Print version ISSN 1806-0013
Rev. dor vol.12 no.4 São Paulo Oct./Dec. 2011
Anatomical variations of the sciatic nerve in a group of Brazilian cadavers*
Joseph Bruno Bidin BrooksI; Cristiano Augusto Cruz SilvaII; Sônia Aparecida SoaresI; Margareth Reiko KaiII; Richard Halti CabralI; Yara Dadalti FragosoI
IMedical Lectures, Departments of
Physiology, Anatomy and Neurology, Universidade Metropolitana de Santos. Santos,
IIDepartments of Neurology and Neurophysiology, Hospital do Servidor Publico Estadual de São Paulo. São Paulo, SP, Brazil
BACKGROUND AND OBJECTIVES: The sciatic
nerve is often involved in conditions of pain. It is a long nerve, prone to
injuries that are the consequence of trauma, inflammation and entrapment. One
possible cause of sciatic pain derives from the piriformis muscle, which maintains
a very close anatomical relationship with the sciatic nerve. The objective of
the present study was to evaluate the characteristics of the sciatic nerve and
its relationship to the piriformis muscle in a group of Brazilian cadavers.
METHOD: Anatomical dissection of 40 human limbs with detailed studies of the sciatic nerve and the piriformis muscle.
RESULTS: Anatomical variations of the relationship between the sciatic nerve and the piriformis muscle were rare. Data on the sciatic nerve length and width showed similar results to those from the literature.
CONCLUSION: The piriformis syndrome is a painful condition considered by many to be associated to anatomical variations of the relationship between the sciatic nerve and the piriformis muscle. The rarity of such variations, in the present series and in the studies published by other groups, suggests that the painful syndrome may often occur without the anatomical variations.
Keywords: Lower limbs, Pain, Piriformis muscle, Sciatic nerve.
The sciatic nerve is frequently involved in daily medical practice of neurology, orthopedics, rehabilitation and anesthesia. The anatomy of the sciatic nerve and its relationship with the piriformis muscle are better studied in cadavers1,2. Recording the findings of such anatomical studies may help in understanding piriformis syndrome, a condition that according to some physicians is well established, but according to others does not even exist3.
Piriformis syndrome is an underdiagnosed cause of gluteus and leg pain, but according to some authors it is vastly overdiagnosed4. The piriformis muscle is closely related to the sciatic nerve, which makes it possible that trauma and inflammation in the piriformis muscle might be clinically represented by sciatic pain5. Identification of the syndrome and accurate diagnosis are usually difficult, especially if the regional anatomy is not known by the physician. Although originally described in 19476, the existence of the piriformis syndrome is still contested by some authors4. However, a very comprehensive recent review of the literature on piriformis syndrome5 has pointed towards confirming the existence of this syndrome as a clinical entity, albeit still somewhat unknown in the medical world.
The piriformis muscle is flat and pear-shaped, originating from the anterior border of the second to fourth sacral segment, from the upper margin of the greater sciatic notch, and from the sacrotuberous ligament7. With the leg extended, the piriformis is mainly an external rotator for the hip, but when the leg is flexed, it is a hip abductor8.
The long and thick sciatic nerve is prone to injuries, and a variety of conditions may originate sciatic pain. One of them seems to be entrapment by the piriformis muscle3. The relationship between the piriformis muscle and the sciatic nerve is variable, since the undivided nerve may emerge above the muscle or through the muscle. The major divisions of the nerve may lie on either side, above or below the muscle9.
The anatomical relationship between the sciatic nerve and the piriformis muscle has been classified using a six-category classification system10. The type "A" relationship is considered to the normal one between the sciatic nerve and piriformis muscle, while types ''B'' to ''F'' are variants that may lead to piriformis syndrome. A clear schematic representation of these types A-F can be found in the recent and detailed work2. This work included a systematic review and meta-analysis of the literature, assessing the prevalence of anatomical variations from 18 anatomical studies on over 6,000 limbs2.
It is important not to confound the A-F classification of the relationship between the sciatic nerve and the piriformis muscle with the A-F classification of sciatic nerve division into tibial and common peroneal nerves1. To make matters more confusing, in approximately 12% of the cases, the common fibular and tibial divisions of the nerve separate proximally to or at the level of the piriformis7. In brief, regarding sciatic nerve division, Group A consists of sciatic nerve division proximally to its exit in the gluteal region. In Group B, it divides in the gluteal region. In Groups C, D and E, it divides in the upper, middle and lower regions of the back of the thigh, respectively. In Group F, the sciatic nerve divides in the popliteal fossa. Once these two classifications are well understood, it is clear that they are totally independent, despite the initial idea that the A-F categories could be the same.
In order to recognize the piriformis syndrome and to be able to address it with proper knowledge, it is important to have good understanding of the anatomy of the region and its variations. Several studies on cadavers have been carried out2 but, unfortunately, data from Brazilian studies are still scarce11.
The aim of the present study was to report on the anatomical findings from Brazilian cadavers and to compare these variations with those reported in the literature.
After approved by the Ethics Committee of Universidade Metropolitana de Santos, SP, Brazil, on October 13, 2009 (2009-18), forty limbs from 20 adult cadavers of mixed ethnic origins were studied (16 males and four females). The cadavers belonged to the Anatomy Department of Universidade Metropolitana de Santos, SP, Brazil. No clinical data on them was available, and therefore, it was not known whether any of them had presented piriformis syndrome when alive.
The cadavers had been kept in 10% formalin, and their gluteal region was dissected using precise surgical instruments. After dissection, the gluteal regions were photo-documented and the following measurements were made using a 0.05 mm precision pachymeter: 1. Relationship between the sciatic nerve and the piriformis muscle; 2. Width of the sciatic nerve at the lower margin of the piriformis muscle; 3. Extra-pelvic length of the piriformis muscle, taking the reference points of the sacrotuberous ligament and the apex of the major femoral trochanter; 4. Extra-pelvic width of the piriformis muscle at the midpoint of the muscle; 5. Distance between the lateral margin of the sacrotuberous ligament and the sciatic nerve margin. For this measurement, the reference point was the lateral margin of the ligament, close to the point of fixation into the sciatic tuber and 6. Distance between the apex of the greater femoral trochanter and the lateral margin of the sciatic nerve.
The results are summarized in tables 1 to 4. Table 1 shows the prevalence of anomalies of emergence and division of the sciatic nerve in relation to the piriformis muscle, comparing the results from the systematic review of the literature with the findings from the present study. In the present study, there was a much higher prevalence (> 80%) of the type A variation, i.e. the "normal" relationship between the sciatic nerve and the piriformis muscle. Table 2 presents the width of the sciatic nerve at the lower margin of the piriformis muscle on the right and left sides, showing a significant difference in nerve width between the two sides. Table 3 shows the extra-pelvic length of the piriformis muscle in the right and left limbs, taking the reference points of the sacrotuberous ligament and the apex of the greater femoral trochanter. Table 4 presents the distance between the lateral margin of the sacrotuberous ligament and the sciatic nerve margin, using the lateral margin of the ligament, close to the point of fixation into the sciatic tuber, as the reference point. Table 4 also shows the distance between the apex of the greater femoral trochanter and the lateral margin of the sciatic nerve.
Figure 1 shows dissection images of the normal and variant relationships between the sciatic nerve and the piriformis muscle.
It is unclear whether the anatomical variations in the relationship between the sciatic nerve and the piriformis muscle are responsible for the pain experienced in piriformis syndrome, since asymptomatic patients might have these variations, while symptomatic patients might not have them7. In fact, there is still a tendency to envisage parallel existence of "sciatic-piriformis anomalies" and "piriformis syndrome". Whether the relationship between nerve and muscle is really cause and consequence of the pain remains to be defined2. However, it is only through detailed study of the regional anatomy that these doubts may one day be clarified.
The present study has confirmed the data in the worldwide literature, with regard to the "normal" type A relationship between the sciatic nerve and the piriformis muscle. This type of relationship, with the sciatic nerve emerging below the piriformis muscle was prevalent in our population, just as it was in all other similar studies that were systematically 2. Variants of this "normal" type A relationship are indeed so rare that hundreds of limbs would have to be studied in order to identify variations B-F in a population. Variations in sciatic nerve width and length are not unknown11, and possibly do not represent an anomaly.
The relatively small number of limbs studied in the present work does not allow for statistical comparisons and conclusions between normal and variant relationships between nerve and muscle. The essence of this study was descriptive, in order to obtain more information on variations of the sciatic nerve anatomy. However, taking into consideration data from the present work and from the literature on the subject, it is fair to say that the piriformis syndrome possibly does not depend on abnormal relationships between the sciatic nerve and the piriformis muscle, or it would be an extremely rare pain-generating condition. In fact, even in a situation of normal relationship between the sciatic nerve and the piriformis muscle, any condition affecting the muscle (e.g., inflammation or trauma) could indirectly affect the nerve. This idea seems to be particularly supported by the good pain relief results achieved when low doses of botulinum toxin are injected into the piriformis muscle of patients with typical signs and symptoms of the piriformis syndrome12.
Piriformis syndrome is a rare pain-generating condition, and only detailed study of sciatic nerve anatomy and its anatomical relationship with the piriformis muscle is likely to shed light on the questions regarding the syndrome. Anatomical variations in the relationship between the sciatic nerve and the piriformis muscle do not seem to be solely responsible for the piriformis syndrome.
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Correspondence to: Presented in August 15, 2011. No conflicts of interest to declare, this work
was carried out without any public or private financial support. * Received from Universidade Metropolitana
de Santos. Santos, SP.
Joseph Bruno Bidin Brooks, MD
Rua da Constituição 374
11015-470 Santos, SP
Phone-Phax: +55 13 3226-3400
Accepted for publication in November 25, 2011.
Presented in August 15, 2011.
No conflicts of interest to declare, this work was carried out without any public or private financial support.
* Received from Universidade Metropolitana de Santos. Santos, SP.