstruction methods were applied in the cadaveric studies. For Objective 1, a total of 64 volunteers and 45 patients were included in the observation and measurement of the femoral nerve and lateral femoral cutaneous nerve (LFCN), and the ultrasound-guided fascia iliac compartment block (FICB) or the femoral nerve block. The studies of this thesis were performed in accord with the institutional ethical guidelines and approved by the Human Research Ethics Committee of the University of Otago for the cadaver study only, the Medical Ethics Committee of Anhui Medical University and the Committee on Medical Ethics of the First Affiliated Hospital of Anhui Medical University, China, for both cadaver and living subject studies.

Results and discussion: Objective 1: The origin and configuration of the NFS of the femoral nerve and the LFCN were defined in the pelvis and upper thigh regions. In the pelvis, the fascia iliaca was originated from the peripheral fascicular aponeurotic sheet of the iliopsoas. The fascia iliaca compartment was a funnel-shaped adipose space between the fascia iliaca and the epimysium of the iliopsoas, had a superior and an inferior opening and contained the femoral nerve and the LFCN but not obturator nerve. In the upper thigh, the fascial lata was originated from the prolongation of the inguinal ligament and the newly identified “iliolata ligaments”. The fascia iliaca and the aponeurotic fibres of the transversus abdominis and the internal oblique abdominis formed a conjoint tendinous sheet under the inguinal ligament. The LFCN pierced the conjoint tendinous sheet and entered into an adipose compartment which was sandwiched in between the fascia lata and the epimysium of the sartorius muscle and then a ligamental canal which was formed by the iliolata ligaments. All these structures were detectable under ultrasound scanning in healthy volunteers. These findings endorse the classical FICB and indicate that the conjoint tendinous sheet can be used as a key ultrasound landmark for the ultrasound-guided FICB. The conjoint tendinous sheet and the ligamental canal of the iliolata ligaments are two major sites susceptible to the entrapment of the LFCN. Based on the fibrous origin and configuration of the NFS, the conjoint tendinous sheet, the sartorius muscle and the anterior superior iliac spine are recommended as the anatomical landmarks to identify and visualise the LFCN with ultrasound and a theoretical strategy for decompression of the LFCN is proposed.

Objective 2: The origin and configuration of the NFS of the proximal segment of the spinal nerve were revealed in the lumbar intervertebral foramina (IVF). The boundaries (particularly the lateral and medial boundaries) and subdivisions of the lumbar IVF were precisely defined. The inferior border of the upper pedicle and the dorsal root ganglion were used to subdivide the lumbar IVF into the supermedial extension, the superior compartment and the inferior compartment. The configuration and localisation of the neurovascular and adipose zones were different between the upper (IVFs L1-2 & L2-3) and lower (IVFs L3-4 & L4-5) lumbar spines. In the upper lumbar spine, the superomedial extension was occupied by the nerve roots and vessels within a dural sleeve and the venous plexus with sparse adipose tissue. The superior compartment was occupied by the nerve roots, dorsal root ganglion, vessels and adipose tissue. The inferior compartment contained a small anteroinferior adipose pocket and a large posterosuperior neurovascular zone and the venous plexus. In the lower lumbar spine, the adipose tissue almost entirely occupied the inferior compartment, and the neurovascular structures were limited within the superior compartment. The adipose zone was gradually tapered and rotated from the inferior compartment to the medial part of the superior compartment and further to the superomedial extension. The findings highlight differences of the fine 3D architecture of the NFS of the proximal spinal nerve and its relationship with surrounding vascular and adipose tissue between the upper and lower lumbar IVFs. The findings may contribute to optimise the surgical approaches through the IVF at different lumbar spinal levels and help to shorten the learning curve of the transforaminal operative techniques.

Objective 3: The origin and configuration of the NFS of the proximal (intracranial) segment of the trigeminal nerve (TN) and its relationship with Meckel’s cave (MC) were studied. The results showed: (1) the trigeminal ganglion was sandwiched in between the arachnoid and dural walls of MC, (2) the rootlets and divisions of the TN were enclosed in the centrally reflected arachnoid sleeves and the peripheral dural sheaths, respectively, and the peripheral dural sheath contributed to the epineurium and perineurium of the division but extended only up to the level of the extracranial exit of the nerves, and (3) the dural wall of MC originated from the meningeal dura and appeared as a half-crescent shape with a wide superficial-lateral edge and a thin deep-medial edge which merges with the endosteal dura of the middle cranial fossa. These findings reveal the fine architecture of the NFS of the TN and its relationship with MC and will contribute to a better understanding of the nature and growth pattern of the TN tumour, and may provide guidance on the precise surgical approach to TN in the future.

Conclusions: The findings of this study disagree with the central hypothesis of this thesis that the NFS of a peripheral nerve has a consistent fibrous configuration along its whole course. The origin and fibrous configuration of the NFS of a peripheral nerve vary in a region-dependent pattern. The results of this study will not only help to optimise PNB techniques, but also provide a better understanding of the minimally invasive surgical approach and the perineurial tumour spread pathway. The results also indicate that for any given peripheral nerve, the NFS of its different segments should be studied specifically in a region-dependent manner.