title: A new method of partial screw augmentation in sheep vertebrae in vitro: biomechanical and interfacial evaluation.
abstract: METHODS
  Partial screw augmentation was carried out by the injection of polymethylmethacrylate (PMMA) through the novel tap into the bone tissue in sheep vertebrae in vitro. Biomechanical tests and microcomputerized tomography analysis were done.
  
  
  OBJECTIVE
  To evaluate the stability of the screw and the interface between the bone and the screw in partial screw augmentation.
  
  
  BACKGROUND
  Augmentation of the pedicle screw with PMMA improves screw stability significantly. However, it results in the full envelopment of the screw by PMMA and the formation of a full "screw-PMMA-bone" interface, which may increase the difficulties in screw removal. Therefore, it is urgent to develop a novel method to augment screw fixation, which avoids full wrapping of PMMA around the screw and facilitates screw removal. We designed a novel tap with an inner hollow and the multiple lateral apertures.
  
  
  METHODS
  A total of 78 fresh adult sheep lumbar vertebrae were randomly divided into 3 groups. No screw augmentation was performed in the control group. The pilot hole was filled with PMMA in the full screw augmentation group, whereas PMMA was injected through the tap into the bone tissue in the partial screw augmentation group. Pedicle screws were ultimately inserted into all the holes. Axial pullout tests, cyclic bending tests, and microcomputerized tomography analysis were carried out 24 hours after screw fixation.
  
  
  RESULTS
  Both partial screw augmentation and full screw augmentation caused significant increase in pedicle screw stability compared with the control group in both the axial pullout tests and the cyclic bending resistance tests. Partial screw augmentation resulted in the formation of a partial "screw-bone" interface and a partial screw-PMMA-bone interface, which seems like 3 anchors or roots extending and fixing in the bone tissue in a cross section of the interface. It was obviously different from the complete screw-PMMA-bone interface with a complete PMMA mantle around the screw in the full screw augmentation group.
  
  
  CONCLUSIONS
  Our proof-of-concept study showed that partial screw augmentation could enhance pedicle screw fixation strength, avoided the full surrounding of the PMMA around the screw, and generated a partial screw-PMMA-bone and a partial screw-bone interface. Partial screw augmentation may be a potential method to augment pedicle screw stability.
authors.affiliation: Institution of Orthopaedics, Xijing Hospital, Fourth Military Medical University, Shaanxi Province, People's Republic of China.
authors.forename: Da
authors.initials: D
authors.lastname: Liu
authors.forename: Zi-Xiang
authors.initials: ZX
authors.lastname: Wu
authors.forename: Ming-Xuan
authors.initials: MX
authors.lastname: Gao
authors.forename: Shi-Yong
authors.initials: SY
authors.lastname: Wan
authors.forename: Lei
authors.initials: L
authors.lastname: Shi
authors.forename: Suo-Chao
authors.initials: SC
authors.lastname: Fu
authors.forename: Jun
authors.initials: J
authors.lastname: Wang
authors.forename: Wei
authors.initials: W
authors.lastname: Lei
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology; D001843:Bone Cements / Q000627:therapeutic use*; D066298:In Vitro Techniques; D016268:Internal Fixators / Q000592:standards*; D023421:Models, Animal; D019904:Polymethyl Methacrylate / Q000627:therapeutic use*; D011475:Prosthesis Failure / Q000009:adverse effects*; D012756:Sheep; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013131:Spine / Q000601:surgery*
chemical_list: D001843:Bone Cements; D019904:Polymethyl Methacrylate