title: Fatigue failure of reverse shoulder humeral tray components of a single design.
abstract: BACKGROUND
  Modularity in shoulder arthroplasty provides surgical flexibility and facilitates less-complex revision surgery. Modular designs must fit in the glenohumeral joint space, necessitating minimal thickness and careful material selection. The potential for fatigue fracture is higher, and fatigue fracture has been experienced by patients. The purpose of this study was to determine the impact of geometry and materials used for modular humeral trays from a single manufacturer.
  
  
  METHODS
  We consecutively retrieved 8 humeral trays of nearly identical designs: 4 Ti-6Al-4V (Ti) and 4 CoCrMo (CoCr). Optical microscopy and scanning electron microscopy were used, along with metallurgical techniques. Finite element and fatigue analyses of the stresses at the humeral tray taper informed observation interpretation.
  
  
  RESULTS
  Two Ti devices were revised for in vivo fracture. Scanning electron microscopy showed cracking in the other 2 Ti trays and no evidence of cracking in the CoCr components. A geometric difference in the CoCr devices resulted in a 25% decreased stress under simulated activities of daily living. Accounting for the tray material properties, the fatigue failure envelope ranged from 1000 to 1 million cycles for Ti and from 30,000 to >10 million cycles for CoCr.
  
  
  CONCLUSIONS
  All Ti humeral tray retrievals fractured in vivo or were cracked at the taper fillet. No CoCr retrievals showed signs of cracking. Finite element and fatigue analyses predict a 10-fold lifetime increase for the CoCr devices compared with the Ti devices. This study shows that fatigue failure is possible for some reverse shoulder components and is likely exacerbated by fillet radius, tray thickness, and material choice.
authors.affiliation: Thayer School of Engineering, Dartmouth College, Hanover, NH, USA. Electronic address: kathleen.a.lewicki.th@dartmouth.edu.
authors.forename: Kathleen A
authors.initials: KA
authors.lastname: Lewicki
authors.affiliation: Thayer School of Engineering, Dartmouth College, Hanover, NH, USA.
authors.forename: Audrey J
authors.initials: AJ
authors.lastname: Martin
authors.affiliation: Dartmouth-Hitchcock Medical Center, Lebanon, NH, USA.
authors.forename: John-Erik
authors.initials: JE
authors.lastname: Bell
authors.affiliation: Thayer School of Engineering, Dartmouth College, Hanover, NH, USA.
authors.forename: Douglas W
authors.initials: DW
authors.lastname: Van Citters
mesh_terms: D000203:Activities of Daily Living; D000368:Aged; D000072228:Arthroplasty, Replacement, Shoulder; D005260:Female; D006801:Humans; D006811:Humerus; D008297:Male; D008875:Middle Aged; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012086:Reoperation; D012785:Shoulder Joint; D000072227:Shoulder Prosthesis; D014025:Titanium
chemical_list: D014025:Titanium
keywords: Shoulder; arthroplasty; fatigue; fracture; humeral trays; rTSA