title: Biomechanical evaluation of lumbar pedicle screws in spondylolytic vertebrae: comparison of fixation strength between the traditional trajectory and a cortical bone trajectory.
abstract: OBJECTIVE In the management of isthmic spondylolisthesis, the pedicle screw system is widely accepted surgical strategy; however, there are few reports on the biomechanical behavior of pedicle screws in spondylolytic vertebrae. The purpose of the present study was to compare fixation strength between pedicle screws inserted through the traditional trajectory (TT) and those inserted through a cortical bone trajectory (CBT) in spondylolytic vertebrae by computational simulation. METHODS Finite element models of spondylolytic and normal vertebrae were created from CT scans of 17 patients with adult isthmic spondylolisthesis (mean age 54.6 years, 10 men and 7 women). Each vertebral model was implanted with pedicle screws using TT and CBT techniques and compared between two groups. First, fixation strength of a single screw was evaluated by measuring axial pullout strength. Next, vertebral fixation strength of a paired-screw construct was examined by applying forces simulating flexion, extension, lateral bending, and axial rotation to vertebrae. RESULTS Fixation strengths of TT screws showed a nonsignificant difference between the spondylolytic and the normal vertebrae (p = 0.31-0.81). Fixation strength of CBT screws in the spondylolytic vertebrae demonstrated a statistically significant decrease in pullout strength (21.4%, p < 0.01), flexion (44.1%, p < 0.01), extension (40.9%, p < 0.01), lateral bending (38.3%, p < 0.01), and axial rotation (28.1%, p < 0.05) compared with those in the normal vertebrae. In the spondylolytic vertebrae, no statistically significant difference was observed for pullout strength between TT and CBT (p = 0.90); however, the CBT construct showed lower vertebral fixation strength in flexion (39.0%, p < 0.01), extension (35.6%, p < 0.01), lateral bending (50.7%, p < 0.01), and axial rotation (59.3%, p < 0.01) compared with the TT construct. CONCLUSIONS CBT screws are less optimal for stabilizing the spondylolytic vertebra due to their lower fixation strength compared with TT screws.
authors.affiliation: Department of Orthopaedic Surgery, National Defense Medical College, Tokorozawa, Saitama; and.
authors.forename: Keitaro
authors.initials: K
authors.lastname: Matsukawa
authors.affiliation: Department of Orthopaedic Surgery, National Hospital Organization, Murayama Medical Center, Musashimurayama, Tokyo, Japan.
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Yato
authors.affiliation: Department of Orthopaedic Surgery, National Defense Medical College, Tokorozawa, Saitama; and.
authors.forename: Hideaki
authors.initials: H
authors.lastname: Imabayashi
authors.affiliation: Department of Orthopaedic Surgery, National Defense Medical College, Tokorozawa, Saitama; and.
authors.forename: Naobumi
authors.initials: N
authors.lastname: Hosogane
authors.affiliation: Department of Orthopaedic Surgery, National Hospital Organization, Murayama Medical Center, Musashimurayama, Tokyo, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Asazuma
authors.affiliation: Department of Orthopaedic Surgery, National Defense Medical College, Tokorozawa, Saitama; and.
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Chiba
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D019544:Equipment Failure Analysis / Q000379:methods; D005260:Female; D020342:Finite Element Analysis; D006801:Humans; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D019637:Orthopedic Procedures / Q000295:instrumentation; D065289:Pedicle Screws; D011475:Prosthesis Failure; D013168:Spondylolisthesis / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D014057:Tomography, X-Ray Computed
keywords: CBT = cortical bone trajectory; FE = finite element; TT = traditional trajectory; cortical bone trajectory; degenerative; finite element analysis; fixation strength; isthmic spondylolisthesis; lumbar pedicle screw; spondylolysis