The piriformis syndrome is buttock pain due to tightening or irritation of the piriformis muscle where it is crossed by the sciatic nerve.
Long-term assessment of clinical outcomes of ultrasound-guided steroid injections in patients with piriformis syndrome. Jeong HS, Lee GY, Lee EG, Joe EG, Lee JW and Kang HS. Ultrasonography. 2015 Jul; 34(3): 206–210.