title: Comparison of 2 kinds of pedicle screws in primary spinal instrumentation: biomechanical and interfacial evaluations in sheep vertebrae in vitro.
abstract: METHODS
  Expansive pedicle screw (EPS) and polymethylmethacrylate-augmented pedicle screw (PMMA-PS) were inserted in sheep vertebrae in vitro and were evaluated by performing biomechanical tests, radiographic examinations and histological observations.
  
  
  OBJECTIVE
  The objective of the study was to compare the biomechanical and interfacial performances of EPS and PMMA-PS in sheep lumbar vertebrae in vitro.
  
  
  BACKGROUND
  It is a great challenge for orthopedic surgeons performing transpedicular fixation in the osteoporotic spine. It was reported that either the EPS or PMMA-PS could increase the screw stability. However, there are no studies comparing the 2 kinds of screws especially in primary spinal instrumentation.
  
  
  METHODS
  A total of 60 sheep lumbar vertebrae were randomly divided into 3 groups. A pilot hole was made in advance in all samples using the same method. Thereafter, the conventional pedicle screw (CPS) was inserted directly into the pilot hole in the CPS group; the hole in PMMA-PS group was first filled with polymethylmethacrylate (PMMA; 1.0 mL) and then inserted with CPS; and the EPS was inserted directly into the vertebrae in EPS group. After a period of 24 hours, biomechanical tests were performed to evaluate screw stability, and x-ray examination, micro-computerized tomography analysis, and histologic observation were performed to evaluate the interface between screw and bone.
  
  
  RESULTS
  Compared with the stability of CPS, those of EPS and PMMA-PS were significantly enhanced. However, no significant differences were detected between the stabilities of EPS and PMMA-PS. The PMMA surrounding the screw blocked direct contact between bone and screw and formed a "screw-PMMA-bone" interface in the PMMA-PS group. There was a "screw-bone" interface in both CPS and EPS groups. Nevertheless, the expanded anterior part of EPS formed a claw-like structure pressing the surrounding bone trabeculae, which made the local bone tissue more compacted and denser than that in the CPS group.
  
  
  CONCLUSIONS
  EPS can enhance the screw stability as markedly as the traditional PMMA-PS in primary surgery, and EPS can form a better immediate interface between screw and bone compared with PMMA-PS. EPS also can effectively avoid thermal injury, leakage, and compression caused by PMMA. A great feasibility was proved in this study to perform comparisons between the 2 kinds of pedicle screws in osteoporotic sheep vertebrae in vivo in the further research. In conclusion, we propose that EPS has a great application potential in augmentation of screw stability in the clinic.
authors.affiliation: *Department of Orthopaedics †Affiliated Stomatological Hospital, General Hospital of Chengdu Military Region, Chengdu, Sichuan Province ‡Department of Orthopaedics, Xijing Hospital, Fourth Military Medical University, Xi'an, Shaanxi Province, China.
authors.forename: Da
authors.initials: D
authors.lastname: Liu
authors.forename: Yi
authors.initials: Y
authors.lastname: Zhang
authors.forename: Wei
authors.initials: W
authors.lastname: Lei
authors.forename: Cai-ru
authors.initials: CR
authors.lastname: Wang
authors.forename: Qing-yun
authors.initials: QY
authors.lastname: Xie
authors.forename: Dong-fa
authors.initials: DF
authors.lastname: Liao
authors.forename: Kai
authors.initials: K
authors.lastname: Jiang
authors.forename: Jin-song
authors.initials: JS
authors.lastname: Zhou
authors.forename: Bo
authors.initials: B
authors.lastname: Zhang
authors.forename: Xian-ming
authors.initials: XM
authors.lastname: Pan
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D015519:Bone Density; D001863:Bone Screws; D021621:Imaging, Three-Dimensional; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000502:physiology*; D008422:Materials Testing; D019637:Orthopedic Procedures / Q000295:instrumentation*; D019904:Polymethyl Methacrylate; D012756:Sheep; D013131:Spine / Q000601:surgery; D055114:X-Ray Microtomography
chemical_list: D019904:Polymethyl Methacrylate