Surgical anatomy of the sciatic nerve and its relationship to the piriformis muscle with a description of a rare variant

T Asmall,G Gunston,Bm Henry,R Venter,K Keet

doi:10.17159/2309-8309/2020/v19n1a5

T Asmall, G Gunston + Show 3 more

Open Access

https://doi.org/10.17159/2309-8309/2020/v19n1a5

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Abstract

Aims: Variation of the sciatic nerve may increase the risk of iatrogenic injury during total hip arthroplasty or arthroscopy, result in failure of peripheral blocks, or be associated with piriformis syndrome. Studies from Africa are scarce, with none to date from South Africa. Thus, the aims were to document the relationship between the sciatic nerve and piriformis muscle, variation in the bifurcation level and in the length of the nerve. Any significant differences between sexes and sides were also investigated. Methods: The lower limbs of 42 cadavers (84 limbs) were dissected and the relationship between the sciatic nerve and piriformis classified according to the patterns described by Beaton and Anson. The region of sciatic nerve bifurcation was documented, and the length of the nerve was measured in individuals with bifurcation in the thigh. Results: The normal relationship between the sciatic nerve and piriformis muscle was present in 64 limbs (76.2%). The bifurcation level of the nerve was variable in more than half the sample. No significant differences occurred in any of the variant patterns or bifurcation regions between side or sex; however, variations were more common in females than in males. The mean length of the sciatic nerve was 133.30±19.33 mm, with no differences in length between sex or side. Conclusion: Variations in the anatomy of the sciatic nerve occurred in up to half of the sample, which may have implications for increased risk of iatrogenic injury in total hip arthroplasty and arthroscopy, piriformis syndrome or sciatic block failure. Level of evidence: Level 4

Highlights

The sciatic nerve (SN) is formed in the pelvis from the ventral roots L4–S3 of the lumbosacral plexus.[1]
An unclassified pattern was observed in one right lower limb (1.2%) of a female cadaver, in which the SN emerged as two separate branches, with the common fibular nerve (CFN) passing beneath the piriformis muscle (PM), while the tibial nerve (TN) coursed beneath the inferior gemellus muscle (Figure 5)
Variant types include the SN passing through the belly of the PM, or the high bifurcation of the SN into the TN and CFN in the gluteal region, with one of these branches possibly coursing through the PM.[6,7]

Summary

Introduction

The sciatic nerve (SN) is formed in the pelvis from the ventral roots L4–S3 of the lumbosacral plexus.[1] The nerve emerges from the greater sciatic foramen, and enters the gluteal region, inferior to the piriformis muscle (PM) It courses over the superior gemellus, tendon of obturator internus, inferior gemellus and quadratus femoris muscles and descends to the ischial tuberosity, passing deep to the gluteus maximus and the long head of the biceps femoris.[2] The nerve enters the posterior thigh and usually divides here into the tibial nerve (TN) and common fibular nerve (CFN). Division typically occurs in the lower third of the thigh, near the apex of the popliteal fossa.[3] this bifurcation level is variable.[4] it passes through the gluteal region, the SN does not give off any branches here. Instead it continues inferiorly to supply motor and sensory innervation to the posterior compartment of the thigh and all compartments of the leg (including all lower limb joints) via its branches.[1,5]

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