Evaluation of the sciatic nerve location regarding its relationship to the piriformis muscle

The localisation of sciatic nerve (SN) is essential for the achievement of several procedures performed in the gluteal region. This study proposed to investigate the location of SN regarding its relationship to the piriformis (PM) by the line joining the posterior superior iliac spine (PSIS), ischial tuberosity (IT) and greater trochanter (GT).

SN-PM relationship was examined in 204 specimens from 102 embalmed cadavers (55 males, 47 females). Distances between PSIS, IT and GT were measured. Midpoints of SN at the lower edge of PM (S1) and IT-GT line (S2) were marked. Perpendicular line from S1 to PSIS-GT (S1-R) and to PSIS-IT (S1-Q), were created and measured. Distances of PSIS-R, PSIS-Q (S1) and IT-S2 were measured and calculated into percentage of PSIS-GT, PSIS-IT and IT-GT lengths, respectively.

Regarding the classification of Beaton and Anson, three types of SN-PM relationship (a, b and c) were obtained. The percentage of type a, b and c was 74.02, 22.55 and 3.43, respectively. Symmetrical SN-PM relationship was found in 75.49%. The mean length of PSIS-IT, PSIS-GT and IT-GT in all types was 129.63 ± 11.89 mm, 151.34 ± 14.78 mm and 73.02 ± 10.20 mm, respectively. A statistically significant difference was found between types a and b (p = 0.013) in PSIS-IT length, whereas mean length of IT-GT and PSIS-GT showed no statistically significant difference between SN-PM types. PSIS-IT line passed SN at the lower edge of PM (S1) in 112 specimens (54.90%). In these cases, S1 and Q were the same point. A statistically significant difference was also found between types a and b (p = 0.023) in PSIS-Q (S1) length. The mean lengths of PSIS-Q (S1), PSIS-R and IT-S2 in term of percentage of PSIS-IT, PSIS-GT and IT-GT line in all types were 60.06 ± 5.90%, 54.19 ± 6.10%, and 37.87 ± 8.27%, respectively. The mean lengths of S1-R and S1-Q were 30.07 ± 8.30 mm and 6.54 ± 7.99 mm. Therefore, SN at S1 could be located at the point of 54.19 ± 6.10% of PSIS-GT length (R) with a distance of 30.07 ± 8.30 mm perpendicular to PSIS-GT line (S1-R). Since the PSIS-IT line did not pass SN at S1 in every case, it might not be suitable for localizing SN at S1. SN at S2 could be located at the point of 37.87 ± 8.27% of IT-GT line. No significant difference was found between types.

Sciatic nerve can be localised by PSIS-GT and IT-GT lines without statistically significant difference between types (a, b, and c) of SN-PM relationship.