Analysis of the causes of subsidence of modern expandable cages for vertebral body replacement in the surgical treatment of thoracolumbar spine injuries

Objective. To analyze the causes of subsidence of modern support cages for vertebral body replacement in the early postoperative period after surgical treatment of thoracolumbar spine injuries.

Materials and Methods. A retrospective analysis of the data of 46 patients operated on in a single surgical session for unstable injuries of the thoracolumbar spine using a telescopic extendable vertebral body cage was performed. The degree of cage subsidence was assessed according to the criteria of Marchi et al.:  penetration of the implant into the body of the adjacent cranial or caudal vertebral by 25% – grade 1,
25–50% – grade 2, 50–70% – grade 3, 75–100% – grade 4. A comparative assessment of demographic, clinical, and radiographic parameters was performed in patients with and without cage subsidence within one year after surgery.

Results. Implant subsidence was detected in 76.5% (n = 13) of patients intraoperatively and in 23.5% (n = 4) after 4 months during an outpatient appointment. Subsidence into the cranial body prevailed (76%, n = 13). The anterior/posterior sequence of surgery stages combined with osteopenia and osteoporosis dominated in the study group (83.3%, n = 10). Quantitative parameters such as age, segmental angle, ROI in HU, surface contact area index, as well as qualitative parameters such as female gender, period of injury, and its low-energy nature had statistically significant differences between the study and control groups (p < 0,05). The augmentation of the screws and the length of fixation did not affect the formation of subsidence, but were associated with its magnitude.

Conclusion. The use of modern expandable body replacement cages for reconstruction of the anterior spinal column leads to their subsidence in some cases. Patient age, female gender, reduced bone density, the area of the bone-implant contact, anterior/posterior stabilization, and the late period of injury significantly affect the formation of subsidence when using expandable vertebral body replacement cages. A mean implant-to-vertebral endplate contact area ratio of less than 0.4 is a promising predictor of subsidence which requires further study.

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