Introduction: Vertebroplasty is used to treat painful osteoporotic vertebral fractures, and involves transpedicular injection of bone cement into the fractured vertebral body. During injection, the fluid cement begins to “harden” to a solid, enabling it to support mechanical load. But the mechanical efficacy of vertebroplasty can be improved by using cements which disperse evenly throughout the vertebral body during injection (1). We hypothesise that a better cement dispersion is obtained with cements that have a slower viscosity increase during hardening. We test this using a numerical model.

Methods: A computer model mimicking the plate- and rod-like morphologies of cancellous bone was loaded into a commercial fluid dynamics package (CFX). During injection, viscosity increased linearly with time to simulate the hardening behaviour of the cement (2). The rate of viscosity increase was altered to mimic the hardening behaviour of 5 different cements, with the rates of increase chosen to encompass the hardening behaviour of commercial vertebroplasty cements (1). Simulations were run for 13 seconds, with cement injection at 1.5 mm/s. Cement dispersion was quantified by the proportion of marrow replaced by cement during injection. Injection pressure was also recorded.

Results: Injection pressure increased with time (p< 0.001), and maximum pressure correlated with the rate of viscosity increase (r2=0.7). The proportion of marrow replaced at the end of the experiment was inversely proportional to the rate of viscosity increase (r2=0.85). Cements with a rapidly increasing viscosity do not fully infiltrate regions of bone with plate-like morphologies, leading to a poorer cement dispersion.

Conclusion: Cements with slower hardening characteristics are dispersed more evenly throughout cancellous bone. Such cements may provide safer and more effective vertebroplasty procedures.

Conflicts of Interest: None

Source of Funding: Bupa Foundation

Introduction: Vertebroplasty is increasingly used in the treatment of painful osteoporotic vertebral fractures, and involves transpedicular injection of bone cement into the fractured vertebral body. Effective infiltration of the vertebral body cancellous bone by the cement is determined by the cement viscosity, and by the permeability of the bone. However, it is unclear how permeability is influenced by regional variations in porosity and architecture of bone within the vertebral body. The aim of the present study was to investigate how permeability is influenced by porosity and architecture of cancellous bone mimics.

Methods: Cylindrical polyamide mimics of two types of cancellous bone structures were fabricated using selective laser sintering (SLS) techniques. Structure A had the rod-like vertical and horizontal trabeculae typical of the anterior vertebral body, while structure B had oblique trabeculae typical of the posterior-lateral vertebral body. Structure B had fewer trabeculae than A. Porosities of 80 and 90% were represented for both structures. Golden syrup, which has a viscosity similar to bone cement1, was injected into the mimics at a constant speed using a ram driven by a materials testing machine. Pressure drop measurements across the mimic, made using a differential pressure transducer, were obtained at five different injection speeds. Permeability of each mimic was calculated from these measurements2. Two more repeat permeability measurements were performed on each mimic.

Results: Repeat measurements were always within 12% of the mean value. For structure A the mean permeabilities were 1.26×10-7 and 1.82×10-7m2 for the 80 and the 90% porosity mimics respectively. The corresponding mean permeabilities for structure B were 1.92×10-7 and 2.86×10-7m2.

Discussion: These preliminary results indicate that higher permeabilities occur in structures with higher porosities, and with structures containing fewer trabeculae that are arranged obliquely. Since permeability is a determinant of cement infiltration, taking into account patient-specific bone architecture parameters may improve the safety and clinical outcome of vertebroplasty. Future experiments will clarify in more detail the architectural parameters that have greatest effect on permeability.