title: Locking Cap Designs Improve Fatigue Properties of Polyaxial Screws in Upper Extremity Applications.
abstract: OBJECTIVE
  This study sought to examine fatigue characteristics of 2 polyaxial locking screw designs: locking cap (LC) and cross-threaded (CT). The goal was to compare LC and CT implants at 0, 10, and 15 degrees of angulation to determine the effect of locking mechanism on screw-plate interface failure. The hypothesis was that LC implants would have superior fatigue properties in comparison to CT designs and that increased angulation of the screw would have a negative impact on the fatigue life of CT implants, but would not have any effect on LC implants.
  
  
  METHODS
  A total of 72 screws were tested in 4 upper extremity implants. Implants were subjected to cyclic shear loads and subsequent ramp to failure. Performance characteristics were statistically compared using nonparametric statistical methods.
  
  
  RESULTS
  Fatigue testing demonstrated that LC designs were consistently able to sustain a significantly higher number of cyclic loads than CT designs. There were no significant differences in the number of cycles sustained by LC designs because of changes in screw angle, but CT implants exhibited decreases in screw stability with increasing angulation.
  
  
  CONCLUSIONS
  Likely because of the spherical screw head geometry, LC fatigue characteristics are not influenced by the orientation of the screw relative to the plate. Application of an LC in the operating room requires additional time, but provides significantly more robust fixation of the screw, especially at oblique angles to the plate and provides a more predictable and consistent biomechanical result.
authors.affiliation: *Biedermann Lab for Orthopaedic Research, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, PA; †Department of Orthopaedic Surgery, Rothman Institute, Thomas Jefferson University, Philadelphia, PA; and ‡Center for Clinical Epidemiology and Biostatistics, Department of Biostatistics and Epidemiology, University of Pennsylvania, Philadelphia, PA.
authors.forename: Surena
authors.initials: S
authors.lastname: Namdari
authors.forename: Samir
authors.initials: S
authors.lastname: Mehta
authors.forename: Ann
authors.initials: A
authors.lastname: Tierney
authors.forename: Michael W
authors.initials: MW
authors.lastname: Hast
mesh_terms: D001863:Bone Screws; D019544:Equipment Failure Analysis; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D006801:Humans; D008422:Materials Testing; D011474:Prosthesis Design; D011475:Prosthesis Failure; D034941:Upper Extremity / Q000601:surgery*