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Adam L. Bartret led a radiological study testing whether variant sciatic nerve routes near the piriformis muscle were associated with piriformis syndrome. The researchers classified anatomy on 783 hip magnetic resonance imaging studies and compared recorded clinical diagnoses. Variants occurred in about 19% of hips, with no statistically significant diagnostic association in this cohort.^[ref]

The study illustrates the distinction between identifying an anatomical variant and showing that it causes pain. A visible difference can be incidental; conversely, this observational result does not establish that every possible compression mechanism is absent.

The piriformis muscle is a deep skeletal muscle connecting the anterior sacrum to the upper femur. It passes through the greater sciatic foramen and helps rotate the extended thigh outward and abduct the flexed thigh. Its position makes it a useful landmark: the sciatic nerve usually emerges below it, although the relationship varies between people.^[ref]

An anatomical variant alone does not establish a cause of pain. Adam L. Bartret and colleagues classified sciatic nerve routes on 783 hip magnetic resonance imaging examinations; variants occurred in about 19%, without a statistically significant association with a recorded diagnosis of piriformis syndrome. This observation separates a visible anatomical difference from evidence that the difference explains symptoms.^[ref]

Piriformis syndrome is a clinical syndrome in which pain, sometimes with numbness or radiating leg symptoms, is attributed to irritation or compression of the sciatic nerve near the piriformis muscle. The nerve and muscle lie close together in the deep buttock region, making a mechanical interaction anatomically plausible.^[ref]

Anatomical plausibility is not proof of the explanation in a particular case. A 2018 study reviewed magnetic resonance imaging and clinical records to compare people with usual and variant relationships between the sciatic nerve and piriformis muscle. It found no significant difference in the recorded frequency of piriformis syndrome, buttock pain or sciatic symptoms between the anatomical groups.^[ref] The study illustrates why an unusual scan finding must be distinguished from demonstrated symptomatic nerve compression.

The sacral plexus is a network of anterior spinal nerve rami, usually incorporating L4–S4 spinal contributions, lying against the anterior surface of the piriformis muscle. It distributes sensory and motor fibres to much of the pelvis and lower limb. Major branches include the sciatic nerve and the pudendal nerve.^[ref]

Its mixed origins explain an otherwise puzzling arrangement: the sciatic nerve carries fibres associated with several spinal nerves, and fibres from one spinal level can leave through different peripheral branches. A nerve plexus is therefore a redistribution network, rather than a new set of neurons inserted into each pathway.

The sciatic nerve is a large mixed nerve connecting the lumbosacral plexus with the lower limb. It passes through the posterior hip region into the thigh and contains the components that continue as the tibial nerve and common fibular nerve. These pathways carry motor signals to skeletal muscles and sensory information from substantial regions of the leg and foot.

Its course relative to the piriformis muscle varies among individuals. A retrospective magnetic resonance imaging study compared common and variant anatomical arrangements and did not find a statistically significant difference in the recorded diagnoses involving pain.^[ref] An unusual anatomical arrangement therefore does not by itself establish that the arrangement is causing symptoms; structure and physiological consequence require separate evidence.