Comparative Study Of Expansive Pedicle Screw And Polymethylmethacrylate-augmented Pedicle Screw In Osteoporosis: Biomechanical And Interfacial Evaluations

The pedicle screw instrumentation has become the most commonly used internal fixation techniques in spinal surgery. With growing aging of population in our country, more and more OP patients with spine disorders undergo spinal surgery with internal fixation. However, osteoporosis (OP) severely influenced binding strength of interface between screw and bone and decrasesed the holding force of screw, which usually resulted in loosening of screw, migration or back-out. Therefor, how to effectively increase stability of pedice screw in OP and prevent lossening of screw has become a pressing tough problem in spine surgery.In previous research, we designed an expansive pedicle screw (EPS) and it was proved that EPS can significantly improve screw stability and effectively reduce the risks caused by increasing diameter and length of screw. Intriguingly, we found from the related literatures that, in spite of risks of thermal injury, leakage and neurologic compression, PMMA is widely used for augmentation of screw in OP. However, there are no biomechanical and interfacial comparisons of the novel-designed EPS and traditional polymethylmethacrylate-augmented pedicle screw (PMMA-PS). Objective: To compare stability and interface of EPS and PMMA-PS through experiments in samples in vitro and in animal in vivo, and to provide sufficient theretical basis for wide application of EPS in clinic.Methods:1) Experiments in vitro. OP biomechanical tests blocks, OP cadaveric lumbar vertebrae, sheep lumbar vertebrae in vitro were all randomly divided into three groups. A pilot hole was prepared using the same method in samples in each group. The conventional pedicle screw (CPS) was inserted directly into the pilot hole in CPS group. In PMMA-PS group, the pilot hole was filled with PMMA followed by insertion of CPS. In EPS group, EPS was inserted directly into the pilot hole and the component elements were assembled to EPS. Twenty four hours after insertion of pedicle screw, X-ray and CT examination and axial pullout tests were performed to all OP biomechanical tests blocks and OP cadaveric lumbar vertebrae, and axial pullout tests, cyclic bending resistance tests, micro-CT analysis and histological observation were performed to all sheep lumbar vertebrae in vitro.2) Experiments in vivo. After successful establishment of OP sheep, sheep lumbar vertebrae (L1-L6) were randomly divided into CPS, PMMA-PS and EPS groups and treated with the same methods in experiment in sheep lumbar vertebrae in vitro. Four sheep were selected randomly and killed at two study periods of 3 months and 6 months after operation respectively and axial pullout tests, micro-CT analysis and histological observation were performed.Results:1) Experiments in vitro. No malpositioned screw and cement leakage were detected and all EPSs were obviously expanding in X-ray and CT examination.â‘ In OP biomechanical tests block, the maximum pullout strength (Fmax) and energy absorbed to failure (E) in PMMA-PS and EPS groups were all significantly higher than those in CPS group, but Fmax and E in EPS group were all significantly lower than those in PMMA-PS group.â‘¡In OP cadaveric lumbar vertebrae, Fmax and EAV in PMMA-PS and EPS groups were all significantly higher than those in CPS group, but there were no significant differences in both Fmax and EAV between EPS and PMMA-PS groups.â‘¢In sheep lumbar vertebrae in vitro, both axial stability and vertical stability of screws in PMMA-PS and EPS groups were significantly enhanced compared with those in CPS group, but there were no significances on both axial stability and vertical screw stability between EPS and PMMA-PS groups. Bone trabeculae wrapped up the screw directly forming a"screw-bone"interface in CPS group. PMMA was found surrounding the screw totally and existing between screw and bone and in cavitas medullaris surrounded screw, which hampered the direct contact between bone and screw and formed a"screw-PMMA-bone"interface in PMMA-PS group. In EPS group, bone trabeculae wrapped up the screw directly forming a"screw-bone"interface. Anterior part of EPS presented an obvious expansion in vertebral body and formed a clawlike structure. The two fins pressed the surrounding bone tissue, which made local bone tissue more compacted and denser than that around anterior part of screw in CPS group.2) Experiments in vivo.â‘ At 3-month and 6-month, Fmax and E in PMMA-PS and EPS groups were significantly higher than those in CPS group; however, there was no significant difference in both Fmax and E between EPS and PMMA-PS groups at two study periods. No significant differences were found in both Fmax and E in CPS and PMMA-PS groups between 3-month and 6-month, but Fmax and E in EPS group at 6-month were significantly higher than those at 3-month.â‘¡At 3-month and 6-month, bone trabeculae wrapped up the screw directly forming a"screw-bone"interface in CPS group. In PMMA-PS group, PMMA was found surrounding the screw totally and existing between screw and bone and in cavitas medullaris surrounded screw, which hampered the direct contact between bone and screw and formed a"screw-PMMA-bone"interface. In EPS group, bone trabeculae wrapped up the screw directly forming a"screw-bone"interface. Anterior part of EPS presented an obvious expansion and formed a clawlike structure. The two fins pressed and stimulated the growth of surrounding bone trabeculae, which made local bone condition (amount and density of bone trabeculae) significantly better than that in CPS group at 3-month and 6-month. From 3-month to 6-month, there were no obvious changes in the bone condition aound screw in CPS and PMMA-PS groups. PMMA was also found existing between bone and screw without any degradation and absorption, which formed a second interface-"screw-PMMA-bone". Newly formed bone wrapped up the expanding part of EPS and grew into the interspace between two fins from 3 months to 6 months, which significantly improved the bone condition and formed a better"screw-bone"interface.Conclusions:1) EPS can significantly increase screw stability and obtain the similar fixation strength of traditional PMMA-augmented pedicle screw.2) EPS can form a significant better interface and bone condition around screw compared with traditional PMMA-augmented pedicle screw, which significantly improve long term stability of screw in vivo.3) EPS can effectively avoid complications caused by using of PMMA such as thermal injury, leakage and neurologic compression and so on. As an effective, safe and easy method, EPS has great potentiality on wide application in clinic.