title: Stability potential of spinal instrumentations in tumor vertebral body replacement surgery.
abstract: METHODS
  The multidirectional stability potential of anterior, posterior, and combined instrumentations applied at L1-L3 was studied after L2 corpectomy and replacement with a carbon-fiber implant.
  
  
  OBJECTIVE
  To evaluate the biomechanical characteristics of short-segment anterior, posterior, and combined instrumentations in lumbar spine tumor vertebral body replacement surgery.
  
  
  BACKGROUND
  The biomechanical properties of many different spinal instrumentations have been studied in various spinal injury models. Only a few studies, however, investigate the stabilization methods in spinal tumor vertebral body replacement surgery.
  
  
  METHODS
  Eight fresh frozen human cadaveric thoracolumbar spine specimens (T12-L4) were prepared for biomechanical testing. Pure moments (2.5 Nm, 5 Nm, and 7.5 Nm) of flexion-extension, left-right axial torsion, and left-right lateral bending were applied to the top vertebra in a flexibility machine, and the motions of the L1 vertebra with respect to L3 were recorded with an optoelectronic motion measurement system after reconditioning. The L2 vertebral body was resected and replaced by a carbon-fiber cage. Different fixation methods were applied to the L1 and L3 vertebrae. One anterior, two posterior, and two combined instrumentations were tested. Load-displacement curves were recorded and neutral zone and range of motion parameters were determined.
  
  
  RESULTS
  The anterior instrumentation provided less potential stability than the posterior and combined instrumentations in all motion directions. The anterior instrumentation, after vertebral body replacement, showed greater motion than the intact spine, especially in axial torsion (range of motion, 10.3 degrees vs 5.5 degrees; neutral zone, 2.9 degrees vs. 0.7 degrees; P < 0.05). Posterior instrumentation provided greater rigidity than the anterior instrumentation, especially in flexion-extension (range of motion, 2.1 degrees vs. 12.6 degrees; neutral zone, 0.6 degrees vs. 6.1 degrees; P < 0.05). The combined instrumentation provided superior rigidity in all directions compared with all other instrumentations.
  
  
  CONCLUSIONS
  Posterior and combined instrumentations provided greater rigidity than anterior instrumentation. Anterior instrumentation should not be used alone in vertebral body replacement.
authors.affiliation: Department of Orthopaedics, Medizinische Hochschule Hannover, Germany.
authors.forename: M J
authors.initials: MJ
authors.lastname: Vahldiek
authors.forename: M M
authors.initials: MM
authors.lastname: Panjabi
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001863:Bone Screws; D002244:Carbon; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000502:physiology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D018583:Pliability; D019736:Prostheses and Implants; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation*; D013904:Thoracic Vertebrae / Q000502:physiology / Q000601:surgery*; D016474:Weight-Bearing
chemical_list: D002244:Carbon