title: Adjustment of suboptimally placed lumbar pedicle screws decreases pullout strength and alters biomechanics of the construct: a pilot cadaveric study.
abstract: BACKGROUND
  Lumbar pedicle screws are placed for internal fixation and help to enhance bony fusion. Optimal screws are medially directed, should be parallel or pointing to the superior endplate, and penetrate 50%-80% of the vertebral body. "Nonparallel" pedicle screws can be inadvertently placed within the confines of the pedicle and vertebral body but are sometimes replaced to obtain a more acceptable postoperative image. A nonparallel (suboptimal) screw is one that is located within the pedicle and body and does not violate bone; however, it is not parallel to the superior endplate. These "cored-out" grooves left in the bone from the initial tap and screw placement may compromise the integrity of the bone and the construct.
  
  
  METHODS
  Dual-energy x-ray absorptiometry scans and L4-5 laminectomies were performed on 6 fresh-frozen cadaveric lumbar spines. We placed 2 optimal pedicle screws in L4, 1 optimal screw in L5, and 1 suboptimal screw in L5 (construct A). Axial rotation, flexion/extension, and lateral bending were tested. The suboptimal screw was repositioned in an optimal trajectory and retested (construct B). Pullout strength was performed on optimal and revised L5 pedicle screws.
  
  
  RESULTS
  The mean axial rotation stiffness was 1.31 N-m/degrees ± 0.22 in construct A and 1.19 N-m/degrees ± 0.17 in construct B (P = 0.023; 95% CI [CI], 0.20-0.02). The mean lateral bending stiffness was 0.015 N/mm ± 0.002 in construct A and 0.016 N/mm ± 0.002 in construct B (P = 0.3; 95% CI, 0.0008-0.001). The mean flexion/extension stiffness was 0.0139 N/mm ± 0.002 in construct A and 0.0126 N/mm ± 0.002 in construct B (P = 0.01; 95% CI, 0.002-0.0004). Axial rotation and flexion/extension stiffness were significantly different between the 2 groups. The mean pullout strength was significantly higher in the nonrevised parallel screw group compared with the reimplanted parallel screw group (906.93 N ± 271.17 vs. 608.32 N ± 207.23, P = 0.031). Dual-energy x-ray absorptiometry imaging demonstrated 4 osteopenic and 2 osteoporotic specimens, although differences in bone mineral density did not play a significant role in assessing either the biomechanical parameters or the pullout strength.
  
  
  CONCLUSIONS
  Great care is warranted in the initial placement of lumbar pedicle screws. Revising a nonparallel screw placement decreases pullout strength and alters biomechanical movements (axial rotation and flexion/extension) in patients with decreased bone mineral density. If a screw is inadvertently placed nonparallel to the endplate but is within the confines of the pedicle and vertebral body with adequate bone purchase, it should not be revised and rather be left in its place.
authors.affiliation: Department of Neurosurgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
authors.forename: Rishi Kumar
authors.initials: RK
authors.lastname: Wadhwa
authors.affiliation: Department of Neurosurgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
authors.forename: Jai Deep
authors.initials: JD
authors.lastname: Thakur
authors.affiliation: Department of Neurosurgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
authors.forename: Imad Saeed
authors.initials: IS
authors.lastname: Khan
authors.affiliation: Department of Orthopedic Surgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
authors.forename: Jeremy
authors.initials: J
authors.lastname: James
authors.affiliation: Department of Neurosurgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
authors.forename: Osama
authors.initials: O
authors.lastname: Ahmed
authors.affiliation: Department of Neurosurgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
authors.forename: Shihao
authors.initials: S
authors.lastname: Zhang
authors.affiliation: Department of Radiology, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Henderson
authors.affiliation: Department of Orthopedic Surgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
authors.forename: Alan
authors.initials: A
authors.lastname: Ogden
authors.affiliation: Department of Neurosurgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA.
authors.forename: Bharat
authors.initials: B
authors.lastname: Guthikonda
authors.affiliation: Department of Neurosurgery, Louisiana State University Health Sciences Center, Shreveport, Louisiana, USA. Electronic address: ananda@lsuhsc.edu.
authors.forename: Anil
authors.initials: A
authors.lastname: Nanda
mesh_terms: D015502:Absorptiometry, Photon; D000368:Aged; D001696:Biomechanical Phenomena; D015519:Bone Density; D002102:Cadaver; D005260:Female; D006801:Humans; D007796:Laminectomy; D008161:Lumbosacral Region / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D010024:Osteoporosis / Q000473:pathology; D065289:Pedicle Screws; D010865:Pilot Projects; D012086:Reoperation; D021801:Zygapophyseal Joint / Q000601:surgery
keywords: Biomechanics; Lumbar pedicle screw; Pullout strength