Adaptation of acrylic photopolymerized resins as model bone cements in total hip arthroplasties

The substitution of poly(methyl methacrylate) (PMMA)-based bone cements with photopolymerized acrylate resins for total hip arthroplasties (THAs) was explored. Using a bis-phenol-A-diglycidylether (bis-GMA)-based resin with a blue photoinitiator, we performed three sets of experiments. Two were parametric studies to evaluate the evolving adhesive strength with both illumination time and inert filler content. The last effort studied typical photocuring exotherms for these resins to compare the risk for thermal necrosis with bone cements. An annular cavity adhesive joint construction was designed between two transparent poly(vinyl chloride) (PVC) cylinders, the annular separation distance being preserved by a plug assembly. This cavity was a model for the cavity between the hollowed out femur and a cemented total hip prosthesis. The cavity was filled with resin and cured using a blue-LED light source rotating within the inner ring. For composite cylinders using cured neat resin, shear strengths were measured to be approx. 120 MPa, much higher than shear strength measurements on bone cements. While the model measurements allowed for determinations of strength in this annular ring configuration, the actual measured strengths seem much larger, due, in part, to off-axis loading, curing shrinkage stresses and uneven bond area. Over 10 min of illumination time was required for measurable strength. Adding fillers to the neat resin led to even higher resin to substrate adhesion, attributed to better overall coverage which if there were small amounts of off-axis loading, would be larger. At least trends could be established. Comparing the photocalorimetry results, typical exotherms were approx. 200 J/g, much lower than published values for mix and set bone cements. While more compliant and weaker than traditional bone cements, further development of photopolymerizable resins may reduce the risk for thermal necrosis if a satisfactory light conveyance technique could be developed.