Piriformis syndrome: anatomic considerations, a new injection technique, and a review of the literature.

BACKGROUND Piriformis syndrome can be caused by anatomic abnormalities. The treatments of piriformis syndrome include the injection of steroid into the piriformis muscle and near the area of the sciatic nerve. These techniques use either fluoroscopy and muscle electromyography to identify the piriformis muscle or a nerve stimulator to stimulate the sciatic nerve. METHODS The authors performed a cadaver study and noted anatomic variations of the piriformis muscle and sciatic nerve. To standardize their technique of injection, they also noted the distance from the lower border of the sacroiliac joint (SIJ) to the sciatic nerve. They retrospectively reviewed the charts of 19 patients who had received piriformis muscle injections, noting the site of needle insertion in terms of the distance from the lower border of the SIJ and the depth of needle insertion at which the motor response of the foot was elicited. The authors tabulated the response of the patients to the injection, any associated diagnoses, and previous treatments that these patients had before the injection. Finally, they reviewed the literature on piriformis syndrome, a rare cause of buttock pain and sciatica. RESULTS In the cadavers, the distance from the lower border of the SIJ to the sciatic nerve was 2.9 +/- 0.6 (1.8-3.7) cm laterally and 0.7 +/- 0.7 (0.0-2.5) cm caudally. In 65 specimens, the sciatic nerve passed anterior and inferior to the piriformis. In one specimen, the muscle was bipartite and the two components of the sciatic nerve were separate, with the tibial nerve passing below the piriformis and the peroneal nerve passing between the two components of the muscle. In the patients who received the injections, the site of needle insertion was 1.5 +/- 0.8 (0.4-3.0) cm lateral and 1.2 +/- 0.6 (0.5-2.0) cm caudal to the lower border of the SIJ as seen on fluoroscopy. The needle was inserted at a depth of 9.2 +/- 1.5 (7.5-13.0) cm to stimulate the sciatic nerve. Patients had comorbid etiologies including herniated disc, failed back surgery syndrome, spinal stenosis, facet syndrome, SIJ dysfunction, and complex regional pain syndrome. Sixteen of the 19 patients responded to the injection, their improvements ranged from a few hours to 3 months. CONCLUSIONS Anatomic abnormalities causing piriformis syndrome are rare. The technique used in the current study was successful in injecting the medications near the area of the sciatic nerve and into the piriformis muscle.