the piriformis muscle ( pm ) is located posterior to the hip joint and passes out of the pelvis through the greater sciatic foramen , dividing it into two topographic areas of utmost clinical importance : suprapiriform and infra - piriform foramina . due to the fact that a large number of invasive medical procedures are performed in the gluteal region , knowledge of the typical and variant anatomical relationships between the pm and sciatic nerve ( sn ) may be crucial to ensure the best outcome . the pm may be also associated with etiopathogenesis of pain syndrome whose symptoms resemble sciatica and which is called piriformis syndrome . the traditional approach describes piriformis syndrome as a neuromuscular disorder classified as compression neuropathy , caused by compression on the sn at the level of the pm [ 25 ] . however , there are some patients with abdominal pain of uncertain etiology ( e.g. , some anatomical variants of vasculature of the abdomen ) that may lead to unnecessary investigation and intervention . piriformis syndrome poses a considerable diagnostic problem , and data on its etiopathogenesis , especially with regard to its anatomical conditions , are still scarce . upon detailed literature review , the authors of the present work have found no anatomy research in which morphometric differences between a group with a typical pm morphology and a typical sn course in the infra - piriform foramen and a group with anatomical variations of both these structures would be statistically analyzed . the article aims at : 1 ) studying the incidence of anatomical variations in the sn course in relation to the piriformis ; 2 ) taking precise anthropometric measurements of selected pm and sn parameters ; and 3 ) comparing the measurement results of the group with a typical sn course and the group with variations of the sn , as well as comparing the results found in male and female limbs . thirty randomized , formalin - fixed human lower limbs of adults of both sexes were studied . the specimens were 14 limbs of women and 16 limbs of men , 13 of which were right limbs and 17 left ones . the bioethics commission of the medical university of lodz issued a consent for the study ( consent no . deep dissection was performed in the gluteal region . upon visualization of the structures located deep to the gluteus maximus muscle special attention was paid to the sn course in the infra - piriform foramen and its relation to pm . the next stage has consisted in precise morphometric measurements of pm parameters ( width  wpm , length of the upper edge luepm , length of the lower edge  llepm ) and sn location from palpable bony landmarks ( distance between the lateral edge of the sn and the greater trochanter  lesn - gt , distance between the medial edge of the sn and the apex of the ischial tuberosity mesn - it ) . when determining the landmarks for measurements , the methodology applied by gvener et al .   has been used , with modifications necessary for the purposes of this study . to complete the data , the following parameters were measured and statistically analyzed : length of the lower extremity ( lle ) from the greater trochanter to the lower edge of the lateral malleolus , thigh length ( tl ) from the greater trochanter to the knee - joint fissure , distance between the posterior superior iliac spine and the greater trochanter ( psis - gt ) , distance between the posterior superior iliac spine and the apex of the ischial tuberosity ( psis - it ) , distance between the apex of the ischial tuberosity and the greater trochanter ( it - gt ) as well as length at which the sn crosses the lower edge of pm ( lsn - lepm ) . the measurements have been taken along straight lines , with stanley powerlock tape rule ( stanley tools product group , new britain , the united states ) and digimatic caliper ( mitutoyo corporation , kawasaki - shi , kanagawa , japan ) . each measurement was taken twice , accurate to within 1 mm ( with the exception of the sn measurements , which were accurate to within 0.1 mm ) ; the average of both measurements was accepted as the final result . individual parameters have been assessed in the anthropometric study and their designations are presented in table 1 . for the purposes of the analysis , the specimens have been divided into two groups : the first one with a typical pm morphology and a typical sn course in the infra - piriform foramen and the second one with variations in the area . distribution normality of the variables has been assessed with the shapiro - wilk test . despite the small size of the sample , deviation from distribution normality has been noted only for the llepm variable ( for both groups ) , and for wpm in the group with a typical pm and psis - gt morphology and in the group with anatomical variations . this has made it possible to use parametric methods for assessment of significance of differences between the groups ( in the case at hand student s t - test ) for the majority of variables . for variables with deviations from the normal distribution the next stage has consisted in assessment of correlations between the variables with a specific correlation coefficient : spearman s rho . upon analysis of the pm morphology after it passes out of the pelvis , three variations of the muscle have been observed . variation i includes the cases with a typical pm morphology , where the muscle is pear - shaped ( figure 1 ) . this pm morphological type has been observed in 21 specimens ( 70% ) , more specifically in 11 male limbs and in 10 female limbs . variation ii covers the cases where pm is divided into two parts with the common peroneal nerve running between them ( figure 2 ) . this variation has been found in six specimens ( 20% ) , three of which are male and three female . one of the cases classified as variation ii has presented a distinct fusion of a tendon of one of the pm parts with the inferior gemellus muscle ( figure 2 ) . variation iii encompasses the limbs in which there is a fusion of pm with the gluteus medius muscle ( figure 3 ) . the variation has been found in three cases ( 10% ) , more specifically in two male limbs and in one female limb . due to the fusion of pm and the gluteus medius , the superior gluteal nerve and superior gluteal vessels run between fibres of the fused muscles . in the study material , a typical course of the sn ( a single trunk in the infra - piriform foramen ) has been observed in 20 limbs ( 66.7% ) , 11 of which are male and nine female ( figure 1 ) . 17 out of the limbs ( 56,6% ) have displayed a typical pm morphology , whereas three of the limbs ( 10% ) have pm fused with the gluteus medius ( figure 3 ) . the variation in which the common peroneal nerve runs through pm ( figure 2 ) was found in six limbs ( 20% )  three male and three female . this variation covered the cases where pm was divided into two parts with the common peroneal nerve running between them . three limbs ( 10% ) , on the other hand , have displayed a variation in which two sn roots merge below the pm inferior edge into one trunk . this morphology has been found in one male limb and two female limbs , with pm displaying a typical morphology . a rare variation with common peroneal nerve running within suprapiriform foramen and tibial nerve running within infra - piriform foramen ( figure 4 ) was found in one male limb ( 3.3% ) . for the purposes of statistical analysis , the study material has been divided into two groups depending on sn relationships to the piriformis . specimens with a typical course of the sn have been classified in the first group . specimens displaying atypical course of the sn in the greater sciatic foramen have been classified in the second group . analysis of the statistical data ( table 2 ) indicates that the groups do not significantly differ in the median , although slightly higher levels have been recorded for the group with the anatomical variations . variations group displayed greater diversity of results ( figure 5 ) . comparing the two populations using an adequate test does not , however , confirm significance of the differences between them from the perspective of the variables at hand . the conducted analysis allows for drawing a conclusion that there is a significant positive correlation between selected piriformis parameters on the one hand , and certain anthropometric measurements on the other , in the typical pm morphology group . the higher ( on average ) psis - it , psis - gt and it - gt , the bigger wpm . analogical , and at the same time stronger , relationships have been observed for llepm , whereas lesn - gt is significantly and positively correlated with psis - it and psis - gt . another significant positive correlation has been observed between lsn - lepm and lle . in the second group these regularities are weaker and not statistically significant . generally , in the group with a typical pm morphology , there has been observed a distinct regularity that a higher level of one parameter corresponds to a higher level of the other . assessment of the relationships between particular pm parameters ( luepm , llepm , wpm ) and selected sn parameters ( lesn - gt , mesn - it ) has revealed slightly different configurations of relationships in the group with a typical course of the sn and the group with atypical course of the sn . only the relationships between lesn - gt on the one hand , and luepm and llepm on the other , as well as between lsn - lepm and wpm can be deemed statistically significant in the group with a typical sn course . in the second group none of the relationships table 3 contains basic descriptive statistics for the analyzed variables in the male specimens group and in the female specimens group . sex - oriented statistical analysis of the gathered material revealed significant statistical differences in sn diameter measured along the lower edge of pm ( the lsn - lepm dimension )  the parameter has been higher to a statistically significant extent in the male limbs . psis - it is another parameter found to be higher to a statistically significant extent in the male group . in the male limbs , there has been a statistically significant positive correlation found between the length of the lower edge of pm ( the llepm dimension ) and the parameters psis - gt , it - gt , lesn - gt , mesn - it and lsn - lepm . in the female limbs only the lesn - gt dimension has correlated to a statistically significant extent with the llepm parameter . in the male limbs group there has been a statistically significant correlation observed between the length of the lower limb ( lle ) and the distance from the lateral edge of the sn to the greater trochanter ( lesn - gt ) . in this group also the lsn - lepm parameter has been found to be positively correlated to a statistically significant extent with the it - gt parameter . in the female limbs group there has been a statistically significant positive correlation observed between the length of the lower limb ( lle ) , as well as the thigh length ( lt ) , and the distance from the sn to the apex of the ischial tuberosity ( the mesn - it parameter ) . a comparison of dispersion of the selected variables in the male limbs group and in the female limbs group has been illustrated in figure 6 . upon analysis of the pm morphology after it passes out of the pelvis , three variations of the muscle have been observed . variation i includes the cases with a typical pm morphology , where the muscle is pear - shaped ( figure 1 ) . this pm morphological type has been observed in 21 specimens ( 70% ) , more specifically in 11 male limbs and in 10 female limbs . variation ii covers the cases where pm is divided into two parts with the common peroneal nerve running between them ( figure 2 ) . this variation has been found in six specimens ( 20% ) , three of which are male and three female . one of the cases classified as variation ii has presented a distinct fusion of a tendon of one of the pm parts with the inferior gemellus muscle ( figure 2 ) . variation iii encompasses the limbs in which there is a fusion of pm with the gluteus medius muscle ( figure 3 ) . the variation has been found in three cases ( 10% ) , more specifically in two male limbs and in one female limb . due to the fusion of pm and the gluteus medius , the superior gluteal nerve and superior gluteal vessels run between fibres of the fused muscles . in the study material , a typical course of the sn ( a single trunk in the infra - piriform foramen ) has been observed in 20 limbs ( 66.7% ) , 11 of which are male and nine female ( figure 1 ) . 17 out of the limbs ( 56,6% ) have displayed a typical pm morphology , whereas three of the limbs ( 10% ) have pm fused with the gluteus medius ( figure 3 ) . the variation in which the common peroneal nerve runs through pm ( figure 2 ) was found in six limbs ( 20% )  three male and three female . this variation covered the cases where pm was divided into two parts with the common peroneal nerve running between them . three limbs ( 10% ) , on the other hand , have displayed a variation in which two sn roots merge below the pm inferior edge into one trunk . this morphology has been found in one male limb and two female limbs , with pm displaying a typical morphology . a rare variation with common peroneal nerve running within suprapiriform foramen and tibial nerve running within infra - piriform foramen ( figure 4 ) was found in one male limb ( 3.3% ) . for the purposes of statistical analysis , the study material has been divided into two groups depending on sn relationships to the piriformis . specimens with a typical course of the sn have been classified in the first group . specimens displaying atypical course of the sn in the greater sciatic foramen have been classified in the second group . analysis of the statistical data ( table 2 ) indicates that the groups do not significantly differ in the median , although slightly higher levels have been recorded for the group with the anatomical variations . variations group displayed greater diversity of results ( figure 5 ) . comparing the two populations using an adequate test does not , however , confirm significance of the differences between them from the perspective of the variables at hand . the conducted analysis allows for drawing a conclusion that there is a significant positive correlation between selected piriformis parameters on the one hand , and certain anthropometric measurements on the other , in the typical pm morphology group . the higher ( on average ) psis - it , psis - gt and it - gt , the bigger wpm . analogical , and at the same time stronger , relationships have been observed for llepm , whereas lesn - gt is significantly and positively correlated with psis - it and psis - gt . another significant positive correlation has been observed between lsn - lepm and lle . in the second group generally , in the group with a typical pm morphology , there has been observed a distinct regularity that a higher level of one parameter corresponds to a higher level of the other . assessment of the relationships between particular pm parameters ( luepm , llepm , wpm ) and selected sn parameters ( lesn - gt , mesn - it ) has revealed slightly different configurations of relationships in the group with a typical course of the sn and the group with atypical course of the sn . only the relationships between lesn - gt on the one hand , and luepm and llepm on the other , as well as between lsn - lepm and wpm can be deemed statistically significant in the group with a typical sn course . in the second group none of the relationships table 3 contains basic descriptive statistics for the analyzed variables in the male specimens group and in the female specimens group . sex - oriented statistical analysis of the gathered material revealed significant statistical differences in sn diameter measured along the lower edge of pm ( the lsn - lepm dimension )  the parameter has been higher to a statistically significant extent in the male limbs . psis - it is another parameter found to be higher to a statistically significant extent in the male group . in the male limbs , there has been a statistically significant positive correlation found between the length of the lower edge of pm ( the llepm dimension ) and the parameters psis - gt , it - gt , lesn - gt , mesn - it and lsn - lepm . in the female limbs only the lesn - gt dimension has correlated to a statistically significant extent with the llepm parameter . in the male limbs group there has been a statistically significant correlation observed between the length of the lower limb ( lle ) and the distance from the lateral edge of the sn to the greater trochanter ( lesn - gt ) . in this group also the lsn - lepm parameter has been found to be positively correlated to a statistically significant extent with the it - gt parameter . in the female limbs group there has been a statistically significant positive correlation observed between the length of the lower limb ( lle ) , as well as the thigh length ( lt ) , and the distance from the sn to the apex of the ischial tuberosity ( the mesn - it parameter ) . a comparison of dispersion of the selected variables in the male limbs group and in the female limbs group has been illustrated in figure 6 . in 1937 beaton and anson introduced a modern detailed classification of sn course in relation to pm . this variation is related to high division of the sn , with the peroneal portion of the nerve emerging between the two parts of pm . in the present study , this variation has been observed in 20% of the limbs ; however , in the literature the incidence of this variation varies . the authors point out that during a surgical procedure aiming at complete decompression of the sciatic nerve , the surgeon should take into account a possibility that another tendon may be located  inferior or deep to the first one . okraszewska et al . recorded the variation in two out of 36 studied limbs of the polish population ( 6% ) , whereas pokorn et al . , in a study conducted on 91 cadavers in the czech population , observed a variation in which pm is perforated by one branch of the sn in 14.3% of the cases . studied 25 male cadavers and found this anatomical relation in six cadavers ( in one case it was bilateral and in five it was unilateral ) . ogengo , in his research on variations of the sciatic nerve in the black kenyan population , found that the tibial nerve passed always under pm ( within the infra - piriform foramen ) , whereas the common peroneal nerve pierced the pm in 7.9% of cases . in a study on 514 limbs chiba found a variation with the common peroneal nerve passing through the pm in as many as 38% of cases , which is the highest rate reported in literature . an atypical course of the sn in which two roots of the sn merge into one trunk below the inferior edge of the pm was observed by ogengo et al . in 4.9% of cases , while okraszewska et al . found it in three out of 36 examined limbs ( 8% of cases ) . this arrangement has been recorded in the present study in 10% of cases . moreover , in literature there are case reports of the sn branches wrapping around the pm in specimens with high division of the sn , so that the tibial nerve runs downwards ( within the infra - piriform foramen ) , and the common peroneal nerve upwards from the pm ( through the suprapiriform foramen ) . this type incidence according to different authors is : 1.5% of the examined cadavers ( ugrenovic et al . ) , 2.4% ( ogengo et al . ) , 6% of the examined specimens ( okraszewska et al . ) , while gvener et al . found it in four out of 25 cadavers ( in two of them it was unilateral and in two it occurred bilaterally ) . a rare case in which three roots of the sn merged into one common trunk only after exiting of the greater sciatic foramen ( with the upper trunk running above and two trunks running below the pm ) was described by nayak et al . . bergman describes a possible fusion of the pm with other muscles , including the gluteus medius , the superior gemellus muscle and in rare cases with the obturator internus . a fusion of the pm with the gluteus medius has been found in three limbs in our specimens , which accounts for 10% of the study material . the matter of differences in the incidence of particular variations of the sn course in relation to the pm between the sexes remains to be settled . okraszewska et al . have recorded no such differences , which is in line with our observations . taking into consideration the clinical problems , it seems probable that occurrence of anatomical differences in the sn course in relation to the pm may , according to some authors , contribute to piriformis syndrome , especially if accompanied by other etiological factors [ 5,1719 ] . chapman and bakkum described a case of a male patient with low back pain and piriformis syndrome symptoms , whose mri revealed an anomaly of the pm  accessory superior bundles of the right piriformis . conservative treatment resulted in improvement , even though the structural cause of the ailment remained . chen , on the other hand , described a case of a 28-year - old female patient with a only recreation of normal relations of the sn to the pm by dissection of the lower head of the piriformis resolved the sciatica symptoms . accessory piriformis muscle as a cause of piriformis syndrome , easily diagnosable with mri , was described by sen and rajesh . a few authors took into consideration anthropometric measurements of the pm in reference to different types of build and sex . gvener et al . presented in 2008 morphometric data on the relationships between the sciatic nerve on the one hand , and the pm and selected bony landmarks in both neutral and test positions on the other hand ; however , the study was conducted solely on male limbs . although it is true that this research was conducted in static conditions due to the specificity of the examined specimens , to the best of our knowledge it is the first work that attempts to present a morphometric analysis of the anatomical relations between the pm and sn with reference to anatomical variations of both these structures . so , in specimens with typical anatomical relations between the pm and the sn , certain regularities have been observed , but in the group of specimens with variations in the pm , on the contrary , the data are more diverse and the correlations between them much weaker . in view of the above , a possibility of considerable deviations from the expected topographic relations should be taken into account when interpreting sciatic neuropathy , but also during any invasive medical procedures in the gluteal region . the research material has also revealed subtle differences in reference to sex . for a majority of parameters there were several differences between the male and female pelvis ( e.g. , the female pelvis is more shallow and the sciatic notches are wider , and the male pelvis is taller and narrower with the acetabulum oriented more laterally . so , slightly different correlations between particular anthropometric parameters in both sexes may result from differences in the structure of the pelvis . results of the conducted research , as well as the scientific literature review , point to the conclusion that there are several anatomical variations of the pm morphology and sn course within the deep gluteal region . there are also statistically significant correlations between some anthropometric measurements in groups of male and female limbs . for a majority of parameters , however , in this study there were no statistically significant differences revealed taking into consideration the location of sn in reference to selected bony landmarks between limbs with a typical sn course and limbs with sn anatomical variations .