title: Biomechanical evaluation of parasagittal occipital plating: screw load sharing analysis.
abstract: METHODS
  Biomechanical evaluation of occipitocervical instrumentation techniques.
  
  
  OBJECTIVE
  Compare methods of occipital instrumentation by quantifying load sharing of occipital screws and measuring motion across instrumented occipitocervical spines.
  
  
  BACKGROUND
  Newer occipitocervical plate/screw systems that attach to longitudinal rods have been developed to improve fixation. These devices place screws in the center of occipital bone or off-midline. Midline plates offer screw purchase in thicker bone. Off-midline systems may increase the effective moment arm for torsional and lateral bending control. Measurement of screw loads within occipital plates is useful for determining optimal plate configuration.
  
  
  METHODS
  Ten cadaveric specimens (occiput-C4) were tested in flexion/extension (FE), lateral bending (LAT), and axial rotation (ROT) over +/-3 Nm pure moment. After intact testing, 4 occipitocervical fixation constructs were tested using washer load cells to assess loading across screws used to fix the plates to the occiput. Parasagittal occipital plates were positioned either convex or concave side facing medially. Each plate was first fixed using 3 screws (rostral, middle, caudal), then with the caudal screw eliminated (simulated failure). Range of motion (ROM) and peak screw loads are reported.
  
  
  RESULTS
  ROM decreased from intact to any of the 4 fusion plate configurations in FE, LAT, and ROT (P << 0.05), but not between plate configurations. Screw load significantly decreased from medially convex to medially concave configurations in LAT, but no significant changes were observed in FE or ROT. With caudal screws removed, middle screws peak loads significantly increased in FE and LAT (P < 0.05), but not ROT.
  
  
  CONCLUSIONS
  Occipital screw placement off-midline improves screw loads under lateral bending forces on occipitocervical constructs, though loads for FE and ROT are unchanged. As screws pullout, the loads may be redistributed, resulting in increased screw pullout forces above. Despite the improvement in screw loads for laterally based plates during lateral bending, overall ROM across the occipitocervical junction is unchanged.
authors.affiliation: Department of Orthopaedic Surgery, William Beaumont Hospital, Royal Oak, MI 48073, USA.
authors.forename: Todd J
authors.initials: TJ
authors.lastname: Frush
authors.forename: Theodore J
authors.initials: TJ
authors.lastname: Fisher
authors.forename: Scott C
authors.initials: SC
authors.lastname: Ensminger
authors.forename: Eeric
authors.initials: E
authors.lastname: Truumees
authors.forename: Constantine K
authors.initials: CK
authors.lastname: Demetropoulos
mesh_terms: D001696:Biomechanical Phenomena; D001860:Bone Plates; D001863:Bone Screws; D002102:Cadaver; D002574:Cervical Vertebrae / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D008875:Middle Aged; D009777:Occipital Bone / Q000503:physiopathology / Q000601:surgery*; D019637:Orthopedic Procedures / Q000295:instrumentation / Q000379:methods; D018583:Pliability; D016059:Range of Motion, Articular; D016103:Spinal Fractures / Q000503:physiopathology / Q000601:surgery; D016474:Weight-Bearing