Novel anatomical findings with implications on the etiology of the piriformis syndrome

Abstract

Purpose: The cause of the piriformis-related pelvic and extra-pelvic pain syndromes is still not well understood. Usually, the piriformis syndrome is seen as extra-pelvic sciatica caused by the entrapment of the sciatic nerve by the piriformis in its crossing through the greater sciatic foramen. However, the piriformis muscle may compress additional nerve structures in other regions and cause idiotypic pelvic pain, pelvic visceral pain, pudendal neuralgia, and pelvic organ dysfunction. There is still a lack of detailed description of the muscle origin, topography, and its possible relationships with the anterior branches of the sacral spinal nerves and with the sacral plexus. In this research, we aimed to characterize the topographic relationship of the piriformis with its surrounding anatomical structures, especially the anterior branches of the sacral spinal nerves and the sacral plexus in the pelvic cavity, as well as to estimate the possible role of anatomical piriformis variants in pelvic pain and extra-pelvic sciatica.

Methods: Human cadaveric material was used accordingly to the Swiss Academy of Medical Science Guidelines adapted in 2021 and the Federal Act on Research involving Human Beings (Human Research ACT, HRA, status as 26, May 2021). All body donors gave written consent for using their bodies for teaching and research. 14 males and 26 females were included in this study. The age range varied from 64 to 97 years (mean 84 ± 10.7 years, median 88).

Results: three variants of the sacral origin of the piriformis were found when referring to the relationship between the muscle and the anterior sacral foramen. Firstly, the medial muscle origin pattern and its complete covering of the anterior sacral foramen by the piriformis muscle is the most frequent anatomical variation (43% in males, 70% in females), probably with the most relevant clinical impact. This pattern may result in the compression of the anterior branches of the sacral spinal nerves when crossing the muscle.

Conclusions: These new anatomical findings may provide a better understanding of the complex piriformis and pelvic pain syndromes due to compression of the sacral spinal nerves with their somatic or autonomous (parasympathetic) qualities when crossing the piriformis.

Keywords: Deep gluteal space problems and piriformis syndrome; Pelvic pain syndrome; Piriformis; Pudendal neuralgia; Sacral plexus.

Fig. 1

Schematic representation of the patterns for the piriformis origin. A Medial pattern with nerve entrapment; B medial pattern without nerve entrapment; C lateral pattern, and D interforaminal pattern (detailed description see in the text). The border of muscle origin is shown on the opposite side with the dashed line: A and B medially to sacral foramina, C laterally to sacral foramina, D in the middle of interforaminal space (interforaminal position)

Fig. 2

Examples of the main patterns for piriformis origin on corresponding anatomical specimens. A Medial pattern with nerve entrapment; B the medial pattern without nerve entrapment; C lateral pattern, and D interforaminal pattern (explanation see in the text).CIA common iliac artery, CIV common iliac vein, EIA external iliac artery, IIA internal iliac artery, IIV internal iliac vein, S1 ventral branches of the sacral spinal nerves 1, S2 ventral branches of the sacral spinal nerves 2, S3 ventral branches of the sacral spinal nerves 3, S4 ventral branches of the sacral spinal nerves 4, S5 ventral branches of the sacral spinal nerves 5, LS lumbosacral plexus, VL5 lumbar vertebrae 5; VS1 sacral vertebrae 1, VS2 sacral vertebrae 2, VS3 sacral vertebrae 3, VS4 sacral vertebrae 4, VS5 sacral vertebrae 5

Fig. 3

Frequency of the piriformis origin patterns. A Frequency of piriformis origin patterns in total (n = 40); B frequency of piriformis origin patterns in males (n = 16); C frequency of piriformis origin patterns in females (n = 24)

Fig. 4

Schematic drawing of topographic relationship between the piriformis and the peripheral nervous network in the pelvis.VL5- vertebra lumbalis 5; VS(1–5)- vertenrae sacralis 1–5; S(1–5) sacral segments of sacral plexus