Of the over 450,000 patients undergoing instrumented spinal fusion surgery each year in the United States,
over 20,000 ultimately require a revision surgery, the majority of which are secondary to insufficient bone
quality. Biomechanical problems associated with insufficient bone quality include pseudarthrosis, insufficiency
fractures of the proximal adjacent vertebra, subsidence of any intervertebral implant-graft construct, and
loosening of an implanted pedicle screw. No objective clinical test is currently available for surgeons to reliably
identify a patient's risk of these adverse outcomes. A bone mineral density test is not enough, as "normal"
bone may still have insufficient quality for high stresses near the implant. The overall goal of this Phase-II
SBIR project is to clinically validate our virtual stress test (VST) technology for pre-operatively
identifying spinal fusion patients who are at high risk of a failed surgery.
VST utilizes the CT scan already acquired for the patient’s pre-operative assessment to construct a virtual,
three-dimensional model of the fusion construct in order to investigate its mechanical response to loading. Our
proposed pre-operative spinal fusion evaluation, based on VST, has three elements:
a) Perform VST to assess pedicle screw loosening, endplate subsidence, and vertebral failure, all specific
to the patient’s vertebrae and spinal fusion surgical plan. From that information, identify patients at high
risk of a failed fusion surgery due to insufficient bone quality;
b) For each vertebral level, specify optimal screw sizing and placement for each pedicle, and measure
bone mineral density (BMD) and vertebral strength;
c) Provide a comprehensive assessment of risk of a future vertebral fracture due to osteoporosis.
Having developed the VST technology in our Phase-I study, this Phase-II study seeks to validate its use in the
largest ever biomechanics study of spinal fusion patients. This case-cohort study will include 1,500 women and
men who underwent spinal fusion surgery at Kaiser Permanente Southern California from 2009–2018, have a
pre-surgical CT scan and at least 90 days of follow-up. Cases (n=500) will be patients with an adverse surgical
outcome that required re-operation after the index procedure. Half of this cohort will be used to optimize the
virtual stress tests for pedicle screw loosening, endplate subsidence, and vertebral failure. Using the remainder
of the cohort, VST will be performed blinded to surgical outcomes in order to test the hypothesis that VST
predicts failed surgery, independent of BMD and other clinical risk factors. If successful, this study would result
in a biomechanics-based clinical tool for pre-operatively evaluating a patient's risk of spinal fusion failure due to
insufficient bone quality. Given the options that now exist for addressing such issues, such as use of anabolic
bone forming agents or prophylactic bone cement injection, such a tool could lead to a reduction in adverse
surgical outcomes and thus fewer re-operations and revisions.