Anatomical study of middle cluneal nerve entrapment.

OBJECT: Entrapment of the middle cluneal nerve (MCN) under the long posterior sacroiliac ligament (LPSL) is a possible, and underdiagnosed, cause of low-back and/or leg symptoms. To date, detailed anatomical studies of MCN entrapment are few. The purpose of this study was to ascertain, using cadavers, the relationship between the MCN and LPSL and to investigate MCN entrapment.

METHODS: A total of 30 hemipelves from 20 cadaveric donors (15 female, 5 male) designated for education or research, were studied by gross anatomical dissection. The age range of the donors at death was 71-101 years with a mean of 88 years. Branches of the MCN were identified under or over the gluteus maximus fascia caudal to the posterior superior iliac spine (PSIS) and traced laterally as far as their finest ramification. Special attention was paid to the relationship between the MCN and LPSL. The distance from the branch of the MCN to the PSIS and to the midline and the diameter of the MCN were measured.

RESULTS: A total of 64 MCN branches were identified in the 30 hemipelves. Of 64 branches, 10 (16%) penetrated the LPSL. The average cephalocaudal distance from the PSIS to where the MCN penetrated the LPSL was 28.5±11.2 mm (9.1-53.7 mm). The distance from the midline was 36.0±6.4 mm (23.5-45.2 mm). The diameter of the MCN branch traversing the LPSL averaged 1.6±0.5 mm (0.5-3.1 mm). Four of the 10 branches penetrating the LPSL had obvious constriction under the ligament.

CONCLUSION: This is the first anatomical study illustrating MCN entrapment. It is likely that MCN entrapment is not a rare clinical entity.