Comparative Studies On Biomechanical Evaluation And Preliminary Clinical Application Of Four Kinds Of Pedicle Screws In Osteoporosis

Background:Osteoporosis, characterized by decreased bone mineral density(BMD) and bone tissue microchitectural deterioration, has been a major threat for aging population in China. Decreases in BMD in osteoporosis reduce the stability of pedicle screw, with the screw loosening and pullout incidence of 0.6%- 12.6%. Therefore, it is of great importance to achieve enhancement of pedicle screw stability and reduction of screw loosening or failure in patients with osteoporosis.Pedicle screw instrument has been widely used as fixation techniques in osteoporotic patients in spinal surgeries. In the present study, we chose four kinds of clinical frequently-applied pedicle screws with different structural design as experimental subjects, namely, single-threaded pedicle screw(SPS), double-threaded pedicle screw(DPS), expandable pedicle screw(EPS) and novel cement-augmented pedicle screw with gradual change side-hole(CAPS). These four types of pedicle screws differ in mechanical performance and screw fixation strength. Furthermore, it is still unclear about their respective mechanical properties and therapeutic efficacy in clinical application. Therefore, this study aimed to compare the stability and fixation strength of these four kinds of pedicle screws under the condition of osteoporosis by the combination of “finite element model- ex vivo biomechanical study- in vivo clinical research”, providing evidence for clinical choice of pedicle screws in osteoporotic patients.Objective:Our study was designed to compare the screw stability and preliminary clinical efficacy of these four kinds of pedicle screws(SPS, DPS, EPS and CAPS) by the usage of finite element method, osteoporotic mechanics module and biomechanical study of postmortem human lumbar spine specimens, and to investigate their clinical application through the analysis of clinical data of osteoporotic patients with spinal implant operation.Methods:1) Based on the statistics used in our experiment, four kinds of pedicle screws models and bone model with screw hole were established with Pro/E software. These three-dimensional geometrical models were introduced into the software of finite element analysis(ANSYS) to constitute pedicle screw-bone model. Model validation was examined by convergence test. The pull-out strength of different screws was measured by stress analysis.2) Osteoporosis biomechanics modules and postmortem human lumbar spine specimens were randomized into four groups with different pedicle screws: SPS, DPS, EPS and CAPS. After routine implantation of pedicle screws, the biomechanical fixation stability of screws was analyzed through X-ray measurement, axial pullout test with indicators of maximum pullout strength(Fmax), energy absorption(E) and stiffness.3) A preliminary clinical study was carried out in 231 osteoporotic patients with spinal implant operations. Patients were randomly divided into SPS group(91 cases), DPS group(41 cases), EPS group(63 cases) and CAPS group(36 cases). Data of operative parameters, radiographic evaluation and questionnaire indicators were collected to compare the biochemical fixation strength, interfacial performance and clinical efficacy of four different pedicle screws.Results:1)Four kinds of “screw-bone” finite element model were established with finite element method. Model validation was examined by convergence test and applied to simulate axial pullout test. When the screw was withdrawn at a constant speed, the maximum principle stress distribution was different in these four kinds of pedicle screws. The maximum principle stress located in the rear of SPS and DPS. In EPS, the maximum principle stress lied in bifurcation of expandable end. While in CAPS, the maximum principle stress located in the interface of bone cement and cancellous bone. Axial pullout tests demonstrated that the stability of pedical screw was CAPS > EPS > DPS > SPS.2) In osteoporosis biomechanics modules, the screw stability of CAPS was markedly higher than that of EPS, DPS and SPS(p < 0.05, CAPS > EPS > DPS > SPS, DPS and SPS with no significant difference). In postmortem human lumbar spine specimens, EPS showed higher screw stabilization than DPS and SPS(p < 0.05, DPS and SPS with no significant difference). In EPS group, the claw-like structure improved the interaction of screw and bone tissue, significantly elevating the stabilization of EPS. There was no significant difference of biomechanical indicators between S-DPS and S-SPS, meaning that the increased screw thread density in DPS did not elevate its stability. In vertebral biomechanical experiments, X-ray results showed that bone cement wrapped up CAPS screw tip to provide fixation effects, improving the stability of CAPS. Our present data showed that CPPS was better than EPS in terms of screw stability, indicating that cement augment in CAPS markedly improved the stabilization property.3) All of the included 231 patients were followed up with the mean follow-up period of approximately one year. Operative data showed that the application of CAPS prolonged the operation time without blood loss increase. Both post-operation VAS and ODI scoring were markedly decreased in four groups as compared with that of pre-operation. Notably, CAPS and EPS showed better improvement compared to SPS and DPS(p < 0.05). Radiographic results revealed that CAPS and EPS kept good fixation location without screw loosening or fracture, with the osseous fusion rate of 96.8% and 100%. However, the loosening incidence of SPS and DPS was 3.3% and 2.3% respectively.Conclusions:1) In osteoporosis, the screw stability and fixation strength of CAPS and EPS were significantly better than that of SPS and DPS, mainly attributed to the bone cement augment of CAPS and screw-bone interface of EPS.2) In osteoporotic patients with spinal implant operation, the application of EPS and CAPS exhibited decreased bone cement leakage or screw loosening incidence and better fixation stability. Our present data showed that EPS and CAPS may serve as novel potential screws for spinal implant operation in osteoporotic patients.