The Biomechanical Study Of A Pedicle Screw Fixation Augmented By PMMA On Fresh-frozen Cadaveric Osteoporotic Vertebrae

ObjectiveThe spinal pedicle screw internal fixation system is widely applied to spine posterior surgery, which plays an important role to the stability of the spine. For posterior spinal stabilization, loosening of pedicle screws at the bone-screw interface is a clinical complication, especially in the osteoporotic population. Axial pullout testing is the standard pre-clinical testing method for new screw designs and different augmentment techniques although it has questioned clinical relevance.The aim of this study was to determine the fixation strength of three current osteoporotic fixation techniques and to investigate whether or not pullout testing results can directly relate to those of the more physiologic fatigue testing.MethodsThirty-eight osteoporotic, human lumbar vertebrae were instrumented with pedicle screws. Before placing pedicle screws, inject bone cement to different parts of the pedicle by enhancer-injector. According to the different parts of the cement injection to the pedicle, the fresh-frozen cadaveric osteoporotic umbar vertebrae specimens are divided into four groups:Group A- non-augmented group (Blank control group); Group B-augment in pedicle group; Group C-augment in vertebral body group; Group D-augment in overall channel group.The bone cement injection site of both sides pedicle is different, on the one side, the pedicle screw undergo pure axial pullout test (5 mm/min). On the other side, the pedicle screw undergo cyclic fatigue test. Cyclic fatigue test was first performed on one pedicle, using a cranial-caudal sinusoidal, cyclic (1.0 Hz) fatigue loading applied at the screw head. The initial compressive forces ranged from 25 to 75 N. Peak force increased stepwise by 25 N every 250 cycles until a 5.4-mm screw head displacement.Results1.Cyclic fatigue test:under the condition of different group pedicle screw head having the same displacement, there is no obvious difference statistical significance when the comparation between group B and group D (P>0.05):There is obvious difference statistical significance when the comparation among group B, group C and group D (P<0.05). When compared to the control group, there is obvious difference statistical significance (P<0.05). When different group pedicle screw head have the same displacement, group B and group D increased fatigue force27% while group C reduced fatigue force 7%.2.Axial pullout test:when compared to the group C, group B and group D increased axial pullout force and the difference has statistical significance (P<0.05); When the comparation between group B and group D, the difference has no statistical significance (P>0.05); When compared to the group A, the difference has statistical significance (P<0.05)Conclusions1. For the osteoporotic spine, pedicle augmentation showed the best biomechanical stability. Although pullout testing was more sensitive, the differences observed were not reflected in the more physiological fatigue testing, thus casting further doubt on the clinical relevance of pullout testing.2.Through the axial pull-out test and cycle fatigue test, the pedicle augmentation can obviously improve the biomechanical stability of the screw. Although overall channel augmentation has the same biomechanical stability with pedicle augmentation, the long-term stability has certain risk due to the existence of the second interface. Therefore, clinical should as far as possible to augment the screw close to the vertebral pedicle.