BIOMECHANICAL EVALUATION OF AN MRI-SIGNAL INDUCING BONE CEMENT FOR VERTEBROPLASTY

Abstract

Introduction: We developed a signal inducing bone cement for surgical interventions under MR guidance. This cement is based on conventional polymethylmeth-acrylate (PMMA), which is mixed with 0.9% saline solution and a contrast agent (CA), or with a hydroxyapatite based bone-filler (Ostim^®, aap Biomaterials, Germany). This signal inducing cement should allow bone filling procedures, like vertebro- and kyphoplasty, under MR guidance in an open Highfield MR Scanner. As we added the signal inducing substances (saline solution, CA, bone substitute) to the PMMA, we changed the biomechanical properties of the cement.

The purpose of this study was to evaluate the biomechanical properties of the signal inducing bone cement for vertebroplasty in a spine model.

Materials/Methods: We placed cadaveric vertebral bodies (n=18, of 4 lumbar spines) between the crosshead and baseplate of a universal testing machine (Zwick^®, Germany) and compressed to failure. Then, we injected cements into the broken vertebral bodies through a transpedicular approach on both sides, under image intensifier control. The so treated vertebral bodies were then tested again in the testing machine. We injected three cements: a conventional PMMA cement (BonOs^®, aap Biomaterials, Germany, 12g PMMA, 5 ml MMA), an NaCl-cement compound (3 ml 0.9% saline solution, 12g PMMA, 5 ml MMA) and a bone substitute-cement compound (3 ml Ostim^®, 12g PMMA, 5 ml MMA). As the CA amount is negligible (< 9μl), it was neglected for these tests. Each cement type was injected in 6 vertebral bodies.

We defined the initial strength (N) of the vertebral bodies as the load at failure, and the strength after treatment as the maximum load, which occurred within the first 6 mm of compression.

Results: The initial strength of the vertebral bodies (n=18) was 4179 N (SD 497 N). The strength after treatment was 7433 N (SD 503 N) for the conventional cements (n=6), 5900 N (SD 376) for the NaCl-cements (n=6), and 7000 N (SD 413 N) for the Ostim^®-cements (n=6).

Discussion: Although the PMMA cement is weakened by dilution with the signal inducing substances (saline solution, CA, bone substitute), the MRI-cements restored the initial strength of the vertebral bodies. The results suggest that these MRI-cements meet the biomechanical requirements for vertebroplasty, and can be used for MRI guided vertebroplasty.