Connecting Spine Instability to Pathological Vertebral Fractures

In new research, a team from Beth Israel Deaconess Medical Center (BIDMC) and Harvard Medical School has looked at the relationship between kinematic instability and pathologic vertebral fractures in patients with metastatic disease of the spine. Their work, “Large Lytic Defects Produce Kinematic Instability and Loss of Compressive Strength in Human Spines,” appears in the May 19, 2021, edition of The Journal of Bone and Joint Surgery. 

When OTW asked why this relationship has not been previously explored, Ron Alkalay, Ph.D., who holds appointments at both facilities, commented, “Historically, research regarding the effect of a lytic lesion on the strength of human spines has been focused on simple mechanical assessment under quasi-static compressive loading.”

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“Our motivation for evaluating the association between the lesion-mediated mechanical instability and the degradation of the spine's strength,” added Dr. Alkalay, “stems from our assumptions regarding the pathomechanics underlying the clinical observation of increased spine instability as a driver for mechanical pain in patients with metastatic disease of the spine.”

For this study, the research team mechanically simulated two patterns of lytic defects on cadaver spines (24 3-level thoracic and lumbar segments) from female donors (aged 47 to 69 years) and assessed them in axial compression (180 N) and axial compression with a flexion or extension moment (7.5 Nm).

The two lytic defects the team tested were: a vertebral body defect, corresponding to Taneichi model C (n = 13); and the model-C defect plus destruction of the ipsilateral pedicle and facet joint, corresponding to Taneichi model E (n = 11).

The team then retested kinematic response and compression strength. According to Dr. Alkalay, “Under a flexion moment, and for both models C and E, the lesioned spines exhibited greater flexion range of motion (ROM) and axial translation than the control spines.”

“Both models C and E caused lower extension ROM and greater axial, sagittal, and transverse translation under an extension moment compared with the control spines…model E, compared with model C, caused significantly greater changes in extension and torsional ROM under an extension moment, and greater sagittal translation under a flexion moment. For both models C and E, greater differences in flexion ROM and sagittal translation under a flexion moment, and greater differences in extension ROM and in axial and transverse translation under an extension moment, were associated with lower compressive strength of the lesioned spines.”

Co-author Mike Groff, M.D. summarized the team’s results to OTW, “We have shown that the extent of destruction of the vertebral body by lytic foci degrades the motion response of the human spine, signifying the degradation of the spines mechanical stability and that this degradation was associated with the loss of the vertebral body spine’s mechanical strength. This study allows clinicians to be more accurate in the assignment of risk for vertebral column collapse for patients presenting with large metastatic lytic defects in the spine.”