Beaton and Anson 18,19
described six anatomical relationships between the sciatic nerve and piriformis muscle using a six category classification system in 1938: (A) the sciatic nerve passing below the piriformis muscle, (B) a divided sciatic nerve passing through and below the piriformis muscle, (C) a divided nerve sciatic nerve passing above and below muscle, (D) an undivided sciatic nerve passing through the piriformis muscle, (E) a divided nerve passing through and above the muscle, (F) an undivided sciatic nerve passing above the piriformis muscle 2,3
). Smoll 5
presented the reported incidence of these six variations in over 6000 lower limbs from multiple cadaveric studies from the late 1800s until 2009. The type A, B, C, D, E and F found in 83.1, 13.7, 1.3, 0.5, 0.08 and 0.08%, respectively 2
. Smoll 5
reported an interesting finding in his review that types D, E and F did not find in four largest cadaver series. Cassidy et al. 2
reported a new variation in 2012 as a subtype of B. We also observed this variation in one pelvis. In this variation the sciatic nerve passed below piriformis muscle but a smaller accessory piriformis muscle with its own separate tendon was present and this accessory piriformis muscle was between the common peroneal and tibial nerves.
Delabie et al. 13 studied with 104 buttocks from 52 randomly selected patients on MRI. They also described the accessory piriformis muscle as bifid and the peroneal nerve passed between two heads in 9.6% of cases.
Smoll 5 reported a systematic review about variations of the piriformis and sciatic nerve with clinical consequence. Thirty anatomical textbooks and six databases were searched to find dissection reports. According to this review, the percentage of anomalies was between 35.8% and 9.5% in gluteal region 5. In our study, division variations of sciatic nerve were determined in 15% of gluteal regions and lower limbs but the division variation in only gluteal region was seen in 10%.
Pokorny et al. 4 determined one of the described variants bilaterally in only one cadaver of 91 cadavers. This variation was type IV. Type IV had the same anatomic features with type B of Beaton and Anson 4,5,18,19. Pais et al. 20 reported three cadavers with anatomical variants of sciatic nerve. Two of them had high division in the inferior portion of gluteal regions on both sides. We also observed a cadaver that had variations bilaterally but the type of variation on the contralateral side was not the same on this cadaver.
In attempts to explain possible causes of piriformis syndrome, studies have been conducted on cadavers to explore the anatomical variations of piriformis muscle and the sciatic nerve. The study of Beaton and Anson in 1938 involving 240 cadavers demonstrated that in 90% of specimens sciatic nerve exited below the piriformis muscle. The remaining 10% of specimens had six different types of anatomical variations 27. Although there is a widely held opinion about the relationship between piriformis syndrome and these anatomical variations, it is not clear yet. The studies of Broadhurst et al. 21 and Kirschner et al. 22 did not support this opinion. Broadhurst et al. 21 described a series of 27 patients undergoing ultrasound imaging of the buttock to see whether or not changes in the piriformis muscle in patients suffering from piriformis syndrome. They determined three variations but all of them were on the unaffected side. Kirschner et al. 22 also reported that some asymptomatic patients presented with these variations and some symptomatic patients did not.
One of the descriptions of the safe region for giving injections into the buttock is superior to a line extending from posterior superior iliac spine to the superior border of the greater trochanter 23. On the right side of a cadaver in present study, two separate nerve divisions passed above and below the piriformis muscle to form the common peroneal nerve (2.5%). The division passing superior to the muscle may be especially vulnerable to injury during intragluteal injections since the nerve was not in the safe region for injections.
The variations of sciatic nerve in gluteal region contribute to nerve palsy during the posterior approach total hip arthroplasty since this surgical approach involves detaching the piriformis tendon and the other short external rotators of the hip from the trochanter and upper thigh. Retraction of piriformis muscle may be responsible of stretching and damage to the nerve 3,24,25. Smoll (2010) mentioned that if type B, D or E variations were present, all or a part of sciatic nerve pierced the piriformis muscle and retraction can cause sciatic nerve compression as the muscle retracts medially, pulling on the nerve 5. The variation observed in present study, passing through piriformis muscle fibers as two separate nerve divisions of common peroneal nerve, was not recorded in the literature previously. The common peroneal nerve in this variation determined in the present study may be also compressed or strangulated by the piriformis muscle fibers surrounding the nerve.
Pais et al. 20 reported in a cadaver the division of the sciatic nerve in its terminal branches inside the pelvis, tibial and common peroneal nerves, just before crossing the greater sciatic notch. The common peroneal nerve exited the pelvis above the piriformis muscle while the tibial nerve coursed deep to piriformis muscle. This variation was type C of Beaton and Anson 18,19. This variation was observed in present study in one cadaver unilaterally, but with a difference. The common peroneal nerve took a division splitting the piriformis muscle, while taking a division from above the piriformis muscle. This kind of high division can result in involvement of only one of the terminal branches in sciatica and may be a reason of atypical sciatic compressive syndrome.
Prakash et al. 9 performed a cadaveric study on 86 lower limbs. The sciatic nerve division in the lower part of the posterior compartment of the thigh was in 40.7%, while the division was in the popliteal fossa in 34.9% and proximal to its entrance in the gluteal region in 16.3%. In the series of present study, the division in the upper part of the posterior compartment of the thigh was differently observed in one lower limb.
The sciatic nerve block in the popliteal fossa is frequently administered for anesthesia and post-operative pain blockage for lower extremity surgery below the knee. The sciatic nerve division in the popliteal fossa and its depth have significant implications for popliteal block 7,15-17. Vloka et al. 15 suggested that insertion of the needle at 100 mm above the popliteal crease was most suitable placement, as a different opinion according to the classical teaching. Consequently, the nerve may divide into its terminal divisions at variable levels extending from sacral plexus till the popliteal fossa. Recognizing these anatomical variations can help clinicians in performing nerve blocks.