INTRODUCTION: The standard technique to treat thoracolumbar spinal instabilities is a posterior stabilization using pedicle screw rod systems. Spinal fusion is often controlled using radiographs or CT-scans, but their reliability regarding misinterpretation is continuously under discussion. A measurement device coupled to spinal implants, that can directly monitor spinal loads, might improve follow-up of different spinal stabilization indications.
MATERIAL AND METHODS: Four strain gauges were applied to conventional carbon fiber reinforced polymer (CFRP) rods for posterior stabilization, forming integrated sensor rods. In combination with pedicle screws, a series of 8 cadaver spine test models were instrumented with the sensor rods, creating a bisegmental thoracolumbar stabilization (Th11-L1). A customized measurement system using an integrated Bluetooth chip transmitted the acquired multidirectional strain data to a computer. To be able to simulate a healing process, a standardized fenestration defect was induced at the middle vertebra at T12 and four silicone mats of different shore hardness (0 ShA, 8 ShA, 30 ShA, 45 ShA) were inserted into the defect with 0 ShA simulating a fractured vertebra. The whole setup was monitored through both, the implant measurement and an optical 3D system. Flexibility tests with pure moments of 5 Nm were applied to the specimens using a custom-made spine tester.
RESULTS: The newly introduced sensor rod system demonstrated reproducible strain data. Even with gross differences in flexibility and bone density between the specimens, significant difference between the maximum strains with each silicone was measured. With increasing hardness of the silicone, the measured strains acting on the rods decreased. An average increase of 365% in strain was observed between the intact state and the sequence after fenestration was performed, instrumented with the least stiff silicone (0 ShA). During the following simulation of the healing process, from 0 ShA to 45 ShA, a significant decrease with an average of 317% in strain was observed.
CONCLUSION: The introduced rod sensor system demonstrated reliable strain measurement for different stable and unstable conditions in a standardized thoracolumbar spine test model. Increasing stability equally reduced strain on the implant. A development of the first prototype with a further miniaturization of the system and integration in the rod is planned, to enable future animal model implantation, generating in vivo data and paving the path for a trial in patients.