Simultaneous measurement of surface deformation across three aspects of the human vertebral body subjected to compressive loads.

Vertebral metastases represent a significant clinical challenge, greatly impacting spinal mechanical stability and affecting patients' quality of life. A non-invasive method to assess the mechanical strength of vertebrae affected by metastatic lesions would be valuable for surgeons in suggesting whether surgical treatment is necessary to preserve vertebral body integrity. This approach involves the use of patient-specific Finite Element Models (FEMs) to make such assessments. The accuracy of the predictions made by FEMs must be validated against experimental data before they can be adopted in clinical practice. An ongoing project at the Medical Technology Laboratory of the Rizzoli Orthopedics Institute aims to perform non-destructive tests on human vertebral bodies to measure the response of the vertebra under simplified, but numerically reproducible, boundary conditions. Although the experimental procedure was found to be suitable for collecting the data needed to validate FEMs, a major issue was the considerable amount of time required to perform the test series on the vertebral body. In fact, with only one Digital Image Correlation (DIC) system, each test had to be repeated three times to measure deformation on three aspects of the vertebral body. Therefore, the laboratory has purchased two new DIC systems. The aim of this study was to implement and verify the feasibility of an experimental protocol involving the use of three DIC systems to simultaneously acquire the deformation on three aspects of the vertebral body in a single test. The first part of the study was focused on setting up the new DIC systems. An optimized speckle pattern, achievable by painting and airbrushing, was created on cylindrical specimens. The DIC parameters were investigated to identify the optimal conditions, balancing measurement error and resolution at zero strain conditions. Once the new DIC systems had been set up, a pilot study was carried out on a vertebral body. Three DIC systems were mounted on a material testing machine to monitor three different aspects of the vertebral body simultaneously. After creating the speckle pattern on the bone tissue surface, a vertebral endplate was fully constrained to a plate mounted on the load cell of a material testing machine. The vertebral body underwent a compressive test which involved the application of two different load levels (-0.6 kN and -1.0 kN). The load was transferred to the vertebral body by applying a homogeneous pressure distribution to the upper vertebral endplate. The described boundary conditions, developed and validated in previous studies, were specifically designed to allow their replication in FEMs. The test showed that it is possible to measure surface deformations on the three aspects despite interference from the illumination of the three systems. The pilot study demonstrated the feasibility of the proposed methodology that allows to carry out each test in a third of the time and does not require repositioning and refocusing the DIC system each time with a potential benefit on strain measurements. The implemented procedure could be applied to a large number of vertebral bodies, facilitating the collection of extensive experimental data needed to validate the predictions of FEMs. This would ultimately support more accurate clinical decision making in the treatment of spinal metastases and help to optimize the management of affected patients.