SCIENTIFIC ARTICLE Surgical evaluation of magnetic resonance imaging findings in piriformis muscle syndrome Hrvoje Ivan Pecina & Igor Boric & Tomislav Smoljanovic & Davor Duvancic & Marko Pecina Received: 28 May 2008 / Accepted: 1 June 2008 / Published online: 12 July 2008 # ISS 2008 Abstract Objective The objective of this study was to evaluate the accuracy of magnetic resonance imaging (MRI) in the diag- nosis of the piriformis muscle syndrome (PMS). Materials and Methods In ten patients, seven female and three male, with a long history of clinical symptoms of the PMS, an MRI was performed as the last preoperative diag- nostic tool. All patients were imaged using 2T MR system (Elscint, Haifa, Israel). Axial and coronal spin-echo, fast spin- echo (FSE), and fat-suppressed FSE-weighted images were made through the pelvic region with 3-mm section thickness and a 0.5-mm gap to show the whole piriformis muscle and the course of sciatic nerve on its way out of the pelvis. A routine examination also included axial fast spin-echo T2, three-dimensional gradient echo. Results In seven cases, an MRI abnormality for the PMS was found. In two women, the MRI demonstrated a bigastric appearance of the piriformis muscle with a tendinous portion between the muscle heads and the course of the common peroneal nerve through the muscle between the tendinous portions of the muscle. In one female patient, the common peroneal nerve passed through the hypertrophied piriformis muscle. In four patients, the MRI showed a hypertrophied aspect of the piriformis muscle and an anteriorly displaced sciatic nerve. All MRI findings were confirmed surgically. In three patients, no apparent abnormalities could be observed, but after a surgical treatment, i.e., a tenotomy of the piriformis muscle and neurolysis of the sciatic nerve, all symptoms disappeared. Conclusion In piriformis muscle syndrome, MRI may dem- onstrate signal abnormalities of the sciatic nerve as well as its relationship with the normal and abnormal piriformis muscle. Keywords Sciatic nerve entrapment . Piriformis syndrome . MRI diagnosis Introduction The sciatic nerve passes through the greater sciatic foramen in the proximity of the piriformis muscle. The piriformis muscle splits the foramen into the suprapiriformis region and the infrapiriformis region or foramen (Fig. 1). The infrapiriformis foramen is triangular in shape and two groups of neuro- vascular structures leave the pelvis through it. The medial group includes the pudendal neurovascular bundle, and the lateral group consists of the sciatic nerve, the inferior gluteal nerve, the posterior cutaneous nerve of the thigh, and the inferior gluteal vessels. The sciatic nerve represents two ter- minal nerves of the sacral plexus, the tibial nerve and the common peroneal nerve, and it enters the thigh as one of the two nerves. A study by Pecina M et al. [1] revealed an in- trapelvic division of the sciatic nerve in 26.5% of the cases, a postforamen division in 4.6% of the cases, a division at the inferior border of the gluteus maximus muscle in 11.5% of the cases, and a division in the thigh in the rest of the cases. Skeletal Radiol (2008) 37:1019–1023 DOI 10.1007/s00256-008-0538-0 H. I. Pecina : I. Boric Department of Radiology, Clinical Hospital “Sestre Milosrdnice”, Vinogradska 29, 10000 Zagreb, Croatia T. Smoljanovic : M. Pecina (*) Department of Orthopedic Surgery, School of Medicine, University of Zagreb, Salata 7, 10000 Zagreb, Croatia e-mail: marko.pecina@zg.t-com.hr D. Duvancic Outpatients Clinic and Clinic for Diagnostic Procedures, Nemetova 2, 10000 Zagreb, Croatia