(B-66) Effect of Tow Sizes on Poly(methyl methacrylate) Bone Cement and Carbon Fiber Composites

Introduction:: Poly(methyl methacrylate) (PMMA), commonly known as bone cement, is FDA approved and commercially available with different trade names and traditionally used to secure implants in various orthopedic and trauma surgeries including knee and hip arthroplasty [1]. However, PMMA is not used as a replacement for metallic implants or for load-bearing bone repair due to its weak mechanical properties. Therefore, several studies investigated incorporating additives such as ceramic, bio-glass, inorganic nanomaterials, and carbon fibers to improve the mechanical properties of PMMA bone cement [2]–[4]. Carbon fiber is a lightweight, high-strength material which is extensively used in aerospace and automotive industries but also recently as medical implants in different form factors [5]. Several aspects of the carbon fiber such as tow size, braid type, and form factor influence the mechanical properties of the final product despite having the same original carbon fiber filament. The objective of this work is to investigate the feasibility of using a carbon reinforced traditional bone cement for targeted orthopedic applications that may require different stiffness and strength depending on the host bone or location. We report and discuss the influence of braided carbon fibers on improving the strength and stiffness of the reinforced bone cement for orthopedic applications.


Materials and Methods:: Carbon fiber and bone cement were provided by MediCarbone (Tucson, AZ) for this study. Biaxially braided carbon fiber in three different tow sizes (3K, 6K, and 12K) and in four different diameters (1”, ¾”, ½” and ¼”) were used in this study. A commercially available bone cement was used and prepared based on the manufacturer instructions. A Teflon mold with 10 mm inner diameter and 200 mm long was used as a mold to provide specimens that are 10 mm in outer diameter and at least 150 mm in length. Carbon sleeve was slid inside the mold and pressed to the edge of the mold before injecting the bone cement. The bone cement was then mixed for 4-5 min, transferred to a 20 ml syringe, and injected into the mold. The mold bottom was sealed until the cement fully polymerized. Once polymerized, the rods were removed from the Teflon mold and tested in flexure using a 4-point fixture on a universal test setup (MTS Criterion® Model 42, Eden Prairie, MN). A sample size of at least four was used for this study. SEM images (Hitachi T4000 tabletop SEM, Tokyo, Japan) of carbon sleeves were taken before and after injection and polymerization of bone cement to analyze the degree of cement impregnation. Degradation of the material was studied by soaking 5 mm diameter and 50 mm long rods in PBS at 37°C for up to 30 days and samples were evaluated in three-point flexure.


Results, Conclusions, and Discussions:: The average flexural stiffness of the 10 mm rods tested are summarized in Table 1 and Figure 1 shows the typical stress-strain profiles of the tested carbon fiber PMMA composite rods made with different grade carbon braids under four-point flexure. Results indicated that stiffness of the carbon reinforced PMMA composite varied from 1 to 3 GPa. A stiffness of 3 GPa for PMMA-based material could be beneficial but values are not in the range of a load-bearing bone (6-20 GPa) where the composite can be used for load-bearing applications and replace metallic implants.

It was noticed that for samples made with a particular tow size, stiffness increased with increasing diameter. The main reason we believe for this trend is that with increasing braid diameter, more stretching is required to fit the braid inside the mold, creating more pores and providing more infiltration of injected bone cement. This increased infiltration provides better carbon to bone cement matrix bonding and hence improved properties (refer to Figure 2). On the contrary, the effect would be opposite if the braid diameter is smaller than that of mold – more compression would be required to fill in the space closing out the pores and decreasing the infiltration and causing inferior bonding between carbon fibers and bone cement.

A secondary result noticed that for a particular braid diameter, increasing the tow size also reduced the elastic modulus of the specimens. A higher tow size has more fibers and therefore more surface area requiring more bone cement for infiltration and difficulties with injecting the viscous traditional free radical bone cement. In fact, the viscosity of bone cement increases with time and the injection and infiltration should be accomplished before the setting. This would be difficult for the larger tow size carbon fiber braids. Therefore, low tow sizes are preferable for composite applications.

The 5 mm and 50 mm pins were fabricated using 3K tow and the samples were tested in three-point flexure after soaking in the 1X phosphate buffered saline for up to 30 days. Results indicated no difference in terms of elastic modulus and flexural strength.


Acknowledgements (Optional): :

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[2] L. Chen et al., “Silicate bioceramic/PMMA composite bone cement with distinctive physicochemical and bioactive properties,” RSC Adv., vol. 5, no. 47, pp. 37314–37322, 2015, doi: 10.1039/C5RA04646G.

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[4] A. V. Boehm, S. Meininger, A. Tesch, U. Gbureck, and F. A. Müller, “The Mechanical Properties of Biocompatible Apatite Bone Cement Reinforced with Chemically Activated Carbon Fibers,” Materials (Basel), vol. 11, no. 2, Jan. 2018, doi: 10.3390/MA11020192.

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