Anatomical variations of the sciatic nerve, in relation to the piriformis muscle

Sciatic Nerve and Its Anatomical Variations: In-Depth Understanding Acquired During Dissection Classes

Knowledge of anatomical variability is extremely important in order to better understand the etiology of pain, if present, or to avoid iatrogenic consequences. Sometimes the anatomical "anomalies" have the same anamnesis but different causes. For example, sciatic neuralgia may be caused by a herniated disc or it may have a different origin. The sciatic nerve (SN), also known as the ischial nerve, is the widest in the human body. This huge peripheral nerve originates from the roots of the lumbosacral plexus (L4-S3) and passes through the great sciatic foramen, under the piriformis muscle (PM). However, there is much variability in the pattern of SNs about the muscle, which has been known since the first half of the 20th century. In the present study, we describe six different case reports of anatomical variations of the SN and its interplay with the PM. The observations were made during dissection classes at the ICLO Teaching and Research Centre (Verona, Italy), on both male and female cadavers aged between 58 and 84 years. The SN was reported as a single and divided nerve into the tibial nerve (TN) and the common peroneal nerve (CPN), passing alone above, below, or between the PM. However, the two parts of the SN may also interact with the PM in different ways, adding to the anatomical variability. A thorough knowledge of the anatomical variations in any part of the human body is extremely important. The various techniques used, from imaging to autopsy or surgery, are also useful in the SN pathway. Thus, the anatomical features and the understanding of each variation are useful for a correct approach that can lead to an effective and correct treatment with a favorable outcome.

Simultaneous Posterior Femoral Cutaneous Nerve and Sciatic Nerve Variations: A Case Report

During the routine dissection of a formalin fixed Caucasian cadaver, a previously unreported variation of the sacral plexus was found in the right gluteal region. The posterior femoral cutaneous nerve was found to pierce the piriformis muscle as opposed to running along its more common course below the muscle. At the same level of the posterior femoral cutaneous nerve, the common fibular nerve also pierced the piriformis muscle, while the tibial nerve passed inferior to the piriformis muscle. No other anatomical variations were found.

Sciatic Nerve Variants and the Piriformis Muscle: A Systematic Review and Meta-Analysis

The present systematic review and meta-analysis provides a comprehensive assessment of the sciatic nerve (SN) variants relative to the piriformis muscle (PM) and compares those variants' prevalence among different geographical populations with respect to gender and laterality. A database search was conducted to identify cadaveric studies pertinent to SN variants relative to the PM. A total of 44 articles were included. The typical morphological pattern (type A, with the SN passing undivided below the PM) was found to be the most common variant, with 90% pooled prevalence. SN variants were more common among East Asians, with a 31% pooled prevalence of total variants. No significant differences were established with respect to gender and laterality. In greater than 10% of the population, the SN coursed through or above piriformis. Patients' epidemiological characteristics may predispose them to certain variants. The common peroneal nerve (CPN) is more susceptible to injury during a total hip arthroplasty or a hip arthroscopy where anomalies are encountered. As anatomical variants are commonly associated with piriformis syndrome, they should always be considered during diagnosis and treatment.

Composite Anatomical Variations between the Sciatic Nerve and the Piriformis Muscle: A Nepalese Cadaveric Study

Piriformis syndrome is a rare syndrome which is one of the main causes of nondiscogenic sciatica causing severe low back pain due to entrapment of sciatic nerve either by the hypertrophy or by inflammation of the piriformis muscle. We have carried out dissection in 20 Nepalese cadavers. Out of 40 dissected gluteal regions, 37 exhibited typical appearance of sciatic nerve, piriformis muscle, and their relations resembling type-a, whereas 3 gluteal regions showed composite structural variations resembling type-b and type-c based on Beaton and Anson's classification. Knowledge pertaining to such variations will be helpful during a surgical intervention in the gluteal region and in turn reduces the risk of injuring these nerves which are more susceptible to damage. Our study reports such variations in Nepalese population which will be helpful during evaluation of the pain induction in various test positions and also useful for analysis of the range of the neurological deficiency in sciatic nerve neuropathies. The present study also explains the basis of the unsuccessful attempt of the sciatic nerve block during popliteal block anaesthesia.

Sciatic nerve and its variations: is it possible to associate them with piriformis syndrome?

ABSTRACT The sciatic nerve forms from the roots of the lumbosacral plexus and emerges from the pelvis passing inferiorly to the piriformis muscle, towards the lower limb where it divides into common tibial and fibular nerves. Anatomical variations related to the area where the nerve divides, as well as its path, seem to be factors related to piriformis syndrome. Objective: To analyze the anatomical variations of the sciatic nerve and its clinical implications. Methods: This was a systematic review of articles indexed in the PubMed, LILACS, SciELO, SpringerLink, ScienceDirect and Latindex databases from August to September 2018. Original articles covering variations of the sciatic nerve were included. The level of the sciatic nerve division and its path in relation to the piriformis muscle was considered for this study. The collection was performed by two independent reviewers. Results: At the end of the search, 12 articles were selected, characterized according to the sample, method of evaluation of the anatomical structure and the main results. The most prevalent anatomical variation was that the common fibular nerve passed through the piriformis muscle fibers (33.3%). Three studies (25%) also observed anatomical variations not classified in the literature and, in three (25%) the presence of a double piriformis muscle was found. Conclusion: The results of this review showed the most prevalent variations of the sciatic nerve and point to a possible association of this condition with piriformis syndrome. Therefore, these variations should be considered during the semiology of disorders involving parts of the lower limbs.

Accuracy and safety of dry needle placement in the piriformis muscle in cadavers

Objectives

The purpose of this anatomic investigation is to (1) establish accuracy of dry needle placement into the medial third of the piriformis muscle as it exits the pelvis from the greater sciatic notch in unembalmed cadaveric specimens, while avoiding puncture of the sciatic nerve, and (2) establish guidelines for dry needle length selection.

Methods

Dry needles were placed in nineteen unembalmed cadaveric posterior hips. Dissection of the posterior hip musculature was performed to confirm location of the needle. A binary decision (yes/no) was made to determine whether the needle reached the piriformis muscle, went through the piriformis muscle, and/or pierced the sciatic nerve. Additionally, mean adipose tissue thickness, gluteus maximus muscle thickness, and perpendicular distance from the needle to the exiting sciatic nerve were recorded.

Results

The needle reached the medial third of the piriformis in 16 out of 19 hips (84.2% accuracy) and never punctured the sciatic nerve. There was a fair (r = 0.493) and good (r = 0.759) correlation between the needle length and the mean fat thickness for the left and right hips, respectively.

Discussion

A physical therapist was able to use bony landmark palpation to locate the piriformis muscle and use estimated adipose tissue thickness to choose a sufficient needle length to reach the medial third of the piriformis muscle. While the needle placement technique was safe and no sciatic nerve puncture occurred, the proximity of the piriformis muscle to the sciatic nerve warrants caution during needle placement.

Level of Evidence

2c.