Advances in the performance and longevity of total joint arthroplasty (TJA) have been enabled by related progress in implant materials, device designs, and surgical techniques. Successful TJA also depends upon adequate bone quality to provide an enduring mechanical foundation. Bone quality can be defined as the ability to repetitively withstand physiologically-relevant loads without excess deformation or fracture. It is now recognized that bone quality encompasses more than just material quantity, i.e. densitometrically-measured bone mass. Bone quality is also determined by: material composition and arrangement, cortical and cancellous structure, and extent of microdamage. These properties, together with the appropriate mass, confer bone with the biomechanical competence needed to meet the repetitive load-bearing demands imposed by total joint implants.

The need for TJA continues to increase in the aging global baby-boomer population. Unfortunately, this group is also experiencing increases in related comorbidities including: osteoporosis, kidney dysfunction, and diabetes, among others. Collectively these three comorbidities afflict more than 74 million Americans, and each is increasing at 2–8% annually. More importantly, each of these comorbidities negatively affects bone quality through alterations in bone turnover independent of bone mass changes commonly associated with these diseases. Specifically, alterations in bone turnover result in abnormal mineral-to-matrix ratios as measured by Fourier transform infra-red (FTIR) spectroscopy (Fig. 1) and altered Young's moduli (shape-independent resistance to deformation) as measured by nanoindentation (Fig. 2). These parameters are related to bones' fracture toughness and load-bearing capabilities, respectively. Also, low bone turnover is associated with mechanically important structural changes, i.e., decreased trabecular thickness (Fig. 3), cortical thickness and cancellous volume. Furthermore, low bone turnover may result in reducing the repair rate of physiologically – induced bone microdamage. This may lead to increases in the number or length of bone cracks, crack coalescence, and ultimately reduced energy needed for fracture.

Therefore, patients needing TJA who also have comorbidities associated with abnormal bone quality are at risk for inferior arthroplasty results. Recognition and treatment of the TJA-relevant biomechanical implications of these comorbidities may help improve outcomes.

Carbon nanotubes are an exciting new type of material and have extraordinary properties (1). A special category of carbon nanotubes (multiwalled or MWNT) is flexible yet have tensile strengths 200 times stronger than traditional carbon fibers (2). Because of their extremely large surface area-to-volume ratio, theory suggests that MWNTs can bond more strongly to polymethylmethacrylate (PMMA) than any other material tested (2). The combination of large tensile strength and strong interfacial (PMMA matrix) bonding suggests that when added to bone cement, MWNTs could bridge and arrest fatigue or impact cracks and thereby favorably improve the clinical performance of bone cement.

The objective of this study was to determine the validity of this hypothesis and whether MWNTs can significantly improve the tensile properties of PMMA. Methods MWNTs (20–30 nanometers in diameter, 20–100 microns long) were grown on a fused quartz substrate by the thermal decomposition of xylene in the presence of a metal catalyst. They are formed in well-aligned mats and grow perpendicular to the walls of a tubular reactor. As a first approach MWNTs were separated and dispersed through the liquid monomer component of PMMA by using an ultrasonic probe. The remaining polymer component was then mixed with this dispersion and the product was used to prepare specimens by casting in molds. Since prior work in other polymer systems (3) indicated that small concentrations of MWNTs could significantly affect a polymer’s physical properties, only fractions (1/16, ¼ and ½) of 1% of MWNTs (by weight) were used to prepare tensile test specimens. Control (0% MWNTs) and experimental (MWNT containing) groups of PMMA specimens were cured in air at room temperature for 7 days and then pulled to failure at 6 mm/min in a protocol conforming to ASTM D638. Maximum load, strength, results a total of 41 specimens have been prepared and tested: 13 controls, 9 with 0.063%, 10 with 0.25%, and 9 with 0.5% (by weight) of MWNTs. Carbon nanotubes improved the tensile load bearing properties of all experimental groups from 17% to 24%, and these values were significant (p=0.01 and p=0.02) for the 0.25% and 0.5% concentrations. The lowest concentration of MWNTs made the smallest improvement (17%) and this was not significant (p=0.07). Scanning electron microscopic examination of the fractured surface revealed nanotubes that were well distributed throughout the matrix.

Discussion: These preliminary results clearly demonstrate that carbon nanotubes can significantly improve the mechanical performance of bone cement. This result is especially encouraging because the MWNTs were added only to the monomer component. Additional performance enhancement may be expected from ongoing work using higher concentrations of MWNTs and their dispersion into the polymer component of bone cement in addition to the monomer component.

Since MWNTs are also electrically conducting and have magnetic properties, MWNTs may also help dissipate the heat generated by polymerization or permit bone cement with an “engineered” mechanical anisotropy. Although static tensile tests are an incomplete measure of bone cement, these preliminary results are very encouraging and motivate continuing study of the more clinically relevant (impact resistance, fatigue properties, etc.) measures of the mechanical performance of MWNT augmented bone cement.