title: Controlled motion with the XL-TDR lateral-approach lumbar total disk replacement: in vitro kinematic investigation.
abstract: OBJECTIVE
  Anterior-approach total disk replacement (TDR) devices are thought to retain close to so-called normal range of motion (ROM); however, they are also inherently unstable due to resection of the anterior longitudinal ligament and annulus. This instability/laxity is manifested as increased neutral zone (NZ) motion. The XL-TDR device (NuVasive, Inc., San Diego, California, United States) is implanted through a lateral approach that preserves the anterior ligamentous and annular structures. This potentially makes the XL-TDR device more stable than those delivered anteriorly. This study investigates XL-TDR biomechanical features in a cadaveric model.
  
  
  METHODS
  Biomechanical evaluation consisting of nondestructive multidirectional testing was performed with the hybrid protocol on six fresh-frozen cadaveric specimens (L2-S1). Motion segment kinematics were obtained using an optoelectronic system. Test conditions were (1) intact spine, (2) XL-TDR at L4-L5, and (3) XL-TDR at L4-L5 with anterior annulus/ligament resected. ROM and NZ were calculated for each condition in each loading direction (flexion-extension total, flexion alone, extension alone, lateral bending, and axial rotation).
  
  
  RESULTS
  Insertion of the XL-TDR device decreased ROM with respect to intact in all directions. NZ in all directions was not statistically different from intact (p < 0.05), although there was a trend toward decreased NZ in flexion (p = 0.078). Removing the anterior ligament/annulus increased ROM significantly with respect to the XL-TDR condition in all directions (p < 0.003). NZ also increased, with the most significant changes in extension, lateral bending, and axial rotation (p < 0.002).
  
  
  CONCLUSIONS
  The kinematics of XL-TDR have demonstrated that the retention of the anterior ligament/annulus has a significant stabilizing effect, diminishing ROM to less than intact, with a more controlled motion (more natural NZ). Removing the anterior ligament/annulus illustrated its stabilizing role, with ROM and NZ increasing significantly. Future studies will investigate the potential benefit of controlled XL-TDR motion on facet kinematics that may have clinical implications related to limiting facet degeneration.
authors.affiliation: Department of Minimally Invasive Surgery, Instituto de Patologia de Coluna, São Paulo, Brazil.
authors.forename: Luiz
authors.initials: L
authors.lastname: Pimenta
authors.affiliation: Department of Research and Testing, NuVasive, San Diego, California, United States.
authors.forename: Alex
authors.initials: A
authors.lastname: Turner
authors.affiliation: Department of Minimally Invasive Surgery, Instituto de Patologia de Coluna, São Paulo, Brazil.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Oliveira
authors.affiliation: Department of Minimally Invasive Surgery, Instituto de Patologia de Coluna, São Paulo, Brazil.
authors.forename: Luis
authors.initials: L
authors.lastname: Marchi
authors.affiliation: Department of Clinical Operations, NuVasive, San Diego, California, United States.
authors.forename: Bryan
authors.initials: B
authors.lastname: Cornwall
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000502:physiology / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000502:physiology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular / Q000502:physiology*; D013123:Spinal Fusion / Q000379:methods; D059145:Total Disc Replacement / Q000379:methods*; D055815:Young Adult