Association Between Lower Instrumented Vertebra Selection and Mechanical Complications After Surgical Correction for Kyphotic Deformity Following Osteoporotic Vertebral Fracture

<b>Background</b>: Kyphotic deformity following osteoporotic vertebral fracture (KDOVF) often requires corrective surgery to restore sagittal alignment; however, mechanical complications, such as proximal junctional failure (PJF) and distal junctional failure (DJF), remain major concerns. Selection of the lower instrumented vertebra (LIV) plays a critical role in balancing mechanical stability and functional preservation; however, the optimal criteria for LIV selection have not been fully established. <b>Methods:</b> This multicenter retrospective cohort study included 52 patients who underwent corrective surgery for KDOVF, with a minimum 1-year follow-up. The patients were classified into a long-fixation group with pelvic fixation (<i>n</i> = 27) and a short-fixation group with lumbar LIV fixation (<i>n</i> = 25). Mechanical complications, radiographic parameters, patient-reported outcomes, and paraspinal muscle fatty degeneration were compared between groups. Subgroup analysis was performed within the short-fixation group to identify the factors associated with DJF. <b>Results:</b> The incidence of PJF was significantly higher in the long-fixation group than in the short-fixation group (37% vs. 8%, <i>p</i> < 0.01), whereas DJF was observed only in the short-fixation group (24%). Within the short-fixation group, patients who developed DJF demonstrated significantly greater preoperative sagittal malalignment, a lower rate of cement-augmented pedicle screw, and more advanced fatty degeneration of the paraspinal muscles. The short-fixation group also showed better postoperative lumbar function. <b>Conclusions:</b> LIV selection in KDOVF surgery is associated with distinct patterns of junctional mechanical complications. Short fixation may be feasible in carefully selected patients who demonstrate preserved compensatory capacity.