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Proximal junctional kyphosis following surgical treatment of global sagittal imbalance : predictive analysis using mathematical and in vitro biomechanical models Murray, Heather Leanne


Introduction: Sagittal realignment using posterior spinal fixation and fusion, with or without corrective osteotomy, is the current treatment for global sagittal imbalance. Patients may develop proximal junctional kyphosis (PJK) through failure of the uppermost instrumented or adjacent vertebra. The effects of surgical and patient variables on the development of PJK have not been studied biomechanically. Objectives: (1) To analyze pre- and post-operative intervertebral loading and the effect of osteotomy location and extensor muscle function on intervertebral spine loading using a 2D equilibrium model of sagittally imbalanced adult spine, and (2) to characterize pure moment loading pathways of multi-segment human cadaveric spines following posterior spinal fixation and in a number of surgical conditions. Methods: (1) Pre- and post-operative lateral radiographic measurements were taken of patients (N=7) and used to predict intervertebral compressive loading patterns. From pre-operative curves, the changes in loading behaviour due to simulated osteotomies and decreasing levels of extensor muscle function were assessed. (2) Six human cadaver five-segment spines in six surgical states were tested in pure flexion-extension bending to represent the post-operative loading of patients without extensor muscle function. Vertebral strain, rod strain, and specimen kinematics were measured and rod loading was used to predict load-sharing between the implant and the spine. Results: (1) Predicted intervertebral compressive loads increased up to 29% after development of PJK. Predicted compressive loads were not notably affected by the chosen level of the osteotomy but increased up to 42% after intra-operative extensor muscle loss. (2) A force couple existed between the vertebral column and the implant, supporting the majority of the applied moment. The additional compressive force on the spine due to the applied moment was predicted based on rod load measurements, found to agree with model predictions. Specimen condition had minimal or no significance on measurements. Discussion: The developed equilibrium model introduced a predictive tool for surgical planning and deformity progression. Predicted intervertebral compressive loads were higher in sagitally-imbalanced spines than asymptomatic spines (96), worsened by loss of extensor muscle function. Simulating muscle loss by applying a pure moment in vitro may provide insight to the resulting additional spine loading.

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