title: Scapula fracture incidence in reverse total shoulder arthroplasty using screws above or below metaglene central cage: clinical and biomechanical outcomes.
abstract: BACKGROUND
  Reverse total shoulder arthroplasty (RTSA) is a viable treatment option for rotator cuff tear arthropathy but carries a complication risk of scapular fracture. We hypothesized that using screws above the central glenoid axis for metaglene fixation creates a stress riser contributing to increased scapula fracture incidence. Clinical type III scapular fracture incidence was determined with screw placement correlation: superior screw vs. screws placed exclusively below the glenoid midpoint. Cadaveric RTSA biomechanical modeling was employed to analyze scapular fractures.
  
  
  METHODS
  We reviewed 318 single-surgeon single-implant RTSAs with screw correlation to identify type III scapular fractures. Seventeen cadaveric scapula specimens were matched for bone mineral density, metaglenes implanted, and fixation with 2 screw configurations: inferior screws alone (group 1INF) vs. inferior screws with one additional superior screw (group 2SUP). Biomechanical load to failure was analyzed.
  
  
  RESULTS
  Of 206 patients, 9 (4.4%) from the superior screw group experienced scapula fractures (type III); 0 fractures (0/112; 0%) were identified in the inferior screw group. Biomechanically, superior screw constructs (group 2SUP) demonstrated significantly (P < .05) lower load to failure (1077 N vs. 1970 N) compared with constructs with no superior screws (group 1INF). There was no significant age or bone mineral density discrepancy.
  
  
  CONCLUSIONS
  Clinical scapular fracture incidence significantly decreased (P < .05) for patients with no screws placed above the central cage compared with patients with superior metaglene screws. Biomechanical modeling demonstrates significant construct compromise when screws are used above the central cage, fracturing at nearly half the ultimate load of the inferior screw constructs. We recommend use of inferior screws, all positioned below the central glenoid axis, unless necessary to stabilize the metaglene construct.
authors.affiliation: Department of Orthopaedic Surgery, Medical College of Georgia at Augusta University, Augusta, GA, USA.
authors.forename: Justin C
authors.initials: JC
authors.lastname: Kennon
authors.affiliation: Department of Orthopaedic Surgery, Medical College of Georgia at Augusta University, Augusta, GA, USA.
authors.forename: Caroline
authors.initials: C
authors.lastname: Lu
authors.affiliation: Department of Orthopaedic Surgery, Medical College of Georgia at Augusta University, Augusta, GA, USA.
authors.forename: Meghan E
authors.initials: ME
authors.lastname: McGee-Lawrence
authors.affiliation: Department of Orthopaedic Surgery, Medical College of Georgia at Augusta University, Augusta, GA, USA. Electronic address: lycrosby@augusta.edu.
authors.forename: Lynn A
authors.initials: LA
authors.lastname: Crosby
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000072228:Arthroplasty, Replacement, Shoulder / Q000009:adverse effects*; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000009:adverse effects*; D002102:Cadaver; D005260:Female; D005593:Fracture Fixation, Internal / Q000379:methods*; D050723:Fractures, Bone / Q000453:epidemiology* / Q000209:etiology / Q000503:physiopathology; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D011183:Postoperative Complications / Q000453:epidemiology* / Q000209:etiology / Q000503:physiopathology; D012086:Reoperation; D012540:Scapula / Q000293:injuries* / Q000601:surgery; D012785:Shoulder Joint / Q000503:physiopathology / Q000601:surgery*
keywords: Reverse total shoulder; arthroplasty; biomechanical; clinical; complication; metaglene; outcomes; scapula fracture