Biomechanical Evaluation At Different Bmd Levels And Preliminary Application In Clinical Practice Of The Expansive Pedicle Screw

Background The use of pedicle screw presents 3-D biomechanical advantages in the spine surgeries of internal fixation. These advantages depend on pedicle screws to retain bony purchase, which will be substantially compromised in the osteoporotic bone. Several previous studies have demonstrated that the fixation strength of pedicle screw can be significantly improved with an expansive design. However, there are some concerns whether this kind of pedicle screw, named expansive pedicle screw, is safe to use in the spine with osteoporosis or severe osteoporosis. Few studies are available to refer the performance of expansive pedicle screw used in the presence of osteoporosis or severe osteoporosis diagnosed according to the clinical criteria. Therefore, our study was initiated and performed in order to elucidate these concerns. Objectives The aim of this study is three-fold: (1) to evaluate whether the fixation strength of the expansive pedicle screw is competent in the human cadaver vertebrae with different levels of bone mineral density and whether the fixation strength can be further improved with the augmentation of a newly developed calcium-based biomaterial; and (2) to measure the micro-architectural indices and mechanical properties of cancellous bone in the vertebral body and to analyze the interrelationships between these parameters and the relationship between them and the corresponding maximum pullout strength of pedicle screws; and (3) to observe the efficacy of expansive pedicle screw in the management of spinal diseases coincided with osteoporosis and to preliminarily make sure of indications for the expansive pedicle screw fixation to provide theoretical and practical evidence for future clinical trials and extensive application of expensive pedicle screw in spine surgeries.Materials and Methods Fresh human cadaver spines were stratified into four levels: normal, osteopenia, osteoporosis and severe osteoporosis, according to the value of bone mineral density. The vertebra was bilaterally instrumented pedicle screws with four protocols, including conventional pedicle screw without augmentation, expansive pedicle screw without augmentation, conventional screw with augmentation and expansive screw with augmentation. The statistical method of"balanced incomplete randomized blocks"was used to assign the pedicles for the four protocols at the same level, resulting in nine screws per group per level. Screw pullout tests were conducted. On the load-stroke curve, the maximum pullout strength, stiffness and energy absorption were determined. The differences between the groups at the same bone mineral density level but different inserting protocols, or between the groups at different bone mineral density levels but same inserting protocol, were compared. Following the pullout tests, the vertebral specimens were collected. Subsequently, cancellous bone cylinders were drilled for micro-CT scanning and compression tests from 6 vertebrae in the groups without augmentation with the new biomaterial. The interrelationships between micro-CT, mechanical parameters themselves and the relationship between them and the corresponding maximum pullout strength of pedicle screws were analyzed. A preliminary clinical study was carried out with prospective design, in which 63 patients, who suffered from different spinal diseases and concurrent osteoporosis, were included. The questionnaires of VAS, ODI and JOA were administered preoperatively and at the latest follow-up to assess the clinical and functional outcomes. Also, the radiographic data were recorded, from which common observations such as the condition of pedicle screws, cage position and the fusion rate of bone graft were taken. Given special measurements were needed for one certain pattern of disease, they were gotten respectively. For example, in order to evaluate the orthopedic results in the treating of spondylolisthesis, the Taillard index (the degree of slip), the slip angle, the average disc height and the lumbar lordosis were measured on the lateral X-ray view pre-and post- operationally, and at follow-up.Results Given the same specimen, the maximum pullout strength and stiffness of expansive screw were significantly higher than those of conventional screw. When the same type of screw was used, the maximum pullout strength of the calcium based cement augmented group was higher than that of the non-augmented group, but the calcium based cement seemed to have very limited impact on improving the stiffness of either conventional or expansive pedicle screw. The pullout strength and stiffness of the expansive screw, augmented conventional screw and augmented expansive screw groups at the osteoporotic level were comparable to those of the conventional pedicle screw group at the osteopenic level. However, under the severely osteoporotic bone environment, the pullout strength of pedicle screw with whatever placement protocol was significantly lower than that of the conventional screw group at the osteopenic level. With the increasing severity of osteoporosis, progressive bone volume loss, mechanical incompetence and micro-architectural deterioration were evident. Significant differences in micro-CT and mechanical parameters were found among the different levels of bone mineral density. Strong correlations were extensively observed among micro-CT or mechanical parameters themselves, between micro-CT indices and mechanical properties. The maximum pullout strength of the both pedicle screws highly correlated with bone volume over total volume (BV/TV), trabecular thickness (Tb.Th) and trabecular separation (Tb.Sp) from micro-CT scanning, and with ultimate stress (σult) and elastic modulus (E) from compression mechanical test. In the clinical trial, the 63 cases were all achieved to be followed up. The average follow-up time was about 22 months (rang 12-44 months) with the minimum of one year. Before surgery, the patients demonstrated severe pain and disability in term of preoperative VAS and ODI. At follow-up assessment, the clinical and functional outcome showed significant improvement in all measurements. The mean VAS and ODI scores were significantly decreased by 72.9% and 66.7%, respectively. According to the results of JOA, an obvious efficacy was shown in 73.0% patients and a moderate efficacy was in 20.7%, so the total effective rate was 93.7 (59/63). Significant differences were found in all the three questionnaire scores between pre-operation and follow-up. There were no instances of screw loosening, pullout, or breakage of the expansive pedicle screws. No measurable subsidence or extrusion of the cages was evident at the follow-up evaluation. According to the radiographic evidence taken at the last follow-up, 61 out of 63/all cases achieved the criteria of solid fusion and the fusion rate is 96.8%. For the eight cases of spondylolisthesis, post-operatively the average slip percentage (Taillard index), slip angle and angle of lumbar lordosis were significantly decreased and the average disc height was significantly improved compared to those of pre-operation. In addition, no significant differences in these measurements were found between post-operation instantly and at the final follow-up.Conclusions Our pullout test results demonstrate that the expansive pedicle screw appears feasible and safe in either osteopenic or osteoporotic spine and that calcium based cement augmentation can further improve the initial fixation strength of expansive pedicle screw. The maximum pullout strength of the pedicle screws highly correlated with some parameters from micro-CT scanning and compression mechanical test. To measure these parameters can help to improve the prediction of the fixation strength of pedicle screw. The preliminary clinical trial has shown that the application of expansive pedicle screw in operative managing spinal diseases with concomitant osteoporosis is a feasible and reliable alternative. This fixation technique can create a favorable biomechanical condition to obtain a solid fusion and to produce good clinical results.In summery, all the results of this study reveal that the expansive pedicle screw is valuable to apply in spine surgery, especially in the presence of osteoporosis.