title: Biomechanical properties of anterior thoracolumbar multisegmental fixation: an analysis of construct stiffness and screw-rod strain.
abstract: METHODS
  Three types of anterior thoracolumbar multisegmental fixation were biomechanically compared in construct stiffness and rod-screw strain.
  
  
  OBJECTIVE
  To investigate the effects of rod diameter and rod number on construct stiffness and rod-screw strain in anterior thoracolumbar multisegmental instrumentation.
  
  
  BACKGROUND
  No studies have been undertaken to investigate the biomechanical effects of rod diameter and rod number in thoracolumbar anterior instrumentation.
  
  
  METHODS
  Ten fresh-frozen calf spines (T13-L5) were used. After intact analysis, a total discectomy and transection of the ALL and PLL were performed at L1-L2, L2-L3, and L3-L4 with intervertebral reconstruction using carbon fiber cages. Three types of anterior fixation were then performed at L1-L4: 1) 4.75-mm diameter single-rod, 2) 4.75-mm dual-rod, and 3) 6.35-mm single-rod systems. Single screws at each vertebra were used for single-rod and two screws for dual-rod fixation. These systems share the same basic design except rod diameter. Nondestructive biomechanical testing was performed and included compression, torsion, flexion-extension, and lateral bending. Construct stiffness and rod-screw strain of the three reconstructions were compared.
  
  
  RESULTS
  The 6.35-mm single-rod fixation significantly improved construct stiffness compared with the 4.75-mm single rod fixation only under torsion (P < 0.05). The 4. 75-mm dual rod construct resulted in significantly higher stiffness than did both single-rod fixations (P < 0.05), except under compression. No statistical differences were observed in rod-screw strain between the two types of single rods, whereas dual-rod reconstruction exhibited less rod-screw strain (P < 0.05).
  
  
  CONCLUSIONS
  For single-rod fixation, increased rod diameter neither markedly improved construct stiffness nor affected rod-screw strain, indicating the limitations of a single-rod system. In thoracolumbar anterior multisegmental instrumentation, the dual-rod fixation provides higher construct stiffness and less rod-screw strain compared with single-rod fixation.
authors.affiliation: Department of Orthopaedic Surgery, Hokkaido University School of Medicine, Sapporo, Japan. odaitr@aol.com
authors.forename: I
authors.initials: I
authors.lastname: Oda
authors.forename: B W
authors.initials: BW
authors.lastname: Cunningham
authors.forename: G A
authors.initials: GA
authors.lastname: Lee
authors.forename: K
authors.initials: K
authors.lastname: Abumi
authors.forename: K
authors.initials: K
authors.lastname: Kaneda
authors.forename: P C
authors.initials: PC
authors.lastname: McAfee
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002417:Cattle; D019245:Compressive Strength; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000601:surgery; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D013904:Thoracic Vertebrae / Q000601:surgery