Reoperation risk by subtype in proximal junctional kyphosis and the impact of osteoporosis treatment in adult spinal deformity surgery.

Background: Mechanical complications after long-segment fusion for adult spinal deformity (ASD) remain a major driver of reoperation. Proximal junctional kyphosis (PJK) comprises heterogeneous morphologic subtypes with potentially distinct clinical courses, yet how subtype relates to reoperation risk and whether bone quality or osteoporosis medications mitigate that risk are not fully defined. Methods: We conducted a single-center retrospective cohort study of consecutive older adults undergoing posterior instrumented fusion for ASD. Patients were classified by PJK subtype and grade using standardized radiographic criteria, and followed longitudinally for the occurrence and timing of reoperation. Bone quality surrogates and osteoporosis therapies, including preoperative teriparatide, were recorded. Multivariable models adjusted for demographic, surgical, and spinopelvic parameters were used to assess associations between subtype, bone quality, medication exposure, and reoperation. Results: Reoperation clustered within specific PJK subtypes, with higher-grade deformity demonstrating a disproportionately greater need for revision. Subtypes characterized by structural failure and junctional collapse showed the strongest association with reoperation compared with alignment-predominant patterns. Lower bone quality correlated with more severe PJK and reoperation, whereas exposure to osteoporosis medication-particularly preoperative teriparatide-was associated with a lower likelihood of reoperation and delayed time to reintervention. Sensitivity analyses yielded consistent effect directions across age, sagittal alignment, and construct length strata. Conclusions: PJK subtype meaningfully stratifies reoperation risk after ASD surgery. Integrating subtype with bone quality assessment can refine perioperative decision-making, identify patients who may benefit from intensified junctional protection, and inform surveillance. Preoperative anabolic therapy emerges as a potentially modifiable factor linked with reduced reoperation, warranting prospective evaluation. These findings support a practical, subtype-guided strategy that couples surgical planning with bone health optimization to reduce failure and reoperation burden.