Biomechanical effects of screw loosening after lumbar PEEK rod and titanium rod fixation: a finite element analysis - Jiang, Guozheng; Wang, Shuyang; Xu, Luchun; Li, Zeyu; Feng, Ningning; Qiu, Ziye; Yang, Yongdong; Yu, Xing (Frontiers in Bioengineering and Biotechnology, 2025)