In vitro study of mechanical properties of cannulated pedicle screws in terms of design variables at patients with osteoporosis

Today, the incidence of osteoporotic cases are increasing due to the increasing of elderly population. One of the most commonly used therapy in the treatment of osteoporotic vertebral fractures is pedicle screw placement. During the use of pedicle screw in these patients with spinal osteoporosis, one of the biggest problem is screw loosening due to the lack of holding strength in the bone-screw interface. Cannulated pedicle screws have been developed and as a reinforcement material such as polymethylmethacrylate (PMMA) cement are utilized to get rid of these problems.In this study, six different cannule types were designed and manufactured. Noncannullated screw was also used in tests as control group. Position, numbers and orientation of holes/slots normal to the main axis of screw shaft were investigated biomechanically. All experimental groups were tested with cement augmentation and also with artificial fusion .Cannule reduces the torsional strength of pedicle screw. Considering the torsional strength and pull-out properties, to obtain the optimum efficiency various design parameters were studied. Torsion and pull-out tests were carried out according to ASTM F 543. Grade 40 polyurethane foam was used to mimic the healthy bone structure in pull-out tests. Also grade 10 polyurethane foam was used to mimic the osteoporotic bones. Same cement type (PMMA) was used in all tests. Single-sided three holes samples which named S3H were showed the best performance in the cement augmentation experiments. On the other hand double-sided three holes samples which named D3H were showed the best performance in artificial fusion groups. As a result of study although in naturel fusion experiments D3H showed the best performance than S3H in cement augmentation experiments, for the realization of the natural fusion needs a long time and in osteoporotic patients we want the immediate performance of the screws, S3H with cement augmentation was evaluated as the best performance.