Detection and prevalence of variant sciatic nerve anatomy in relation to the piriformis muscle on MRI

Morphologic and morphometric bilateral analysis and sexual dimorphism in sciatic nerves of adult cadaveric specimens in Uganda

BACKGROUND

The Sciatic nerve (SN) exhibits distinct sex and side-related differences, which have significant implications for clinical practice. The study investigated the sex and side-related morphologic and morphometric variations of the nerve using cadavers.

METHODS

This is a cross-sectional cadaveric study involving 62 Ugandan cadavers. Continuous variables were reported using descriptive statistics and discrete variables were reported as percentages. Ordinary two-way ANOVA was used to compare the dimensions and proportion of the patterns of the SN.

RESULTS

The study identified six categories of exit patterns of the SN, type A ("Below and undivided") occurred in a majority of cases (62.9%). A penta-furcate branching pattern dominated the whole population. Bifurcate termination pattern was found in most SNs (90.3% and 87.1% for right and left limbs respectively) while the rest have the trifurcate termination pattern, with no side or sex-related variations. The average dimensions of SN were within normal ranges, and showed no side-related differences but with a sex difference (significantly higher in males than females), mean length of the SN in centimetres (length A: Males, right limbs = 30.58 ± 9.00; left limbs = 31.30 ± 6.20; Females, right = 26.07 ± 6.58; left = 26.30 ± 5.56). The difference in the length "A" for the males left limb and females right limb was statistically significant with a p-value of 0.0195.

CONCLUSIONS

Most of the examined SNs showed normal anatomical characteristics with rare cases of sex-related dimorphism in the termination level and morphometry (length and diameter) of the nerve. The observed sexual dimorphisms in sciatic nerves are of clinical and surgical interest; hence, suggesting the need for further investigations in different populations, especially using advanced techniques such as ultrasonography anatomic techniques.

MR Imaging of Entrapment Neuropathies of the Hip

Entrapment neuropathies of the hip (ENH) can occur due to a variety of causes with clinical symptoms that may mimic musculoskeletal disorders. Etiologies include entrapment in a fibromuscular canal, tethering due to posttraumatic fibrosis and extrinsic compression from muscle hypertrophy or a mass. Magnetic resonance (MR) imaging enables detection and characterization of peripheral nerve pathology. In addition, MR imaging can impact both diagnostic judgment as well as therapeutic management (nonoperative and operative management) of patients with ENH. This review article will summarize the role of MR imaging in detection, characterization, and management of nerve entrapments around the hip joint.

Ultrasonographic visualization of anatomical variations of the supraclavicular nerves

Typical anatomy of the supraclavicular nerve (SCN) is described as originating from the cervical plexus and dividing into medial, intermediate, and lateral branches. The SCN is vulnerable to injury during clavicular surgery, leading to altered sensation post-operatively. There is also increasing interest in anesthetizing the SCN in shoulder or clavicular surgery. Utilizing a high-frequency (20 MHz) ultrasound probe, 20 healthy volunteers were scanned, giving data for 40 SCNs. For each nerve, anatomical course and branches were graphically plotted using a custom Python 3.8.12 program and Microsoft Excel. Of 40 nerves, only 19 (47.5%) demonstrated a typical course, with the rest showing considerable variability of branching patterns. Crossing branches (CBs) were found in 24 (60%) with a total of 54. Just over half (29, 54.7%) of these crossed the clavicle lateral to its midpoint, with 32 (59.6%) CBs having a diameter of ≥25% compared to that of the SCN main trunk. The distance from the mid-clavicular point at which the branches crossed the clavicle was recorded. This study demonstrated that over half the SCNs had atypical branching patterns with intra-volunteer variability. Preoperative mapping may be useful in preventing injury and subsequent numbness.

Innervation patterns of hamstring muscles, including morphological descriptions and clinical implication

PURPOSE

The hamstrings muscles are innervated by sciatic nerve branches. However, previous studies assessing which and how many branches innervate each muscle have yielded discrepant results. This study investigated the innervation patterns of hamstrings.

MATERIALS AND METHODS

Thirty-five cadaver limbs were investigated. The average age of subjects was 78.6 ± 17.2 years, with 48.6% male and 51.4% female, while 57.1% were right limbs and 42.9% left. The sciatic nerve, hamstrings and associated structures were dissected. The number of nerve branches for each muscle and the level where they penetrated the muscle were recorded.

RESULTS

The sciatic nerve was connected by a fibrous band to the long head of the biceps femoris. This muscle was innervated by either one or two branches, which penetrated the muscle into its superior or middle third. The short head of the biceps femoris was innervated by a single nerve that usually penetrated its middle third, but sometimes inferiorly or, less commonly, superiorly. The semitendinosus was always innervated by two branches, the superior branch penetrating its upper third, the inferior mostly the middle third. The semimembranosus usually was innervated by a single nerve branch that penetrated the muscle at its middle or lower third. Four specimens revealed common nerves that innervated than one muscle.

CONCLUSIONS

We have characterized hamstring innervation patterns, knowledge that is relevant to neurolysis, surgery of the thigh, and other procedures. Moreover, a mechanical connection between the sciatic nerve and biceps femoris long head was identified that could explain certain neuralgias.

The Association between Anatomical Variants of Musculoskeletal Structures and Nerve Compressions of the Lower Limb: A Systematic Review and Meta-Analysis

Objective: The aim of this study was to describe the main anatomical variants and morphofunctional alterations in the lower limb that compress surrounding nervous structures in the gluteal region, thigh region, and leg and foot region. Methods: We searched the Medline, Scopus, Web of Science, Google Scholar, CINAHL, and LILACS databases from their inception up to October 2023. An assurance tool for anatomical studies (AQUA) was used to evaluate methodological quality, and the Joanna Briggs Institute assessment tool for case reports was also used. Forest plots were generated to assess the prevalence of variants of the gluteal region, thigh, and leg. Results: According to the forest plot of the gluteal region, the prevalence was 0.18 (0.14-0.23), with a heterogeneity of 93.52%. For the thigh region, the forest plot presented a prevalence of 0.10 (0.03-0.17) and a heterogeneity of 91.18%. The forest plot of the leg region was based on seven studies, which presented a prevalence of 0.01 (0.01-0.01) and a heterogeneity of 96.18%. Conclusions: This review and meta-analysis showed that, in studies that analyzed nerve compressions, the prevalence was low in the thigh and leg regions, while in the gluteal region, it was slightly higher. This is mainly due to the PM region and its different variants. We believe that it is important to analyze all the variant regions defined in this study and that surgeons treating the lower limb should be attentive to these possible scenarios so that they can anticipate possible surgical situations and thus avoid surgical complications.

Fibular (peroneal) neuropathy

Fibular neuropathy has variable presenting features depending on the site of the lesion. Anatomical features make it susceptible to injury from extrinsic factors, particularly the superficial location of the nerve at the head of the fibula. There are many mechanisms of compression or other traumatic injury of the fibular nerve, as well as entrapment and intrinsic nerve lesions. Intraneural ganglion cysts are increasingly recognized when the mechanism of neuropathy is not clear from the medical history. Electrodiagnostic testing can contribute to the localization as well as the characterization of the pathologic process affecting the nerve. When the mechanism of injury is unclear from the analysis of the presentation, imaging with MRI and ultrasound may identify nerve lesions that warrant surgical intervention. The differential diagnosis of foot drop includes fibular neuropathy and other neurologic conditions, which can be distinguished through clinical and electrodiagnostic assessment. Rehabilitation measures, including ankle splinting, are important to improve function and safety when foot drop is present. Fibular neuropathy is less frequently painful than many other nerve lesions, but when it is painful, neuropathic medication may be required. Failure to spontaneously recover or the detection of a mass lesion may require surgical management.

Neurovascular structures to avoid during musculoskeletal ultrasound-guided intervention in the extremities

Ultrasound is a useful modality for guiding injections and other procedures in the extremities. Its portability and the ability to adjust the probe and needle in real time, as well as lack of radiation exposure make it preferable for many routine procedures. However, ultrasound is highly operator dependent and a firm understanding of regional anatomy is important, including neurovascular structures that are in close proximity during many of these procedures. Knowledge of the characteristic location and appearance of neurovascular structures in the extremities allows safe needle advancement and prevention of iatrogenic complications.

Non-osseous considerations in diagnostic imaging for pelvic and acetabular trauma

High-quality imaging is crucial for orthopedic traumatologists in the evaluation and management of pelvic and acetabular fractures. Computed tomography (CT) plays an essential role in the diagnosis and treatment of patients with these complex injuries. A thoughtful evaluation of associated soft tissues can reveal additional details about the patient and their injury that may impact treatment. This review aims to highlight soft tissue findings that should be identified when evaluating the initial diagnostic imaging after pelvic and acetabular trauma.

Variant Sciatic Nerve Anatomy in Relation to the Piriformis Muscle on Magnetic Resonance Neurography: A Potential Etiology for Extraspinal Sciatica

OBJECTIVE

To assess the prevalence and clinical implications of variant sciatic nerve anatomy in relation to the piriformis muscle on magnetic resonance neurography (MRN), in patients with lumbosacral neuropathic symptoms.

MATERIALS AND METHODS

In this retrospective single-center study, 254 sciatic nerves, from 127 patients with clinical and imaging findings compatible with extra-spinal sciatica on MRN between 2003 and 2013, were evaluated for the presence and type of variant sciatic nerves, split sciatic nerve, abnormal T2-signal hyperintensity, asymmetric piriformis size and increased nerve caliber, and summarized using descriptive statistics. Two-tailed chi-square tests were performed to compare the anatomical variant type and clinical symptoms between imaging and clinical characteristics.

RESULTS

Sixty-four variant sciatic nerves were identified with an equal number of right and left variants. Bilateral variants were noted in 15 cases. Abnormal T2-signal hyperintensity was seen significantly more often in variant compared to conventional anatomy (40/64 vs. 82/190; p = 0.01). A sciatic nerve split was seen significantly more often in variant compared to conventional anatomy (56/64 vs. 20/190; p < 0.0001). Increased nerve caliber, abnormal T2-signal hyperintensity, and asymmetric piriformis size were significantly associated with the clinically symptomatic side compared to the asymptomatic side (98:2, 98:2, and 97:3, respectively; p < 0.0001 for all). Clinical symptoms were correlated with variant compared to conventional sciatic nerve anatomy (64% vs. 46%; p = 0.01).

CONCLUSION

Variant sciatic nerve anatomy, in relation to the piriformis muscle, is frequently identified with MRN and is more likely to be associated with nerve signal changes and symptomatology.

Determination of a neurologic safe zone for bicortical S1 pedicle screw placement

BACKGROUND CONTEXT

Lumbosacral fixation is commonly used for the management of lumbosacral instability. As the sacrum mainly consists of cancellous bone, bicortical fixation, in which the pedicle screw penetrates the anterior sacral cortex, can help increase the strength of fixation. However, this method carries a risk to the L5 nerves which lie anterior to the sacrum at this level.

PURPOSE

The goal of this study is to determine a safe zone for the placement of S1 pedicle screws to decrease the likelihood of L5 nerve injury.

STUDY DESIGN

Retrospective imaging review.

PATIENT SAMPLE

This study evaluated imaging data of patients who underwent lumbar spine magnetic resonance imaging (MRI) at our institute between September 1, 2020 and September 1, 2021.

OUTCOME MEASURES

T1-weighted axial MRIs were measured at the level of S1 pedicle screw placement. The space medial and lateral to the L5 nerve root on the anterior sacrum were measured and defined as safe zones. Additionally, the nerve width and sacral lengths were measured at this level.

METHODS

The distribution of the measurements were evaluated to determine a medial and lateral safe zone, as well as the average nerve width at the level of S1 pedicle screw placement. Correlation analysis was performed to determine a relationship between safe zone sizes and sacral size.

RESULTS

A total of 400 MRIs were analyzed. The average medial safe zone measured was 32.8 mm (95% CI: 32.2-33.4) with no nerves lying within 22.3 mm of the midline sacrum. The average lateral safe zone measured was 17.7 mm (95% CI: 17.1-18.2), with no nerves within 5.3 mm of the lateral border of the sacrum. The average nerve root width was 6.2 mm (95% CI: 6.13-6.34). An increased sacral length was associated with a larger medial (p<.001) and lateral (p<.001) safe zone.

CONCLUSIONS

Our study revealed lateral and medial safe zones for the placement of S1 pedicle screws to avoid iatrogenic nerve injury in a retrospective cohort of 400 patients. There were no L5 nerve roots found within 22.3 mm of the sacrum's mid-axis or within 5.3 mm of the sacrum's anterolateral border. These defined safe zones can be used during pedicle screw planning and placement to decrease the risk of injury to the L5 nerve root.

Novel anatomical findings with implications on the etiology of the piriformis syndrome

PURPOSE

The cause of the piriformis-related pelvic and extra-pelvic pain syndromes is still not well understood. Usually, the piriformis syndrome is seen as extra-pelvic sciatica caused by the entrapment of the sciatic nerve by the piriformis in its crossing through the greater sciatic foramen. However, the piriformis muscle may compress additional nerve structures in other regions and cause idiotypic pelvic pain, pelvic visceral pain, pudendal neuralgia, and pelvic organ dysfunction. There is still a lack of detailed description of the muscle origin, topography, and its possible relationships with the anterior branches of the sacral spinal nerves and with the sacral plexus. In this research, we aimed to characterize the topographic relationship of the piriformis with its surrounding anatomical structures, especially the anterior branches of the sacral spinal nerves and the sacral plexus in the pelvic cavity, as well as to estimate the possible role of anatomical piriformis variants in pelvic pain and extra-pelvic sciatica.

METHODS

Human cadaveric material was used accordingly to the Swiss Academy of Medical Science Guidelines adapted in 2021 and the Federal Act on Research involving Human Beings (Human Research ACT, HRA, status as 26, May 2021). All body donors gave written consent for using their bodies for teaching and research. 14 males and 26 females were included in this study. The age range varied from 64 to 97 years (mean 84 ± 10.7 years, median 88).

RESULTS

three variants of the sacral origin of the piriformis were found when referring to the relationship between the muscle and the anterior sacral foramen. Firstly, the medial muscle origin pattern and its complete covering of the anterior sacral foramen by the piriformis muscle is the most frequent anatomical variation (43% in males, 70% in females), probably with the most relevant clinical impact. This pattern may result in the compression of the anterior branches of the sacral spinal nerves when crossing the muscle.

CONCLUSIONS

These new anatomical findings may provide a better understanding of the complex piriformis and pelvic pain syndromes due to compression of the sacral spinal nerves with their somatic or autonomous (parasympathetic) qualities when crossing the piriformis.

The absence of piriformis muscle, combined muscular fusion, and neurovascular variation in the gluteal region

The gluteal region contains important neurovascular and muscular structures with diverse clinical and surgical implications. This paper aims to describe and discuss the clinical importance of a unique variation involving not only the piriformis, gluteus medius, gluteus minimus, obturator internus, and superior gemellus muscles, but also the superior gluteal neurovascular bundle, and sciatic nerve. A routine dissection of a right hemipelvis and its gluteal region of a male cadaver fixed in 10% formalin was performed. During dissection, it was observed a rare presentation of the absence of the piriformis muscle, associated with a tendon fusion between gluteus and obturator internus, and a fusion between gluteus minimus and superior gemellus muscles, along with an unusual topography with the sciatic nerve, which passed through these group of fused muscles. This rare variation stands out with clinical manifestations that are not fully established. Knowing this anatomy is essential to avoid surgical iatrogeny.

Reunification of a split sciatic nerve

Sciatic nerve (SN) variaitons can result in interesting clinical presentations. We identified a SN variant that does not fit into preexisting classification schemes. In an adult male cadaver, the SN was found to divide proximally and partly exit through the piriformis muscle. Distal to the piriformis, the two parts of the SN were reunited. Although apparently extremely rare, such a finding should be added to the archives of anatomical variations.

Deep gluteal syndrome as a cause of posterior hip pain and sciatica-like pain

Deep gluteal syndrome is an increasingly recognized disease entity, caused by compression of the sciatic or pudendal nerve due to non-discogenic pelvic lesions. It includes the piriformis syndrome, the gemelli-obturator internus syndrome, the ischiofemoral impingement syndrome, and the proximal hamstring syndrome. The concept of the deep gluteal syndrome extends our understanding of posterior hip pain due to nerve entrapment beyond the traditional model of the piriformis syndrome. Nevertheless, there has been terminological confusion and the deep gluteal syndrome has often been undiagnosed or mistaken for other conditions. Careful history-taking, a physical examination including provocation tests, an electrodiagnostic study, and imaging are necessary for an accurate diagnosis. After excluding spinal lesions, MRI scans of the pelvis are helpful in diagnosing deep gluteal syndrome and identifying pathological conditions entrapping the nerves. It can be conservatively treated with multidisciplinary treatment including rest, the avoidance of provoking activities, medication, injections, and physiotherapy. Endoscopic or open surgical decompression is recommended in patients with persistent or recurrent symptoms after conservative treatment or in those who may have masses compressing the sciatic nerve. Many physicians remain unfamiliar with this syndrome and there is a lack of relevant literature. This comprehensive review aims to provide the latest information about the epidemiology, aetiology, pathology, clinical features, diagnosis, and treatment. Cite this article: Bone Joint J 2020;102-B(5):556-567.

Update in the evaluation of peripheral nerves by MRI, from morphological to functional neurography

Imaging studies of peripheral nerves have increased considerably in the last ten years. In addition to the classical and still valid study by ultrasound, new neurographic techniques developed from conventional morphological sequences (including 3D isotropic studies with fat suppression) are making it possible to assess different peripheral nerves and plexuses, including small sensory and/or motor branches, with great precision. Diffusion-weighted sequences and diffusion tensor imaging have opened a new horizon in neurographic studies. This new approach provides morphological and functional information about the internal structure and pathophysiology of the peripheral nerves and diseases that involve them. This update reviews the different MR neurography techniques available for the study of the peripheral nerves, with special emphasis on new sequences based on diffusion.

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.

A Rare Anatomical Variant of Unilateral Piriformis Muscle Agenesis: A Case Report

Anatomical variation of neuromuscular structures of the gluteal region is common. The piriformis muscle, in particular, has an important relationship with the sciatic nerve and may be associated with distinct clinical conditions. We report an incidental finding of unilateral piriformis muscle agenesis diagnosed on computed tomography and magnetic resonance imaging, a rare anatomical variant of the gluteal region.

Entrapment of the Sciatic Nerve Over the Femoral Neck Stem After Closed Reduction of a Dislocated Total Hip Arthroplasty

Sciatic nerve injury is a rare but potentially extremely disabling complication of posterior dislocated total hip arthroplasty. Initial closed reduction is recommended followed by a careful neurovascular examination. This procedure and the following stability testing are usually safe and typically associated with a very low complication rate. We report the case of sciatic nerve entrapment around the neck of the femoral stem after closed reduction of a posteriorly dislocated total hip arthroplasty. Immediate postreduction palsy led to surgical exploration, identification, neurolysis of the sciatic nerve and safe reduction was performed. Patient outcome was marked by complete sensitive sciatic nerve recovery, but complete loss of motor sciatic nerve function. This case highlights the importance of careful postreduction neurovascular assessment and prompt surgical exploration when indicated.

Type II sciatic nerve variant: an unexpected interventional hazard

A 12-year-old girl presented with a 1-week history of right hip pain. MRI revealed a lesion in the incompletely fused posterior triradiate cartilage of the right acetabulum with imaging characteristics of an osteoid osteoma. The surgeon requested guidewire placement under CT guidance for lesion drilling. The CT approach was planned to avoid the usual course of the sciatic nerve, and the lesion was successfully removed and confirmed at histology to be an osteoid osteoma. At a follow-up visit, MRI showed no recurrence but demonstrated an abnormal common peroneal nerve from the sciatic notch to the popliteal fossa with denervation oedema in the tibias anterior and peroneal musculature. Review of the imaging showed a bifid piriformis muscle and type II sciatic nerve variant, with the common peroneal nerve component exiting the sciatic notch between the muscle bellies. The nerve's unexpected superolateral position placed it adjacent to the course of the guidewire, resulting in a presumed thermal injury at the time of drilling. This unusual case highlights the importance of the pre-procedural documentation of sciatic nerve variants in the planning of pelvic intervention.

Quantitative magnetic resonance (MR) neurography for evaluation of peripheral nerves and plexus injuries

Traumatic conditions of peripheral nerves and plexus have been classically evaluated by morphological imaging techniques and electrophysiological tests. New magnetic resonance imaging (MRI) studies based on 3D fat-suppressed techniques are providing high accuracy for peripheral nerve injury evaluation from a qualitative point of view. However, these techniques do not provide quantitative information. Diffusion weighted imaging (DWI) and diffusion tensor imaging (DTI) are functional MRI techniques that are able to evaluate and quantify the movement of water molecules within different biological structures. These techniques have been successfully applied in other anatomical areas, especially in the assessment of central nervous system, and now are being imported, with promising results for peripheral nerve and plexus evaluation. DWI and DTI allow performing a qualitative and quantitative peripheral nerve analysis, providing valuable pathophysiological information about functional integrity of these structures. In the field of trauma and peripheral nerve or plexus injury, several derived parameters from DWI and DTI studies such as apparent diffusion coefficient (ADC) or fractional anisotropy (FA) among others, can be used as potential biomarkers of neural damage providing information about fiber organization, axonal flow or myelin integrity. A proper knowledge of physical basis of these techniques and their limitations is important for an optimal interpretation of the imaging findings and derived data. In this paper, a comprehensive review of the potential applications of DWI and DTI neurographic studies is performed with a focus on traumatic conditions, including main nerve entrapment syndromes in both peripheral nerves and brachial or lumbar plexus.