Abstract
Summary Statement
There are no standardised methods for assessing the cement flow behaviour in vertebroplasty. We propose a novel methodology to help understand the interaction of cement properties on the underlying displacement of bone marrow by bone cement in porous media.
Introduction
Concerns related to cement extravasation in vertebroplasty provide the motivation for the development of methodologies for assessing cements (novel and commercially available) and delivery systems. Reproducible and pathologically representative three-dimensional bone surrogates are used to understand the complex rheology underlying the two-phase flow in porous media.
Patients & Methods
The bone surrogates were achieved by first developing CAD models then manufacturing the physical models through a suitable rapid prototyping technique. MicroCT 100 (Scanco Medical, Switzerland) was used to assess the variability in the model morphology (n=8). Contact angle measurements were performed on the material to compare the surface wettability to that of bone. The surrogates were filled with bone marrow substitute (Carboxymethyl cellulose 2.5 % in water, 0.4 Pa.s) then 5 ml of silicone oil (Dow Corning Corp. 200® Fluid, 60 Pa.s) was injected at a constant flow rate (3mL/min) using a syringe pump. The injection was radiographically monitored and the video sequences were captured. Experiments were repeated three times. The displacement of the syringe plunger and the force applied on the plunger were recorded. Image processing was performed on the video sequences to quantitatively describe the resulting flow patterns and calculate parameters including the time of leakage and the mean spreading distance.
Results
The variability in the model morphology was very low with a strut thickness of 0.253 ± 0.010 mm and a pore spacing of 0.83 + 0.01 mm. The surface wettability was very similar between all materials with a contact angle around 65°. The measured displacement of the syringe plunger confirmed the flow rate to be constant at 3 ml/min. The peak injection pressure was 0.443 ± 0.013 MPa which is well below the reported clinical measurement of injection pressure during vertebroplasty1. Anterior oil leakage occurred at 34.6 ± 4.71 seconds. The oil never reached the posterior wall and the mean spreading distance at the end of the injection was 23.39 ± 1.11 mm.
Discussion/Conclusion
These complex three-dimensional bone surrogates provide a clinically relevant representation of the in vivo situation in terms of geometry, porosity and permeability. They overcome limitations of previous models by being constant in terms of both porosity and geometry which is crucial to reduce the variability, render the experiments reproducible and shift the focus onto understanding the cement flow behaviour. The proposed methodology will help study cement-fluid interaction to get better representation of in vivo cement flow patterns and provide a tool for validating computational simulations. Funding was provided by the EU under the FP7 Marie Curie Action (PITN-GA-2009-238690-SPINEFX).
