title: Biomechanical evaluation of monosegmental pedicle instrumentation in a calf spine model and the role of fractured vertebrae in screw stability.
abstract: BACKGROUND
  Monsegmental pedicle instrumentation (MSPI) has been used to treat thoracolumbar fractures. However, there are few reports about the biomechanical characteristics of MSPI compared with traditional short-segment pedicle instrumentation (SSPI) in management of unstable thoracolumbar fractures, and the influence of vertebral fracture on screw stability is still unclear.
  
  
  METHODS
  This study was to compare the immediate stability between MSPI and SSPI in management of unstable L1 fracture, and to evaluate the role of fractured vertebrae in screw stability. Two studies were performed: in the first study, sixteen fresh calf spines (T11-L3) were divided into two groups, in which unstable fractures at L1 were produced and then instrumented with MSPI or SSPI respectively. The range of motion (ROM) and lax zone (LZ) of specimens were evaluated with pure moment of 6 Nm loaded. The second study measured and compared the pullout strength of screws inserted in to 16 intact and fractured vertebrae of calf spines (L1-3) respectively. The correlation of pullout strength with load sharing classification (LSC) of fractured vertebrae was analyzed.
  
  
  RESULTS
  No significant difference in the ROM and LZ of the destabilized segments after fixation between MSPI and SSPI, except in axial rotation of ROM (P < 0.05). After fatigue cyclic loading, the MSPI showed a significant increase of ROM during lateral bending and axial rotation (P < 0.05); however, there were no significant differences in the LZ during all loading models between groups (P > 0.05). The mean pullout strength of pedicle screws in fractured vertebrae decreased by 13.7%, compared with that of intact vertebrae (P > 0.05), and had a low correlation with LSC of the fractured vertebrae (r = 0.293, P > 0.05).
  
  
  CONCLUSIONS
  MSPI can provide effective immediate stability for management of unstable thoracolumbar fractures; however, it has less fatigue resistance during lateral bending and axial rotation compared with SSPI. LSC score of fractured vertebrae is not a major influence on the pullout strength of screws.
authors.affiliation: Department of Spine Surgery, the First Affiliated Hospital and Orthopedic Research, Institute of Sun Yat-sen University, Guangzhou, China.
authors.forename: Fuxin
authors.initials: F
authors.lastname: Wei
authors.affiliation: Department of Spine Surgery, the First Affiliated Hospital and Orthopedic Research, Institute of Sun Yat-sen University, Guangzhou, China.|The medical school of Shenzhen University, Shenzhen, China.
authors.forename: Zhiyu
authors.initials: Z
authors.lastname: Zhou
authors.affiliation: Department of Spine Surgery, the First Affiliated Hospital and Orthopedic Research, Institute of Sun Yat-sen University, Guangzhou, China.
authors.forename: Le
authors.initials: L
authors.lastname: Wang
authors.affiliation: Department of Spine Surgery, the First Affiliated Hospital and Orthopedic Research, Institute of Sun Yat-sen University, Guangzhou, China. gzsyliu@qq.com.
authors.forename: Shaoyu
authors.initials: S
authors.lastname: Liu
authors.affiliation: Department of Spine Surgery, the First Affiliated Hospital and Orthopedic Research, Institute of Sun Yat-sen University, Guangzhou, China.
authors.forename: Rui
authors.initials: R
authors.lastname: Zhong
authors.affiliation: Department of Spine Surgery, the First Affiliated Hospital and Orthopedic Research, Institute of Sun Yat-sen University, Guangzhou, China.
authors.forename: Xizhe
authors.initials: X
authors.lastname: Liu
authors.affiliation: Department of Spine Surgery, the First Affiliated Hospital and Orthopedic Research, Institute of Sun Yat-sen University, Guangzhou, China.
authors.forename: Shangbin
authors.initials: S
authors.lastname: Cui
authors.affiliation: Department of Radiology, the First Affiliated Hospital of Sun Yat-sen University, Guangzhou, China.
authors.forename: Ximin
authors.initials: X
authors.lastname: Pan
authors.affiliation: Department of Spine Surgery, the First Affiliated Hospital and Orthopedic Research, Institute of Sun Yat-sen University, Guangzhou, China.
authors.forename: Manman
authors.initials: M
authors.lastname: Gao
authors.affiliation: The medical school of Sun Yat-sen University, Guangzhou, China.
authors.forename: Yajing
authors.initials: Y
authors.lastname: Zhao
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000592:standards*; D002417:Cattle; D023421:Models, Animal; D016103:Spinal Fractures / Q000473:pathology / Q000601:surgery*; D013131:Spine / Q000473:pathology / Q000601:surgery