title: [Biomechanical study on effects of bone mineral density on fixation strength of expansive pedicle screw].
abstract: OBJECTIVE
  To evaluate the fixation strength of expansive pedicle screw (EPS) at different bone mineral density (BMD) levels, further to provide theoretical evidence for the clinical application of the EPS in patients with osteoporosis.
  
  
  METHODS
  Fresh human cadaver spines (T12-L5 spines) were divided into 4 levels: normal BMD, osteopenia, osteoporosis, and severe osteoporosis according to the value of BMD, 12 vertebra in each level. Conventional pedicle screw (CPS) or EPS was implanted into the bilateral vertebra in CPS group and EPS group, respectively, 12 screws in each group per BMD level. Screw pullout tests were conducted. The maximum pullout strength, stiffness, and energy absorption were determined by an AG-IS material testing machine with constant rate of loading in a speed of 5 mm/min.
  
  
  RESULTS
  With the decline of BMD from normal to severe osteoporosis level, the maximum pullout strength and the stiffness correspondingly declined (P < 0.05). In CPS group, the energy absorption gradually decreased (P < 0.05); in EPS group, significant difference was found between other different BMD levels (P < 0.05) except between normal BMD and osteopenia and between osteoporosis and severe osteoporosis (P > 0.05). At the same BMD level, the maximum pullout strength of EPS group was significantly larger than that of CPS group (P < 0.05); the stiffness of EPS group was significantly higher than that of CPS group (P < 0.05) except one at normal BMD level; and no significant difference was found in the energy absorption between 2 groups (P > 0.05) except one at osteopenia level. No significant difference was found in maximum pullout strength, stiffness, and energy absorption between EPS group at osteoporosis level and CPS group at osteopenia level (P > 0.05); however, the maximum pullout strength, stiffness, and energy absorption of EPS group at severe osteoporosis level were significantly lower than those of CPS group at osteopenia level (P < 0.05).
  
  
  CONCLUSIONS
  Compared with CPS, the EPS can significantly improve the fixation strength, especially in patients with osteopenia or osteoporosis.
authors.affiliation: Department of Orthopedics, Orthopedic Office of Research of Chinese PLA, Lanzhou General Hospital of Lanzhou Military Command, Lanzhou Gansu, 730050, P R China.
authors.forename: Mingxuan
authors.initials: M
authors.lastname: Gao
authors.forename: Xusheng
authors.initials: X
authors.lastname: Li
authors.forename: Ping
authors.initials: P
authors.lastname: Zhen
authors.forename: Zhigang
authors.initials: Z
authors.lastname: Wu
authors.forename: Shenghu
authors.initials: S
authors.lastname: Zhou
authors.forename: Qi
authors.initials: Q
authors.lastname: Tian
authors.forename: Wei
authors.initials: W
authors.lastname: Lei
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D015519:Bone Density; D001863:Bone Screws; D002102:Cadaver; D005260:Female; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D006801:Humans; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000473:pathology / Q000601:surgery*; D008297:Male; D008422:Materials Testing; D008875:Middle Aged; D010024:Osteoporosis / Q000473:pathology / Q000601:surgery*; D013904:Thoracic Vertebrae / Q000473:pathology / Q000601:surgery*