Mechanical properties and spreading characteristics of bone cement for spinal applications

Osteoporotic vertebral compression fractures can be successfully stabilized through vertebroplasty, a percutaneous injection of bone cement directly into the cancellous bone found in the vertebral body. Severe complications of this procedure can occur if bone cement leaks out of the targeted vertebra and enters the vascular system or spinal canal. Fractures of adjacent level vertebras may also be a consequence of the procedure since it leads to increased stiffness in the bone. In this work, a range of custom made polymethylmethacrylate (PMMA) bone cements for use in vertebroplasty were prepared in order to study the impact on handling characteristics, rheological and mechanical properties of three variables: the liquid to powder ratio, the concentration of crosslinker (ethylene glycol dimethacrylate) and the concentration of initiator (benzoyl peroxide). Bone cement containing the mean of these variables was used to study the spreading characteristics when injected at different viscosities in an artificial vertebral model made of rigid polyurethane foam. The spreading patterns were scanned using computed tomography and quantitatively described using the indicators circularity and mean cement spreading distance. Stiffness of the resulting foam/cement construct was also evaluated. In order to tailor PMMA bone cement for use in bone weakened by osteoporosis, an attempt was made to produce porous cement with lower Young’s modulus by adding various amounts of Castor oil. An evaluation of the particle release during curing of the porous cement was also made, as an excessive release of particles would limit its clinical use. Results show an increase in setting and dough times when increasing the liquid to powder ratio. A decrease in setting and dough times was noted when increasing the amounts of crosslinker and initiator. Peak polymerization temperature was found to increase with increased liquid to powder ratio, as well as when increasing the amounts of crosslinker and initiator. Addition of crosslinker was found to have the largest positive effect on cement strength. A higher liquid to powder ratio led to a decrease in Young’s modulus. Quantification of cement spreading showed a slight increase in circularity with increased viscosity, while no clear trend could be seen in measuring the mean cement spreading distance. A 10-fold increase in Young’s modulus was found when comparing PMMA filled samples of polyurethane foam with pure foam samples. Adding Castor oil to the cement led to significant lowering of both Young’s modulus and strength. The Young’s modulus was reduced from 1355 MPa for regular cement to on average 566 MPa when 30 vol% oil was added. The ultimate compressive strength decreased from 98 MPa for regular cement to on average 27 MPa with an addition of 30 vol% oil. No significant difference in particle release during curing was found when comparing regular cement with cement mixed with Castor oil. Careful consideration should to be taken when designing any new formulation of PMMA bone cement. Rheological and mechanical properties are key factors for a successful intervention and for the integrity of the treated and adjacent level vertebras. Rigid polyurethane foam was found to provide a realistic and simple model for simulating cement flow in cancellous bone, as well as a base for evaluating the mechanical effects of vertebroplasty. It was also found that generating pores in the cement by adding Castor oil is an effective way of lowering the modulus, making it more compliant with osteoporotic bone.Validerat; 20101217 (root