Inverse bone remodeling patterns in isthmic lumbar spondylolisthesis: differential changes in vertebral body and pedicle bone quality and their association with pedicle screw loosening

Bone quality changes in isthmic spondylolisthesis (IS), especially within the pedicle, remain poorly understood. This study investigated regional bone quality alterations in IS using DXA and CT-derived Hounsfield unit (HU) measurements and evaluated their association with pedicle screw loosening. Adult patients with single-level L5–S1 IS treated with posterior lumbar interbody fusion were retrospectively reviewed. Bone quality was assessed using DXA T-scores and CT-based vertebral body and pedicle HU values. Multivariable logistic regression and ROC analyses were performed to identify factors associated with radiographic screw loosening and assess discriminatory performance. DXA showed no significant difference in bone mineral density between patients with IS and controls. However, CT revealed an inverse regional remodeling pattern at the slipped segment, with increased vertebral body HU values but decreased pedicle HU values. Lower pedicle HU, lower vertebral HU, and older age were independently associated with radiographic screw loosening. Pedicle HU showed a numerically higher discriminatory ability than vertebral HU for radiographic screw loosening, with an exploratory Youden-derived cutoff value of 115.5 HU. IS demonstrates inverse regional bone remodeling characterized by increased vertebral body HU but decreased pedicle HU at the affected segment. Because reduced pedicle bone quality was more closely associated with radiographic screw loosening, pedicle HU may serve as a clinically relevant surrogate marker for preoperative assessment of screw–bone interface risk in patients with IS.