title: Effect of off-axis screw insertion, insertion torque, and plate contouring on locked screw strength.
abstract: OBJECTIVE
  This study quantifies the effects of insertion torque, off-axis screw angulation, and plate contouring on the strength of locking plate constructs.
  
  
  METHODS
  Groups of locking screws (n = 6-11 screws) were inserted at 50%, 100%, 150%, and 200% of the manufacturer-recommended torque (3.2 Nm) into locking compression plates at various angles: orthogonal (control), 5-degree angle off-axis, and 10-degree angle off-axis. Screws were loaded to failure by a transverse force (parallel to the plate) either in the same ("+") or opposite direction ("-") of the initial screw angulation. Separately, locking plates were bent to 5 and 10-degree angles, with the bend apex at a screw hole. Locking screws inserted orthogonally into the apex hole at 100% torque were loaded to failure.
  
  
  RESULTS
  Orthogonal insertion resulted in the highest average load to failure, 2577 ± 141 N (range, 2413-2778 N), whereas any off-axis insertion significantly weakened constructs (165-1285 N, at 100% torque) (P < 0.05). For "+" loading, torque beyond 100% did not increase strength, but 50% torque reduced screw strength (P < 0.05). Loading in the "-" direction consistently resulted in higher strengths than "+" loading (P < 0.05). Plate contouring of 5-degree angle did not significantly change screw strength compared with straight plates but contouring of 10-degree angle significantly reduced load to failure (P < 0.05).
  
  
  CONCLUSIONS
  To maximize the screw plate interface strength, locking screws should be inserted without cross-threading. The mechanical stability of locked screws is significantly compromised by loose insertion, off-axis insertion, or severe distortion of the locking mechanism.
authors.affiliation: *Department of Orthopaedic Surgery, Vanderbilt University School of Medicine, Nashville, TN; and †Department of Orthopaedic Surgery, Washington University School of Medicine, St. Louis, MO.
authors.forename: Bethany
authors.initials: B
authors.lastname: Gallagher
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Silva
authors.forename: William M
authors.initials: WM
authors.lastname: Ricci
mesh_terms: D001860:Bone Plates; D001863:Bone Screws; D019544:Equipment Failure Analysis; D011475:Prosthesis Failure; D013314:Stress, Mechanical; D019415:Torque