Bone Cements for Orthopedic Applications

Poly(methyl methacrylate) (PMMA) bone cements have been vastly used in orthopedic surgery for augmentation and fixation of total-joint prostheses. The material also provides an interface responsible for stabilization and transferring of mechanical loads between implant and bone. Even though the mechanical and physical properties of acrylic powder-liquid cements are well understood and described in the literature, there is a vast interest in the development of new cement formulations to overcome a number of drawbacks still associated with the application of this material in vivo.

Areas of interest to be explored in bone cements include:

  • Incorporation of novel antibiotics and bioactive compounds.
  • Improved formulations with optimal antibiotic release for two-stage surgeries.
  • Highly viscous and pseudoplastic formulations for use in the augmentation of spinal fractures.
  • Calcium phosphate containing formulations with adequate handling, injection and mechanical performance.
  • Resorbable formulations for use as bone fillers.

The two-solution bone cement (TSBC) has been developed as a potential alternative to the traditional powder-liquid cement formulations. The TSBC system has the advantage of being pre-mixed in cartridges not requiring mixing of the components or waiting for delivery. Pre-mixing is advantageous because it enables the components of the mixture to reach full swelling while being doughy and highly viscous from the beginning of the mixing process. The TSBC system exhibits a combination of physical and mechanical properties that are of clinical relevance for orthopedic, and specifically, spinal applications. The highly viscous TSBC system has been observed to provide improved interdigitation with cancellous bone, and its superior pseudoplasticity simultaneously facilitates flow through needles while inhibiting deleterious extravasation by viscosity recovery at the delivery sites (Figure 1).

Figure 1. Simulated vertebroplasty in cadaver vertebrae using TSBC. (left) Fluoroscopic view of cannula placement via pedicle; (center) bone cement injection (mass of cement is indicated by the arrow); and (right) cross section of a VB treated with 5 mL of TSBC prepared with PMMA microspheres. Cement was injected in the VB immediately after mixing through a mixing nozzle using a 12-gauge and 15-cm length delivery needle. High viscosity allows for improved interdigitation and minimal risk of extravasation. VB: vertebral body. Reference: Rodrigues et al. Spine J 2011;11(5):432.

Some of the limitations of the original TSBC included short setting-time, high residual monomer content, and lack of bioactivity. We are currently working on improving each of these drawbacks by adding high concentrations of calcium-phosphate fillers in the cement formulation (Figure 2).

Figure 2: Morphology of calcium-phospate containing TSBCs prepared at 50wt% filler concentration. (left) HA-containing TSBC after 35 days of immersion in phosphate buffered saline medium (PBS); (center) cancellous bone from an extracted implant for comparison with cement porosity; and (right) Brushite-containing TSBC after 35 days immersion in PBS. The increased porosity mimics bone topography, which will allow for improved bone integration.